Reducing N Loss with AgResearch

Nitrogen losses to the environment through the grazing of pasture and crops can be high in Otago/Southland, especially in winter and on heavy, poor draining soils. Losses can be reduced by on-off grazing, feed pads, herd homes and the like. Nitrification inhibitors may also have a place in the cool of the south. Each potential remedy has its own set of limitations and problems.

The intensification of dairying in Otago and Southland over the past decade or so has led to the increased use of urea-boosted pasture and forage crops to provide winter feed for larger herds. Wintering cattle on such crops causes considerable pugging and soil damage. This leads to an increase in leaching of nitrogen into ground and surface waters and in the emission of nitrous oxide, a potent greenhouse gas.

The largest source of lost N is urine because it is deposited in comparatively large amounts onto small areas. The amount of N exceeds the capacity of soil and plants to absorb it, especially in cold weather, and the excess is volatilised or leached away. Topdressing with urea in anticipation of feed shortages exacerbates the problem by increasing dry matter production and N uptake, which leads to an increase in the amount of N recycled to pasture as urine-N, and consequently to higher N leaching and N2O emission loss. In addition, higher inputs of N fertiliser increase direct N2O emissions that occur immediately following fertiliser application.

Dr Ross Monaghan, a scientist at AgResearch Invermay, has been part of a group studying N leaching in the south in an attempt to define losses and establish best practices for mitigation.

The area studied was the impact of N leaching from dairy pastures on groundwater quality. The conclusion was that fertiliser N applications could be considered sustainable at no more than 200 units/ha/year. This figure includes the N in effluent applications as well as fertiliser N.

A later study looked at nitrification inhibitors – chemical compounds that delay the transformation of ammonium ions into nitrate in soils and can reduce the risk of nitrate leaching and nitrous oxide emissions from urine and N fertilisers. Most nitrate is leached from urine patches from autumn and winter grazings, and Dr Ross Monaghan found that strategic applications of inhibitor around these periods could reduce such losses by at least a third. Similar reductions were achieved in nitrous oxide losses for most of the year, but the economics of regular applications of inhibitor are debatable. The longevity of inhibitor effectiveness decreases with increasing soil temperature, so southern NZ is likely to be the most successful region in which to use it, according to Ross.

“There is some argument among scientists at present as to whether the production benefits are as large as claimed” he says.

“In Southland we would say that the cost of the product is almost offset by the small increase in pasture yield and it’s also a benefit in terms of greenhouse gases because it reduces the emissions of nitrous oxide.”

The third area of nitrogen studied has been leakages from wintering systems. Ross says that in terms of N losses per unit area, forage crops leach N at about 3 to 4 times as much as pastures, which indicates a problem for parts of Southland where herds are wintered on free draining soils.

“We have also looked at whether the DCD inhibiter could manage those sorts of losses but the short answer is that it is not very effective in that sort of scenario,” he says. “That led us on to the more recent research around the off paddock wintering systems available for the southern dairy herds – wintering barns, herd homes, feed pads, on-off grazing using stand-off pads, off-farm grazing, etc. – and there is a lot of work going on in that area. It’s clear that in the cool, wet south on heavy soils there are production and environmental advantages in getting cows off those paddocks for at least some of the winter.”

Most farms show damage from autumn and winter pugging that soils take some time to fully recover from, and hence there is a considerable reduction in pasture productivity and a financial incentive to reduce treading damage.

However, DairyNZ analyses show that most herd shelters are costly and may increase wintering costs by $10 – $20 per cow per week, so widespread adoption will not happen rapidly. Ross says that many farmers are looking for more affordable systems that will help avoid high leakage from winter forage crops.

“Whatever you come up with, it is important that it doesn’t compromise cow welfare and that you manage it well. This will mean collecting and storing all the effluent from it and holding it for application when soils start to dry out and can utilise it,” he says.

“Good management is just as important if you are sticking with winter forage crop grazing. The preliminary evidence we have is that the way you set the paddock up and the way you graze it – protecting the gullies, swales and near-stream areas, back fencing and all those sorts of little things – they add up to an essential set of activities that help you really minimise the farm’ environmental footprint.”

“Gullies are critical source areas for sediment and phosphorus (P) losses, and we encourage farmers to really think about their management and protect them rather than try to get every last bite off them.”

“Reducing winter damage to soils under either pasture or crops by any of these means will certainly help mitigate nutrient and sediment losses and may also be the cheapest means of increasing total feed supply on southern South Island farms.”