Development of indicators to quantify nitrogen rejections of grazing dairy cows according to fertilisation types

Nitrogen (N) rejections are a problem of importance in cattle production. In pasture, the amounts of N rejected by the cattle are large since grass N content exceeds the ‘animals’ requirements. The present study aims to quantify, with dairy cows in a rotational
system, N rejections from urine. The trial paddocks were fertilized with compost, slurry or mineral N and grazed during 7 days by 35 dairy cows in late lactation. Milk urea concentration was determined in tank milk samples every day. Urine samples were taken from each cow at the 3rd and the 5th day after the entry in the grazing plot. Urine N and creatinin were determined to quantify urine N excretion. These observed values were compared to urine N excretion data obtained from dry matter intake and grass N content. The mean observed urine N excretion of 312 g N day-1 was similar to the mean calculated urine N
excretion of 330 g N day-1. The correlation between calculated urinary N excretion and observed urinary N excretion was significant (P<0.001; r2= 22.4%) and the correlation between observed urinary N excretion and milk urea content tended to be significant (P<0.10; r2 =60%). From these data, it appears that urine N excretion prediction can be more precise with tank milk urea than with urine N excretion calculated from N intake. This research has to be continued and to be repeated during other grazing periods and with cows at different lactation periods to validate the results.