The Nature of Early-Season Infertility in Sheep

Abstract

Breeding records of the University of Wisconsin Hampshire and Shropshire flocks for the years 1947 through 1952 were analyzed. The average fertility from first service for the years in which breeding was begun August 1 was significantly lower than when breeding was delayed until September 1. The fertility results from first service in both the Hampshire and Shropshire ewes were analyzed by dividing them into two-week intervals according to breeding dates. In general, fertility increased with the advance of the season up to October 1, a highly significant change. Estimates of fertilization rate and embryonic death were made in 1952. Embryonic death was estimated by comparing 18-day non-return rates in the University of Wisconsin Hampshire and Shropshire flocks with the fertilization rates (on a ewe basis) in western ewes bred to the same purebred stud rams. Ram fertility was significantly lower before September 16 than afterwards. Of the 77 western test ewes from which ova were recovered, 64.3% failed to have any fertilized ova before September 16 compared with 34.3% for the remaining period. All ova recovered from 16.7% of the ewes bred before September 16 and 2.9% bred thereafter were classified as morphologically abnormal (P>.06), accounting for part of the fertilization failure. The remainder in each case may be attributed to poor sperm. The estimated embryonic death (100 minus the ratio of the percentage of 18-day non-returns to the percentage of ewes with some fertilized ova) before September 16 was 28.6% as compared with 9.9% for the remaining period. The differences between the 18-day non-return rates and the fertilization rates for the ewes bred before September 16 and for those bred later were not significant. Embryonic death appeared to be greater during the first 18 days following service (20.4%) than during the remainder of gestation (9.4%). No significant correlations were found between semen quality, as evaluated by various tests, and fertility.