Possible correlated or pleiotropic effects on food intake, growth rate and carcass traits (meat quality and carcass composition traits) were investigated for the prolificacy related Booroola gene (FecB). This gene was introgressed in a Texel sheep population by systematic backcrossing with sires carrying the Booroola gene. In a 3-year experiment 273 spring-born male lambs were offered concentrates ad libitum, after weaning. Lamb carrier effects and dam carrier effects for the FecB gene were estimated, accounting for year, backcross generation, slaughter weight, weaning age, concentrate type, rearing code, age of the dam and litter size as fixed effects. Carriers of the FecB had a higher dressing percentage (+1·15%), longissimus muscle depth (+0·26 mm to +0·7 5mm) and cross-sectional area (+0·98 cm2). However the gene also had a positive effect on subcutaneous fatness (+0·12 points), KKCF (+30 g) and a substantial positive lamb carrier effect on carcass fat tissue content (+3·09%), which reflects a relative increase in fatness of 11·9%. The FecB gene had also very small, significant effects on some meat colour traits. The dam carrier effects of the Booroola gene were negatively associated with food efficiency and with longissimus muscle depth (–1·38 to –2·25 mm). The increased fatness of lambs could imply that for optimal use of FecB, non-carrier terminal sires should be used on heterozygous dams.

Ovulation rate, embryo survival, lamb production, lamb survival and milk production of Awassi and BooroolaAwassi crossbred ewes, kept indoors, were compared. Awassi were non-carriers while Booroola × Awassi (F1) and about half of 3/4 Awassi-1/4 Booroola (BQ) ewes were heterozygous at the FecB gene. Mean ovulation rate increased by 1·5 to 1·6 corpora lutea per ewe ovulating and prolificacy by 0·7 lambs born per ewe lambing in Fj and BC1 (B+) ewes as compared with Awassi. Embryo survival rates in BC1 ewes with two, three and four ovulations were 0·83, 0·68 and 0·71, respectively. Lamb survival rates at 1 day of age were 0·93, 0·90 and 0·77 and average birth weight was 4·9, 4·0 and 3·0 kg for lambs born as singles, twins and triplets, respectively. Average milk production of the Awassi was 506 I per ewe per lactation. F1 and BC1 ewes produced respectively, proportionately 0·48 and 0·63 of the Awassi milk production and there was no significant difference in milk production between BC1,(B+) and BC1(++) ewes. The relatively low milk production of the Booroola Awassi crosses suggests that heterosis and recombination effects on milk production were negative. It is concluded that incorporation of the B allele per se can increase lamb production in the Awassi without affecting its milk production.