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.
Prostaglandin analogue (ICI 8o 99 6) was administered ( 100 tig IM) on day 10 or I of the oestrous cycle in 8 Romanov (R), 10 Romanov X Prealpes (R X P), 8 Prealpes (P), 8 Ile-de-Fvance (IF)ewes. Ovulation rate before treatment was 2 . 9 (R), 2 . 9 (R X P), 1 . 7 (P) and 1 . 5 (IF). The mean ovulation rate in the induced cycle was not significantly different from that in the cycle before treatment (R = 2 . 7 ; R X P = 2 . 4 ; P = z.o ; IF = 1 .6). Delay (hr) between PG injection and onset of oestrus was significantly (P < o.oi) shorter in the Romanov that in the other genotypes R = 3 6 (range = 34 -40 ) ; R x P = 43 ( 3 8-52 ) ; P = 4 3 (32-50) ; [44][45][46][47][48][49][50][51][52][53][54]. Mean duration of oestrus (hr) was longer in the breed of high prolificacy (R = 53 ; 1 R X -P=45;.P=39;7f==3i); this was also the case in the cycle before PG treatment.It appears that luteolysis induced by this dosage of PG analogue is equally effective in animals with 1 , 2 or 3 corpora lutea ; but it is also evident that the delay between injection and onset of oestrus can be breed dependent.
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