Sweet corn growing

Date: Aug 2007   Author: Clarrie Beckingham

Introduction

The sweet corn industry in New South Wales is expanding - benefiting from increasing domestic consumption, export development and import replacement. Also, it is an attractive crop for producers to grow because:

  • the plant grows quickly and is considered a valuable rotational crop;
  • farming operations can be mechanised.

Most sweet corn in New South Wales is grown for the processing sector, ending up on the supermarket shelves as products, which include canned kernels, frozen cobetts and frozen kernels.

Planting usually commences in spring when soil temperatures reach 14–16°C. In warmer coastal and inland districts, a longer growing season allows two crops to be planted each year compared with one crop in the cooler Tablelands and Slopes districts.

Successful sweet corn production relies on:

  • management planning well in advance of sowing (for instance, planting to ensure that harvesting meets the most profitable market niche and processor schedules);
  • thorough research to identify market requirements, impediments to production and, especially, product specifications;
  • good cultural management tailored to suit particular cultivars, with special attention given to harvesting technologies, establishment, plant population, fertiliser, irrigation, soil management and pest and disease control.

The sweet corn industry has recently achieved important advances in production and marketing through benchmarking, crop monitoring and adopting quality management. There are some important developments that demonstrate the confidence and progressive attitude of the industry.

These include:

  • development of export markets, particularly to Asia;
  • a steady increase in total production and ‘per hectare’ production;
  • rising ‘per capita’ consumption;
  • adoption of world’s best practice by the industry, which is already identified as a technology and market innovator;
  • new supersweet cultivars are expected to lead to increasing demand and a strengthening of the industry’s position in the marketplace;
  • development of new cultivars that show greater insect and disease tolerance and product style.

Sweet corn classification*

  • Class : Monocotyledon
  • Order : Gramineae (Grass family)
  • Family : Corn
  • Genus/species : Zea mays var. rugosa

*Source: Australian Vegetable Growing Handbook 1999

Corn is thought to have originated in Mexico. The ancient Inca, Mayan and Aztec civilisations are credited as being the first to domesticate corn as a crop. Archaeological excavations in Mexico unearthed wild corn cobs which were dated about 5000 BC, and cultivated corn cobs about 2000 BC.

Sweet corn differs from other corns (field maize, popcorn and ornamental) because the kernels have a high sugar content in the milk or early dough stage.

Industry statistics

Sweet corn is grown in most Australian States, with New South Wales producing just over half of the national production. The dominant position of New South Wales and its processing industry is expected to continue. Figure 1 shows the major sweet corn growing areas in Australia.

Table 1 shows the level of Australian production in 2002.

 NSWQldVic.Tas.WASANTAust.
Tonnes 41 377 6905 30 226 106 1197 656 N/A 80 467
Hectares 2408 1697 743 321 163 96 6 5434
Average yield (t/ha) 14.2 8.7 9.9 16.7 10.2 13.5 1.6 11.9

Source: Australian Bureau of Statistics (ABS)

Figure 2

Yields: In New South Wales and Queensland, a yield of 17 tonnes per hectare (t/ha) is average for processing crops, with the range being 12–22 t/ha. Yields for fresh market crops range from 600 to 1500 cartons per hectare (ctn/ha) with an average of 1000 ctn/ha. One 18 L carton holds about 9 kg or 24 to 30 whole cobs.

Australians are also eating more sweet corn. As Figure 2 shows, per capita consumption jumped 75% throughout the 1980s and early 1990s. This per capita consumption is, however, much lower than in the United States, where each American eats an average of 12 kg of sweet corn a year. In the United States, sweet corn is ranked the third most popular vegetable for consumption, while in Australia it is ranked 10th.

Most of the sweet corn eaten in Australia is grown here. Occasionally, though, processors will import sweet corn to overcome a local shortfall. Imported products are usually frozen or canned. Table 2 gives an indication of the proportion of imports versus exports in the sweet corn trade.

Table 2. Estimated sweet corn production and use in Australia (tonnes)

  Commercial
production
ImportsExports
1995–96 81 898 18 576 n/a
1991–92 50 934 17 718 710
1982–83 31 718 80

Source: ABS

Australia also exports an increasing amount of frozen or canned kernel, frozen cob, long-life vacuum-sealed cobs and fresh cobs. The important export markets are Japan, South-East Asia and Europe. The United States and Japan have been the most important growth areas for Australian exporters.

The processing industry

About 80% of sweet corn production goes to the processing sector rather than the fresh food market. Significant recent developments in this sector have been:

  • internationally competitive products;
  • adopting best practice and international benchmarking programs;
  • adopting quality management systems.

Growers and processors actively support research and development.

Simplot Australia Pty Ltd is the only processor left in NSW, located at Bathurst. Sweet corn production for the Simplot factory is centred at two main growing areas of NSW - the Central West and the Riverina (see Table 3).

Table 3. Processing production in New South Wales

Company Production areas SowingHarvestProducts Main cultivars
Simplot Australia Pty Ltd (Bathurst) Bathurst Nov–Dec Mar–Apr Frozen and canned Jubilee and some supersweet
Cowra Oct–Jan Jan–May    
Riverina Sept–Jan Dec–Apr    

In 2002, the New South Wales sweet corn gross value of production for processing crops was estimated to be valued at $7.0M.

Sweet corn is processed into the following products:

  • Frozen
    • cobetts (portions of whole cobs)
    • whole cobs
    • whole kernels
  • Canned
    • whole kernels
    • cream style
  • Other
    • soups
    • mixed vegetable packs
    • Cornjacks®
    • long-life vacuum-packed cobs (using a special laminated plastic bag to withstand heat during processing).

Kernel products are the most popular processed lines, with frozen whole cob and cobett products expected to improve their market share.

Cob quality is important for processors. Cultivars and cannery intakes are assessed for:

  • mechanical damage
  • field yield
  • percentage trash (husk and stalk)
  • percentage end-fill
  • percentage insect and disease damage
  • maturity
  • cob colour, flavour, texture and appearance
  • percentage moisture
  • cob weight, length and diameter
  • pollination in the field
  • trueness to type
  • importantly, suitability for processing machinery.

Kernel characteristics, both visual and internal, are also taken into account during cultivar assessment and factory intake.

Baby corn

Baby corn is harvested and processed by hand from cobs while very young. Harvesting takes place usually at or within 2 days of silks emerging. Cob size is usually 4–10 cm long and 1–1.5 cm in diameter.

The production of baby corn is very labor-intensive, and investigations into mechanisation of harvesting and processing are underway.

Selecting the best cultivars for baby corn production is important. Factors considered when selecting a cultivar include high cob numbers, disease resistance, high yield, good row arrangement and a strong yellow cob colour. Both su and sh2 types are used. In Thailand, where there is a successful baby corn industry, plants are usually spaced 45 cm × 1 m.

The fresh market industry

Figure 3

Sweet corn for the fresh market has traditionally been grown in major vegetable production areas close to wholesale markets. In New South Wales, the Sydney Basin (Hawkesbury-Nepean Valley), the Hunter Valley and coastal districts are long-established growing areas. Inland processing crop areas also produce for the fresh market.

