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Ovarian Response of Cows Following Pregnant Mare Serum Gonadotrophin Treatment during Two Successive Estrous Cycles

Department of Animal Science, University of Guelph, Guelph, Ontario, Canada

ABSTRACT

Thirteen cows (8 Holsteins, 3 Ayrshire, 2 Shorthorn; 3 to 7 years old) were treated with pregnant mare serum gonadotrophin (PMSG) during two successive estrous cycles. The gonadotrophin was injected on Day 16 of the cycle (estrus = Day 0) and again on Day 16 of the nest cycle (10 cows) or 21 days after previous PMSG injection (3 cows). Doses of PMSG injected during the first cycle were 3,000 IU (6 cows: Group A) and 2,000 IU (7 cows: Group B). These doses were reversed for the individual cow during its next estrous cycle. Cows were slaughtered 10 to 14 days after the second PMSG treatment. Response to PMSG was evaluated as number of ovulations and total follicular response (calculated by the sum of ovulations plus follicles greater than 10 mm diameter). Mean ovulation rate and mean total follicular response were 19.5 (±7.80) and 20.7 (±7.96) for Group A cows and were 10.7 (±4.72) and 29.1 (±11.32) for Group B. Two cows in Group B had not ovulated before slaughter; mean ovulation rate of the remaining 5 cows was 15.0. Substantial superovulation responses may be obtained following the second of two PMSG treatments during two successive estrous cycles.

Present Address: Division of Forestry and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061.