Heat distribution in ewes during an autumn service: Effect of ram introduction

Abstract

The ram effect in sheep has been known for some time, but more information is available in anestrous ewes than in cycling. This work analyzed the evolution of estrus presentation in 182 adult ewes of the Border Leicester (BL, n=49), Ideal (n=68), and Milschschaf (MIL, n=65) breeds during an autumn breeding season. The study was conducted on a commercial farm located in Paysandú, Uruguay (32° 31’ S, 58° 08’ W; 130 m above sea leve), using natural mating in the pen with harness-equipped rams. Marked ewes were considered in estrus. Estrus records were taken over 28 days, divided into 4 weeks. Only ewes marked for the first time were considered. The weekly estrus percentage was estimated as the number of ewes in estrus / (total number of ewes – number of ewes marked in the previous week) * 100, so the denominator differed each week. The data were analyzed using the GENMOD procedure (SAS), considering the binomial nature of the variable and including in the model the effects of breed, week, and the interaction between both factors. Regardless of breed, the proportion of ewes showing estrus in the 2nd week was higher than in the other weeks (32.3%^b, 74.1%^a, 43.3%^b, 41.1%^b, for the 1st, 2nd, 3rd, and 4th weeks, respectively; P<0.05). The percentage of MIL ewes in estrus in the 2nd week (86.1%) was higher (P<0.05) than that of the other breeds (BL: 69.4%, Ideal: 68.2%, P>0.1). The data suggest that during the breeding season, the introduction of rams tends to concentrate estrus in the 2nd week and that this clustering is more pronounced in the MIL breed. This information may support breeding season planning.