Winter-grazed forage crops play an integral role in New Zealand farm systems, especially in the cooler climes. They offer flexibility in grazing management, while keeping cost structures low. However, it is becoming increasingly evident that winter-grazed forage crops represent a hot spot, a critical source area, for nitrogen leaching.
With this in mind, we commissioned AgResearch to investigate the use of our nitrification inhibitor, DCn, for targeted application on winter-grazed forage crops.
In a study conducted in Southland, small plot trial work over three years showed that nitrate leaching loss from early winter grazing of forage crops was high, averaging 55 kg N/h/year. This was about five times higher than the loss seen on the adjacent pastoral milking platform.
Even though the area occupied by the winter-grazed forage crop was only 10-15% of the total farm, it accounted for around 40% of the total farm system nitrate leaching.
However, the study showed that applying DCn within one week of grazing the forage crop reduced nitrate leaching by approximately 20%, conserving more soil nitrogen in the system to contribute to growth once pasture was re-sown in spring.
Subsequent trial work, which included measurement of pasture production following regrassing of a winter-grazed forage crop, showed that areas treated with DCn grew around 13% more pasture dry matter than untreated areas. If we assumed 80% pasture utilisation and allowed a return of $6/kg MS, then one application of DCn to a winter-grazed forage crop could result in a net benefit of $340/ha.
Although sheep distribute urine more evenly than cattle, and their urine has a lower nitrogen concentration, nitrate leaching under intensive sheep-grazed sytems over winter can be surprisingly high.
AgResearch studies showed that, where sheep-grazed forage systems were treated wtih DCn, there was evidence that a 24% drop in nitrate leaching could be achieved, although there was a lot of variability between samples, so it was difficult to draw any firm statistical conclusions.
In the same study, AgResearch found that cattle-grazed forage crops treated with DCn showed a 35% reduction in nitrate leaching.
Given the large amount of nitrogen leaching from these systems, this represents a considerable opportunity for farmers to capitalise on, in the subsequent spring recropping or regrassing phase.
