Abstract #444

# 444
Effect of MGA versus CIDR estrus synchronization on estrus response and pregnancy rates in 311-d-old beef heifers.
Hazy R. Nielson*1, Rosemary V. Anderson2, Rick N. Funston1, 1University of Nebraska, West Central Research and Extension Center, North Platte, NE, 2Anderson Ranch, Whitman, NE.

A study compared the effect of melengestrol acetate (MGA)-PG and 14-d controlled internal drug release (CIDR)-PG estrus synchronization protocols on estrus response and pregnancy rates of 311-d-old Angus-based, crossbred heifers (n = 153). Fall-born heifers, at 10 mo of age, were randomly assigned to 1 of 2 estrus synchronization protocols in the spring (2 replications/treatment). Heifers in the MGA protocol received MGA for 14 d fed through the diet beginning on d 0 of the synchronization treatment period. Heifers in the CIDR treatment received the same diet as MGA heifers and were implanted with a CIDR (Eazi-breed CIDR) on d 2 of the treatment period and removed on d 16. Following estrus synchronization, heifers from both treatments were combined and received a single PG (Lutalyse) injection on d 32. Heifers with activated heat detection aids (Estrotect) were AI 12 h following observation. All data were analyzed with the GLIMMIX procedure of SAS (SAS Inst. Inc., Cary, NC). Group BW was measured at weaning (198 kg) and before breeding (273 kg). Pre-breeding BW was 50.1% of predicted mature BW. Heifer age at breeding was not different (P = 0.12) between MGA and CIDR treatment groups. Percentage of heifers demonstrating signs of estrus was similar (P = 0.42) between synchronization treatment groups (CIDR vs. MGA, 71.5 vs. 77.4 ± 1.0%). Heifers not expressing estrus were not given an opportunity to become pregnant and removed from the herd. Pregnancy rates to AI of heifers expressing estrus (n = 115) were similar (P = 0.27) between CIDR and MGA synchronization treatment (46.3 vs. 36.1 ± 6.8%). Bulls were placed with heifers at a 1:25 ratio 10 d following AI. Final pregnancy rate was also similar (P = 0.96) between CIDR and MGA treatment groups (51.0 vs. 51.5 ± 7.4%). Heifer BW at pregnancy diagnosis was not different (P = 0.45) between CIDR and MGA treatment groups (325 vs. 321 ± 3.4 kg). The numerical 10% decrease in AI pregnancy rate in MGA compared with CIDR synchronization is not significant but is of interest. Approximately half of these 311 d old heifers exposed to AI and bulls became pregnant.

Key Words: beef heifers, estrus synchronization, heifer development