Pedigree and early performance records from lambs on three New Zealand research stations, involving 51 station-years of data (55 146 recorded lambs born), were analysed in order to study genetic and environmental factors affecting perinatal and preweaning survival. Mean birth weights at each site were very similar, being 4.26 kg at Woodlands, 4.22 kg at Tokanui, and 4.32 kg at Rotomahana Station. Total survival rates to weaning (preweaning survival) across sites averaged 79.0, 78.9, and 80.1%, respectively. Analyses of variance for perinatal survival and preweaning survival showed significant effects of contemporary group of lamb, age of dam, and birth rank-sex combinations, whilst linear and curvilinear effects of birth weight were also significant. Optimal birth weights were 4.36 kg and 4.77 kg for maximal perinatal survival and maximal preweaning survival, respectively. Heavy singles and twins were at higher risk of not surviving, but light singles and twins were also at risk. At birth, survival was consistently lowest from lambs out of 2-year-old dams, whilst preweaning survival was lower amongst lambs from 2-and 5-year-old dams, and higher in lambs from 3-and 4-year-old dams.Estimates of total heritability (direct + maternal + direct-maternal covariance) for perinatal survival as a transformed (logit) trait were 0.055 (Tokanui and Woodlands data), and 0.105 (Rotomahana data). A00029 Received 7 June 2000; accepted 31 August 2000Corresponding estimates for preweaning survival were 0.031 and 0.101, respectively. Maternal genetic variances for perinatal and preweaning survival as logit traits were 1.5-5 times the size of the lamb's additive genetic variance. Our analyses confirm previous low genetic parameter estimates for lamb survival.

Results on the body and carcass composition of 7885 lambs sired by 371 rams of 15 different breeds mated to Romney cast-for-age ewes are given. Trials were run at the Ruakura Agricultural Centre and the Manutuke Research Station from 1963 to 1972. Lambs of the different breeds were slaughtered at the same average age so that differences between breeds/crosses in liveweight and carcass weight reflected differences in growth rate. Ranked from heaviest to lightest mean age-adjusted carcass weights, lambs sired by the Dorset Horn, Poll Dorset, Hampshire, Border Leicester, Suffolk, Dorset Down, South Suffolk, and South Dorset Down produced the heaviest carcasses, and those sired by the Cheviot, Southdown, English Leicester, and Ryeland were intermediate. The Lincoln, Merino, and Romney sired carcasses were lighter. The longer-woolled Romney, Merino, and Lincoln had lowest dressingout percentages based on full liveweights and the short finer-wooled breeds such as the Southdown, Dorset Down, and Poll Dorset/Dorset Horn dressed 2-3% higher. When compared at the same carcass weight, lambs sired by the Southdown followed by the Ryeland had the fattest carcasses and those A94091Received 26 September 1994; accepted 27 April 1995 1 Deceased sired by the Suffolk, Cheviot, and Dorset Horn produced the least fat, leanest carcasses. The Southdown, Dorset Horn, South Dorset Down, and Poll Dorset crosses had the largest eye muscle areas for carcasses of similar weight.

Phenotypic and genetic parameters for clinical footscald and footrot were investigated in a pedigree flock of New Zealand Romney sheep inspected during outbreaks over six years. At inspections, each sheep's overall status with regard to footscald and footrot was summarized on a combined severity scale (Rating). Sheep were also categorized in terms of two binomial variables specifying the presence or absence of footscald or footrot (FI) and the presence or absence of footrot only (FR). A trinomial variable specified no infection, infection limited to footscald, or footrot with or without footscald (FSR), all variables being derived from the Rating value. Notwithstanding marked annual variations in prevalence of FI, F1 prevalence increased with age among ewes, and a higher prevalence of FI among male than female lambs was noted. Birth rank, birth date and dam's age had no significant effect on Rating in lambs. Repeatability of ewe Rating over successive annual inspections was low. Heritabilities of Rating, FI, FR and FSR, estimated by half-sib analyses were 0.14, 0.28, 0.17 and 0.25, respectively. Heritability of Rating estimated from dam-offspring regression was 0.12. Production traits did not appear to be genetically associated with liability to footrot. Selection of the top 5% of progenytested sires for breeding could be expected to reduce the prevalence of FI by approximately 45% per generation.

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Two sets of correlated responses were studied in five Romney selection lines and their controls. The lines were from one experiment selecting for yearling body weight or yearling fleece weight since 1967, from a second experiment selecting for weaning weight or yearling fleece weight since 1973, and from a third experiment selecting for yearling body weight since 1973. Overall, the five selection lines were evaluated whilst being run together with controls at a single site. The responses were faecal egg count (FEC) after weaning, using lamb crops born in 1990-93, and food intake using 1993-born 7-month-old ram lambs. Four or five new sires were used per line per year, with 1471 and 232 animals being recorded for FEC and food intake respectively. The three body weight lines were each 20-22% heavier than their respective controls in March (6 months of age) and May. Average FEC over this time period was significantly greater by 63% in one of these three lines whereas there was no difference from the control in the other two body weight lines. The two fleece weight lines were 10-11% heavier in March-May, with 34-35% heavier yearling fleece weights, than their respective controls, whereas average FEC was significantly greater (by 35 and 46%) in both selection lines. Significant correlated increases in estimated food intake were recorded in all three body weight lines and in one of the two fleece weight lines, and these increases were proportional to the liveweight response. The implications of the correlated responses and their variability are discussed.

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The proportions of cuts from 7885 lamb carcasses sired by 15 different ram breeds, reared on pasture at two locations over 10 years, were measured and examined for breed differences. Although highly significant breed differences were found, their magnitude was small reaching a largest difference between crosses of 1.22% for the leg and 1.26% for the loin compared at the same carcass weight. Compared at the same time of slaughter (age), the maximum differences between the progeny of different ram breeds were 1.02% for the leg and 1.06% for the loin. Wether lambs had a slightly higher proportion of forequarter cuts. Sire breed had a major effect on the proportion of lambs reaching the fatter P grade (class) with the meat breeds Southdown, Dorset Down, South Dorset Down, Hampshire, and South Suffolk sires producing 68-76% P carcasses and the wool breeds such as the English Leicester, Merino, Romney, and Lincoln producing 18-23% of P carcasses. Lambs sired by the Poll Dorset, Dorset Horn, Suffolk, Ryeland, Border Leicester, and Cheviot produced 49-62% P carcasses.

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