THE RELATIONSHIPS BETWEEN SEASONAL-VARIATIONS IN THE CONCENTRATION OFUREA IN BULK MILK AND THE PRODUCTION AND FERTILITY OF DAIRY HERDS

The concentration of urea was measured in bulk milk samples taken on s even occasions during a period of 13 months from 1,658-1,815 of the 2, 100 suppliers of milk to a dairy. The mean concentration of urea was s ignificantly higher when the cows were grazing than when they were hou sed, but there were no significant variations within either of these p eriods. In herds with very low milk urea concentrations while they wer e housed (< 2.4 mmol/l) the mean increase during the grazing season wa s approximately 2.0 mmol/l, whereas in herds with very high urea conce ntrations while they were housed (> 6.0 mmol/l) there was a slight dec rease in urea concentration during the grazing season. The annual milk yield of herds with low urea concentrations was significantly lower t han the yield of herds with intermediate or high concentrations. The i nterval between calving and first insemination was significantly longe r in herds with low milk urea concentrations, but these herds also had a higher 56-day non-return rate.