Some effects of nutrition and management on the fertility of dairy cattle.

When 24 cows were classified according to whether they had a better or worse energy status (defined as a plasma beta-hydroxybutyrate level below or above 15 mg/100 ml and a plasma glucose level above or below 55 mg/100 ml) the median interval between calving and the onset of cyclicity was shorter and conception rates were better for the cows with a better energy status at seven and 14 days after calving. This was not repeated when the cows' energy status was assessed as better at either 21 days after calving or at the time of service. In the same group of cows there was a positive correlation between the loss of condition score in early lactation and beta-hydroxybutyrate levels. In a further trial 15 cows fed a silage-based diet during the dry period had better conception rates and greater luteinising hormone responses to buserelin than 14 cows fed a straw-based diet. The straw-fed cows lost more weight before calving and had significantly higher non-esterified fatty acid levels. After calving both groups of cows were fed the same diet but the cows fed straw in the dry period produced less milk and lost less weight than the cows fed silage. Dry cow nutrition affected subsequent fertility and production. The non-esterified fatty acid level in the last week or two of pregnancy is suggested as a practical indication of the level of dry cow nutrition and its probable effect on productivity. No relationships were observed between plasma urea concentrations and the fertility of the cows.