Mating performance of paddock-mated rams. I. Changes in mating performance, ejaculate characteristics and testicular size during the joining period

Abstract

A series of field experiments was undertaken to examine the possible relationship between the major factors thought to influence the paddock-mating performance of rams (sperm producing capacity, sperm output and mating behaviour) and flock fertility. Also the effect of mating on these factors was examined. Six mating groups were used, four ram groups were subjected to conventional paddock-mating conditions, using paddocks ranging in size from 2.5 to 30 ha and were joined for 6 to 7 weeks to 110 (two rams), 112 (two rams), 204 (three rams) and 336 ewes (six rams) respectively at a rate of 7.6, 10.3, 6.3 and 6.5 g testicular tissue per ewe. Rams of the remaining two groups, of three rams each, were selected for High (High TV) and Low (Low TV) testicular volume and were joined to 152 and 150 ewes each at a rate of 5.5 and 4.1 g testicular tissue per ewe for 16 days, following which they were immediately rejoined with a second group of 140 ewes each for 34 days. In all groups the proportion of ewes mated during the first 17 days was high (87–92%); this level of sexual activity had no apparent adverse effect on the fertility of the rams as reflected by percentage of ewes returning to first service. Semen collections were taken from the Working rams and the Control rams prior to, during and after the joining period. Semen samples were evaluated for the ejaculate quantity parameters of volume, sperm density, and total number of sperm, and for the ejaculate quality parameters of motility score, % live sperm and % morphologically abnormal sperm. For the paddock mated rams, volume, density, and total sperm per ejaculate were high during the pre-joining period with mean values ranging from 0.79–1.10 ml, 3.16 · 10 9–4.25 · 10 9 sperm/ml and 3.59 · 10 9–4.09 · 10 9 sperm respectively. The values decreased to a stable minimum level during the peak mating period with mean values ranging from 0.35–0.49 ml, 0.97 · 10 9–1.29 · 10 9 sperm/ml and 0.37 · 10 9–0.56 · 10 9 sperm respectively, and increased to near pre-joining levels during the latter stages of the joining period. For all mated rams combined, there were no significant relationships between prejoining testicular size and ejaculate quantity characteristics recorded during the joining period. At male to female breeding ratios of 2.0% it does not seem warranted to select for increased testicular size prior to joining. The effects of mating on semen motility, % live sperm and % morphologically abnormal sperm were not as clear cut as those on ejaculate volume or density. Motility score for mated rams followed a pattern similar to that for ejaculate density, but no consistent pattern was observed in the percentages of live or abnormal sperm. The relationships between the characteristics of ejaculates collected during the period of high mating activity and flock fertility were not significant for individual semen characteristics or for all characteristics combined. Incorporating the estimated number of services per ewe did not significantly improve this relationship. Testicular volume and scrotal circumference decreased significantly to 56–86% and 80–92% of pre-joining levels respectively in all the mated ram groups excepting Low TV rams. The rate of decrease was linearly related to pre-joining testicular size of the rams and was not solely attributable to liveweight change. It was concluded that mating had a direct and adverse effect on the testicular size of rams which appears to be accelerated by the sudden decrease in liveweight often experienced by paddock-mated rams.