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

We hypothesized that heifers in diestrus at the beginning of a Syncro-Mate- B (SMB) regimen would have higher pregnancy rates to AI than heifers not in diestrus and that administration of a PGF(2 alpha) analogue 11 d before a SMB regimen would increase pregnancy rates to AI. In both replicate years o f Exp. 1, heifers (n = 150) were classified by stage of the estrous cycle a t the beginning of a SMB regimen (d 0). Following implant removal (d 9), he ifers were artificially inseminated 12 h after the onset of estrus (95.5% i n estrus by 72 h). Blood samples were collected for progesterone (P-4) anal ysis on d 0, 9, and 20. Pregnancy rates did not differ between yr 1 and 2. Pregnancy rate for heifers classified in diestrus (53.6%; n = 69) was highe r (P = 0.06) than for heifers in metestrus (43.7%; n = 48). Pregnancy rate for proestrus (44.4%; n = 18) heifers was not different from that for heife rs in the metestrus or diestrus groups. Mean plasma Pq concentration was af fected by both treatment and day. Pregnancy rate was higher (P < 0.01) for heifers with P-4 > 1 ng/mL plasma (51.6%; n = 120) than for heifers with P- 4 less than or equal to 1 ng/mL plasma (23.3%; n = 30) on d 0. In Exp. 2 be ef heifers (Santa Cruz; n = 195) were allotted to two treatments. Heifers ( n = 98) in the control group were administered a conventional SMB treatment . Heifers (n = 97) in the PGF group were injected with PGF(2 alpha) 11 d (d -11) before a SMB regimen. Progesterone concentration was determined from blood samples collected on d -11, -2, 0, and 9. All heifers were artificial ly inseminated 48 to 50 h after implant removal. At the beginning of the SM B regimen (d 0), a greater (P < 0.05) percentage of PGF (74.2%) than of con trol heifers (59.2%) were in diestrus (P-4 > 1 ng/mL). Mean P-4 concentrati on was not affected by treatment or day x treatment but differed (P < 0.05) among days. Pregnancy rate of cycling heifers was similar for PGF (36%) an d control heifers (35.9%). Pregnancy rate was higher (P < 0.01) for heifers with P-4 > 1 ng/mL plasma (37.6%) than for heifers with P-4 less than or e qual to 1 ng/mL plasma (18.5%) on d 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 fertili ty warrant investigation.