Summer legume forage crops: cowpeas, lablab, soybeans
Series: Agfact P4.2.16 Edition: Second edition Last updated: 29 Sep 1999
Cowpeas (Vigna unguiculata) and lablab (Lablab purpureus) are fast growing, annual, summer forage legumes. They are excellent quality crops for fattening both sheep and cattle, and are also regarded as good feed for milking cows. In a crop rotation program, they can significantly improve soil nitrogen levels by nitrogen fixation or by incorporation in soil as a green manure crop. Cowpeas also offer the alternative of grain production. Both cowpeas and lablab are tolerant of drought and heat. Soybeans (Glycine max) are also very useful forage and hay crops in some situations.
In NSW, cowpeas and lablab are adapted to slopes and plains with a minimum annual rainfall of 500 mm, and to coastal or irrigated areas. Soybeans are used mainly as forage crops on the coast. Traditionally, these crops have been grown in the northern half of NSW, where summer rainfall is more predominant. Each crop is suited to specific situations and has special requirements. Table 1 compares the characteristics of each crop.
Table 1. Characteristics of summer forage legumes
|Total forage production||*||**||***|
|Suitability to light sandy soils||*||***||**|
|Suitability to heavy clay soils||***||*||***|
|Potential carrying capacity||**||**||***|
|Tolerance to trampling||*||**||***|
|Recovery after grazing||*||**||***|
|Tolerance/resistance to root rot||***||*||***|
|Leaf disease susceptibility||***||**||*|
|Susceptibility to insect attack||***||**||*|
|Late autumn growth||*||**||***|
|Potential for grain production after grazing||*||***||*|
|Tolerance to heat & moisture stress||*||***||***|
|Suitability for sheep||**||***||***|
|Suitability for cattle||**||***||***|
|Suitability for hay and silage||**||**||**|
|Availability of seed||***||***||**|
|Key: high ***; medium **; low *|
Cowpeas and lablab do very well on a wide variety of soils-from light, sandy soils through to well-drained, heavier-textured soils. Lablab's performance on heavy soils is greatly superior to that of cowpeas. Both require well-drained soils, although lablab has better resistance to phytophthora root rot. Soybeans are well-adapted to loams, heavier clay loams and heavy, self-mulching clay soils. They are superior to cowpeas and lablab where soils become waterlogged.
Cowpeas. Caloona and Poona are the main forage varieties grown, especially in inland districts, whilst Red Caloona is the variety favoured in coastal districts. Red Caloona has good resistance to phytophthora root rot, but it matures more quickly than Caloona and Poona and may not produce as much forage. Caloona is the better variety for inland districts. However, seed sources are often mixed as there is no certification scheme in NSW. There are around 15,000 seeds/kg in each of these varieties.
Lablab. Rongai and Highworth are the two main commercial forage varieties in NSW. Both are very similar in appearance and agronomic performance. Rongai has the longer growing season. It has white flowers, and more obvious hairs on the leaves than does Highworth. Seeds are tan-coloured and average around 4000 seeds/kg. In NSW, Rongai is normally killed by frosts before it can flower. Highworth matures more quickly, usually flowering in early to mid-April, three to six weeks earlier than Rongai. It has purple flowers and more erect growth. This variety has smaller black seeds, averaging around 5000 seeds/kg.
These lablabs will flower, but do not complete seed development in northern areas of NSW as the plants are usually killed by frost before the pods develop. Seed of these varieties has to be obtained from Queensland each season.
In addition to these, Koala is a quick maturing grain type which can be used as a dual purpose grazing/grain type. However, it produces only around 70% of the dry matter of Rongai and Highworth.
Endurance is a perennial lablab variety which has been released as a short-term legume pasture plant. It has persisted for 2–4 years in Queensland and dry matter production has been around 70% of Rongai. It is hard seeded and needs to be scarified before sowing (5500 seeds/kg).
Both lablab and cowpeas do best under warm, humid conditions, with temperatures between 20°C and 30°C. They both display outstanding tolerance to periods of heat and moisture stress.
