ABSTRACT
This experiment was a continuation for the study on crossbred ewes treated by injection with 160 mg/ewe rbST at 14-day interval during pre- and post-partum period, where lambs were injected after weaning (2 months old) by 80 mg rbST/ lamb at 14- days interval till puberty. Growth performance and puberty of male and female lambs were studied.
Live body weight (LBW) of treated male and female lambs was significantly heavier (P<0.05) than untreated lambs in the control group.
Results showed early signs of puberty in treated male lambs, where injection of 80 mg rbST/ ram lamb reduced age (P<0.05) at the three stages of puberty (I, II and III) comparing to control. Moreover, average LBW was significantly higher (P<0.05) in treated ram lambs than those in control group at II and III stages of puberty. Scrotal circumference was  not affected by rbST treatment at all stages of puberty. However, puberty characteristics were more pronounced during the II and III stages, where ram lambs in treated group had significantly (P<0.05) younger age and heavier weights compared with control. Ejaculate volume, percentages of initial gross motility, sperm livability and abnormality percentage improved (P<0.05) by injection of rbST, while sperm cell concentration was not affected. Moreover, the overall plasma testosterone concentration was higher (P<0.05) in treated ram lambs (3.03± 0.4 ng/ml) compared to control group (1.85± 0.4 ng/ml).
In ewe lambs, injection of rbST reduced age (P<0.05) at puberty by 60 days, while LBW at puberty in both treated and control group was almost the same. The plasma progesterone concentration during the pre- pubertal period was not affected by rbST treatment. In conclusion, administration of 80 mg rbST/ lamb at 14-day interval starting at 2 month of age improved growth performance, puberty characteristics and semen physical characteristics of male rams. Meanwhile, slight improvement was noticed on female lambs.