The relationships between seasonal variations in the concentration of urea in bulk milk and the production and fertility of dairy herds.

The concentration of urea was measured in bulk milk samples taken on seven 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 significantly higher when the cows were grazing than when they were housed, but there were no significant variations within either of these periods. In herds with very low milk urea concentrations while they were housed (< 2.4 mmol/l) the mean increase during the grazing season was approximately 2.0 mmol/l, whereas in herds with very high urea concentrations while they were housed (> 6.0 mmol/l) there was a slight decrease in urea concentration during the grazing season. The annual milk yield of herds with low urea concentrations was significantly lower than the yield of herds with intermediate or high concentrations. The interval between calving and first insemination was significantly longer in herds with low milk urea concentrations, but these herds also had a higher 56-day non-return rate.