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The influence of sex and gonadectomy on liveweight growth and its components was examined in a comparative slaughter experiment using Southdown – Romney cross lambs run under New Zealand pastoral feeding conditions.Twenty ram lambs were castrated and 20 ewe lambs spayed at docking, at approximately 4 weeks of age. Ten ram and 10 ewe lambs were killed at this age and 5 lambs from each of the four ‘sexes’ (entire and gonadectomized, male and female) were killed 4, 8,12 and 24 weeks after docking.Live-weight gain was higher for males than females and higher for entire than gonadectomized lambs, with some evidence of a greater depression in growth associated with castration than spaying. Effects on carcass weight followed those on live weight; weights for entire rams, wethers, entire and spayed ewes were 16.92, 14.72, 14.02 and 12.46 kg. respectively at approximately 7 months of age.

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Factors associated with mortality in 7,727 lambs born to 7,091 twoto five-year-old ewes over the 9 years 1959-67 were studied. The lambs were from two Romney flocks and first-, second-, third-, and fourthcross Border Leicester x Romney flocks. Lamb survival rate (lambs weaned as a percentage of all lambs born) increased with increasing age of dam for both single-and multipleborn lambs. Variation in survival rate between flocks and between female and male lambs was large. In 60% of the dead lambs, 44.6% of single-born lambs died of dystokia and 15.1 % from physiological starvation. Of the multiple-born lambs autopsied, 16% died from dystokia and 41.7% from starvation. Infections accounted for 11.6% and pre-natal deaths for 10.3% of the remaining deaths. Most of the deaths occurred within 3 days of birth, and relatively more single-than multiple-born lambs died at birth. Analysis of variance of birth weight showed that first-cross lambs were the heaviest and that birth weights decreased with interbreeding, to the fourth-cross lambs. Male lambs were 0.5 Ib heavier than females and singles 2.3 Ib heavier than twins. Lamb birth weight increased from two-year-old to five-year-old dams. Survival rate was related to birth weight. In single-born lambs survival rate was highest in lambs of about average birth weight and decreased with lambs of lower or higher birth weights. In multipleborn lambs survival rate was lowest with lambs of low birth weight and increased with increasing birth weight. Lamb mortality was highest in the earliest-born lambs.

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