Growth, endocrine profiles and reproductive responses of ewe lambs after medium and long-term treatment with ovine growth hormone

Two experiments were conducted to examine effects of exogenous ovine growth hormone (oGH) on growth and reproductive traits of ewe lambs. In the first trial, 30 Debouillet ewe lambs (4 months old) received either 0 or 2.5 mg, s.c. of oGH (Day 0 = first day of 98-day treatment). Ovarian cyclicity was determined by monitoring serum progesterone. Efficiency of gain (first 50 days of treatment) was more (P < 0.10) desirable in oGH-treated animals, but did not differ (P > 0.20) between groups during the last 48 days of treatment. Serum GH in alternate-day samples was elevated five-fold (P < 0.01). First estrus occurred 10 days earlier (P = 0.14) in oGH-treated ewe lambs. In a second trial, 45 ewe lambs were evenly divided into three groups receiving 0 mg of oGH (CON; 50 injections), 2.5 mg of oGH (GH98, 50 injections) or 25 injections containing 2.5 mg of oGH followed by 25 injections of 0 mg of oGH (GH48) on alternate days for 98 days before a breeding season. Ewe lambs receiving GH48 had higher (P < 0.05) gains the first 24 days than those receiving CON or GH98. Ewes receiving GH48 demonstrated first estrus (P = 0.06) 22 days before control ewes and 28 days before GH98 ewes. Other reproductive traits did not differ (P > 0.25). Serum GH was greatly elevated by injections of exogenous oGH, but neither serum insulin nor prolactin was affected. Short-term elevation of serum GH resulting from exogenous oGH injections marginally enhances reproductive and growth responses, but does not induce major changes in these traits in ewe lambs after 4 months of treatment.