“If you overestimate your nitrogen leaching then you may introduce costly mitigation measures to your farm to lower it that you don’t need to do,” Ballance Agri-Nutrients science extension officer Jim Risk said.
“But if you underestimate it you will think you are okay and then find out later you need to reduce it to comply with council regulations and have not enough time to do so.”
He said the figures coming out of Overseer were only as good as those going in.
“It’s rubbish in, rubbish out so having accurate monthly data is crucial.”
Risk said the five factors for nitrogen loss were animals, effluent, fertiliser, feed and drainage.
“On a dairy farm, the cow’s urine is the greatest source of nitrogen with a typical dairy cow urinating two litres of urine on 0.2m2 which penetrates to a depth of about 10mm in less than a minute. A urine patch works out to be the same as if you were putting between 600 to 1000kg N/ha on.”
At high stocking rates, urine patches could overlap, causing even higher concentrations of nitrogen.
Risk advised farmers to value effluent and use it wisely.
“If you had to put the equivalent on in fertiliser as what spreading your effluent on your farm gives you it would probably work out about $15,000 to $20,000 so it has significant value.
“But you have to apply it so you don’t lose those nutrients. Leaching happens when effluent is applied to wet soils and at too high a rate for the soils to absorb it.
“If your own soils are too wet and your effluent pond is full consider spreading it on your runoff or even selling it.”
Nitrogen fertiliser should only be spread in small amounts and on actively growing pastures which could take it up and use it.
“Small, regular applications of nitrogen fertiliser means the nitrogen is much more likely to be used by plants than lost by leaching.”
He warned farmers not to overlook bought-in feed and the protein content of all feeds offered to cows.
“The higher the protein in the feed the more nitrogen will be in the cow’s urine and so the more likely that there will be nitrogen losses.”
He said when figures were put into Overseer, divide the farm into blocks based on effluent application and soil types.
“This will change your figures and give you a more accurate estimate of your nitrogen losses. Soil types such as stony soils, sands, gravels and pumice are more prone to leaching than denser soils. Rainfall also comes into it but that can’t be controlled. What can be controlled is where some farm activities are done. Think about where cows are stood off and where crop paddocks are to minimise drainage from soil types that are prone to leaching.”
Participants at the workshop told how they were working on reducing nitrogen losses by not growing winter crops on wet soils, extending areas on the farm where effluent could be spread, building feedpads to stand cows off to stop urine patches on sodden soils, and grazing pastures carefully to prevent pugging and possible leaching.
DairyNZ’s catchment engagement leader Julia Christie said dairy farmers shouldn’t just be looking at the figures coming out of Overseer for their farm.
“It’s not just about getting nitrogen losses down to a number that’s required. You should be thinking about your long-term goals. You should be asking yourself how you want to farm in the future.”
She urged farmers to form catchment groups and to become involved with looking after their local waterways.
“Get to know your nutrient budget, find out how you can make a difference.”
She said all dairy farmers needed to work together to meet national and local government expectations for clean water.
“Every improvement you make on your farm increases the sustainability of the whole dairy industry.
“It’s really up to all of us.”
