Effects of month of breeding and feed level on ovulation and lambing rates of Panama ewes.

Abstract

Approximately 300 mature Panama range ewes were used in 1966 and 1967 to study the effects of season of year (September and January), feed level (60, 90 and 140% N.R.C.) and changes in feed level (90 to 140, 90 to 60% N.R.C. vs. no change) on ovulation rate. In 1968 and 1969 approximately 380 similar ewes were used to study an additional season (September, November and January) and the effects of the above variables on number of lambs born in addition to ovulation rate. Feed levels were changed to 65, 85 and 105% of N.R.C. in the second study. In the first study, ovulation rate increased during September independent of feed level followed by a seasonal decline which was affected differentially by feed level resulting in significant effects of both season and feed level on ovulation rate. A change in feed level on January 9 resulted in a significant change in ovulation rate observed on February 9. The effects of season and feed level in the second study were very similar to the first study. Ovulation rate went up seasonally from September 1 to early November followed by a general seasonal decline through to the end of the study on February 4. There was an apparent but nonsignificant increase in ovulation rate due to increase in feed level between September 1 and October 4. Feed levels near the peak of the breeding season (November 1 to December 4) had very little effect on ovulation rate. However, following a seasonal decline in ovulation rate, feed level between January 1 and February 4 had a significant effect on ovulation rate (1.78, 1.81 and 1.50 CL/ewe on 105, 85 and 65% N.R.C, respectively). Similarly, the percentage of ewes lambing was significantly affected by feed only in January when the resulting lambing rate was the lowest (P<.05). Although lambing rate was not significantly affected by treatment, the differences consistently reflected a positive relationship between feed level and lambing rate. This apparent effect of feed level was greatest early and late (September and January) in the breeding season and lowest during midseason (November). The significant ovulatory response to feed level in January appeared to be offset by a poorer embryo survival (lambs born per C.L.) in the groups on the intermediate and high levels of feed in January as compared to September. Lamb production was consistently higher from September breeding due primarily to more ewes lambing and lower embryo mortality. It is concluded that, when the seasonal stimulus for ovulation is high, differences in nutrition have little influence on ovulation rate.