Fresh market production varies from year to year, making up between 10% and 20% of total production in New South Wales.

The Sydney Market at Flemington is the major centre for marketing fresh produce in Australia. The supply of fresh corn to the Sydney Market has increased significantly each year as the popularity of supersweet corn rises. Figure 3 shows the intake of sweet corn at the Sydney Market from 1991–97. Table 4 shows there has been an overall increase of 143% in intake since 1993.

Table 4. Intake of sweet corn (tonnes) at the Sydney Market, by State

State19932000
NSW 1594 2096
Vic. 565
Qld 2787 10 739
SA 10 47
WA n/a
Total 4956 12 882

Containers used for fresh market sweet corn include:
Half cartons: 18 L, 9 kg
Cartons: 32 L, 18 kg
Bags: 18 kg

Prices and supply

Figure 4

For up-to-date prices and details on deliveries to the Sydney Market (previously known as Flemington Market), contact the Sydney Market Reporting Service on (02) 9746 3437 or 0416 108 639.

Observations by the Sydney Market Reporting Service show that supply can fluctuate with weather and competition from other marketing channels. Figure 4 illustrates the fluctuations which can occur over a 1-year period.

Information is available (at the following links) on the prices and throughput of sweet corn at the Sydney Market for the year 2000 and for the months January to July of 2001. This information has been compiled by Ausmarket Consultants from information collected by the Sydney Market Reporting Service:

Typical sowing times in the Sydney Basin for fresh market crops

Plant in mid-August to mid-October, which is usually the cut-off date for disease-susceptible cultivars. Resistant cultivars can be planted until mid-February. In favourable seasons, harvesting can be extended into late May.

Sequential planting for the fresh market is usually planned to ensure a regular supply of cobs over an extended marketing period.

In the Sydney Basin, foggy, misty mornings followed by bright, sunny days can predispose kernels to rupture, especially in some supersweet cultivars. Also, some newer cultivars may be susceptible to mosaic virus disease. It is important to thoroughly research the features of a new cultivar before planting, and then trialling it in a small way before undertaking a larger commercial planting.

Typical sowing times for inland fresh market crops

Typical sowing times for inland fresh market crops are similar to those listed for processing production in Table 3.

Consumer preferences

The introduction of supersweet (extra sweet) cultivars, both uniform golden yellow and the bicolour white–yellow cultivars, has created new local and export market opportunities. Supersweets are reported to comprise 90% or more of fresh market sales.

(On left) Bicolour supersweet cultivars are popular in important Asian markets.
(On right) The line-up of all-yellow kernel cultivars is increasing as supersweet types become more popular.

In Australia, consumer surveys have shown that older generations prefer normal sweetness, and young generations prefer supersweets. Industry sources predict that per capita consumption of supersweets will increase. Supersweets (such as sh2 types) offer improved keeping quality by maintaining their sweetness for a longer time after harvest.

However, careful choice and local trialling of cultivars is important, as yield and cob quality can vary between cultivars and districts.

Compared with standard sweet corn cultivars (su types) that can lose their crispness and sweetness a few days after harvest, supersweet corn kernels contain more sugar and convert it to starch less rapidly. Sh2 supersweet corn cultivars are less creamy than su sweet corn cultivars.

Baby corn is also becoming more popular, with most supplies coming from Thailand.

Consignment and transport

Before consigning fresh market sweet corn, it is necessary to be sure of buyers’ specifications for product, packing, handling and cool-chain management.

Fresh sweet corn is highly perishable in hot weather. For every 5°C increase in pulp temperature, sugar breakdown to starch doubles, meaning that sweet corn can completely lose flavour in a short period of time.

Within a few days at ambient temperatures, husks also dry out very quickly, silks dry and darken, and cobs ‘sweat’ and caramelise, resulting in significant quality deterioration.

The recommended pulp temperature throughout the cool chain (farm to consumer) is 0–2°C. Growers, packers, market agents and retailers should use temperature probes to monitor pulp temperatures. Sweet corn should also be stored in high humidity (98%) to optimise shelf life.

Domestic market

Fresh corn is almost exclusively supersweet in the major markets. Good quality, attractive packaging and presentation has proved popular in the Sydney Market, where a 95% increase in receivals has been recorded between 1994–95 and 1996–97.

Presentation is important and buyers look for quality and evidence of good packing. Usually, there are 24 or 30 cobs (with shanks trimmed) in a 9 or 10 kg 18 L white polystyrene carton (shown at right ). Growers can also use 15 kg bags or 500 kg bins. Cobs need to have a short stalk and the husks should be fresh, clean and green. Prepacks of poly or microwaveable trays of 2–4 cobs per pack are also sold with 12 trays per 18 L cellopack carton. In the prepacks, cobs are almost completely dehusked and trimmed or ‘topped and tailed’. There should be no insect or disease damage. Complete kernel fill to the tip of cobs is important for appearance, and cobs must be uniform and a good size.

Sweet corn sold in wholesale markets in New South Wales must be labelled or branded with the packer’s name and address.

Most consumers buy on looks, and sweetness is not a priority. Supersweet consignments are, however, increasing in volume. While bicolours may taste better, they traditionally have not sold as well as a uniform, golden-yellow-coloured cob, as consumers generally think that the white kernels indicate that the cob has ‘gone off’. Market promotion may change this trend. Ice topping attracts a premium, and consignments are best pressure-cooled, ice-packed and consigned in refrigerated trucks to ensure pulp temperatures remain at 0–2°C.

Export market

The supersweet cultivars have increased sweet corn’s popularity in many countries. This has opened up new export development opportunities, especially in Japan where fresh packs and long-life vacuum packs are popular. Bicolour and supersweet cultivars are preferred. However, buyer preferences can vary, and research of markets and product specifications is important. Export agents—Austrade, grower organisations, and NSW Department of Primary Industries (NSW DPI)—are sources of assistance for local growers and processors.

A good reference publication for anyone considering exporting sweet corn is Vegetables to Asia, NSW Agriculture Seminar, available from the Gosford Horticultural Institute, Narara, on (02) 4348 1900.

Economics

An example of a gross margin for a Western Australian supersweet crop for fresh market export is shown in Table 5 and has been extracted from the Agriculture WA publication Vegetable Budgeting Handbook for the Manjimup District by P. Gartrell (October 1997). The information is based on a 10 hectare operation. The price is based on prefreight, pregrading and prepackaging costs.

Table 5. Gross margin, costs and returns

Income 14 400 kg (5400 kg is wastage) @ $0.69c $10 000
Costs Growing 30.0% $1 416
  Repairs, maint., fuel 30.7% $1 452
  Labour 30.0% $1 419
  Packing, grading, freight 4.8% $228
  Interest, commission 4.5% $214
Gross margin a. Income $10 000
  b. Direct costs $4 728
  c. Gross margin (a – b) $5 272

Sensitivity analysis

The effect of ‘yield’ and ‘price’ on ‘gross margin’ in Table 6 shows the effect on ‘gross margin per hectare’, given variations in seasonal and market conditions impacting on both yield and price as indicated.

