New research has exploded the ‘‘simplistic’’ assumption that the more dairy cows on a farm the higher the nitrate leaching. It also suggests an increase in cows could actually reduce nitrate leaching.
But there are major ‘‘buts’’ and conditions attached to the finding, which is not a silver bullet for dairying environmental risk management, warns lead author of the report on the research, DairyNZ principal scientist John Roche.
However, he said, the research conclusion, now the subject of a submission to the world’s most prestigious dairy science forum, Journal of Dairy Science, was ‘‘groundbreaking in that it flies in the face of what has been simplistically assumed – that more cows equals more nitrate leaching’’.
The big buts or provisos are that nitrate leaching decreases in a higher stocking rate scenario provided feed is not purchased to keep cows milking longer into autumn, and only if the number of lactating cows is decreased between the high-risk leaching autumn time of March to June.
The conclusion of the research, a collaboration principally between DairyNZ and AgResearch, is important for dairyfarmers because it gives them more options to manage their farm systems to achieve profitability as well as providing another tool to manage environmental regulatory compliance, Dr Roche said.
‘‘What it is pointing to, and we have good component research undertaken by AgResearch that supports this, is that surplus nitrogen in the diet through the autumn period is a key contributor to our environmental footprint. This is one way of doing it (managing footprint).
‘‘Higher stocking rates facilitate better pasture management and we know that pasture utilisation is strongly linked to profitability. What this means is, we can have very profitable systems and manage our environmental footprint as well.’’
Dr Roche said the assumption that more cows per hectare equalled more nitrate leaching was because people have confused stocking rate with intensification of farming via purchased feed.
‘‘In many of those situations we can see an increase in nitrate leaching with increasing stocking rate," Dr Roche said.
"So there has been a push from some quarters that we need to remove cows from the land for a period of time, or we need to put down concrete or increase effluent-collection facilities to reduce the amount of time those cows are on pastures during that critical time of the year, which is now March through to about June.
‘‘In fact it hasn’t been the increase in stocking rate that has done that, it’s more likely to have been the purchase of feed in at the same time. A classic example of people adding two and two and not getting four.’’
The research was based on a pastoral, and virtually ‘‘closed’’, feed system, he said.
Young stock were reared off-farm in the model. Increasing the stocking rate meant rearing more heifers, which increases nitrate leaching. ‘‘But this pales into insignificance in comparison with the effect of the (number of cows) on the milking platform,’’ he said.
The key time for a dairy farm to contribute to nitrate leaching is through urine patches during March, April, May and June.
‘‘That’s believed to be because during that time the grass growth rate slows down but you’ve still got high levels of protein in the grass and therefore you get high concentrations of nitrogen in the urine patch and the grass is not able to take it up. It’s just prior to the time when rainfall exceeds evapotranspiration so you get downward movement of the water through the soil and you get loss of nitrate from that.’’
Dr Roche said the way a farmer could manage high stocking rates systems when not buying-in feed was to identify poor-producing cows early in the season, for culling around Christmas or early January. Also to be identified are cows that are not pregnant, which should be off the farm by early February. Based on the remaining cow numbers, the farmer could then set up the farm system for the following season.
This would involve drying off early cows that need to gain condition and drying off the remainder of the herd more quickly than if the farm was buying-in feed.
‘‘So by doing that your stocking rate is actually declining by default at the critical time of the year and particularly the food demand of the herd is declining,’’ Dr Roche said.
A cow in-milk would eat 15kg of dry grass a day. A dry cow at the same time would be eating less than 8kg.
‘‘So by drying off sequentially you have a greater proportion of your herd actually dry and nitrogen surplus to their requirements is far less and therefore there is less nitrogen coming out in urine, less nitrogen in every urine patch, and the nitrogen coming out in urine is spread over a greater area because the more cows the more urine patches," he said.
‘‘We believe that’s the reason (for the research finding).’’
Dr Roche said further research by DairyNZ and AgResearch was planned ‘‘to determine why we have seen what we have seen’’.
‘‘But what is very, very clear is that nitrate leaching does not go up with the stocking rate. If anything, the results of this study suggests it goes down. But this is no silver bullet.’’
But for sensitive farming areas of the country where agriculture will be limited by the amount of nitrogen that can be leached, the research finding is important, Dr Roche said.
‘‘For sensitive zones, the profitability of a dairy business in that environment will be defined by the amount of money they make per kilogram of nitrate leached is what will define their profitability.
‘‘What we are seeing with an increased stocking rate is greater pasture growth, greater pasture utilisation, so we’re getting more pasture harvested.
‘‘But the key thing is that extra pasture is harvested in the springtime when there is very limited risk of nitrate leaching. So the increase in milk production is before Christmas and then after Christmas we are sequentially drying off the herd and reducing the nitrate being deposited in urine.’’
So what is the optimal stocking rate under the new research finding model?
Dr Roche said from a productivity perspective and taken across different land classes, the optimum stocking rate, established by research 17 years ago, is about 80kg of live-weight per tonne of feed available.
‘‘That means in the lower/central Waikato we were talking stocking rates of about 3 to 3.3 cows/hectare as being optimal for an all-grass system. If people were bringing in feed, they would increase the stocking rate in terms of cows per hectare, but the 80kg (ratio) would remain the same.’’
However, the ground has shifted with the increased focus on environmental management and brought-in feed so an optimum stocking rate taking into account productivity and environmental considerations has not yet been defined.
‘‘It’s very dependent on where you are and the sensitivities of that catchment. But in a nitrate sensitive zone, for example, it appears your profitability per kilogram of nitrate leached actually increases with the stocking rate, as long as feed is not purchased to keep cows milking longer.’’
Dr Roche said the reason for that is most of the milk sold from higher stocking rate farms is produced before Christmas.