Pregnancy rate in beef heifers after synchrony to random or programmed estrous cycles

Abstract

We hypothesized that heifers in diestrus at the beginning of a Syncro-Mate-B (SMB) regimen will have higher pregnancy rates to AI than heifers not in diestrus and that administration of prostaglandin F2. (PGF2.) I I d prior to a SMB regimen will increase pregnancy rates to Al. In experiment 1, heifers (n=150) were categorized by the stage of the estrous cycle at the beginning of a SMB regimen (day 0). Following implant removal (day 9), heifers were artificially inseminated 12 h after the onset of estrus. Blood samples were collected for P4 analysis on days 0, 9, and 20. Estrous response rate was 95.5%. Pregnancy rate among heifers classified to be in diestrus (DI; 54%; n--69) was higher (P <.05) than heifers in metestrus (MET; 44%; n=48). Pregnancy rate among proestrus (PRO; 44%; n=l 8) heifers was not different than heifers in MET or DI. Mean plasma P4 concentration was affected by treatment and day. Pregnancy rate was higher (P <.05) for heifers with P4 > I ng/ml plasma (52%; n=120) than heifers with P4 < I ng/ml plasma ( 25%; n=30) on day 0. In experiment 2, beef heifers (Santa Cruz; n=195) were allotted into one of two treatments. Heifers (n=98) in the control (CON) group were administered a conventional SNM treatment. Heifers (n=97) in the PGF group were injected with PGF2. I I d (day-I 1) before a SMB regimen. Progesterone concentration was determined from blood samples collected on day-I 1,-2, 0, and 9. All heifers were artificially inseminated 48 to 50 h after implant removal. At the beginning of the SMB regimen (day 0), a greater (P <.05) percentage of PGF (74%) than CON heifers (59%) were in diestrus (P4 > I ng/ml). Mean P4 concentration was not affected by treatment or day x treatment, but differed (P < .05) wrong days. Pregnancy rate of cycling heifers was similar for PGF (36%) and CON heifers (36%). Pregnancy rate was higher (P <.05) for heifers with P4 > I ng/n-d plasma (3 8%) than for heifers with P4 < I ng/n-d plasma (I 5%) on day 0. These results support the hypothesis that fertility is enhanced when a progestin synchrony regimen is initiated during diestrus, but methods to program estrous cycles to increase fertility warrant investigation.