Ram mating behavior after long-term selection for reproductive rate in Rambouillet ewes1

Abstract

Mating behavior is known to be heritable in several species, but it is not known if selection schemes for ewe reproductive traits affect mating behavior of rams. Therefore, our objectives were to determine if divergent (high vs low index) selection of a female reproductive trait (lambs born divided by age of ewe minus one) affects mating behavior patterns of male offspring during training to service an artificial vagina (AV) in a less than natural setting (Exp. 1) or during serving capacity tests (SCT) in simulated natural mating conditions (Exp. 2). The method for evaluating male mating behavior was a series of serving capacity tests to estimate sexual performance. For Exp. 1, five, 30-min observations were conducted to assess the mating behavior of 29 rams (22 mo old) being trained to service an AV. In Exp. 1, percentages of rams mounting (73 vs 36%) and ejaculating (67 vs 29%) differed (P < 0.05) between rams from high and low Rambouillet ewe selection lines, respectively. In Exp. 2, ram classification consisted of exposing each 22- to 24-mo-old ram (n = 48) to three unrestrained ewes in estrus for 18, 30-min tests. Any ram that had not mounted or ejaculated during SCT was evaluated for sexual orientation in a 30-min preference test in which each ram had access to both restrained estrual ewes and restrained rams. In Exp. 2, there was no difference (P > 0.2) in percentages of rams mounting (92 vs 78%) and ejaculating (88 vs 74%) between rams from high and low Rambouillet ewe selection lines, respectively. There was no difference (P > 0.33) between high and low ewe selection line rams for mounts or ejaculations for 18 SCT. Only one male-oriented ram was identified, which was from the high line. In Exp. 2, six SCT were necessary to obtain 95% reliability in sexual performance scores. Results of these studies indicate that long-term selection of ewes for litter size did not alter ram sexual performance after SCT under simulated natural mating conditions or affect incidence of male-oriented rams. Mating behavior observed during training to service an AV was lower for rams from ewes of the low selection line than for rams from ewes of the high selection line. We concluded that long-term selection for reproductive rate in ewes did not alter mating behavior patterns of male offspring; however, test conditions, such as restrained and unrestrained ewes and number of serving capacity tests, may affect conclusions in studies that evaluate sexual performance of rams.