MPA impregnated intravaginal sponges for the induction of oestrus in anoestrous Border Leicester × Merino ewes. 2. Maiden ewes

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In the spring of 1965 flocks of 18-month-old (flock 1) and 12-month-old (flock 2) anoestrous maiden ewes were treated for 13 days with intravaginal sponges impregnated with 17a - acetoxy 6a - methylpregn - 4 - ene - 3,20 - dione (MAP). Each ewe was given an injection of 1000 i.u. Pregnant Mare Serum Gonadotrophin (PMSG) at the time of sponge removal with half the ewes of each flock receiving a further injection of 1000 i.u. PMSG 16 days later. In the period 2-8 days after sponge removal, 45.9 and 57.1 per cent of the ewes in flocks 1 and 2 respectively were served, and of these 5.9 and 17.9 per cent respectively lambed. The second injection of PMSG produced a second oestrous period, 17-25 days after sponge removal, in 89.5 and 70.8 per cent of the ewes of flocks 1 and 2 respectively, and of these, 64.7 and 29.4 per cent, respectively, lambed. The additional administration of PMSG significantly increased the number of lambs born in flock 1 (P<0.01) but not in flock 2. Where a single injection of PMSG was used, failure to obtain a satisfactory lambing response was associated with a high incidence of 'silent heat' at the first induced ovulation, and a failure to promote regular sexual activity. No lambs were born in the untreated control groups until eight weeks after those of the treated groups.