Comparison of Various Methods of Estrus Detection in Synchronized Virgin Beef Heifers

Methods of heat detection were compared in the Mid- Crest Area Cattle Evaluation Program (MACEP) heifer development program in the 1998-breeding season. A total of 189 heifers from thirteen consignors entered the program on November 10, 1997. These heifers were condition scored, hip height measured, weighed, disposition scored, booster vaccinated, and treated for parasites at the time of arrival. Determination of the heifer’s mature weight was made and a target of 65% of their mature weight at breeding was established. The ration was designed to meet this goal. The heifers were kept in a dry lot until all heifers were AI bred once. The heifers were periodically weighed and condition scored to monitor their gains and the ration was adjusted as needed. The estrus synchronization program consisted of an oral progesterone analog for 14 days; 17 days after completion of the progesterone analog treatment a single injection of prostaglandin was given and the heifers were then estrus detected. Two concurrent methods of estrus detection were utilized: 1) Ovatec â electronic breeding probe (probe), 2) HeatWatchâ estrus detection system (HW), and 3) a combination of probe and HW. Probe readings were obtained each 12 hours and the heifers were continuously monitored for estrus activity using the HW system. The probe was used as the primary estrus detection method and the HW system was used as a back-up system. Those heifers that did not demonstrate any estrus signs prior to 96 hours post prostaglandin treatment were mass inseminated at 96 hours. Post AI breeding, 151 of the heifers were placed on pasture and ran with clean-up bulls for 60 days. The remaining heifers left the program after the AI breeding was completed. Pregnancy to the AI breeding was determined by ultrasound on June 29, 1998. Results from using both probe and HW were 60% pregnant by AI, probe alone was 32% pregnant by AI, and HW alone was 27% pregnant by AI. The result of mass insemination was 20% pregnant by AI.

The Use of a Vaginal Conductivity Probe to Influence Calf Sex Ratio via Altered Insemination Time

One hundred eighty-nine mixed breed beef heifers from 13 consignors enrolled in the MACEP heifer development project were utilized in this study. Heifers were synchronized by feeding 0.5 mg melengestrol acetate (MGA) per head per day for 14 days followed by an injection of prostaglandin F2a (PGF2a; 25 mg Lutalyse®) 17 days after the last MGA feeding. Each heifer was fitted with a Heatwatch® transmitter on the morning of PGF2a administration to facilitate detection of estrus. Vaginal conductivity measurements were taken using an Ovatec® probe every 12 hours for 96 hours beginning at the time of PGF2a injection. Heifers randomly assigned to produce a female calf were inseminated near the onset of estrus (as indicated by probe values of £ 55 on the decline). Heifers randomly assigned to produce a male calf were inseminated approximately 24 hours after the onset of estrus (as indicated by probe values of ³ 60 on the incline). All heifers not inseminated by 96 hours after PGF2a were mass inseminated in an attempt to impregnate as many heifers as possible. Heifers that were diagnosed as pregnant as a result of the artificial insemination were subjected to ultrasonography for fetal sex determination. Only 70 of the 189 heifers (37.0%) exhibited estrus according to Heatwatch® and incidence of estrus was influenced by heifer average daily gain, reproductive tract score, and disposition score. Heifers receiving a disposition score of 3 (78.7) had a higher (P<.05) probe reading at AI than those receiving a disposition score of 1 or 2 (70.8 and 72.5, respectively). Heifers with probe readings at insemination of 80 - 84 and > 84 had lower (P<.05) pregnancy rates to AI (13.6 and 0.0%, respectively) than heifers with probe readings in the ranges of < 60, 60 - 64, 65 - 69, 70 - 74, and 75 - 79 (35.7, 40.9, 31.4, 35.3, and 26.9% respectively). Heifers that were bred when probe values were increasing had a lower (P<.05) percentage of male fetuses (34.4%) than those bred during a period of decreasing probe values (69.2% male fetuses). These results demonstrate that a vaginal conductivity probe may be a useful tool to determine an insemination time that could potentially alter calf sex ratio.