Evaluation of Three Estrous Synchronization Protocols in Beef Heifers

Abstract

Objectives of this study were to evaluate synchronization, conception, and pregnancy rates of heifers synchronized with melengestrol acetate (MGA)-prostaglandin F2α(PGF2α,), Select Synch, or Select Synch preceded by MGA (MGA-Select Synch). Heifers in the MGA-PGF2αgroup (n = 209; BW = 378kg) received MGA (0.5 mg/ d per heifer) for 14 d and PGF2α(25 mg) 19 d later. Select Synch heifers (n = 213; BW = 374kg) received gonadotropin-releasing hormone (GnRH; 100 μg) followed by PGF2α(25 mg) 7 d later. The MGA-Select Synch heifers (n = 210; BW = 373kg) were fed MGA (0.5 mg/d per heifer) for 7 d, GnRH (100 μg) the day following the last MGA feeding, and PGF2α(25 mg) 7 d after GnRH. More (P<0.01) heifers were in estrus 1 to 4 d before PGF2a administration in both the Select Synch (20%) and MGA-Select Synch (24%) groups than in the MGA-PGF2α(4%) group. Pregnancy rates for heifers in estrus early (d 1 to 4 before PGF2α) were greater (P<0.05) for both Select Synch (55%) and MGA-Select Synch (63%) compared with MGA-PGF2αheifers (18%). Synchronization rate (detected after PGF2α) was greater (P<0.01) for MGA-PGF2αheifers (86%) compared with Select Synch (66%) and MGA-Select Synch (68%) heifers; however, conception rate did not differ (P=0.13) and averaged 72, 63, and 62% for MGA-PGF2α, Select Synch, and MGA-Select Synch heifers, respectively. Select Synch (52%), MGA-Select Synch (58%), and MGA-PGF2αprotocols (61%) provided similar (P=0.18) overall AI pregnancy rates; however, more heifers were in estrus before PGF2αadministration in protocols using GnRH.