Reproductive development during winter and spring of Merino ram lambs grown at three different rates

Abstract

Twenty-four spring-born Merino ram lambs which had grown too slowly to reach puberty in their first autumn of life here studied from 5 months of age onwards. At 7 months of age, when their mean weight was 25.5 kg, groups of eight rams were allocated to three feeding levels, 130% (group L), 165% (group M) and 200% (group H) of their energy maintenance requirement. Development of testes, penis, sexual activity and ejaculate characteristics was assessed at 2-week intervals for the next 22 weeks. The ram lambs grew at 83.8, 130.3 and 182.8 g per day for groups L, M and H respectively. Groups M and H reached full penis development 4 weeks earlier than group L. At the end of the experiment groups L, M and H had mean testis volumes of 312.5, 368,7 and 437.5 ml (P < 0.05), respectively. For groups L, M and H, respectively, at the end of the experiment, the mean sperm density was 1.47 x 109/ml, 2.07 x 109/ml and 2.51 x 109/ml (P < 0.05). The total numbers of sperm per ejaculate were significantly higher (P < 0.05) in the H group than in the M and L groups. Other ejaculate characteristics (ejaculate volume, motility score, percentage of live and abnormal sperm) were not significantly different between the groups. Libido, assessed in 5-min and 20-min tests, tended to increase during the experiment, but there were no significant differences in libido score at any time of testing. Thus in Merino rams which had failed to reach pubertal weights in their first autumn, more generous subsequent feeding allowed puberty to be reached when appropriate weights were achieved in the following winter and spring.