Table 6. Gross margin sensitivity ($/ha)

Marketable yield (kg/ha) Average price ($/kg)
$0.56 $0.63 $0.69 $0.76 $0.83
12 240 2195 3045 3895 4745 5595
14 400 3272 4272 5272 6272 7272
16 560 4348 5498 6648 7798 8948

Cultivars

Development of improved varieties (also known as ‘cultivars’) is an industry priority. Processors and fresh market buyers have specific characteristics they see as important. Before planting, growers should ensure that the cultivar they have chosen meets market requirements.

All commercial sweet corn is of hybrid cultivars with the following attributes:

  • high yield
  • good kernel set
  • sweetness depending on gene type
  • resistance to disease.

A total of 34 varietal characteristics are used to evaluate cultivars.

Processing cultivars

Special sweet corn hybrids have been developed for processing. These hybrids have:

  • a small core
  • a deep kernel
  • an even shape to facilitate shelling
  • a blunt tip to lessen wastage.

Cobs used for freezing need to look attractive with good colour and straight rows. The kernel should be tasty and have a thin, tender seed coat or pericarp.

Tillers or suckers on plants are not usually desirable, and plants should set cobs at least 60 cm high to make mechanical harvesting easier. If more than one cob is formed, it is best if the cobs mature at the same time. Husk cover should be sufficient to discourage insect attack but loose enough when processed to strip easily at the factory. Processors dislike cultivars with non-white silks, as these look unattractive in the canned product.

Other characteristics considered important include maturity, seed and plant performance.

Fresh market cultivars

For the fresh market, it is particularly important for the cultivar to maintain its quality after cutting or harvesting, and a tight, attractive green-coloured husk is desirable. This is in contrast to processors’ requirements and highlights the need for growers to carefully select cultivars according to why and where it is to be grown.

Standard or normal sugary (su gene)

These are most commonly grown for processing. Typically, these corns have more vigorous seedling vigour than supersweets and also have a shorter harvest window of 2–5 days and shorter shelf life of 2–4 days, depending on growing conditions and post-harvest handling.

Supersweets (sh2 and se genes)

An increasing range of high-sugar cultivars are available and these are becoming more popular with consumers. These new cultivars can be described as ‘supersweet’, ‘bisweet’ or ‘extra sweet’. Seed for these high-sugar cultivars is described as ‘shrunken’, ‘sugar enhanced’ or ‘sweet’.

Most popular as fresh market cultivars and increasingly of interest to processors, the seedlings of Supersweets are usually less vigorous because of low stored starch levels in the seed. Supersweets have a longer harvest window of 5–12 days and a shelf life of 4–10 days, depending on growing and post-harvest handling. In the United States, supersweets account for 20% of processing cultivars and all fresh market cultivars planted. In New Zealand, supersweets make up half of total production.

History of supersweets

Dr J. R. Laughnan at the University of Illinois first described a field corn with more sugar than normal sweet corn. This high-sugar corn grew from seed that was generally light in weight with a shrivelled appearance. The gene that produced this mutant corn was named ‘shrunken’.

‘Shrunken two’ (sh2) gene

‘Shrunken two’ (sh2) cultivars produce corn with a sugar content up to twice that of standard corns at peak maturity. The sh2 corns may also have a higher sugar content than a standard cultivar 7 days after harvest.

The sh2 corns have a tough pericarp (seed coat) that gives the corn a crisp texture when eaten.

‘Sugary enhanced’ (se) gene

Dr A. M. Rhodes, also from the University of Illinois, later produced a breeding line that was very sweet with a tender or creamy texture. Described by Rhodes as ‘sugary enhanced’ (se) this corn did not have to be isolated from other standard sweet corns. Se cultivars are believed to be easier to grow than sh2 types; however, the tender skin of se hybrids could predispose them to diseases.

Other genes and combinations

Plant breeders are also combining se and sh2 types to suit specific sugar level requirements and the need to avoid isolation from standard corns. Other genes developed include brittle and sweeter types.

Isolation requirements

It is important to prevent starchy kernels being produced by cross-pollination of supersweet cultivars and standard sweet corns or other corns. Isolation from standard corns can be achieved by planting supersweets at least 400 m from standard sweet corns or by staggering plantings to have at least 14–21 days between pollination times. It is therefore vital to know the pollination times or flowering times for different cultivars.

Note: All sweet corn must be isolated from field corn (maize), popcorn or other corn types.

List of cultivars

Table 7 lists the cultivars commonly used for processing (P) and fresh markets (F). This list will no doubt change as some lines fall out of favour and new ones replace them. Contact the seed companies listed in the table for more details on the full range of cultivars available.

Table 7. Sweet corn cultivars grown in New South Wales in 1998

VarietyType Seed company Maturity (days)(ii) Comments
Empire
(P)(i)
Standard
su
Lefroy Valley 98 Used for processing. High yields and sugar levels. For freezing and canning.
Jubilee
(P)
Standard
su
Syngenta 90–96 Main processing variety. Good tip fill and tender pericarp. High yields.
Heritage (P) Standard
su
Syngenta 90–96 Important processing variety with high yields and rust tolerance.
Miracle
(F)
Standard
su
Lefroy Valley 75 Fresh market. Has some good cold tolerance.
Punch
(P)
Standard
su
Lefroy Valley 98 Popular in New Zealand for frozen cobetts. For freezing and canning.
Golden Millennium
(P)
Standard
su
Syngenta 92–98 Vigorous high-yielding, with tolerance to rust and turcica.
Rosella 425
(P)
Standard
su
Lefroy Valley 93 Mid to full season. Cold tolerant, rust tolerant. For canning or freezing.
Tungsten
(P)
Standard
su
Lefroy Valley 90–96 Full rust resistance, good colour and cob characteristics. For canning and freezing.
Challenger
(F, P)
Supersweet
sh2
Yates Vegetable Seeds 80–85 Yellow kernels, good yields. Resistant to Northern Leaf Blight. Suitable for freezing.
Florida Staysweet
(F)
Supersweet
sh2
Pacific Seeds 80–85 Supersweet with good shelf life and uniform cobs for fresh market.
Florida Supersweet
(F)
Supersweet
sh2
Pacific Seeds 80–85 Cob characteristics of Florida Staysweet plus rust resistance.
Gladiator
(F)
Supersweet
sh2
Lefroy Valley 90–98 Popular late season cultivar and is also grown full season.
Goldensweet
(F)
Supersweet
sh2
Lefroy Valley 85–97 Semi-processor. Main variety in the domestic fresh market. Main season. Rust resistant.
Headstart
(F)
Supersweet
sh2
Lefroy Valley 72–78 High vigour, high quality yellow supersweet. Seven days earlier than Goldensweet.
Honeysweet
(F)
Supersweet
sh2
Lefroy Valley 80–85 Earliest maturing supersweet. Excellent vigour.
Sovereign
(F, P)
Supersweet
sh2
Syngenta 86–90 Good early season cultivar. Suit processing and prepacking. Rust and turcica tolerant.
Krispy King
(F, P)
Supersweet
sh2
Syngenta 86–90 Good early season cultivar. Suit processing and prepacking. Turcica tolerant.
Pacific H5
(F, P)
Supersweet
sh2
Pacific Seeds 90–95 Resistant to Johnson Grass mosaic virus. Expected to dominate in Queensland for fresh market. Has performed well in the Sydney Basin area.
Punchline
(F, P)
Supersweet
sh2
Yates Vegetable Seeds 80 Processor and fresh market. Ideal for early sowing due to good germination characteristics.
Shimmer
(F, P)
Supersweet
sh2
Yates Vegetable Seeds 85 New cultivar. Strongly resistant to rust. Increasing area sown for processing.
Sunsweet
(F)
Supersweet
sh2
Henderson Seeds 95 Resistant to rust.
Dynasty
(F, P)
Supersweet
sh2
Syngenta 90–92 One of the most popular cultivars in the US.
Dominion
(F, P)
Supersweet
sh2
Syngenta 90–92 Resistant to rust. Superior quality supersweet.
Flair
(F)
Supersweet
sh2 bicolour
Lefroy Valley 95–100 Replaced Golden Pearl. Fresh market.
Madonna
(F)
Supersweet
sh2 bicolour
Yates Vegetable Seeds 85 Larger cob size than Cabaret.
Snogold
(F)
Supersweet
sh2 bicolour
Lefroy Valley 90–95 Fresh market. Popular as export variety into Asia. Early and main season. Australia’s first supersweet white and yellow bicolour sweet corn.
Cabaret
(F, P)
Supersweet
sh2 bicolour
Yates Vegetable Seeds 83 Main season type. Resistant to Northern Leaf Blight.

