Agresearch scientists Tony van der Weeden, Seth Laurenson, and Dave Stevens, who are all based at Invermay, told Southland and Otago farmers at a seminar on managing farm systems to protect soils and reduce greenhouse gas admissions there were more potential problems in autumn than spring or summer for nitrous oxide and nitrate losses.
“Nitrous oxide is the second only to methane as the most important greenhouse gas for New Zealand agriculture,” van der Weeden said.
“When there is less air in our soils due to moisture or compaction, it is released into the atmosphere from urine patches. It has 300 times the global warming potential compared with carbon dioxide.
“When soil is dry the losses are usually quite low and we want farmers to start thinking of ways to minimise grazing when pastures are saturated.”
He said using feedpads and barns not only minimised pasture damage but also reduced greenhouse gas emissions.
“Although there is the potential to cause problems when effluent from pads is collected in ponds and then spread onto paddocks.
“Nitrous oxide can be released from ponds and if effluent is spread onto water-logged soils the losses will still occur. It’s called pollution swapping.”
The Invermay scientists are trying to put dollar figures on the cost of damaged pasture following grazing during wet conditions compared with the cost of feedpads and feeding cows on them during lactation, hoping that what is good for farmers’ bottom line is also good for the environment.
“We’re trying to model the cost of the loss of grass through pugging but there are a lot of factors such as soil water concentration, the stocking rate, the length of grazing time, the covers at the start of grazing, and how long it takes for paddocks to recover, which is affected by weather after the pugging event,” Laurenson said.
The capital cost of building a feedpad, maintaining it, feed costs, and returning effluent to paddocks also differed from farm to farm.
‘Nitrous oxide can be released from ponds and if effluent is spread onto water-logged soils the losses will still occur. It’s called pollution swapping.’
He said trial work using cows fitted with GPS collars at Telford near Balclutha had shown cows could eat their intake between milkings in four to five hours and could then be stood off without having to be fed supplement.
“Even at night cows still walk a lot and cause pugging damage.”
Although nitrogen losses occurred through the year, it was urine from autumn grazing that potentially had the most risk of entering the atmosphere and waterways.
“Nitrogen losses can be far greater then as plants take up less nitrogen during cooler temperatures so farmers needed to be careful when spreading effluent from ponds during autumn and also late applications of urea.
“Especially nitrates. Urine patches from the autumn stay in the soil and are not used by the plants so when it rains during winter they end up in waterways.”
However, farmers were less likely to stand cows off during the autumn as paddocks were generally dryer.
Many farmers at the Milton seminar were concerned regulations in the future would force them to keep cows on pads, even when no pugging would occur, to stop nitrogen leaching.
“We want to stay grass farmers,” one farmer said. “We’re worried the goal posts on how much nitrogen leaching we are allowed are going to continually shift.”
The scientists wanted to hear how farmers currently restricted grazing when soils were wet.
“We need the practical stuff from you,” van der Weeden said.
Phone apps, emails, and other systems to warn farmers when the risk of nitrogen losses because of high soil moisture content and low soil temperatures was high, were discussed.
The scientists’ work, funded through the government’s Global Research Alliance, is ongoing and a further seminar will be held in a year’s time.
DairyNZ has resource books on standoff pads and nutrient management available through their website www.dairynz.co.nz or by phoning 0800 4DairyNZ (0800 4 324 7969)
