Relation between nutrition, performances and nitrogen excretion in dairy cows

Reducing N excretion from individual cows is one way among others to better cope with the problem of the negative contribution of the dairy herd to th e "Nitrogen cycle" on the farm. The objectives of this paper are first to q uantify the effects of the main forage systems and protein feeding level on the amount of N excreted in relationship to their simultaneous effects on animal performances and efficiencies and then to examine the particularitie s of the grazing situation. N excretion depends primarily on the level of N intake i.e. on forage species, fertilisation, growth stage and protein sup plementation and therefore varies between the main usual forage systems fro m 90 to 150 kg N per cow per year (i.e. 12 to 20 kg N per ton of milk). A s imple method is proposed to calculate the load of excreta N from a dairy he rd according to the specific pattern of feeding practices over the year. Th e effects of the level of metabolic protein supply over a wide range of die tary concentrations (80-125 g PDI per UFL) were analysed from a set of 5 fe eding trials. Through that range, excreta N were largely increased and prod uctive responses were also important (but without any residual effect), not only for milk yield (+15 to +30%) but also for milk protein concentration (+2 g.kg(-1)) and feed efficiency (+10%). The simultaneous increase observe d in feed intake (+1 to +3 kg DM) accounted for half of the productive resp onses and could explain why the nutritive balance was hardly affected by pr otein levels, even in early lactation. Most of the productive parameters re sponded to increasing PDI levels according to laws of decreasing return tha t are given in the text. On the contrary, the relative N losses (excreta N per milk N) decreased curvilinearly with decreasing PDI levels reaching a m inimal plateau. The concentration of 100 g PDI per UFL appears as a common key value for both phenomena: higher PDI levels results only in small incre ases in productive performances whereas N losses increase sharply, and the reverse occurs with lower PDI levels. At grazing, the level of N fertilisat ion, through the increase in sward yield and N content, is the main determi nant of productive performances and losses of excreta N per ha. The other f actors of sward valorisation such as stocking rate only have a moderate eff ect whereas the effect of concentrate supply could be low or high according to protein content. Total grazing days per ha is an integrative parameter that accounts quite well for all these factors since it reflects both sward yield and herd valorisation conditions. Roughly, each extra 100 grazing da ys induced by higher fertilisation increases N flows by 10-15 kg.ha(-1) as milk and by 70-80 kg.ha(-1) as excreta.