THE EFFECT OF STOCKING RATE ON SOIL SOLUTION NITRATE CONCENTRATIONS

Economically viable and productive pasture-based milk production systems in South Africa are the mainstay of meeting the growing need for milk and dairy produce. However, pasture-based systems also carry a high risk of an environmental impact which may increase as new systems are developed. Within such systems, which can be highly intensive, one is concerned about the effect of potential nitrogen (N) losses to ground and surface waters. This demands an appraisal of N flows within such systems. This was the topic of the study of Dr J. McCarthy and colleagues in Ireland. The results were published in the Journal of Dairy Science, Volume 98 of June 2015, pages 4211 to 4224, using the title: The effect of stocking rate on soil solution nitrate concentrations beneath a free-draining dairy production system in Ireland.

The authors conducted a 3-year (2011 to 2013) whole-farm system study on a free-draining soil type that is highly susceptible to N loss under temperate maritime conditions. In the study, soil solution concentrations of N from three spring-calving, grass-based systems designed to represent three alternative whole-farm stocking rate treatments were compared. These were a low (2.51 cows per hectare), medium (2.92 cows per hectare) and a high stocking rate (3.28 cows per hectare). Each stocking rate treatment had its own farm area, containing 18 paddocks and 23 cows. Nitrogen loss from each treatment was measured using ceramic cups installed to a depth of one meter to sample the soil water. The annual and monthly average nitrate, nitrite, ammonia and total N concentrations in soil solution collected were analyzed for each year using repeated measurements over time. Subsequently, and based on the biological data collated from each farm area treatment within each year, the efficiency of N use was evaluated with an N balance model.

If expressed on a similar N input base, the results showed that by increasing stocking rate increased grazing efficiency and milk production per hectare resulted. In contrast, stocking rate had no significant effect on soil solution concentrations of nitrate, nitrite, ammonia, or total N. The N balance model evaluation of each treatment, incorporating input and output data, indicated that the increased grass utilization and milk production per hectare at higher stocking rates resulted in a reduction in N surplus and increased N use efficiency.

The results are exciting as they suggest the possibility of sustainable intensification of pasture-based dairy systems because, at the same level of N inputs, increased stocking rate had little effect on N loss. The results, furthermore, suggest that greater emphasis should be put on increased pasture production and utilization under grazing, to further improve the environmental impact of grazing systems.