Soybeans. These are useful forage and hay crops in coastal districts. Any of the grain varieties recommended for a district can be used, but the later maturing types are preferred for grazing. All grain types are suitable for haymaking. The best quality hay is made from soybean crops when pods are half filled. In all production areas, crops intended for grain are sometimes cut for hay after poor pod set, or when the relative return for hay is higher than expected returns from grain i.e. drought.
Cowpeas and lablab are best sown into a well-prepared, fallowed seedbed that has a good depth of subsoil moisture (at least 75 cm). Seed should be sown at a depth of 4 to 6 cm into moist soil with good seed-soil contact.
In some situations, seed can be roughed into cereal stubble and double cropped effectively. However, crop establishment and production will be much more dependent on good seasonal conditions following sowing.
Conventional or 'no till' sowing equipment is suitable for sowing all these forage legumes. Press wheels can improve crop establishment.
Good 'no till' or direct drilling techniques can be used with all three-forage crops. All show good seedling vigour and establish well where weed competition is controlled before sowing with suitable knockdown herbicides.
Windblasting on sandy soils can be a problem, but is almost completely overcome by direct sowing into cereal stubbles or by furrow-sowing seed on 35 cm row spacings without covering harrows. When using furrows on sloping country, ensure that sowing is on the contour to prevent erosion.
Cowpeas and lablab can be sown when danger of frost is over, and when soil temperatures reach a steady 18°C at sowing depth at 9 am Eastern Standard Summer Time (E.S.S.T.) over three or four consecutive days. In most districts except the tablelands, these legumes can be sown for forage from mid-October to early January. The earlier sowings usually produce the most feed.
The best sowing time for soybeans is mid-November to December.
Cowpeas. Sow cowpeas at 10 to 14 kg/ha of good quality seed under dryland conditions, with 18 to 35 cm row spacings, to give a plant population of 90,000 to 130,000 plants/ha (9 to 13 plants/square metre). In irrigated areas and higher rainfall districts, rates can be increased to 20 kg/ha (190,000 plants/ha).
Lablab. Sow lablab at 15 to 20 kg/ha under dryland conditions, to achieve a plant population of 40,000 to 60,000 plants/ha (4 to 6 plants/square metre), and up to 30 kg/ha in irrigated or high rainfall areas (95,000 plants/ha).
Row spacings from 18 to 90 cm are suitable for lablab. Wide rows allow better stock access and reduce trampling effects without any reduction in yields.
Soybeans. Soybeans should be sown at around 40 to 50 kg/ha (depending on seed size) in areas with high rainfall.
Use the following formula to calculate the required seeding rate (kg seed/ha):
|Seeding rate =||Desired plant population/ha|
Seeds/kg x germination % x establishment %
What seeding rate is required to establish 100,000 cowpea plants/ha?
|Number seeds/kg =||15,000|
|Seed germination % =||90%|
|Establishment % =||70%|
|Required seeding rate =||100,000
|=||10.5 kg seed/ha|
What seeding rate is required to establish 60,000 Highworth lablab plants/ha?
|Number seeds/kg =||5000|
|Seed germination % =||90%|
|Establishment % =||70%|
|Required seeding rate =||60,000
|=||19 kg seed/ha|
Seed must be inoculated before sowing with the correct strain of inoculant to ensure good nodulation and nitrogen fixation in the soil.
Cowpeas should be inoculated with Group I, lablab with Group J and soybeans with Group H. When inoculating, always use a methyl cellulose glue slurry, rather than a water only slurry, to increase the survival of the inoculant. Use 5 g of methyl cellulose per litre of water for the normal glue slurry method or 15 g methyl cellulose when lime pelleting seed.
Lime pelleting is not normally required, except on problem acid soils. However, a light covering of microfine lime may have the advantage of drying the seeds and so prevent them sticking together following inoculation. There is no demonstrated advantage in lime pelleting soybeans on acid soils.
