Effect of Season on Reproduction of Rambouillet Ewes

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Abstract
The effect of season of year on overall reproductive efficiency of Rambouillet sheep was investigated utilizing 539 aged ewes in two replicates during a 2-year period. The ewes were maintained in drylot on the same ration for a period of 60 days prior to and during 6-week mating periods starting March 21, June 21, September 21 and December 21. The occurrence of estrus was recorded daily by use of painted rams. An observational laparotomy was performed on a sample half of each group 4 days after estrus, or at the end of 6 weeks if no estrus was recorded, to obtain ovulation rates. A nonsignificant difference was observed in the number of ewes showing estrus with a low of 84.5% in March and a high of 99.1% in December. Ovulation rates of 105.6, 140.8, 175.4 and 151.9 were recorded for the four mating periods, respectively. These values represent a percent of the total ewes available in each group. Similar values for lambs born were 83.9, 96.5, 126.8 and 135.2, respectively. The highest ovulation rate occurred at the September mating, while the highest lambing rate followed the December mating. It is suggested that length of the photoperiod was the major factor affecting the occurrence of estrus and ovulation rate, but that lambing results, particularly at the June and September mating periods, were substantially modified by high environmental temperatures. Copyright © 1965. American Society of Animal Science . Copyright 1965 by American Society of Animal Science