Relation between nutrition, performances and nitrogen excretion in dairy cows

Abstract

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 the "Nitrogen cycle" on the farm. The objectives of this paper are first to quantify 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 particularities of the grazing situation. N excretion depends primarily on the level of N Intake I.e. on forage species, fertilisation, growth stage and protein supplementation and therefore varies between the main usual forage systems from 90 to 150 kg N per cow per year (i.e. 12 to 20 kg N per ton of milk). A simple method is proposed to calculate the load of excreta N from a dairy herd according to the specific pattern of feeding practices over the year. The effects of the level of metabolic protein supply over a wide range of dietary concentrations (80-125 g PDI per UFL) were analysed from a set of 5 feeding trials. Through that range, excreta N were largely increased and productive 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 observed in feed intake (+1 to +3 kg DM) accounted for half of the productive responses and could explain why the nutritive balance was hardly affected by protein levels, even in early lactation. Most of the productive parameters responded to increasing PDI levels according to laws of decreasing return that 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 minimal 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 increases in productive performances whereas N losses increase sharply, and the reverse occurs with lower PDI levels. At grazing, the level of N fertilisation, through the increase in sward yield and N content, is the main determinant of productive performances and losses of excreta N per ha. The other factors of sward valorisation such as stocking rate only have a moderate effect 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 days induced by higher fertilisation increases N flows by 10-15 kg.ha-1 as milk and by 70-80 kg.ha-1 as excreta.