It is advisable to inoculate only enough seed for each day’s planting. Inoculate seed the day before sowing and allow it to dry overnight. Store inoculated seed under cool conditions out of sunlight.
All these forage legumes nodulate readily when properly inoculated with the correct strain of rhizobia.
The production of nodules depends on the efficiency of inoculation, soil moisture, soil temperature, nutrition and soil acidity. Nodulation appears to improve with good soil moisture and lower soil temperatures. Poor nodulation sometimes occurs with increasing soil temperature and low soil moisture.
When well-nodulated, these forage legume crops can fix around 20 to 140 kg residual nitrogen/ha in the soil. Such levels of nitrogen fixation represent the equivalent of around 50–300 kg urea fertiliser/ha.
The amount of soil nitrogen fixed varies widely, but the effect on following cereal crops is usually dramatic.
The nett soil nitrogen fixation of a well-nodulated crop will depend on the initial soil nitrogen level, crop dry matter production, and grain yield, as well as the factors mentioned above.
Location, soil type and history of fertiliser application will determine fertiliser needs.
Phosphorus. Phosphorus is the main nutritional requirement and should be applied at sowing.
- Soils with low to medium levels of phosphate, require 125 to 150 kg single superphosphate/ha, or the equivalent in concentrated fertiliser. On coastal soils, up to 250 kg single superphosphate/ha may be required. Up to 200 kg single superphosphate/ha can be sown down the same sowing tube as the inoculated seed.
- Highly fertile, self-mulching clays and alluvial soils which are high in phosphate do not generally require fertiliser.
Potash. Potash may be required on coastal soils. Determine this by a soil test.
Molybdenum. Molybdenised superphosphate should be applied to sandy, acid soils once every four to five years.
Zinc. Zinc deficiency can occur on some alkaline, self-mulching, clay soils that have high levels of phosphorus. Plants with zinc deficiency have leaves with yellowing between the veins. It can be remedied by applying zinc either to the soil or as a foliar application to the plant.
Herbicides and weed control
Although these legume crops are for grazing, summer grasses and broadleaf weeds can be a problem, particularly when they emerge with the crop. It is important to select paddocks with low populations of broadleaf weeds (such as Bathurst and Noogoora burrs and Datura).
Most summer grasses-such as barnyard grass, black grass, spiny burr grass and liverseed grass-can be controlled with pre-emergent herbicides. There are limited options for post-emergent control of broadleaf weeds in cowpeas or lablab. Several are registered for use in soybeans, but are generally too expensive to use on forage crops.
Cowpeas, lablab and soybeans are highly sensitive to the phenoxy herbicides such as 2,4–D, M.C.P.A., 2,4–D–B, Tordon-50-D® and dicamba. Do not apply these herbicides to or near these crops, as severe damage will occur.
For further information on herbicides in these crops, see Weed control in summer crops.
Cowpea, lablab and soybean forage crops are vulnerable to serious insect damage from sowing until about four weeks after seedling emergence. Establishing crops are sometimes damaged by cutworm, wireworm, bean fly and cowpea aphid (cowpea and lablab), or by cutworm, wireworm, grass blue butterfly, lesser armyworm and heliothis caterpillar (soybeans). In inland areas, cowpea crops appear more sensitive to attack by black cowpea aphid than does lablab. Control may be warranted in some situations. Once established, the crops may be damaged by various caterpillars feeding on the growing points and leaves of plants, and by locust swarms.
If cowpeas or Koala lablab are allowed to recover after grazing for grain production, they must be closely monitored for thrips and mirids when budding and flowering, and for heliothis when flowering and podding. They also need to be examined for green vegetable bug and brown bean bug.
For further information see Insect and mite control in field crops.
Apart from phytophthora root rot (Phytophthora vignae) in cowpeas, diseases are not a major problem in these crops.
Leaf diseases (such as bacterial scorch) can affect cowpeas in some seasons and these are usually best controlled by grazing. Powdery mildew (Oidium sp.) occasionally affects cowpea crops in autumn and can result in premature leaf drop. Palatability does not appear to be affected.
