Under the duration-controlled grazing regime cows are grazed on pasture over two four-hour intervals per 24-hour period, giving them sufficient opportunity to consume their daily pasture intake. The remainder of the time is spent away from the paddock using an appropriate standoff facility.
The theory is simple. As urine spots are a major source of nitrogen leaching in New Zealand dairy farming systems, if cows spend less time in the paddock, with less opportunity to deposit urine, this should drive a reduction in nitrogen leaching.
While standing off the cows still generate excreta but this is captured and stored as effluent, and returned to soils evenly in ideal environmental conditions at relatively low nitrogen concentrations.
Dr Christine Christensen, a research officer at Massey University, spent three years researching the concept for her PhD thesis with a three-year plot-based field trial at Massey University as the testing ground. She tested a duration-controlled regime – four-hour grazings after morning and afternoon milkings – against a standard routine of a seven-hour day grazing and a 12-hour night grazing. Cows in both treatment groups were offered 5-6kg drymatter (DM) of pasture on the plots, with another 2-3kg DM of supplement. Pasture production and nitrate leaching were measured for each trial plot.
Nitrate leaching from the duration-controlled plots was measured as being 43%, 65% and 53% less than that measured on the standard grazing plots over the three-year trial period – an average annual reduction of 52%. In terms of nitrate leaching, duration-controlled grazing had the most impact in late summer through autumn (February to May).
“The pasture simply hasn’t got time to take up enough nitrogen before the onset of winter drainage,” Christensen said.
In winter and early spring, duration-controlled grazing minimised treading damage and preserves soil structure.
A farm-scale systems trial based around the duration-controlled framework has kicked off at Massey’s No. 4 Dairy. The 200-cow freestall barn is part of the Pastoral 21 research programme which will provide the foundation for the duration-controlled herd. This herd will run alongside a herd managed along standard practice. There will be clear decision rules under the duration-controlled regime, tailored to the difficult Tokomaru Silt Loam soils.
Soil moisture deficit will be driving decisions over the winter and spring, when soil preservation and minimising treading damage is the aim. A deficit of less than 1.5mm (effectively saturation point) sees the cows off pasture, in the barn. A deficit of between 1.5mm and 5mm translates to spending the night in the barn while grazing pasture during the day. If the soil moisture deficit is more than 5mm, Christensen said the cows will spend as much time as possible in the paddock.
Over late summer and autumn, when the aim will be reducing nitrate leaching, the duration-controlled grazing regime will be adopted – four-hour grazings, twice daily, with the rest of the time spent in the barn.
One of the other critical success factors is the application of the slurry collected from the barn. The key philosophy is to get the slurry on early, when the soil moisture deficit exceeds 6mm and the tractor and slurry wagon can get on the paddock.
“Get it on as early as you can when the pasture is screaming out for it – then you get your pasture growth response over spring.”
Application rates will be low, just 1-2mm. As the season progresses, slurry will only be applied when the grass is actually growing. It will not be applied if the soil moisture deficit gets above 75mm.
Currently, there’s some reduction in the nitrate leaching value modelled in Overseer if duration-controlled grazing is adopted year-round. However, more research over a wider range of soil types, such as freer draining stony Canterbury soils, will be required before the modelled effects match the effects measured at No. 4 Dairy.