(i) P = processing, F = fresh market. (Most common uses.)
(ii) Maturity varies depending on time of planting and weather conditions.
(iii) All cultivars listed have yellow kernels except for bicolours which have white and yellow kernels. Cultivars with kernels entirely white are also available and are sweeter than bicolours.
(iv) Cumulative heat units required to grow different temperate cultivars may be listed as a guide to predict flowering or maturity in technical notes distributed by seed companies. Heat units can be calculated by the following formula on a daily or hourly basis:

Heat units = Tmin – Tmax 10°C (base temp.)
2

where T is in °C.

Growing sweet corn

The following information is typical for many sweet corn crops. Supersweet cultivars are reported to require more exacting management because of lower seedling vigour (see ‘Supersweet management’ for more specific information).

Successful sweet corn field production begins with good seed quality, correct plant populations and good weed control. Irrigation and nutrition will also play a critical role throughout the life of the crop.

Climate

Sweet corn is a warm-season, frost-sensitive crop with a preferred growing season temperature of 15–32°C. Sweet corn is very similar to the field maize plant and, provided irrigation water is available, can be grown almost anywhere in New South Wales. It is ready to pick in 75–105 days, depending on the cultivar, locality and sowing time. Planting time is strongly influenced by the temperature of the soil at sowing depth, which should be at least 12°C and increasing at 9 am.

Ideally, however, soil temperatures should be between 15°C and 35°C, with a minimum for supersweet of 14–16°C.

Frosts can limit the start of the season, as they damage young seedlings—particularly the growing tip which is most susceptible. The crop must be harvested before the first frost.

Hot, dry, windy conditions at flowering time, when temperatures exceed 35°C, can severely stress plants and disrupt pollination and seed set. Timing of irrigation can also influence pollination and seed set so that the endfill on cobs is affected.

Coastal sowings are usually early, as sweet corn is prone to leaf diseases which can develop after Christmas and cause serious losses. However, recent trialling and commercial plantings of tolerant/resistant cultivars has overcome this problem.

Soils

Sweet corn grows best in deep, fertile, loam soils, supplied with adequate nutrients. The well-drained, alluvial soils usually found along rivers and creeks are ideal for sweet corn.

Visually check the soil profile well before planting. Rootzone depth is critical to success and should preferably be a minimum of 50 cm because sweet corn roots can grow to 1.2 m. Deep ploughing before planting to break up shallow hard pans and compaction layers is important for successful plant growth.

Preparation

Obtain soil samples from a depth of 0–15 cm, 2–5 months before planting, to determine chemistry and fertiliser requirements, particularly for lime/dolomite and superphosphate.

Careful consideration must be given to soil and seed management at sowing time. A light harrowing at sowing time can be used to remove dry soil crusts and allow sowing into moist, friable soil underneath the crusts.

A well-prepared, finely tilled seed bed is essential to:

  • assist crop establishment
  • kill germinating weeds
  • provide good soil moisture at planting time
  • suit the particular irrigation method being used.

If the soil is dry, it should be watered before sowing to ensure that the seed is sown into moisture.

Typically, soils are initially deep-ripped with strong chisel-type ploughs, then ploughed to a shallower depth with tyned implements and disc ploughs.

Soil preparation should commence 2–5 months before planting, and superphosphate and/or lime and manure requirements can be applied and incorporated 6–8 weeks before planting.

A soil pH of 5.5–6.5 (CaCl2) is preferred. (Note: The CaCl2 (calcium chloride) method of pH testing gives a slightly lower reading than the 1:5 soil:water technique.)

Rotations

Sweet corn is an important rotational crop. In a rotation, it helps reduce weed and disease build-up in other vegetables, particularly brassicas, and is a useful stock stubble feed.

Improved soil management, direct drilling, minimum tillage and permanent beds are popular practices in the Riverina. These techniques may result in cost savings.

Plant population and spacings

It is very important to have well-filled rows and, at planting time, plant population must be carefully considered. Decisions about plant and row spacings will be affected by:

  • the desired market
  • type of machinery used (especially for harvesting)
  • irrigation technology
  • cob size desired
  • pest and disease control.

Seed size varies, so it is better to discuss the desired plant population (plants per hectare) rather than the seed rate (kilograms per hectare). The ideal population depends on the cultivar and growing conditions. It ranges from only 35 000 plants per hectare under marginal conditions to 50 000–65 000 (typically 55 000) plants per hectare for heavily fertilised and well-managed irrigated crops. Very high plant populations may produce smaller cobs. A study by Falivene (1995) indicated that moderate increases of plant population from 50 000–70 000 plants per hectare increases yield with minor effect on cob size.

Seed companies advise planting 10% extra seed to allow for seedling failure.

A plant population of around 50 000 plants per hectare may be recommended for supersweet sh2 bicolour corn for export, with the following spacings used:

  • plants 22.4 cm apart with 81 cm between rows;
    OR
  • plants 18 cm apart with 1 m between rows.

Sprinkler irrigation

Where sprinkler irrigation is used, the rows are usually 75–96 cm apart. Typical spacings may be:

  • 50 000 plants per hectare:
    • plants 27 cm apart with 75 cm between rows, e.g. for inland processing crops;
      OR
    • plants 21 cm apart with 96 cm between rows;
  • plants 25–35 cm apart with 80 cm between rows, e.g. for coastal fresh market crops;
  • 40 000 plants per hectare:
    • plants 33 cm apart with 76 cm between rows.

