Quantitative genetic aspects of breeding for resistance to gastrointestinal parasite i small ruminants

The aim of this thesis is to explore some of the quantitative genetic aspects of resistance to gastrointestinal nematode parasites in small ruminants. The focus is on animals in temperate areas such as Northern Europe and consequently on the parasite species which poses the main production losses in this area, Teladorsagia circumcincta. Most of the published genetic parameter estimates in the literature for resistance to nematode parasites are for sheep, while there is a distinct lack of estimates for goats. A genetic analyses of parasitological and production data from a cashmere goat population (kids) was performed. The data were collected over a time span of four years from a farm in Scotland. The heritability of the indicator trait of resistance faecal egg count (Fec) was estimated to be 0.17 while that of the mean of several measurements was 0.32. The heritability of fibre traits was in excess of 0.5, while the heritability of live weight was 0.22. The genetic correlations between Fec and the production traits were slightly positive but not significantly different from zero. The phenotypic correlations were very close to zero. Fec data collected over four years from a commercial flock of Scottish Blackface in Scotland were analyzed using random regressions analyses to estimate the genetic and phenotypic parameters of the flock and how they change over time. The random regression model gave at least as good description of the data as univariate models fitted at individual time points. The added benefit of random regression analysis was that it allowed heritabilities and correlations to be interpolated for time points when data were not available, thereby enabling sampling time strategies to be determined. Genetic correlations between samples taken from 14 to 24 weeks of age were all greater than 0.8. The distribution of Fec in sheep has been found to generally fit the negative binomial distribution, with a small proportion of animals shedding most of the parasites. This fact could be utilised for separating or culling the most parasitised animals, as an immediate control measure. By means of computer simulation, a scenario of exploiting this particular distribution of Fec combined with selection over ten years in a closed flock was examined, in silico. Different management scenarios and different culling/separation scenarios were explored. The impact on lamb performance after ten years of separating the worst animals based on Fec did not exceed a 4% improvement in live weight in any scenario. The effect of culling was higher but there is a trade off by the fact that there is a profit loss due to the loss of animals. Thus there is relatively low merit in using, additionally to selection, separation/culling to help control the impact of nematode parasites. Protein supplementation has been proposed as a means of helping to reduce the impact of gastrointestinal parasites. The interaction of genotype and nutrition and the effect of different levels of protein supplementation on estimated genetic and phenotypic parameters for a flock of lambs were examined using computer simulation, for artificial and natural challenge scenarios. In the artificial challenge scenario the correlations between Fec and production traits became stronger as dietary protein level was reduced and the parasitic challenge was increased. There was little discernible pattern for natural challenge. It may be concluded that the predicted genotype x environment interaction is of little practical significance with respect to challenge level and dietary protein content. These results contribute to our understanding of the genetics properties of the resistance to gastrointestinal nematodes of small ruminants. They will allow more effective design of breeding strategies for nematode resistance in small ruminants. In particular, this thesis has demonstrated that breeding schemes for improving nematode resistance in small ruminants are predicted to be robust with respect to a) age of sampling b) nutritional regime (protein) and c) management (culling or separation).