Abstract:
In an experiment at Cowra Agricultural Research Station, New South Wales, five drops of Border Leicester (BB), Merino (MM), Border Leicester x Merino (BM) and Merino x Border Leicester (MB) lambs have been compared to estimate hybrid vigour effects on lamb survival and growth, and on the wool and body measurements of ewe hoggets. Hybrid vigour has been estimated as the percentage superiority of the crossbreds 1/2(MB + BM) over the average of the purebred 1/2(BB + MM) flocks. Hybrid vigour was observed for lamb survival, with the crossbreds superior by 10 per cent in lambs weaned per lamb born (P < 0.05). The crossbred lambs were also heavier at birth (4 per cent) and weaning (6 per cent) and crossbred wether lambs were 11 per cent heavier at slaughter (all P < 0.05). The crossbred ewes were significantly superior to the average of the purebred flocks in both greasy (8 per cent) and clean fleece weight (12 per cent), liveweight (12 per cent) and chest depth (5 per cent) (all P < 0.05). There was no evidence that the crossbreds were superior in any component of wool production per unit area of the skin ; fibre length, fibre diameter or follicle density. Other evidence from this experiment indicates that the breeds and crosses differ in their susceptibility to pneumonia and that there may be heterosis for resistance. This situation could give rise to heterosis for production characters and the heterosis estimates obtained may only be applicable to first-cross flocks derived from Border Leicester studs which are susceptible to pneumonia. Lamb survival to weaning was higher for purebred Merinos (88 per cent) than for purebred Border Leicesters (73 per cent, P < 0.05). The Border Leicester lambs were heavier at all ages and in all years, although the magnitude of their superiority varied significantly between years (P < 0.05). This breed x year interaction was also significant for clean fleeca weight, but the average production of the two groups was similar over the five years. Variation, measured as the coefficient of variation, was generally lower within the crossbred flocks for those characters for which hybrid vigour effects were observed, but these differences were not statistically significant.