Incidences phytotechniques et zootechniques d'une réduction ou d'une suppression de la fertilisation azotée sur des prairies pâturées par des vaches laitières

Abstract

Phytotechnical and zootechnical effects of a nitrogen fertilizer reduction or suppression on pasture grazed by dairy cows. Three levels of intensification were compared for a permanent pasture grazed over 6 consecutive years in a rotational grazing system. The three levels of intensification were as follow: a high nitrogen fertilizer input system with 136 kg N.ha -1 and with 3.35 cows.ha -1 - H-N group -, a system with a moderate fertilizer input in which nitrogen fertiliza- tion was reduced by 33% but with the same stocking rate - M-N group - with 91 kg N.ha -1 along with 3.35 cows.ha -1 and a system with no fertilization associated with a reduced stocking rate - 0-N group - with 2.51 cows .ha -1 . There were no effects of the treatments on grass character- istics - e.g. post grazing height, percentage of refusals, total nitrogen content, fiber content. The absence of nitrogen fertilization reduced the pre-grazing height and DM yield of the pasture. The clover content was reduced in the first and sixth years in nitrogen fertilized groups. The individual milk yield was not affected by the treatments (15.4 kg .d -1 ) but the milk yield per hectare was largely affected by the stocking rate. The protein content and cell count were unaffected by the treatments but butter fat content was higher in the 0-N group than in the other two groups (4.10 vs. 3.90%; P < 0.05). The average daily liveweight gain was not affected by the treatments (0.23 kg .d -1 ). The plasma and milk urea contents were significantly higher in fertilized groups than in 0-N group (164 vs. 151 mg N.l -1 in plasma and 194 vs. 175 mg N.l -1 in milk respectively; P < 0.05). Daily nitrogen excretion per animal was similar for both treatments but when expressed per ha and year, the excretion was higher in the fertilized groups. Thus, it has appeared that a 33% reduction of nitrogen fertilization with a sim- ilar stocking rate allowed to maintain performances either on an individual animal or a per hectare basis. Grazing management with no nitrogen fertilization but with an adjusted stocking rate induced a reduction in performances expressed per hectare but maintained similar individual performances as with high input systems. Furthermore, there were both a higher nitrogen efficiency and a reduction in nitrogen excretion per hectare. dairy cows / nitrogen fertilization / grazing / milk yield / nitrogen balance