Flood irrigation

In the Riverina, twin rows are commonly used on beds that range from 1.5–2.0 m, centre to centre. Plants are approximately 25 cm apart in rows. Further inland, where other types of harvesting equipment operate—sometimes with flood irrigation—row spacings of 95–100 cm are used.

Sowing

Sowing is usually done with a precision row crop planter, with an air seeder the most popular type of planter. Tractor-mounted equipment can today perform the following functions in one operation:

  • sow
  • fertilise
  • apply insecticide crop protectants for seed and seedling
  • apply herbicide.

The more even the seed drop, the better, as this results in a uniform crop. A uniform crop results in improved pest and disease control, and more consistent cob quality and yield at harvest time. Sweet corn seed is smaller than maize and its appearance varies according to cultivar, but it is usually wrinkled.

Depending on grade, there are from 4200–7000 seeds/kg. Seed rates vary from 11–18 kg/ha, depending on seed quality and plant population. At 5000 seeds/kg, about 11.5 kg of good quality seed should be sown to establish 50 000 plants per hectare. Planting plates must be selected to suit seed size and seed is planted about 3.5–5.0 cm deep. A shallower sowing (2.5 cm) may be recommended if the soil is cold, when sowing supersweets, or for low-germination types.

Pre-emergent herbicides should be applied in a band at sowing and before seedling emergence.

Seed should be fungicide-treated for disease, particularly for boil smut and damping-off diseases. When planting sweet corn seed with a plate seeder, remember to consider seed shape (round, flat) and plate changes. Avoid irregular plant spacing and planting depths. Planting speed should be adjusted according to planter design, seed type and soil conditions.

Crops like canola (Brassica family) that may come before sweet corn in a rotation are heavy feeders of sulphur and can produce isothiocyanates (ITC) that have a bio-fumigation effect. ITC can:

  • suppress phosphorus-fixing mycorrhiza fungi in the soil;
  • suppress germination of sweet corn.

Normally, a 2–3 week break between canola and sweet corn is recommended to avoid ITC effects.

Fertiliser requirements and nutrition

To calculate the fertiliser requirements for sweet corn, take into account:

  • soil and tissue analysis;
  • local experience;
  • paddock history;
  • nutrient removal by crops (see below) (Note: nutrients in plants can be ploughed back into the paddock);
  • fertiliser preference and type;
  • cultivar;
  • soil type.

Leaching and fixation of nutrients need to be considered and specialist advice is recommended for interpreting soil test results.

Nutrient removal

A 28 t/ha crop of sweet corn typically removes:

  N P K
Cobs: 110 kg/ha 16 kg/ha 60 kg/ha
Plants: 200 kg/ha 24 kg/ha 150 kg/ha
Total: 310 kg/ha
or 11 kg/t
40 kg/ha
or 1.4 kg/t
210 kg/ha
or 7.5 kg/t

American research has shown that:

  • 60% of nitrogen is taken up in the 2 weeks before and 2 weeks after tasselling;
  • less than 15% of the total nitrogen requirement is taken up during the first 4 weeks of growth, and high concentrations of fertiliser in the root zone promote strong growth. Amount and timing are therefore important influences on crop performance.

It is important that soil moisture levels are always at an optimum level for best nutrient usage, uniform crop growth and maximum cob quality.

Seedling vigour will be dependent upon good fertiliser and irrigation management. Fertiliser at planting time should be banded and placed where seedling roots will be, for instance 5 cm to the side and below the seed. Do not place fertiliser in contact with seed. Therefore, the preferred fertiliser strategy (if not using fertigation) is to apply basal fertiliser at planting followed by two sidedressings. Fertiliser sidedressing should be done with care to prevent root pruning at a critical growth stage.

Nitrogen, phosphorus and potassium are in most demand by sweet corn. The micronutrients molybdenum, boron, zinc and sulphur are also important. An application of micronutrients should be considered before planting, with levels monitored during plant growth—particularly of zinc.

Some typical fertiliser programs

Central West

Processing crops, standard cultivars:

  • Preplant:
    80 kg N/ha as anhydrous ammonia.
  • Planting:
    200–250 kg/ha of NPK compound, e.g. 11:15:9.
  • Sidedress:
    50–120 kg N/ha.

OR

  • Preplant:
    15–40 kg N/ha (up to 75 kg)
    10–20 kg P/ha
    0–30 kg K/ha.
  • Sidedress:
    50–120 kg N/ha applied 30–40 days after sowing or when crop is 30–50 cm high, depending also on sap analysis.

Coastal areas

Compound fertilisers are popular when fields have cropped heavily in the past. For example, a complete NPK mixture could be applied at 550 kg/ha at planting time. Otherwise N and P starter fertilisers are used.

Broiler litter/manure is more commonly available in coastal areas and can be used as a preplant basal fertiliser, applied 3 weeks before sowing, especially if manures are fresh. A rate of 5–10 t/ha (maximum 15 t/ha) is recommended for ‘cured’ manure.

Irrigation

Corn requires 500–600 mm of moisture as supplementary or full irrigation for successful production. Crop development and soil water should be carefully monitored. Precise soil moisture measuring equipment can help farmers make decisions about irrigation. Sweet corn should not suffer any setback to growth due to significant fluctuations in the soil moisture level.

It is particularly important for moisture to be adequate from the early tasselling stage, when wilting can reduce yields by up to 20%, and up until harvesting.

Irrigation water is usually applied by furrow or with overhead sprinklers. As a general guide, between 4–10 ML/ha (depending on district, 5–7 ML in warm areas) or 25–50 mm per week is used to grow a crop in New South Wales, with some moisture usually provided by rainfall. Sweet corn is more sensitive to moisture stress than field maize. Depending on crop developments, application rate, weather conditions and soil type, an irrigation interval of 5 days may be necessary when the need for irrigation is high.

Sweet corn will not stand ‘wet feet’, so poorly drained, compacted and heavy clay soils should be avoided.

Irrigation design, management and scheduling should pay particular attention to soil type.

Irrigation tips

Research has shown:

  • Accurate irrigation scheduling for sweet corn can reduce irrigation costs and help meet yield goals.
  • Allowing no more than 50% depletion of available soil water usually will avoid yield-limiting water stress.
  • Critical growth stages are GS 3–7, that is, 3 weeks before silking to 2 weeks after silking. See figure 5.

Weed control

Normally, weeds are easily controlled in sweet corn crops by a range of options available using herbicides and cultivation. Poorly managed weed populations can harbour pests and diseases that can restrict plant growth and cob quality. Rotation of crops and thorough ground preparation will minimise weed problems. Inter-row cultivation (‘scuffling’) is usually necessary once or twice within 30 days of planting to break soil crust, and later can be combined with fertiliser sidedressing and hilling-up. Cultivation needs to be shallow, as sweet corn roots are near the surface and must not be damaged.

For advice on herbicides, contact your local district horticulturist. The herbicides selected will depend on types of weeds to be controlled, management preferences and whether plant-back time must be known to prevent damage to the next crop to be planted in that field.

Supersweet management

Johnston (1995) advises that supersweet sh2 cultivars are more exacting in their cultural requirements.

