Back Reducing Nitrogen Leaching

Date: 12 May 2014

During the past decade there has been an increasing focus on improving the quality of the water in our lakes, rivers and streams. Currently regional councils throughout the country are either working on, or rolling out plans which set nitrogen leaching targets for each catchment area. Like it or not, this legislation is set to have a big impact on the way dairy farmers can operate in the future.

Overseer is the management tool which is being used to predict on-farm nitrogen (N) leaching. This computer model calculates and estimates the nutrient flows in a productive farming system.It identifies risk for environmental impacts through nutrient loss including run off and leaching.

DairyNZ suggest that if Overseer predicts you are within 6 kgN/ha of the target, you just need to make your system more efficient but if you are more than 6 kgN/ha above the target you need to look at your whole farm system.

While many factors contribute to on-farm nitrogen leaching, cow urine is the main villain responsible for 69% of the nitrogen loss from a typical dairy farm1.
Assuming there is a best-management fertiliser policy in place, farmers who are well above the N-leaching target can decrease N-leaching by either decreasing the number of cows (and therefore the amount of urine excreted) per hectare, or by moving to a more intensive system which uses a number of strategies to decrease the impact of urinary nitrogen.

These strategies include:

1. Decreasing the amount of nitrogen in cow urine. When dietary N exceeds animal requirements, a high proportion (typically 60-70%) of the excess N intake is excreted in the urine. The more surplus protein a cow eats the more urinary nitrogen she excretes. In early lactation a high producing cow requires around 18% crude protein but ryegrass-clover pasture can contain more than 30% crude protein. Diluting the excessive dietary protein in pasture by feeding a low crude protein supplement (e.g. maize silage or maize grain) will decrease the amount of nitrogen in cow urine.

2. Growing crops which utilise soil nitrogen. Maize has an effective rooting zone of 150-180 cm depth. This allows it to capture nutrients from depths 2-3 times greater than most pasture species. New Zealand research has shown maize crops can be grown in high fertility dairy paddocks (including effluent paddocks) without the need for any artificial fertiliser. The maize plant effectively "mines" the soil nutrients (including nitrogen) which have fallen below ryegrass roots. The net result is a reduction in nitrogen leaching and the provision of low cost maize silage.

3. Collecting and distributing cow urine over a large area. Dairy cows are estimated to urinate 10-12 times per day. Each time they deposit around 2 litres of urine on an area that is just 0.2 m2 in size. The volume of nitrogen in a urine patch (1,000 kgN/ha) is greater than the annual plant uptake (300-700 kgN/ha/year) and this results in the build-up of soil nitrogen2. Leaching losses from urine patches are high, especially in the winter when rainfall is high. Keeping cows off pasture during wetter months, collecting effluent and applying it evenly over a large area at an appropriate rate and time will greatly reduce N-leaching on many farms.

The challenge for many will be how to reduce N leaching while at the same time maintaining (or improving) profitability. The good news is it can be done and there are a number of farmers who are already doing it. While mitigation strategies such as feed pads and wintering barns are a significant capital cost, they deliver a number of benefits in addition to reducing nitrogen leaching. I'll discuss this in more detail in my next article.

1 Agresearch data for a dairy farm producing 850kgMS/ha using 100kg fertiliser N, effluent applied to land.
2 Moir et al. 2011. The spatial coverage of dairy cattle urine patches in an intensively grazed pasture system. Journal of Agricultural Science 149: 473-48

Ian Williams

Forage and Farm System Specialist