Target forage crops for nitrate leaching

9 June 2011

Target forage crops for nitrate leaching

Applications of the nitrification inhibitor DCn to winter-grazed forage crop areas can reduce nitrate leaching by as much as 30 per cent, retaining nitrogen in the soil for future pasture or crop growth, an AgResearch study shows.

The research, which has been running for more than three years, shows that there is the potential for nitrate leaching from winter forage crops to be as much as five times higher than from adjacent dairy pasture.

Aaron Stafford, Ballance’s Science Extension Manager says the work has produced some exciting results.

‘The winter-grazed forage crop, which occupied 10-15 percent of the dairy farm area, was responsible for about 40 percent of the total farm system nitrogen (N) leaching.

‘Effectively, the forage crop was acting as a critical source area for N-loss within the larger farm system. Because the amount of N being leached under grazed forage crops is typically quite large, the additional soil N that can be “conserved” with DCn is potentially quite large, providing a good opportunity to turn this into extra feed in the subsequent pasture renewal or cropping phase.’

An initial study indicated that DCn applied within a week after grazing a forage brassica crop reduced nitrate leaching by around 20 percent. Measurement of pasture production following regrassing of the winter-grazed forage crop showed that areas treated with DCn grew about 13 percent more pasture dry matter.

In a two-year field trial on free-draining pumice soils the impact of DCn was even greater. It showed that for a kale crop grazed in June, N leaching losses were extremely high (130-170 kg N/ha) due to the large winter drainage.

‘The use of DCn immediately after grazing reduced nitrate leaching by about 30 percent, meaning that an extra 40-50 kg N/ha was available in the soil the following spring. That’s N that would otherwise have been leached.’

These measurements of N leaching were taken by analysing soil solution collected in ceramic suction cups, backed up at the same time by measurements of the actual amount of soil mineral N. So far, trials have been conducted in Southland and the Central Plateau.

Mr Stafford says there is no reason to limit the use of DCn to forage crops grazed by dairy cattle, as nitrate leaching under intensive sheep-grazed winter forage crops can also be surprisingly high.

‘Remember, DCn is not a “silver bullet” that will completely eliminate nitrate leaching. Farmers should also consider other management strategies such as time-restricted grazing and use of a stand-off pad to reduce the amount of urine being deposited in the paddock.
Note: DCn contains the active ingredient dicyandiamide (DCD) in a granular form.
ENDS

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