Lablab is relatively free of fungal diseases. Ascochyta leaf spot (Ascochyta phaseolarum) and sclerotinia stem rot (Sclerotinia sclerotiorum) have been recorded in Queensland. Sclerotinia stem rot can be a major problem in soybeans where crop rotation with resistant crops is not used.
Charcoal rot can affect all crops, but is usually not serious.
Place in crop rotation
Apart from their high value as forage crops, these legumes are included in crop rotations to build up soil nitrogen as well as to break weed and disease cycles. They are particularly useful for building up fertility in country that has been run down from overcropping. Provided crops are properly nodulated, cowpeas and lablab can fix 20 to 140 kg residual nitrogen/ha into the soil. This can give a significant bonus to later cereal crops in the rotation.
These legume crops should be rotated with a grass crop to minimise disease buildup and take advantage of increased soil nitrogen.
These forage legumes can produce good yields of high quality dry matter.
Under dryland conditions, yields of cowpeas have ranged from 500 kg dry matter (DM)/ha to over 4000 kg DM/ha under favourable conditions. Production per season is usually in the 2000 to 3000 kg DM/ha range. Yields of up to 8000 kg DM/ha have been recorded in irrigated areas.
Lablab has also produced from around 500 kg DM/ha to over 5000 kg DM/ha, with irrigated areas recording yields of up to 14 000 kg DM/ha.
Soybeans have yielded up to 10,000 kg DM/ha in irrigated areas.
In general, a lablab or cowpea crop of full canopy and approximately 0.5 metres in height will yield around 3500 to 4500 kg DM/ha (15,000 to 20,000 kg fresh material/ha). Leaf and leaf stalk make up about half of this. Only the leaf portion should be considered in determining the feed availability of the crop.
Cowpeas and lablab show little difference in dry matter production up to the first grazing, after the first 10 to 12 weeks of growth. However, during late summer and autumn, lablab usually produces more total dry matter because of its higher growth rates in autumn, superior tolerance of trampling and better survival and recovery after grazing.
Cowpeas, lablab and soybeans provide high quality forage suitable for growing and fattening stock, as well as feeding lactating dairy cows. These legumes generally produce a forage very high in crude protein, low in fibre, high in digestibility and high in metabolisable energy. Metabolisable energy is the energy value of the feed. It is measured as megajoules per kilogram of dry matter (MJ/kg DM). Table 2 shows a comparative range of feed values for these forage legumes.
Crude protein. Crude protein levels of leaves and shoots are usually over 20 per cent, depending on the crop's stage of growth and seasonal conditions. Stems contain only about 10 per cent crude protein. Crude protein levels can be equal to that of lucerne and greatly superior to most tropical grasses and forage sorghums.
Digestibility. The digestibility of these forage legumes is about 50 to 56 per cent on a whole-plant basis, and appears to vary little with the crop's age or with changes in environment. The leaf is much more digestible than the stem. Leaf parts have 60 to 75 per cent digestible dry matter (DDM %) while stems have been analysed as 50 to 55 DDM %. Animal intake declines as leaf availability declines. Stock selectively graze the leaf parts, with cattle observed to consume 80 per cent and sheep 60 per cent more leaf material than stems. This highlights the importance of the leaf component for yield, quality and animal production.
Table 2. Forage legumes showing range of feed values
|Lablab - leaf only
Lablab - stem only
Lablab - whole plant
|Cowpeas - leaf only
Cowpeas - stem only
Cowpeas - whole plant
|Soybeans - leaf only
Soybeans - stem only
Soybeans - whole plant
|Lucerne @ 10% flower
Forage sorghum - 80 cm regrowth
Source: Feeds Evaluation Unit, NSW Agriculture. Data derived from crops grown under different conditions.
Muldoon, D. K. (1985). Australian Journal of Experimental Agriculture 25: 417-423.
* Protein is measured as crude protein. More than 11 % is required to fatten stock.
