PERFORMANCE OF CROSSBRED PROGENY OF TRANGIE FERTILITY MERINO AND BOOROOLA MERINO RAMS AND POLL DORSET EWES .1. LAMB BIRTH-WEIGHT, SURVIVAL AND GROWTH

Merino rams of the Trangie Fertility and Booroola strains were joined to Poll Dorset ewes in single sire mating groups over 3 years. Forty s ires produced a total of 1250 lambs in 885 litters from 451 ewes. Lamb birth weight averaged 4.1 kg, 80.8% of lambs survived to 3 days of ag e and growth rates to weaning (mean 93 days) averaged 289 g/day. Lambs with Trangie Fertility sires grew 15 g/day faster and were 1.3 kg hea vier at weaning than those lambs with Booroola sires (P<0.001). There were no effects of sire strain on birth weight or lamb survival. Birth weight increased with dam liveweight in mid-pregnancy (P<0.001), and weaning weight and growth rate increased with dam liveweight at joinin g (P<0.001). Survival of lambs was predominantly a function of birth w eight. Lambs weighing 4 kg at birth from primiparous dams had survival rates of 76% compared with 88% for lambs from multiparous darns (P<0. 01). No lamb under 2.0 or over 6.3 kg survived, and 48% of deaths occu rred within 1 day of birth. Dystocia, particularly of heavy, single-bo rn lambs, caused 53% of observed lamb deaths. For the traits birth wei ght, lamb survival, weaning weight and growth rate, the direct heritab ilities were 0.24 +/- 0.10, 0.05 +/- 0.05, 0.19 +/- 0.10 and 0.12 +/- 0.08 respectively; the corresponding maternal heritabilities were 0.08 +/- 0.05, 0.05 +/- 0.04, 0.05 +/- 0.05 and 0.07 +/- 0.05 respectively ; and the common environmental effects among litter mates (c(2)) were 0.57 +/- 0.08, 0.47 +/- 0.08, 0.15 +/- 0.08 and 0.13 +/- 0.08 respecti vely.