The effect of additional feeding on the fertility of high-yielding dairy cows.

One hundred and ninety three pairs of cows were selected from 38 herds, and the numbers of each pair were assigned at random to experimental and control groups. Each control cow was fed according to its yield with the feedstuffs normally used by the herd. Each experimental cow was fed in the same way and, in addition, received 500 g of a feed additive twice daily from the 40th day of lactation until first insemination or to the 75th day of lactation if no visible oestrus had been observed between days 40 and 75. One kg of the feed additive contained glucogenic substances (glycerol and Ca-propionate) equivalent to 1.90 moles of glucose, 13.6 MJ metabolizable energy, and 180 g digestible crude protein. There were no significant differences between the groups with respect to their mean daily milk yield, their plasma glucose concentrations at first insemination, or the interval between calving and first insemination. However, the experimental cows had a significantly shorter interval between calving and last insemination (mean difference 11.0 days). The pregnancy rate was significantly higher among cows with high than among cows with low plasma glucose concentrations. There was also a tendency towards a reduced pregnancy rate among cows with either high (greater than 7.0 mmol/litre) or low (less than 4.0 mmol/litre) milk urea concentrations.