Effects of Origin and Environment on Reproductive Phenomena in Rambouillet Ewes. I. Breeding Season and Ovulation

Abstract

A 2-year experiment was conducted to study the effects of location (Dubois, Idaho and McGregor, Texas) and source of ewe (Texas or Northwest) on reproductive phenomena of Rambouilett sheep. A seasonal estrual pattern which was highly correlated with ovulation rate was observed at both locations each year (r =0.89,0.93,0.99,0.89). However, the incidence of both estrus and ovulation during April through August (anestrum) the first year in Idaho was higher in the ewes originating in Texas than in those originating in the Northwest (55 vs. 14, P< .01 and 57 vs. 14, P< .01, respectively). Ewes from both origins located in Texas tended to show less marked seasonally in the occurrence of estrus during the first year than did similar ewes from the corresponding origins located at Idaho. However, the incidence of ovulation in the Texas ewes located in Texas was higher (P< .01) during March through August than in the Northwest ewes located in Texas. Also the incidence of estrus in Northwestern ewes located in Texas dropped dramatically at least a month earlier than their counterparts in Idaho and increased to a relatively high level again at least 2 months earlier than at Idaho. A similar but much less marked difference in the incidence of estrus occurred in ewes of Texas origin at the two locations. The second year, ewes located in Idaho with a Texas origin followed almost the identical estrous pattern of the Northwest ewes at the same location. At Texas, the Northwestern ewes went into anestrum 1 month earlier than the Texas ewes and stayed in anestrum about 1 month longer on the average (P< .01). The most striking effect was the difference in breeding season between the two locations. The Northwestern ewes went into and out of anestrum at least 2 months earlier in Texas than in Idaho (P< .01). The Texas ewes in Texas went into anestrum about a month and a half earlier and out of anestrum nearly 3 months earlier than their counterparts in Idaho after 1 year of acclimatization (P< .01). Northwest ewes at either location showed a higher ovulation rate during the fall and winter than the Texas ewes (P< .01). The ovulation rate of all ewes located at Idaho pooled over both years was significantly higher (P< .02) than that for all ewes located at Texas. The data appear to suggest that number of hours of darkness is a primary factor in controlling reproductive phenomena in sheep. However, this effect is not clearcut as certain observations remain unexplained.