  • Soil at planting needs to be finely tilled and trash-free, with:
    • good soil moisture and soil contact to initiate germination;
    • soil temperature 14–16°C minimum.
  • Sowing:
    • depth of 25–40 mm
  • Fertiliser:
    • Basal at sowing time, 40 mm to one side and 60 mm below seed. Apply:
      50 kg N/ha
      30 kg P/ha (Note: phosphorus in some districts will be applied before sowing)
      60 kg K/ha
      1 kg Zn/ha
    • Sidedress (21 days after emergence) when 60% of nitrogen requirement is utilised. Apply:
      120 kg N/ha
      5 kg P/ha
      40 kg K/ha

      and, 35 days after emergence, apply:
      30 kg N/ha
      20 kg K/ha (avoid K if levels are very high in soils).

Root development

Remember: Supersweets have a smaller, weaker root development. Phosphorus and ample moisture are very important for developing secondary roots.

Irrigation

Do not allow plants to stress at the following times:

  • at weeks 4–6, when cob initiation is underway;
  • during pollination;
  • during hot dry spells.

Integrated Pest Management (IPM)

Integrated Pest Management (IPM) is a management strategy that uses the available pest management tools most effectively. For a pest problem to arise, three conditions need to be met:

  1. a pest is present
  2. a crop is susceptible to attack
  3. environmental conditions favour pest increases.

The benefit of IPM is the management of pests economically, with a minimal use of pesticides and reduced insecticide resistance.

There are four components essential to any IPM program:

  1. accurate pest identification and an understanding of pest life cycles and habits;
  2. crop monitoring;
  3. threshold levels;
  4. effective management methods.

1. Accurate pest identification and an understanding of life cycles and habits

In an IPM system, it is important to correctly identify the pest, and, with information on the pest’s life cycle, it allows for targeting a particular management tool at the most appropriate lifestage of the pest.

2. Crop monitoring or scouting

Crop monitoring allows for early warning of the presence of all three conditions for pest problems, and for remedial action to be taken if necessary. It allows growers NOT to apply pesticides if the pest is not present in numbers that warrant control. It allows growers the option of applying ‘softer’ pesticides if beneficial insect populations are high. Basically, it gives the grower a greater range of management options to choose from so that the most appropriate tool can be used.

Crop monitoring requires attention to:

  • record keeping
  • water
  • nutrients
  • growth and development
  • pest and disease
  • root zone.

Crop monitoring is a guaranteed checklist that enables a grower to record, and control, crop performance.

An impressive range of technology is available to assist with crop monitoring and grower observation. Experience and an ability to interpret information is important.

2.1 Record keeping

Records must be kept of all activities including yields, times, dates, paddocks, results, cultivars, pest control measures, weather conditions and growth stages.

There are a number of options for collecting, setting out and storing of records. Computer technology is becoming more commonplace. A notebook is the bare minimum requirement.

Records can be used to make more informed decisions when:

  • monitoring crops
  • checking past performance of crops
  • predicting or estimating.

2.2 Water

Irrigation water supply is possibly the most important factor limiting production.

When irrigating, two important questions a grower should ask are:

  • When does my crop require water?
  • How much water should the crop receive?

The answers to these questions can come from three sources:

  • Soil-based information. This technology works well, using, for example:
    • soil auger
    • tensiometer
    • Neutron Probe®
    • EnvironScan®
    • The Gopher®.
  • Crop-based information. Visual checking of crops has been the traditional approach and remains important when crosschecking with other techniques, for instance tensiometer and evapotranspiration.
  • Weather-based information. These techniques estimate water use of crops by computer and manual measurement of crop evapotranspiration (ETO—the ‘O’ refers to a reference crop).

Monitoring crop water requirements is best achieved by a combination of techniques, for instance tensiometer or probe with ETO and visual checking.

Water quality is also important and should be monitored for possible pesticide ‘degradation’ effects as well as irrigation.

2.3 Nutrients

Nutrient monitoring is an important guide to crop health and production capabilities. To be effective, a nutrient monitoring program assesses plant tissue and soils by sampling.

When to sample. The important periods that should be considered are:

  • preplant (soils and water only)
  • development or growing period (tissue)
  • production period (tissue).

Sampling procedures. It is essential to know the correct sampling procedures, for example:

  • Plant analysis:
    • type of tissue to collect, and timing
    • how to handle and dispatch samples
    • how to interpret results.

    The recent release of the field sap test kits has enabled ‘on the spot’ field testing for nitrogen and potassium.

  • Soil analysis considers:
    • what depths to sample, and area
    • how to handle and dispatch samples
    • how to interpret results.

It is important to have samples analysed by NATA-accredited laboratories.

What to collect. Sample according to soil type, cultivar and age. If testing for disorders, take an apparently normal, healthy sample as well. All samples should be clearly labelled and a record should be kept of the details of the sampling, such as time and location.

The results of analysis and the ability to draw conclusions are only as good as the sample taken, how representative they are, and how they are collected, handled and dispatched.

Plant analysis or soil analysis. The advantages of plant analysis are as follows:

  • fertiliser programs can be evaluated;
  • disorders can be diagnosed; and
  • the grower can evaluate existing nutrient levels and plan for next season’s production, and be assured that things are all right.

The advantages of soil analysis are as follows:

  • fertiliser programs can be evaluated;
  • a guide to crop nutrition; for instance, nutrient requirement, correction of deficiencies, and avoiding toxicities in crops.

Analysis of nutrients in the crop and soil can be crosschecked to confirm that the nutrient levels that are in the crop are related to growth. The cost of analysis is low but the benefits are considerable.

If in doubt about procedures, ask your adviser, company representative or fertiliser dealer.

2.4 Growth and development

Figure 5

Visual checking and actually measuring growth and development can be useful and is often overlooked.

It is important to know the typical sizes for growth phases and timing for the cultivars, and preferably record them.

If growth is not normal in a field or section of a field, early detection of a possible problem and search for a cause is easier when comparisons are made with normal crop growth.

A planning calendar can be prepared listing typical growth stages, timing and planned activities, such as plant analysis and strategic watering.

A tape measure will be useful for measuring shoot length, leaf dimensions and canopies.

2.5 Pest and diseases

Pest and disease monitoring is becoming more popular as IPM programs develop. It is important to have information on pest and disease life cycles and be able to identify pests and diseases that attack sweet corn so that action can be taken at a time when plants are most susceptible to pests or disease, and control is most economical.

Monitor or check crops up to three times per week by walking through the crop in a ‘Z’ or ‘W’ or ‘figure 8’ pattern, and inspecting at least 20 plants in a thorough fashion. This involves checking the top and bottom of plants, the underside of leaves and even the root systems to allow early detection and control of potential problems.

2.6 Root zone monitoring

Often, only above-ground parts of the crop are monitored. On occasions, it is necessary to check below the soil surface. Problems of soil-borne pests and diseases, and soil degradation, for instance compaction from machinery or livestock, can be more easily detected by checking below the soil surface. If necessary, pull problem plants up and check roots for signs of problems.

