EFFECT OF NORGESTOMET TREATMENT AFTER INSEMINATION ON THE CALVING RATE OF POSTPARTUM SUCKLED BEEF-COWS

Two experiments were conducted on postpartum suckled beef cows synchro nized with Syncro-Mate B and artificially inseminated approximately 48 h after implant removal. In Exp. 1, cows (greater than or equal to 42 d postpartum at the timed AI) were randomly assigned to treated (n = 101) and control (n = 85) groups on d 12 after the timed AI. Treated c ows received norgestomet/silicone implants that were left in situ for 9 d. Norgestomet treatment had no effect (P > .25) on the calving rate s from the initial timed AI or from the return estrus. Nonpregnant nor gestomet-treated cows returned to estrus in a more (P < .05) synchroni zed manner than the nonpregnant control cows. In Exp. 2, early postpar tum cows (< 42 d postpartum at the first Al; n = 30) were included and all 118 cows (88 cows were 2 42 d postpartum) received norgestomet/si licone implants as in Exp. 1. Of the 30 early postpartum cows, eight ( 19 to 41 d postpartum at the time of the first AI; mean = 29.3 d) calv ed to the first AI and nine calved to the second synchronized estrus. The calving dates at the next calving season for these 17 cows (57% of the cows in this group) was advanced an average of 46 d (319-d calvin g interval). The calving rates for the two timed insemination periods were similar (P > .25) for early and later (2 42 d postpartum) postpar tum cows. Treatment with norgestomet implants on d 12 through 21 had n o detrimental effects on established or subsequent pregnancy, synchron ized the return estrus of nonpregnant cows, and was efficacious in est ablishing pregnancy early postpartum.