Reproduction in the Yearling Ewe as Affected by Breed and Sequence of Feeding Levels. I. Effects on Ovulation Rate and Embryo Survival

Abstract

Ovulation rate and embryo survival were studied at slaughter in 128 yearling ewes of mixed breeding in 1955 and in 330 grade Hampshire and grade Columbia ewes in 1956 and 1957 combined. Straight-bred and reciprocal cross-bred fetuses were produced by mating purebred Hampshire and Columbia rams to the grade Hampshire and Columbia ewes. The ewes in each year were subjected to various sequences of alfalfa-grass hay only or the same hay plus grain (full and limited levels of feeding, respectively) from the beginning of the experiment in mid-winter until they were slaughtered in the next fall or winter. Breeding was begun in October. Those animals that were better grown out as indicated by greater body weight shed the greater number of ova in each of the three years. This was true whether variation in body weight was that which existed between ewes within the breed-feed sub-groups or whether it was produced by long time differences in feeding levels. Flushing (full feeding for approximately 3 weeks prior to breeding) seemed to increase ovulation rate in 1957 only. No appreciable increase in weight was shown, hence temporary improvement in the nutritive state of the animal appears to be responsible. Virtually all of the prenatal deaths seemed to occur by 25 days after breeding. Full feeding was apparently detrimental to embryo survival in one out of the three years. A greater number of embryos from Columbia dams survived than from Hampshire dams. This was true whether the fetuses were crossbred or straight-bred and was accounted for almost entirely by a better maternal “environment” provided by the Columbia dams. There was little additively genetic variation between embryos in survival rate. Copyright © . .