Effect of ram exposure on out-of-season reproductive performance of young maiden Merino rambouillet ewes

A trial was conducted to evaluate the ram effect stimulus on the possibility of advancing puberty of ewe lambs as well as on their reproductive performance at two stages of the anestrous season (February to June) in central Mexico (25 degrees 51' NL). A total of 80 single Merino Rambouillet ewe lambs, 11-13 months-old and 49.0+/-6.6 kg live weight, were randomly assigned to one of two experimental breeding groups: at the middle (M; March-April) or the end (E; May-June) of the natural anestrous season. Ewes were isolated from rams since they were born and kept under stall-fed conditions. During March and May, ewes were exposed within breeding group to two Merino Rambouillet rams fitted with a marking harness, remaining together for a 45 d-period. To determine ovarian function, blood samples were taken from each ewe to quantify serum progesterone concentrations by RIA. Ewes with less than 0.5 ng mL(-1) and depicting non-estrual behavior 16 days prior to ram exposure, were classified as inactive. Only 17.5% and 10% (P>0.05) of the M and E groups were defined as reproductively active. From the inactive ewes, 15.1% (M-group) and 77.7% (E-group) (P<0.01), responded to the ram effect, displaying their first estrus 26.6+/-4.8 and 22.6+/-3.8 days after ram exposure (P>0.05), respectively. The observed fertility at first estrus in the M and E groups was 78.5 and 100%, respectively. After 32 days following ram exposure, another 17 and 5 ewes in the M and E groups displayed estrous behavior and had 64.7 and 100% fertility rates (P>0.05), respectively. Results of this study confirm the possibility of advancing puberty in Merino Rambouillet ewes by means of the ram effect. This practice generated up to 100% fertility rate, 92.5% lambing rate, and 1.16 lambs per lambed ewe that has reached 49.0+/-6.6 kg live weight, proportionately 0.6 to 0.7 of their mature body weight. Ewes bred at the end of the anestrous season (May-June) had a higher response to the ram effect stimulus and a better reproductive performance than March-bred ewes. Improving the reproductive performance of earlier breeding schemes of young ewes by means of the ram effect could be of economic significance.