3. Threshold levels

The level of insect activity that requires action to prevent economic loss is called the threshold level. Threshold levels will vary depending on the district and whether the crop is destined for processing or the fresh market. Contact local advisors and consultants for more information.

4. Effective management methods

These methods should include the purchase of disease-free seed, good spray management, crop hygiene, rotation of crops, use of varietal resistance and timeliness of operations. All of these are considered best management practices.

Pests

The main pests of sweet corn are as follows.

  • In the rootzone and young plants:
    • wireworms
    • false wireworms
    • cutworms
    • black beetles
    • army worms.
  • In silks and cobs:
    • Helicoverpa armigera (heliothis/corn earworm) is the primary sweet corn pest.
    • Corn aphid (Rhopalasiphum maidis) can transmit maize dwarf mosaic virus. This virus is not normally seen in crops in New South Wales. Rarely do aphids cause direct feeding damage. However, they are a problem for fresh export of sweet corn, as colonies are often found in the cob sheath.

Management of heliothis

Figure 6

Heliothis (Helicoverpa armigera) is the most common and difficult pest of sweet corn. The following are some guidelines for management:

  • Design a management action program.
  • Ensure uniform crop growth by using good production practices from planting time.
  • Monitor crops from planting to harvest. For heliothis, an important time is 2–3 weeks before tasselling, through to 100% brown silks. Figure 6 shows the critical growth periods for heliothis attack in processing sweet corn crops. Crops should ideally be checked 2–3 times per week during this time using a systematic protocol such as the one suggested below. In some areas there may be a commercial crop scout who will monitor your crop on a fee-for-service basis. In some cases, processing companies or the like will employ someone to scout crops that they have contracted. In other cases, growers will scout their own crops. If a scout is not available in your area, you may attract one to start a business if there is sufficient demand, usually requiring a number of growers to want their services.

The scouting protocol for corn is still being developed. The current recommendation is as follows.

At emergence, up to 1–2 weeks prior to silking

  • Monitor weekly.
  • Monitor for soil insects immediately after sowing and at crop emergence.
  • Check for heliothis and any grub by examining four consecutive plants at five dispersed sites in the crop (walk a ‘Z’ or ‘W’ through the crop). Look for eggs and grubs. Check the whole plant, and especially check in the boot, or the tassel when first growing out of the boot.

Pre-silking and harvest

  • Monitor 2–3 times per week.
  • Check four consecutive plants at five dispersed sites, focusing on silks and the area above and below the silks (±10 cm). Look for heliothis eggs and grubs. Check a total of 40 silks (two silks per sample plant if possible.)

(Right) The author scouts for heliothis eggs.

Note: Keep in mind the harvest time and that there are strict withholding periods following pesticide use.

WARNING

Pesticide residues may occur in animals treated with pesticides, or fed any crop product, including crop waste, that has been sprayed with pesticides.

It is the responsibility of the person applying a pesticide to do all things necessary to avoid spray drift onto adjoining land or waterways.

ALWAYS READ THE LABEL

Users of agricultural (or veterinary) chemical products must always read the label and any Permit before using the product, and strictly comply with the directions on the label and the conditions of any Permit. Users are not absolved from compliance with the directions on the label or the conditions of the Permit by reason of any statement made or not made in this publication.

Spraying sweet corn

  • Spraying corn for heliothis will depend on location, time of sowing, markets, past experiences, pesticide selection, insect activity and scout decisions. Correct timing, coverage of target, and droplet size are vital to the effectiveness of spraying, both at ground level (photograph) and from the air. For more information on using pesticides in an IPM program, contact your local advisory services.
  • Time the first spray to prevent grubs from growing too big (longer than about 5 mm); otherwise, control will be difficult and damage high, especially when insect resistance is a problem. The timing of the first spray is a management decision based on information available and perceived risk (or cost penalty). It is necessary to have a preferred spray/control option with a back-up alternative.
  • Keep good records of all major events and management activities.
  • Understand pesticides (including new chemistry and biopesticides) and try to develop some knowledge of:
    • pesticide families, including the importance of rotation and how to use each group for maximum effect;
    • ovicide, contact, systemic, residual and biopesticide actions;
    • the effect of growth dilution at various crop stages;
    • withholding periods of pesticides;
    • environmental impact (for instance drift, groundwater contamination).
  • Mechanically control heliothis pupae using the following methods:
    • plough in stubble immediately after harvest;
    • plough early in spring.
  • Other important factors in control are timeliness of operations based on monitoring, good coverage of target areas with sprays, and use of biological agents such as parasites, predators and viroid agents.

Diseases

The primary diseases of sweet corn can be divided into three types:

  • Fungal diseases, which include head smut, boil smut, downy mildew, northern (turcica) leaf blight, maydis leaf blight, seedling diseases (pythium and fusarium), stalk rots (fusarium), ear rots (fusarium) and storage rots.
  • Bacterial diseases such as soft rot of plant tops.
  • Viral diseases including maize dwarf mosaic virus, wallaby ear, and sugarcane/Johnson grass mosaic virus.

Diseases such as rust and northern leaf blight can also occur and cause considerable damage to sweet corn. Hybrid cultivars which are resistant/tolerant to these diseases are available. Ensure weed populations are kept in check and/or monitored particularly for aphids, which can cause infection.

Ensure seed is treated with recommended fungicides, and have a clear understanding of boil smut quarantine areas and requirements. For more information, refer to Agnote DPI-104 Boil smut—an update for sweet corn, maize and popcorn industries.

Maturity testing

To guarantee optimum harvest time and desired cob characteristics, the maturity of sweet corn can be assessed in the following ways. If possible use a combination of methods.

Moisture level

Within 2 weeks of harvest the moisture level in cobs will begin to decline, usually at a rate of 0.25–1.00% per day depending on weather conditions, cultivar and management.

A procedure used by the processing industry to monitor and measure cob moisture accurately is as follows:

  1. Randomly select and dehusk 15 cobs and, using a sharp knife, remove a slice of kernel material down the entire length of one side of each cob.
  2. Put the kernel material in a blender until the kernels become a smooth homogeneous fluid or paste, depending on the maturity of the corn.
  3. Separately weigh two glass Petri dishes containing a circle of ‘Whatman No. 4’ ashless filter paper. Record the weight of each to two decimal places. Label the dishes ‘1’ and ‘2’.
  4. Weigh 5 g of corn slurry onto the filter paper in each dish, smearing it thinly over the filter paper. For each dish, record the total weight of the dish (with filter paper) plus sample to 0.01 g.
  5. Place both dishes in the microwave and turn on for 2 minutes.
  6. After the microwave stops, turn the filter paper upside down, and operate the microwave for another 2 minutes.
  7. After the microwave stops, remove the Petri dishes from the microwave with tongs, and weigh to two decimal places. Record weights.
  8. Return dishes to the microwave and repeat steps 6 and 7 until two consecutive weighings are only 0.01 g apart for each dish.
  9. Calculate the moisture content of each sample using the formula below. Average the two results, recording the final figure.

