Semester

Spring

Date of Graduation

2001

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Animal and Nutritional Sciences

Committee Chair

Paul E. Lewis.

Abstract

The quantity of progesterone (P4) delivered by a new P 4 releasing intravaginal insert (polycapralactone [PCL] insert), was inadequate to increase serum concentrations of P4 to that observed in cycling ewes during the luteal phase. Percentage of ewes in estrus was greater in P4 treated than in control ewes. Estrous response was greater after treatment with P4 for 5 than for 12 d. Progesterone increased the percentage of ewes lambing to both service periods by 20 percentage points. Prolificacy to the first service period in ewes treated with FSH was greater than the prolificacy in control and all ewes lambing to the second service period. Treatment with P4 for 5 d was as effective as for 12 d to induce fertile estrus in FSH-treated anestrous ewes.;In the second experiment, a 5 d pre-treatment with P4 prior to ram introduction induced estrus in anestrous ewes. The estrous response varied quadratically with time after ram introduction, and the conception rate varied quadratically with the time of observation of onset of estrus. Progesterone treatment increased the percentage of ewes lambing and the number of lambs born per ewe exposed over that observed in ewes introduced to rams alone. Prolificacy to the first service tended to be greater in FSH treated ewes than in ewes receiving P4 alone and all ewes lambing to the second service period.;In the third experiment, ram introduction by itself but not P4 pre-treatment alone induced an increase in LH secretion, follicular development, an LH surge and ovulation in fall-born ewe lambs. Progesterone delayed the RI LH surge, an effect that was prevented by the administration of estrogen 24h after P4 withdrawal. Progesterone pre-treatment for 5 days prior to ram introduction resulted in the expression of estrous behavior in a small percentage of fall-born ewe lambs. Estrogen increased the percentage of P4 pre-treated ewes displaying estrus. Poor reproductive performance of fall-born ewe lambs treated with P4 and exposed to rams during anestrus is not due to the inability of rams to induce ovulation but may be related to a reduced secretion of or sensitivity of behavioral centers to estrogen.

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