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Computer Modeling of Sheep Reproduction: I. An Algorithm for Quantifying Anestrous Cycles in Ewes¹

Purdue University and U.S. Department of Agriculture, West Lafayette, IN 47907

Abstract

An algorithm that mathematically describes annual estrous activity in ewes is given. The function fitted as the ordinate is similar to a normal distribution function related to date as the abscissa. The data used were collected from literature on different breeds and from different locations of the world; all studies had been specifically conducted to study estrous activity in ewes in these locations. Parameters estimated in an iterative procedure from the algorithm were length of time during which estrous cycles occurred (SA), a measure of asymmetry (SD), day of mid-estrous (DE), highest incidence of estrus (MX) and lowest incidence of estrus (MN), day of lowest incidence of estrus (DN) and day of mid-anestrous season (DA). Incidence of estrus is defined as the proportion of ewes exhibiting an active estrus. These parameters were estimated for several breeds and locations. The data, as collected, were inadequately cross-classified to allow for unconfounded estimation. However, analyses of fitted parameters that describe the curves gave quantitative results about the influences of breed of sheep, location, latitude and altitude and indicated which parameters were associated with these factors. Breeds differed significantly for SA, DE, MX and DA. Locations differed for DE, MX and DA. Latitude was associated with differences in DE, DN and DA with altitude also associated with differences in DE and DA. There were no significant effects on SD even though breeds differed sizably in SD.

¹ Journal Paper No. 9996 of the Purdue Agric. Exp. Sta. Joint research contribution from the Dept. of Anim. Sci., Purdue Univ. and the Anim. Genet. Res. Unit, ARS, USDA.

² Present address: Roman L. Hruska U.S. Meat Anim. Res. Center, ARS, USDA, Clay Center, NE 68933.