** Acid Detergent Fibre. This is an estimate of the fibre component of the plant. Generally the lower the figure the better.
+ Digestible Dry Matter %. This is the proportion of dry material in the plant that the animal is able to digest. The higher the figure the better.
++ Metabolisable Energy is the energy value of the feed. It is measured as megajoules per kilogram of dry matter (MJ/kg DM). Feed must provide 10 to 12 MJ/kg DM for stock to fatten.
Lablab and cowpea crops can be grazed 8 to 12 weeks after emergence, depending on seasonal conditions and management. Crops should be allowed to reach a full canopy and a height of about half a metre before grazing. Cowpeas give better regrowth if grazing is delayed until flowering. Soybeans regrow poorly after grazing, so delay grazing as long as possible, until just before pods are half-filled.
Lablab can be grazed up to three times, cowpeas usually two to three times and soybeans usually once, depending on when the crop was sown, seasonal conditions and grazing management.
Heavy, prolonged grazing of cowpeas and lablab should be avoided. Remove stock as soon as the leafy portion of the plants has been eaten. If these crops are not grazed too hard, and the stems and basic plant skeletons are left intact, there will be maximum recovery and regrowth and you will obtain several grazings. Do not cut lablab or cowpeas if recovery is required. Grazing or cutting the crop lower than 15 cm will kill many plants, thus lowering plant density and production.
Stock numbers are usually adjusted to suit crop growth and to allow four to eight weeks' grazing. For crops to provide this length of grazing, stock numbers will usually be 15 to 25 sheep/ha or 1.5 to 2.5 steers/ha. Lablab will provide grazing three to four weeks longer into autumn than cowpeas or soybeans.
Strip grazing will help to obtain more efficient use of forage.
Cowpeas, lablab and soybeans have a low bloat risk for cattle, although some losses have been recorded, mainly where hungry stock have been introduced onto these crops. As a standard husbandry practice, livestock should be vaccinated with 5 in 1 or pulpy kidney vaccine before introducing them into these crops. Vaccination is especially important for recently purchased stock and for stock coming off dry or poor quality feed.
Cowpeas can cause photosensitivity around the face and ears in a small percentage of sheep, particularly crossbred lambs, but this is not considered a major or regular problem. Lablab and soybean appear to be free of this problem.
Stock often take a few days to acquire the taste for cowpeas and lablab when first introduced to them. Animals will wander around the paddock preferring to graze any grasses and weeds for up to a week, but then adapt quickly. However, they more readily accept soybeans.
Mature grain crops of soybean damaged by rain should not be grazed by sheep or fed to them, because they may be infected with the fungus, Phomopsis, which produces toxins that cause symptoms similar to lupinosis.
Weight gains for lambs and steers are comparable with those on lucerne.
Sheep. In one trial on cowpeas at Dubbo*, lambs averaged 197 g/head/day weight gain over a 30 day grazing period. At Coonamble+, first-cross lambs grazing cowpeas gained an average of 177 g/head/day over a 60 day period. They gained an average 198 g/head/day during the first month and declined to 159 g/head/day over the last 30 days, probably due to a reduced availability of leaf material.
Source: R. Kearins, former District Livestock Officer. Dubbo.
+Source: T. Paton, former District Livestock Officer, Coonamble.
Cattle. Commercial steers on lablab in the Merriwa/Quirindi districts, have had weight gains ranging from 0.5 to 1.2 kg/head/day, depending on age and condition of stock. Many graziers have recorded weight gains in the order of 0.7 to 1.0 kg/head/day over a 60 day grazing period. Sheep and lambs have also performed well on lablab.
Estimated carrying capacity
A lablab or cowpea crop of full canopy and about 60 cm high will produce around 3000 kg/ha of total dry matter.
Only the leaf portion, which is approximately half the total crop weight, is considered when estimating carrying capacity. This means that 1500 kg/ha of dry matter is available for grazing.
