PSXII-8 - Effects of pre-synchronization with short-term Melengesterol Acetate in nulliparous beef heifers

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 prior to 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 7d prior to the start of the 7-day Co-synch + CIDR program (PRE, n=31); and 2 cc Lutalyse followed by MGA feeding for 7d followed 10d 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 cc IM injection of GnRH and placing a CIDR into the vagina. The CIDRs were removed 7 days later and accompanied by 2cc IM injection of PGF_2α. An injection of GnRH occurred 60-66 hours following PGF_2α 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, d0, d7, and d9) 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 d0 than PRE heifers. At d7 there was a tendency (P=0.07) for PRE+10 heifers to have higher levels of progesterone than PRE, but did not differ by d9 (TAI, P=0.36). TAI conception rates in heifers in the PRE treatment group (63%) tended (P = 0.09) to be higher compared to 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 prior to TAI synchronization can increase conception rates early in the breeding season in beef heifers.