The regularly changing and currently high price of nitrogen fertiliser is making dairy farmers look very closely at the economics of topdressing with it to grow extra grass pasture.

Research over the years has shown that when we apply nitrogen (N) to grass based pastures, we generally get a response ranging anywhere from about seven to 25 kilograms dry matter (DM/kg) of N applied.

The factors controlling the size of the response to N include the amount of N already available in the soil, temperature, available soil moisture and any other limiting plant nutrients such as potassium or sulphur.

Urea is the cheapest source of nitrogen fertiliser, with a recent North Coast quote putting it at $695 per tonne bag, excluding GST. At 46 per cent N this puts urea’s cost at $1.51/kg N.

Using the above response range, this translates to a cost of around six to 22 cents/kg of extra DM grown.

At only 50pc utilisation, this equates to 12 to 44 cents/kg of DM consumed, but farmers who are good enough to get 75pc pasture use reduce this cost to around eight to 29 cents/kg DM consumed.

This highlights the importance of good management to get high pasture use.

One kg of pasture DM consumed should convert to at least a litre of milk currently valued at around 48 cents for many North Coast dairy farmers.

Comparing the cost of using N to grow extra grass with the cost of purchased feed, lucerne hay at $250/tonne equates to 28 cents/kg DM, while grain at $400/tonne translates to 45 cents/kg DM.

These figures show that the use of N fertiliser to grow extra grass to turn into milk is still a viable proposition, providing pasture use is good and the grass response to N is not being limited by poor growing conditions.

Growing N fertilised ryegrass dominant pasture during the cool season is a reliable way to get good quality feed early in the season and also during cold spells in winter.

The main alternative to the high N rye approach is greater use of clover.

To get white clover or Persian clover to make their best contributions in a mixed ryegrass and clover pasture, we need to cut back the amount of N fertiliser used - so that the ryegrass does not dominate the clover.

This lower N fertiliser input reduces cost, but usually results in a longer wait to the first grazing and less pasture growth in the vital winter period.

The clovers are less reliable than N top-dressed ryegrass in cold frosty winters, but they come into their own in good spring conditions.

The other thing with the clovers is that they are more sensitive than ryegrass to low soil pH and marginal levels of soil phosphorus and potassium.

Contact Kerry Moore, Kyogle, (02) 6632 1900.