Rearing of dairy cattle 8. Relationships of dietary energy intake, changes in live weight, body condition and fertility

Abstract

In each of 2 years, two groups of 25 British Friesian heifers were given, respectively, a moderate- and a low-energy diet for a 12-week period. Mean live-weight gains of 0·25 kg/day and 0·45 kg/day were recorded for the moderate-energy diet, and losses of 0·22 kg/day and 013 kg/day for the low-energy diet. Heifers were inseminated artificially at oestrus at approximately the mid-point of the 12-week period, this oestrus being the next observed after synchronized oestrus, which followed luteolysis induced by cloprostenol injections. With few exceptions, heifers that gained weight were in good or medium condition at insemination and those that lost weight were in medium or poor condition.Highest pregnancy rate (0·78) was recorded for heifers that gained weight and were in medium condition at insemination. For heifers losing weight, pregnancy rate dropped to 0·46 (P < 0·05) due to reproductive failure occurring before the 25th day after insemination. Only in heifers in poor body condition was there a significantly higher proportion failing to ovulate normally (P < 0·001). Among heifers gaining weight, those failing to become pregnant to the first insemination were approximately lOOg/kg heavier than the pregnant ones (P<0·05).Irrespective of gain or loss of weight, and of body condition at insemination, embryo losses did not occur later than the 25th day after insemination, except that five heifers that showed oestrus when pregnant (according to plasma progesterone levels) and were reinseminated then showed evidence of embryo loss.