EFFECT OF HORMONE PRETREATMENT OF PREPUBERTAL SHEEP ON THE PRODUCTIONAND DEVELOPMENTAL CAPACITY OF OOCYTES IN-VITRO AND IN-VIVO

Twenty 3-6-week-old Merino lambs were given either 3, 1 or 0 treatment s of 50 mu g oestradiol benzoate and (48 h later) a 1.5 mg Norgestamet implant left in situ for 9 days (3-, 1- and 0CYCLE+G). On Day 7 after the last implant insertion, and on the same day for 0CYCLE+G, each la mb received 400 I.U. pregnant mare serum gonadotrophin and 6 mg follic le-stimulating hormone (FSH). The reproductive tracts were removed for oocyte collection 24 h after FSH. Reproductive tracts were also colle cted from 16-24-week-old lambs (n = 31) (0CYCLE-G). The number of antr al follicles per ovary was similar for the 3-, 1- and 0CYCLE+G treatme nts. Similar rates of in vitro maturation and monospermic fertilizatio n were obtained for all groups. The proportion of blastocysts per clea ved oocyte was higher for 1CYCLE+G (50.5%) than for 3CYCLE+G (32.9%), 0CYCLE+G (24.3%), and 0CYCLE-G (11.8%) (P < 0.05). Viable fetuses were obtained at Day 93 of pregnancy after transfer of embryos from all tr eatments. These results indicate that a single treatment with oestroge n and progesterone, prior to gonadotrophin stimulation, will increase the yield and developmental capacity of oocytes from prepubertal sheep .