Document Type

Dissertation - University Access Only

Award Date

1996

Degree Name

Doctor of Philosophy (PhD)

Department / School

Animal Science

First Advisor

Herley L. Miller

Abstract

A two year project was conducted to evaluate the administration of MGA + PGF2α on onset of estrus, days to first AI, conception rate and endocrine hormone profiles. The 1994 trial used 64 multiparous beef cows which were at least 30 days postpartum and stratified by age, breed, and days postpartum into two groups: a treatment group (n=32) which received MGA orally for 14 days and a control group (n=32) which did not receive MGA. Cows in both groups were injected with PGF2α 17 days after the last MGA feeding date and bred A.I. following observed estrus. The number of days from the start of MGA feeding to first detected estrus was lower for treated compared to control cows (19.50 ± 3.70 and 34.31 ± 3.28, respectively, P = .01). The average number of days from calving to first A.I. service was not different between groups (P = .30). The average number of days from first PGF2α injection to A.I. date also was not different between groups (P = .11). A greater number of animals showed cyclic activity in the treatment group compared to the control group. (P =.025). Melengestrol acetate increased conception rate to the first AI service (P =.05). In the 1995 trial 100 multiparous cows were used to evaluate the administration of MGA- PGF2α on conception rate, onset of estrus, and days to 1st AI. Cows were divided into two groups, a longer postpartum group (LPP) > 30 d postpartum (n=72) and a shorter postpartum group (SPP) < 30 d postpartum (n=28). The animals in the trial received the same ration and breeding procedure as previously described for 1994. Number of animals pregnant by AI was 42 of 50 for treated and 23 of 50 for control animals (P = .02). Number of animals conceived during the breeding season was 45 of 50 for treatment and 41 of 50 for control cows (P = .65). In the LPP group, number of days to 1st estrus following MGA withdrawal was lower for treated compared to control cows (53.72 ± 3.21 vs 62.21 ± 3.24, P = .051). Average number of days to 1st AI was lower for treated compared to control cows (70.34 ± 1.98 vs 79.39 ± 1.11, respectively; P = .01). In the SPP group, the number of days to 1st estrus was lower for treated compared to control cows (48.57 ± 4.36 vs 69.57 ± 4.31, respectively; P = .01). Average number of days to 1st AI was lower for treated compared to control cows (63.45 ± 3.91 vs 85.92 ± 3.94, respectively; P = .01). In the 1995 trial 20 multiparous cows were bled hourly for 48 hr. to evaluate the administration of MGA- PGF2α on blood serum hormone profiles of P4, E2, and LH. The animals were divided into two groups: a treatment group (n=10) which received MGA orally for 14 d and a control group (n=10) which did not receive MGA. Animals from each group were further divided into two groups of five animals each based on being cyclic or non-cyclic as determined by P4 and heat data. The animals in the trial received the same ration and breeding procedure as previously described for 1994. Serum P4 levels were lower in the cyclic treatment (CT) group compared to the cyclic control (CC) group (.208 ± .02 ng/mL vs .465 ± .02 ng/mL, respectively; P = .01). Serum estrogen-17β levels between CT (15.77 ± 1.98 pg/mL) and CC (10.81 ± 2.05 pg/mL) were different (P = .048), also E2 levels were different between NCT group (15.47 ± 1.98 pg/mL) compared to NCC group (9.10 ± 1.98 pg/mL, P = .038). Mean LH levels for CT were higher than for CC group during the entire bleeding period (7.48 ± .36 ng/mL vs 3.74 ± .37 ng/mL, respectively; P = .001). Mean LH levels were higher for the NCT group (6.46 ± .36 ng/mL) than the NCC group (3.22 ± .36 ng/mL, P =.001). In conclusion the MGA + PGF2α combination may be the estrous synchronization treatment of choice, particularly when treating mixed groups of cyclic and anestrous cows. How MGA effects the hypothalamic-hypophysial axis is debatable.

Library of Congress Subject Headings

Beef cattle -- Reproduction
Progestational hormones, Synthetic
Prostaglandins

Format

application/pdf

Publisher

South Dakota State University

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