Graduate Thesis Or Dissertation
 

Synchronization of estrus and subsequent conception rates in dairy heifers and rats following treatment with melengestrol acetate

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  • Two experiments were conducted to study the effects of melengestrol acetate (MGA) on inhibition of estrus, post-treatment conception rates in heifers and rats, and MGA given during gestation on the reproductive performance in rats. MGA was administered orally to Holstein heifers and by subcutaneous injection to Sprague- Dawley rats. Twenty-four Holstein heifers (Experiment 1) were used in the study and each animal in the treatment group (12 heifers) received 1.0 mg. of MGA daily for 14 consecutive days. The treatment was initiated irrespective of the stage of the estrous cycle. Following withdrawal of treatment, one-half of the animals from each of the control and treatment groups were inseminated twice at the first post-treatment estrus at approximately 12-hour intervals, the other half received single inseminations. Conception rates were based on pregnancy diagnosis at 45 or more days following the last service. Estrus was inhibited during the period of treatment in all animals in the treatment group. Withdrawal of treatment resulted in an effective synchronization of estrus and ovulation,and 83.3% of the treated animals were in heat within a 48-hour period. The difference between the number of MGA treated animals showing estrus within 3-6 days following treatment withdrawal and the number of control animals in heat during the corresponding period of time, was highly significant (P < 0.005). Treatment resulted in a significantly (P < 0.05) lower conception at the first post-treatment insemination for the treatment group compared to that of the controls (8.33 vs. 58.33 %). No significant difference in conception rates was observed between single and double inseminations at the first estrus following treatment withdrawal. Conception rate for the treatment group at the second estrus following treatment (54.54%) was similar to that of the control animals at the first estrus following cessation of treatment (58.33%). Although an equal number of animals from each group conceived following two or more services, the average number of services per conception for the treatment group (2.64) was significantly higher (P < 0.05) than that for the control group (1.54). Forty-eight sexually mature Sprague-Dawley female rats with body weights ranging from 225 to 260 gm. were used in the rat study (Experiment 2). The animals were assigned at random to three treatment groups. Two of the groups received an initial daily treatment of 0.01 mg. and 0.10 mg. of MGA (in 0.1 ml. of propylene glycol) for seven days, respectively. The control group received 0.10 ml. of propylene glycol only, for the same duration. Following withdrawal of treatment, the animals were placed in breeding cages and allowed to mate during a breeding period of ten days, each cage contained four female and two male rats. Starting on day 7 of pregnancy, one-half of the animals from each group were injected daily with 0.10 mg. of MGA for 14 consecutive days. Cyclic changes in the vaginal cytology was not inhibited in control animals and the group of animals receiving 0.01 mg. of MGA daily during the treatment period. Inhibition of cyclic changes during the period of treatment in all animals receiving daily injections of 0.10 mg. of MGA were observed. The animals in this group showed estrus on an average of 4.46 days following withdrawal of treatment. No significant differences in conception rates among the groups were detected following a breeding period of ten days. Analysis of data on growth response in the dams and their reproductive performance did not indicate any significant interaction between initial and post-breeding treatments. Post-breeding treatment resulted in significant (P < 0.01) depressing effects in mean weight gains during experimentation and an increase in mean gestation length as compared to controls. Animals receiving treatment during gestation also had a significantly lower (P < 0.05) number of pups per litter and a significantly lower (P < 0.01) mean litter weight. Pups from mothers receiving treatment during gestation had significantly lower (P < 0.01) birth weights and also registered a significantly higher (P < 0.01) percent mortality compared to controls.
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