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Nitrogen fertilisers: when a booster becomes a maintenance thing

By Bruce McCorkindale

Traditionally famers and their advisers have seen nitrogen fertilisers as a booster to increase pasture growth at key times and boost growth of crops and supplementary feeds.

What happens if a farm system changes significantly; for example, a large number of dairy cows are wintered on crops every year and requires considerable balage and hay?

The situation could be that there are many paddocks regularly having hay and balage removed, but never having those crops fed back out on them because the supplements are being used on crop paddocks.

Even if the farmer is being very conscientious about replacing the minerals removed – such as phosphorus, potassium, and sulphur– and some lime to add calcium, the equation for nitrogen loss is often overlooked.

Consequently, soil nitrogen drops and pasture species that are more competitive under a low-nitrogen environment thrive, resulting in a significant change in pasture species.

A crop of 15-20 bales per hectare of balage will remove about 60-80 kg of nitrogen.

Other sources of nitrogen loss throughout the year, including atmospheric losses from urine patches and leaching, will take the annual loss to around 110-120 kg of nitrogen per hectare (N/ha).

Another problem is in reduced nitrogen fixation by clover. The action of closing a paddock to produce balage tends to increase grass competition with clover. As a result, annual nitrogen fixation suffers, to approximately 35-60 kg N/ha.

If the removal of hay or balage happens infrequently, the paddock will be able to recover within a couple of years.

However, when it occurs more frequently, application of nitrogen fertilisers after the supplements have been removed should be considered as a maintenance rather than booster fertiliser to return available soil nitrogen to a level that maintains the desirable balance of pasture species.