The following calculation can help determine a crop's carrying capacity:
|Crop total yield||3000 kg/ha DM|
|Leaf portion (50%)||1500 kg/ha DM|
|Livestock utilisation %
(allowance for trampling and spoilage)
|Cattle dry matter requirement
(3% of liveweight)
|a 300 kg steer would require |
about 10 kg of DM/day
Therefore crop carrying capacity =
|Feed (leaf) available x Utilisation %
Cattle dry matter requirement/day
|=||number of steer grazing days|
|1500 kg DM/ha
|=||90 steer grazing|
For a grazing period of 60 days, the average number of 300 kg steers that can be run on this crop is:
Cowpeas. In favourable seasons, early sown cowpeas can recover after grazing to produce a grain crop. Graze lightly, keeping as much of the plant frame as possible, and close up crops by early to mid-February. Caloona and Poona crops usually begin flowering in late February to March.
However, sowings for grain only are best made in December to early January. Crops are best harvested with an open front header using low drum speed and wide concave settings. Rotary drum headers give the best results.
Cowpeas are normally harvested in May to June, after several frosts-which are usually required to dry the vines.
Grain yields from these dual purpose cowpeas are very variable, but usually range between 0.4 and 1.0 t/ha.
The remaining stubble does not create any problems when double cropping with cereals.
Lablab. The quick maturing variety Koala can be grazed lightly and treated similarly to Caloona/Poona cowpeas, so that the basic plant frame is left and stock removed by early February.
In northern NSW, variety Highworth will produce a grain crop only rarely, when the onset of frosts is delayed until late June. Generally, Highworth flowers too late (early to mid-April) to produce grain, the plants being killed by frosts during May.
Soybeans. Soybeans do not recover for grain production after grazing.
Hay and silage
Cowpeas, lablab and soybean make excellent quality hay that compares favourably with lucerne hay (see table 3). A conditioner should be used to split or crack the thick stems. Crude protein levels generally range from 15 to 20 per cent with metabolisable energy (ME) values of 8 to 9.5.
Unlike haymaking species such as lucerne, these forage legumes will not recover after being cut low.
Soybean hay damaged by wet weather should not be fed to livestock because it may contain toxins from the fungus Phomopsis.
Lablab, cowpeas and soybeans can produce good quality silage, particularly when mixed with forage sorghums or millets.
Like all legumes, these crops have very low soluble sugar levels which can prevent good fermentation and production of silage. To produce good quality silage from pure crops of these summer legumes, it is necessary to cut and wilt the crop to 30 to 35 per cent DM before harvesting for ensiling. Cutting and wilting 18 to 24 hours before ensiling will allow the concentration of soluble sugars in the plants to increase and therefore ensure satisfactory fermentation of silage.
Table 3. Average nutritive value of hay
(% dry matter)
|Lucerne||90||15 - 20||8.5 (8 - 9.8)|
|Soybean||90||17||8 - 9|
Source: Feeds Evaluation Unit, NSW Agriculture
Excellent silage can be made by sowing cowpeas or lablab with forage sorghum or millet. These mixtures will also need to be wilted before ensiling.
When sowing cowpeas or lablab with forage sorghum or millet, sow three quarters of the recommended seeding rate for cowpeas or lablab with one half the recommended rate for forage sorghum or millet. This mixture also makes excellent hay.
For good quality hay, cowpeas are best cut around mid-flowering, but lablab can be cut any time after 12 weeks' growth. Cut soybeans before the pods are half filled.
The contribution of the following officers is acknowledged: Warren McDonald, Pasture Specialist, Tamworth; Ross Watson, formerly District Agronomist, Scone; Peter Desborough, Research Agronomist, Grafton; Chris Bourke, Senior Research Scientist (Poisonous Weeds), Orange; Bill Noad, Agricultural Resource Officer, Dubbo for editing.
Pasture improvement may be associated with an increase in the incidence of certain livestock health disorders. Livestock and production losses from some disorders are possible. Management may need to be modified to minimise risk. Consult your veterinarian or adviser when planning pasture improvement.
Author: Colin Mullen