PASTURE YIELD AND ENVIRONMENTAL EFFECT OF NITROGEN FERTILIZATION.

The response of pasture yields to nitrogen (N) fertilization is a long-standing topic of debate. In dairy-pasture systems, N application is often thought to be directly proportional to pasture yield. The question is: is it. This was evaluated using data from 153 pasture camps over five years. Fertilizer application rates were grouped into three treatments: <200, 200-350 and >350 kg N per hectare (ha) and the herbage yield response over the five years was recorded.

There were no differences between treatments in total annual herbage yield, suggesting that the about 200 kg N per ha per year was probably sufficient. Nitrogen level had a weak but significant negative correlation with total annual yield and only accounted for 6% of the yield variation. Because of the lack of herbage yield response with the higher N applications, N use efficiency was the best at the low level of application. Coinciding results showed that spring and summer account for the highest yields; the warmer and moist conditions favouring N mineralisation in the soil. Therefore, farmers need to consider time of year and plan their monthly or seasonal fertilizer application accordingly, to account for peak N mineralization rates.

From these results, a second question emerges: if there is little response at high N application levels, what are the environmental consequences of the N not used in herbage growth. This was tested in a three year study on rotationally grazed pastures. Different levels of N fertilizer were applied, namely 0 (N0), 220 (N20), 440 (N40), 660 (N60) and 880 (N80) kg N per ha per year. The additional N-input from the manure of the grazing cows was estimated as 450 kg N per ha per year on all treatments. The environmental efficiency was estimated by the carbon footprint (CF), the pasture camp-N-balance and the whole-farm N-balance. For the CF the pasture milk yields from the fertilizer treatments were calculated according to the forage intake, the net energy lactation content of the forage and the stocking rate.

The average pasture camp-N-balance over the three years for the N0, N20, N40, N60 and N80 were -119, +86, +299, +501 and +706 kg N per ha per year respectively. The highest CF (~2.7 CO2eq per kg energy corrected milk [ECM]) were associated with the N80 treatment and the lowest with the N0 and N20 treatments (~1.4 kg CO2 per kg ECM). The farm-N-balance as well as the N-footprint were positively correlated with increasing N-fertilizer rates. Methane from enteric fermentation (~44% ± 2.7) and nitrous oxide (N2O) (~15% ± 3.1) emissions were main contributors to the CF. It was concluded that fertilizer N applied at a level of 220 kg N per ha per year (N20) in addition to the animal excreta N is sufficient to ensure adequate pasture yields of about 20 ton DM per ha per year to achieve a pasture milk yield of about 17 ton ECM per ha and a low CF. This is in line with the results above.

References:

 M. P. Phohlo, P. A. Swanepoel & S. Hinck, 2021. Excessive nitrogen fertilization is a limitation to herbage yield and nitrogen use efficiency of dairy pastures.  Sustainability 13, x. https://doi.org/10.3390/xxxxx.

H.P.J. Smit, T. Reinsch, P.A. Swanepoel, C. Kluß & F. Taube, 2020. Environmental impact of rotationally grazed pastures at different management intensities in South Africa. Sustainability 12, x. doi/www.mdpi.com/journal/sustainability.