Segregation analyses were applied to data from an experimental sheep flock to investigate the presence of a major gene affecting litter size. The data set contained 14 years of litter size data, with up to five parities per ewe, from Cheviot sheep carrying the putative Thoka fecundity gene from Icelandic sheep. Segregation analyses were performed using a Markov chain Monte Carlo method implemented using Gibbs sampling. Uniform priors were initially used for estimating variance components, the gene effect and fixed effects in the data. Genotypes in the base generation were assumed known based on the use of the imported Icelandic donor semen from the founder rams. The use of alternative priors (naive and inverse-gamma distributions) for the variance components did not significantly affect the results, demonstrating the data to be sufficiently powerful for the analyses used. Segregation analyses detected a major gene for litter size in the Thoka Cheviot flock increasing litter size by 0.70 lambs per ewe lambing for a single copy of the gene. When the analysis was repeated without fixing the genotypes in the base population, the analyses predicted a different genotype than that previously used for one of the founder rams and suggested the major gene to be segregating in the Cheviot founder animals prior to the introduction of the Thoka rams. A liability threshold analysis was also applied to the data. As identified in other studies, the threshold analysis overestimated the heritability, but the estimated major gene effect was not significantly different from other analyses. The results confirm the segregation of the Thoka gene in a Cheviot flock and highlight the statistical method as a useful tool for identifying carrier animals to be used for future matings.

The putative Thoka gene, with large effects on fecundity, originated in Icelandic sheep. The gene was introduced to the UK in 1985 through a programme of crossbreeding and established in Cheviot sheep (Russel et al ., 1997). Ewes have been retained in the flock as putative Thoka gene carriers if they have lambed in each of the first three years and had at least two sets of twins. Progeny tests on a separate population of ewes have been used on two occasions to identify rams believed to carry the gene. Despite this complex breeding programme, the actual segregation of a gene for fecundity has yet to be unambiguously demonstrated in this flock. The purpose of this study is to use complex segregation analysis to demonstrate the existence of this gene, estimate the size of its effect and frequency of the favourable allele within the population.

The Thoka gene for fecundity, a gene which originally occurred in Icelandic sheep, was introduced to the UK in 1985 and through a programme of crossbreeding was established in Cheviot sheep (Russel et al, 1997). Ewes are now retained as "Thoka carriers" only if they have lambed in each of the first three years and had at least two sets of twins. The gene has been introduced into separate North and South Country Cheviot (NCC and SCC) lines which are now phenotypically indistinguishable from purebred animals and contain proportionately less that 0.2 of the Icelandic genotype. The purpose of this analysis was to determine the increase in fecundity in the two breeds and whether or not the reproductive response to the incorporation of the Thoka gene differs between NCC and SCC breed types.

AbstractIn a 2 × 2 × 2 experiment, 296 Scottish Blackface ewes were fed to achieve one of two levels of body condition (high: condition scores ^2·75; low: condition scores ^2·50) by 2 weeks before joining with rams. Half of the ewes of each condition score were fed dried grass pellets ad libitum and 300 g hay per head daily, between 2 weeks before and 8 days after joining, while the remaining ewes were offered a live-weight maintenance ration comprising 750 g pellets and 400 g hay per head daily. From 8 days after joining the intake of all ewes was gradually changed, over a 2-week period, to 1250 g pellets and 400 g hay per head daily. Two days before joining, half of the ewes of each condition/intake group were passively immunized against testosterone. A total of 99 ewes were slaughtered 18 days after joining and their ovulation rate at the first mating determined. Ovulation rates were significantly affected by immunization (immunized: 2·31, control: 1·86, pooled s.e. = 0·055; P < 0·01) and food intake (ad libitum: 2·24, maintenance: I 92, pooled s.e. = 0·055; P < 0·05) but not by body condition at mating (high: 2·15, low: 2·02, pooled s.e. = 0·059; P > 0·05). Lambing rates were dependent on treatment (immunized: 1·82 control: 1·62, pooled s.e. = 0·044; P < 0·01), food intake (ad libitum: 1·85, maintenance: 1·59, pooled s.e. = 0·043; P < 0·01) and body condition (high: 1·82, low: 1·62, pooled s.e. = 0·044; P < 001). In ewes in a high body condition at mating, the effects of intake and immunization were additive but in ewes in low condition, immunization induced little improvement in performance in ewes fed ad libitum. Circulating antibody titres were lower in ewes fed ad libitum than in ewes offered a maintenance ration (1: 416 v. 1: 534, pooled s.e. = 14; P < 0·001), but ovulation rates and litter sizes were not related to titres.

ABSTRACTThe effect of injection of 0, 0·5, 1·0, 1·5, 2·0 or 5·0 × standard doses of anti-testosterone antiserum on the reproductive performance of Scottish Blackface ewes was investigated using a total of 423 ewes with body condition scores ranging from < 2·00 (low) to > 2·75 (high). Increased mean litter sizes and lambing rates were obtained in ewes in all levels of body condition following immunization with a range of antibody doses (P < 0·05). However, it is concluded that there is an intermediate optimum dose, probably approximately 1·5 × standard doses, for ewes of this breed, at a non-synchronized mating. The limiting effect of body condition on reproductive performance could not be completely overcome by increasing the antibody dose. Circulating antibody titres were directly related to the dose administered but there was no direct relationship between circulating titre and reproductive performance.

ABSTRACTThe effect of passive immunization against testosterone on reproductive performance was investigated in 180 mature Scottish Blackface ewes in different levels of body condition at mating. Ewes were fed to achieve a wide range of condition (scores 1·5 to 3·0) by 2 weeks before a synchronized mating. During the week before mating, 90 ewes were each injected with 32 ml of a neat anti-testosterone antiserum (1: 96 000 initial titre) and 90 ewes were untreated. At about 55 days after mating, 50 immunized and 50 control ewes were slaughtered and the numbers of corpora lutea and embryos present were recorded. The proportion of immunized ewes that conceived during two periods of mating was in excess of 0·9 in all condition categories. By contrast, untreated ewes in the low condition category (score =£ 2·00) had significantly lower conception rates than ewes in higher condition categories. Immunized ewes with condition scores of 2·50 or less at mating had significantly higher ovulation rates, litter sizes and lambing rates compared with control ewes, while immunized ewes with scores of 2·75 or higher were not significantly different from control ewes in these measurements. There was no effect of immunization on embryo mortality rate, lamb sex ratio, lamb birth weight, or lamb survival rate in the 1st week after birth.