Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/20/2000
Publication Date: 6/20/2000
Citation: FUNSTON, R.N., ANSOTEGUI, R.P., THOMPSON, B.D., LIPSEY, R.J., ROPP, M.D., GEARY, T.W. EVALUATION OF THE MELENGESTEROL ACETATE/PROSTAGLANDIN (MGA/PGF) AND MGA/SELECT SYNCH ESTROUS SYNCHRONIZATION PROTOCOLS IN BEEF HEIFERS. WESTERN SECTION OF ANIMAL SCIENCE PROCEEDINGS. 2000. v. 51. p. 140-142. Interpretive Summary: This study evaluated a new protocol to replace the traditional melengestrol acetate/prostaglandin (MGA/PGF) for synchronizing estrus in beef heifers. The traditional MGA/PGF protocol is a 31 day protocol that requires daily consumption of MGA for 14 days and an injection of PGF 17 days after the last feeding of MGA. The length of the traditional protocol requires extra planning, and it is often difficult to ensure consistent consumption of MGA for 14 days. We evaluated the addition of 7 days of MGA feeding one day before initiation of the Select Synch protocol (MGA/Select Synch). Synchronization, conception and pregnancy rates of beef heifers that received either the MGA/PGF (78%, 59%, and 46%, respectively) or MGA/Select Synch (80%, 57%, and 45%, respectively) protocol were similar. The MGA/Select Synch protocol may be a viable system for producers desiring a more manageable synchronization system.
Technical Abstract: Synchronization, conception and pregnancy rates of beef heifers receiving either the Select Synch protocol preceded by 7 d of MGA (MGA/SS) or the traditional MGA/PGF protocol were evaluated. Crossbred beef heifers from one location (n=796) were randomly assigned to treatment and AI sire. Heifers in the MGA/SS group (n=402) were fed MGA (.5 mg/hd/d) for 7 d, received an injection of GnRH (100 mcg) the day following the last MGA feeding and an injection of PGF (25 mg) 7 d after GnRH. Heifers in the MGA/PGF group (n=394) received MGA (.5 mg/hd/d) for 14 d followed 17 d later by an injection of PGF (25 mg). Heifers were observed for estrus continuously during daylight hours beginning 1 d before through 5 d after PGF treatment. Heifers received AI according to the AM/PM rule. Pregnancy status was determined by ultrasound approximately 42 d after AI. Mean body condition scores and weights at time of breeding for the MGA/SS and MGA/PGF heifers were 6.3 and 6.4, and 374 and 380 kg, resp. Synchronization rates were not different (P>.1) between the MGA/SS (85%) and MGA/PGF (83%) treated heifers. Conception and pregnancy rates to AI were 58 and 48% for the MGA/SS heifers and 59 and 49% for the MGA/PGF heifers, resp. (P>.1). Five percent of the MGA/SS and 4% of the MGA/PGF heifers were detected in estrus the day before PGF. By 56 h following PGF, 70% of the MGA/SS and 59% of the MGA/PGF heifers had been observed in estrus. Approximately 90% of synchronized heifers were in estrus by 80 h following PGF in both treatments. In summary, short-term (7 d) MGA feeding preceding the Select Synch protocol produced similar synchronization, conception and pregnancy rates as the traditional MGA/PGF protocol.