Conception rates in synchronized beef cows with and without monensin feeding.

Abstract

Two groups of Hereford and two groups of Hereford crossbred beef cows were each divided into herds of 74 to 76 cows. Forty nine to 51 cows in each herd were synchronized with the Syncro-Mate-B (SMB) treatment and inseminated once between 48 and 52 hr after removal of the implant. The other 25 cows served as controls and were bred naturally. All herds received 2.27 kg/head/day of a corn-based supplement plus corn silage fed ad libitum for 15 days before and 20 days after the scheduled insemination date. In addition, one herd of each breed group was fed 250 mg of monensin/head/day during the same period. Overall conception rates for control and SMB-treated cows were 77 and 84.3%, respectively, and were not significantly different. Conception rates for the SMB-timed insemination were lower (P < .10) for Herefords than for crossbreds, averaging 34.9 and 48.5%, respectively. There was no difference in conception rates due to monensin treatment. Rumen samples taken 3 to 4 hr after feeding showed a lower ratio of acetate to propionate for Herefords fed monensin as a result of a lower proportion of acetate and a higher proportion of propionate in those animals.