% Moisture =

(Wt dish+ paper+wet sample) – (Wt dish+paper+dried sample) × 100
(Wt dish+paper+wet sample) – (Wt dish+paper)   

Sugar reading: degrees Brix

Obtain kernel liquid from cob samples, as outlined above for measuring cob moisture. A Brix refractometer (photograph) can be used to measure the sugar level of the kernel sample (the photograph shows a digital refractometer at top, and a popular hand-held unit at bottom). Sugar content in sweet corn cobs is considered by industry to be the major factor determining eating quality.

Table 8. Typical moisture and sugar levels of sweet corn types at harvest*

Type Moisture level Sugar level (°Brix)
Supersweets (sh2 lines, including bicolours) for processing and fresh market Range 75–85% Range 14–22
Majority 76–81% Majority 16–18  
(Processing 16–79%) Aim for 18  
Standards (su lines, e.g. Jubilee) for processing and fresh market 71–73% 11–12

*Note: It is important to consult with buyers, processors and seed companies for specific requirements. Moisture and sugar levels may vary with cultivars.

Degree days

The ‘number of days’ is temperature-dependent. Some 95-day corn crops have been known to take 120 days when growing into a colder frost-free autumn/winter period.

Visual assessment of cobs

Seek an opinion from a company representative, adviser or neighbour, and note their advice.

Cobs

Randomly assess cobs in a field. When silks are browned off and dry, mature cobs usually have the largest diameter.

It is usually 3–4 weeks from onset of silking to harvest.

If flag leaf and the husk lose their bright green, fresh appearance, the cob will be overmature. Confirm by checking kernels.

Kernels

Internally, mature kernels when punctured have a milky consistency. Immature kernels have a clearer colour, and tip fill is not complete (NB: Poor pollination may also be a cause of incomplete tip fill of cobs). Maturity indicator in the Hawkesbury is the colour of the kernels, from whitish to golden yellow.

Sensory assessment

Taste-testing the cobs close to harvest provides a good opportunity to monitor cob development and develop a keen palate for optimum maturity.

Harvesting

Maturity of cobs varies from 70–100 days depending on cultivar, climate (temperature) and soil moisture.

Corn for processing

The New South Wales sweet corn processing crop is usually harvested mechanically by six-row self-propelled harvesters. Bulk handling at harvest time allows processors to maximise factory throughput and control cob quality parameters.

The harvester snaps off cobs, complete with husks, and loads them into a trailed field bin holding 7 tonnes. When full, the bin hydraulically tips the corn into a truck ready for delivery. These trucks hold 23 tonnes. A six-row harvester will harvest upwards of 35 tonnes per hour in a good average crop. Quality deteriorates rapidly after harvesting, and processing must begin as soon as possible after harvest. Long road hauls in hot weather are undesirable.

Su cobs for processing are mature when moisture content reaches between 72% and 74%, and sh2 cobs between 76% and 79%. Silks will normally have appeared 21–28 days earlier and will have dried off. The kernel will be at the full milk to very early dough stage. In cool, moist conditions the crop matures slowly and a delay of 2–4 days in harvesting will have little effect on quality.

Some cultivars dry more slowly than others. In hot, dry weather, however, moisture content of the kernel can fall very rapidly, and if the corn is to make top grade it must be harvested within 24 hours of reaching 72% moisture. Sweet corn for freezing should generally be harvested at 72% moisture.

Sweet corn at 70–71% is satisfactory for canning as whole kernels, but below this it is suitable only for creamed corn. It is unusual for material with a moisture content below 68% to be used by the processor. As the moisture content falls, the starch percentage rises and the grain gets a rather floury texture.

Processors link planting and harvest time to precise schedules to suit factory requirements.

Fresh market corn

Corn for the fresh market is generally harvested by hand or machine at a higher moisture content of 76–80%, and the kernels at the top of the cob are 75% full. Excess leaf is trimmed and the cob checked for insect damage prior to harvesting. When hand harvesting, the cob is removed from the plant with a cane knife and is usually sold with the husk on. Excessive flag and shank on the cob is undesirable, as husk freshness is maintained with moisture taken from the kernel. Picking may take place over several days, as the cobs seldom ripen evenly. If the dried silk pulls off readily it is an indication that a helicoverpa grub is in the cob and therefore the cob may not be suitable for sale or it may need trimming.

Kernels at the tip of supersweet cobs may not have fully coloured at harvest and this should not stop harvest operations as kernels will colour up when cooked.

Rapid transport and cooling of fresh market crops is important to maintain cob quality. Hydrocooling and pressure cooling techniques are preferred for immediate removal of field heat. Night and early morning harvesting is preferred.

The stubble residue after harvesting provides useful cattle fodder, and, if not ploughed in, is generally grazed unless burnt (due to disease risks) or ploughed down (for heliothis insect control). In inland districts, corn is often followed by grazing oats, and rotations usually include lucerne and perhaps a vegetable crop such as tomatoes.

In the Hawkesbury/Nepean area, the stubble is usually ploughed in soon after harvest and the soil prepared for a rotation crop such as brassicas or lettuce.

Conclusion

Sweet corn is a crop showing increasing potential. Exports by the fresh and processing sectors are developing, especially to Asian market places where sweet corn popularity amongst consumers is increasing.

Australian consumption of sweet corn and supersweet market share is also expected to increase, and there is the opportunity for import replacement.

Sweet corn is an attractive crop for growers to grow as its cultural requirements are well understood, the crop grows quickly and all operations can be fully mechanised for processing. Australian growers achieve yields equal to or better than world’s best 25 t/ha.

By improving management and adopting innovation in areas like Integrated Pest Management on the farm, Australian ‘average per hectare’ yields can be expected to increase. Increased planting densities and improved soil management options, such as direct drilling and minimum tillage, show promise, and adoption by producers is expected to increase.

The processing sector has undertaken important initiatives aimed at becoming more internationally competitive.

References and further reading

Claxton, R. A. 1976, Sweet Corn: A Million Dollar Industry in New South Wales, NSW Department of Agriculture Bulletin.

Falivene, S. 1995,  ‘Improving the International Competitiveness of the Processing Sweet Corn Industry in New South Wales’, Final Report, HRDC Project VG 227.

Johnston, P. 1995, ‘Supersweet Management’, Proceedings of Sweet Corn Integrated Pest Management, NSW Agriculture (now NSW DPI), Cowra, NSW.

Lorenz, O. & Maynard, D. 1980, Knotts Handbook for Vegetable Growers, 4th edn, John Wiley & Sons.

NSW Agriculture et al., 1998, Maize Check, NSW Agriculture (now NSW DPI), and Department of Natural Resources and Energy, Victoria.

Pullar, D. et al. 1993, Winning the Race: Being Internationally Competitive, A case study of the Australian sweet corn industry, Case Study No. 7.

Acknowledgment

The assistance and comment from the following people is gratefully acknowledged:

Simplot Australia Pty Ltd , Bathurst Field Service Staff.
Mr Peter Johnston , Formerly Technical Sales Representative, Snowy River Seeds.
Mr Leigh James , District Horticulturist (Vegetables), NSW DPI.
Dr Sandra McDougall , Technical Specialist Vegetables, NSW DPI.