Effect of a simultaneous reduction of nitrogen fertilization and stocking rate on grazing dairy cow performances and pasture utilization

Three levels of nitrogen fertilization and stocking rate corrected to obtain the same level of herbage allowance were applied to permanent and sating meadows in Normandy over 5 years: high = 320 kg N/ha/year and 2.5 cows/ha; medium = 100 and 2.0; low = 0 and 1.7. Each year, 18 dairy cows were assigned to each treatment for the whole grazing season. Weather conditions in the first 2 years only permitted 90 days of gazing in the spring. Three full grazing seasons (190 days) followed. Reducing nitrogen fertilization systematically reduced the biomass available per hectare and per cycle (2540, 2230 and 1760 kg DM for the H, M and L levels, respectively), the CP content (225, 177 and 158 g/kg DM, respectively) and the digestibility of the grass available (0.74, 0.71 and 0.70 units, respectively). In spite of the total suppression of nitrogen fertilization, the annual productivity of the meadow reached 8.5 DM under low treatment and did not decrease throughout the 5 years of the experiment. In spite of a small decrease in the nutritive value of the grass and with a same mean of herbage allowance, the average individual milk yield, the butter fat and protein contents of the five springs and of the last 3 years (27.3 and 22.1 kg/day, respectively), were not different under all three treatments. There were inter-annual differences in treatment response, especially in the spring, associated to wider variations of available grass under the low treatment. Supplementation of 100 kg N/ha/year in the medium protocol reduced that inter-annual variability. Milk yield per hectare decreased in the same proportions as the stocking rate and on average amounted to 15.6 kg milk/ha less for each kg of nitrogen suppressed from 320 to 100 kg N/ha/year and to 19.0 kg milk/ha for each kg of nitrogen suppressed from 100 to 0 kg N/ha/year. These results are to be corrected for local agro-climatic potential because their dependence on soil nitrogen supply increases as nitrogen fertilization is reduced. ((C) Elsevier/Inra.).