PSXII-8 Effects of Pre-Synchronization with Short-Term Melengesterol Acetate in Nulliparous Beef Heifers

Abstract

Abstract Conception rates in nulliparous beef heifers can be greatly influenced by their ability to reach puberty at or prior to the initiation of a breeding season. Previous research has analyzed the efficacy of utilizing both long and short-term exposure to a source of progesterone via a controlled internal drug release insert (CIDR) prior to estrous synchronization with timed artificial insemination (TAI) and found improved conception rates. The purpose of this study was to evaluate the effects of feeding melengesterol acetate (MGA) immediately and 10 days before estrous synchronization. Ninety-three crossbred heifers (395.67 ± 5.37 kg) were blocked by BW, genetics, and reproductive tract scores and allotted to 3 treatments. The 3 treatment groups were: 7-day Co-synch + CIDR without presynchronization (CON, n = 31); 2cc PGF2α (Lutalyse) followed by melengesterol acetate feeding for 7 d before the start of the 7-day Co-synch + CIDR program (PRE, n = 31); and 2 mL Lutalyse followed by MGA feeding for 7 d followed 10 d later by the start of the 7-day Co-synch + CIDR program (PRE+10, n = 31). The 7-day Co-synch + CIDR protocol was initiated on d 0 by administering a 2 mL IM injection of GnRH and placing a CIDR into the vagina. The CIDRs were removed 7 days later and accompanied by 2 ml IM injection of PGF2α. An injection of GnRH occurred 60 to 66 hours following PGF2α at TAI with frozen semen from a single bull. Ten days after TAI, heifers were exposed to a bull. Estrotect patches were applied throughout the study to assess estrous behavior. Ovaries were visualized 24 hours post TAI to determine whether ovulation occurred. Pregnancy was determined on days 40, 64, and 109 post TAI via transrectal ultrasonography. Blood was collected via jugular venipuncture (d -19, MGA initiation and termination, d 0, d 7, and d 9) and serum progesterone determined. Performance data were analyzed using the MIXED procedure and conception data were analyzed using the GLIMMIX procedure of SAS. Heifers in the PRE+10 treatment group had higher levels of progesterone (P = 0.04) at d 0 than PRE heifers. At d7 there was a tendency (P = 0.07) for PRE+10 heifers to have greater concentrations of progesterone than PRE, but did not differ by d 9 (TAI, P = 0.36). TAI conception rates in heifers in the PRE treatment group (63%) tended (P = 0.09) to be greater compared with the controls (35%) and not different from PRE+10 (43%), with no differences in season-long pregnancy rates (P > 0.15). Presynchronization with MGA immediately before TAI synchronization can increase conception rates early in the breeding season in beef heifers.