Jp. Kastelic et al., MELENGESTROL ACETATE AND ESTRADIOL-17-BETA FOR SYNCHRONIZING ESTRUS AND IMPROVING REPRODUCTIVE-PERFORMANCE IN BEEF-CATTLE, Canadian journal of animal science, 77(4), 1997, pp. 683-688
Four experiments were conducted to investigate the use of melengestero
l acetate (MGA) and estradiol-17 beta (E-17 beta) for synchronizing es
trus and improving reproductive performance in beef cattle. Synchroniz
ed pregnancy rates were higher (P < 0.05) in heifers fed MGA after AI
than controls (4 of 9 versus 0 of 7, respectively), but were not diffe
rent for cows (20 of 37, 54% versus 23 of 40, 58%). Estrus rate (perce
ntage of cows detected in estrus) was higher (P < 0.05) for cows treat
ed with E-17 beta + MGA compared with those treated with cloprostenol
alone (85.0 versus 65.9%). Synchronized pregnancy rates were not signi
ficantly affected by feeding MCA or grain only after detected estrus a
nd Al (20 of 31, 64% versus 17 of 30, 57%). For 20 cows not detected i
n estrus and bred 100 h after cloprostenol treatment, pregnancy rate w
as higher (P < 0.05) for those fed MGA after AI than controls (6 of 11
, 54% versus 1 of 9, 11%). Feeding MGA after breeding may help to impr
ove pregnancy rates.