In the spring of 1965, anoestrous lactating Border Leicester X Merino ewes were treated for 13 days with intravaginal sponges impregnated with 17a - acetoxy 6a - methylpregn - 4 - ene - 3,20 - dione (MAP) 7-11 weeks after lambing. Each ewe received 1000 i.u. Pregnant Mare Serum Gonadotrophin (PMSG) at the time of sponge removal. Half of the ewes received another 1000 i.u. PMSG 16 days after sponge removal. In the 2-4 days after sponge removal 80.5 per cent of the treated ewes exhibited oestrus, and 75.8 per cent of these were pregnant at autopsy 15 weeks later. The mean number of foetuses per pregnant ewe was 2.46. It was not until approximately 15 weeks after the period of treatment that a similar number of untreated ewes had exhibited oestrus. The second injection of PMSG had no effect on either the number of ewes pregnant or on the number of foetuses present at autopsy. This was largely due to the high rate of conception at the first induced oestrus.

The spring reproductive activity of ewes following a late winter lambing, and varying duration of suckling or lamb weaning age of 6-13 weeks, was investigated in 321 Booroola Merino x Dorset (BD) and 543 Trangie Fertility Merino x Dorset (TD) ewe records over 2 years and 238 Border Leicester x Merino (BLM) ewes in the second year. The ewes were joined about 12 weeks post-partum in November for 6 weeks, following lambing in July-August to conform to an %-monthly lambing regime. In the first year, ewes were run with vasectomised rams and oestrus, ovulation rate, and interval to first observed oestrus were determined under a simulated joining regime. In the second year entire rams were joined in spring following July-August lambing and oestrus, pregnancy, fetal number, lambs born per ewe joined and lambing interval were analysed. Amongst ewes rearing lambs, longer suckling significantly increased the interval to first oestrus and the lambing interval (P<0.01), but had no effect on the percentage of ewes expressing oestrus, percentage of pregnant ewes or lambs born. Significantly fewer ewes that lambed but failed to rear any lambs (LL), than ewes rearing lambs expressed oestrus, ovulated and became pregnant. This resulted in considerably lower lambing rates for LL ewes (95.8 v 137.2% lambs born, P<0.01). Reproductive activity was not greatly affected by ewe liveweight or liveweight gain during joining. BD and TD ewes had hgher pregnancy and lambing rates and a shorter lambing interval than BLM ewes. Ovulation rate and fetal number increased with the number of lambs being reared (0.20 � 0.08 and 0.11 � 0.04 per lamb, P<0.05). The results show that high pregnancy and lambing rates can be achieved from natural joining in the spring following late winter lambings within an 8-monthly lambing regime, when lambs are weaned prior to joining.

No abstract

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.

Weaned Dorset x Border Leicester-Merino lambs were rotationally grazed at 62, 86 and 11 1 lambs per hectare on irrigated areas of either hybrid forage sorghum or paspalum-white clover pasture. No differences in quantity of available forage were measured between the forages but the protein content of the paspalum-white clover pasture, at 11 per cent, was 49 per cent higher than that of Sudax. Protein content of both pastures increased with increased stocking rates. The range in growth rate of the lambs between the lowest and the highest stocking rates was from 68.4 to -5.8 g per day for paspalum-white clover and 43.9 to -27.7 g per day for Sudax. The biological and economic potential of each forage type is discussed.