Estrous Synchronization Programs for Heifers

S. Whisnant and J. Cassady

Abstract

Three estrous synchronization protocols were tested in beef heifers of varying breeds. These methods were variations on the melengestrol acetate (MGA, Pfizer) and prostaglandin F2 a (PGF) protocol. All groups were fed 0.5 mg per head per day of MGA in a grain carrier for 14 days. Nineteen days after the cessation of MGA feeding all heifers received an injection of PGF. Group 1 received no further treatment and heifers were bred by AI approximately 12 hours after detected estrus. The other two groups were given an injection of a gonadotropin releasing hormone (GnRH) product 12 days after the cessation of MGA. Heifers in the second group were checked for estrus twice daily and if detected in estrus were bred by AI approximately 12 hours after detection. Those not seen in estrus by 72 hours were given an injection of GnRH and inseminated at that time. Group 3 had all heifers bred by AI at 72 hours after PGF. Heifers were given an injection of GnRH at 72 hours also.  Pregnancy rates to the estrous synchronization protocols were determined by calving dates. More heifers in groups 2 (56%) and group 3 (46%) became pregnant to the synchronized breeding than in the control group (23%). The majority of heifers in group 2 were bred at 72 hours without being seen in estrus. These results indicate that timed artificial insemination is possible in heifers using the protocol of group 3.

Introduction

Many of the estrous synchronization programs used in mature cows do not work as well for heifers. This is especially true for programs involving timed artificial insemination (TAI). One protocol that has been used with success in heifers is the combination of melengestrol acetate (MGA) and prostaglandin F2 a (PGF). In this protocol, MGA is fed to heifers for 14 days. Then 17-19 days after the cessation of MGA feeding PGF is given and heifers are bred after estrus detection. Recently it was reported that administration of 100 ug of gonadotropin releasing hormone (GnRH) 12 days after the cessation of MGA (one week before the injection of PGF) resulted in a tighter estrus synchrony in heifers. The objective of the current experiment was to determine if this protocol could be combined with another injection of GnRH at 72 hours after PGF and used for TAI of heifers thus eliminating the need for estrus detection.

Materials and Methods

Beef heifers of various breeds (13-15 months of age) located at the North Carolina Department of Agriculture Tidewater Research Station (Plymouth), Upper Piedmont Beef Cattle Research Station (Reidsville), Butner Beef Cattle Research Station (Butner) and the North Carolina Department of Agriculture Cherry Hospital Research Farm (Goldsboro) were used in this experiment. Heifers were randomly assigned to one of three insemination protocols. All heifers were fed 0.5 mg per head per day of MGA for 14 days and all received an intramuscular (im) injection of PGF (25 mg Lutalyse, Pfizer) 19 days after the cessation of MGA feeding. Heifers in group one received no further treatment and were bred by AI after detected estrus after the PGF injection. The other two groups received 100 ug of GnRH (Fertagyl, Intervet Inc., Millsboro, DE) im 12 days after the cessation of MGA feeding which was seven days before the PGF administration. Heifers in group 2 were checked for estrus twice daily after the PGF administration and inseminated approximately 12 hours after being detected in standing estrus. Those heifers not seen in estrus were injected with GnRH and inseminated at 72 hours after PGF. In group 3 all heifers were given another injection of GnRH and inseminated 72 h after the PGF injection (TAI) without estrus detection. Calving date was used to determine which heifers conceived to the synchronized breeding. Heifers calving within 291 days of PGF administration were assumed to have conceived at first breeding.

Results and Discussion

Five heifers at one location had late term abortions. Those heifer’s records were excluded from analysis. Of the remaining 121 heifers, 89 calved. Treatment (P < 0.73) and location (P < 0.28) did not affect calving rate. Groups 2 (56%, P < 0.01) and 3 (46%, P < 0.03) had a greater proportion of calves born within 291 day of PGF administration than did Group 1 (23%). Groups did not differ for proportion of heifers calving within 309 days of PGF administration (P < 0.17). The majority of group 2 heifers were inseminated at 72 hours after PGF without being detected in estrus. Therefore many of them were in fact given the group 3 treatment. This further strengthens the conclusion that timed AI as used for group is successful for heifers. Timed AI using the above protocol was an effective means of increasing the proportion of calves born early in the breeding season.

Figure 1.