B.J. Hayes¹ , S.P. Miller² and J.W. Wilton^2
1Department of Maths and Computing, Central Queensland University, Australia
and Meat Quality CRC, Tropical Beef Centre, Australia
²Centre for Genetic Improvement of Livestock, Dept. of Animal and Poultry Science, University of Guelph

Summary

Introduction

Crossbreeding systems use between breed non-additive variation (heterosis), but little effort has been made to use within breed non-additive effects when designing breeding programs. One way to use dominance within breed genetic variation, is to predict the performance of mating pairs. Dominance can be predicted in progeny from potential matings based on dominance observed in similar past matings, using knowledge of relationships. Breeding schemes can be developed which make matings to maximise the performance of future progeny, taking advantage of this within breed dominance variation as well as the heterosis observed between breeds.

The objectives of this study were to 1) develop breeding strategies which take advantage of heterosis and within breed dominance variation to maximise merit of future progeny; and 2) evaluate the performance of these strategies in livestock herds with a range of values for heterosis, additive variation and dominance variation.

Materials and Methods

The "super breed" model, originally proposed by Paul VanRaden at USDA., was expanded to include both additive and dominance genetic variation both between and within breeds. This model consists of considering all animals in the population despite their breed composition as belonging to one "super breed" and pure breeds are considered as inbred lines within the super-breed. Heterosis is then modeled as a recovery from inbreeding depression through a regression on the level of inbreeding. This model allowed prediction of both between breed dominance effects, which are accommodated through the regression on inbreeding, and individual (within breed) dominance effects.

This model was used to predict the performance of future progeny from potential matings of male and female candidates. Two types of breeding strategies were developed; selection followed by mating strategies, and mate selection strategies. Beef producers are familiar with the first type of strategy. The second strategy involves determining the potential merit of progeny from the potential matings of all males and females in the target herd, considering additive effects, heterosis, as well as within and between breed dominance predicted with the super breed model. The best matings can then be chosen. The performance of the two types of breeding strategies were evaluated at a range of values of dominance variance. A number of small herds (all with 20 animals) were used as target herds.

Results and Discussion

Figure 1 shows the performance of selection and mating strategies at different values of dominance and additive variance (p, where p is the frequency of the dominant allele at a biallelic loci with an additive and dominance genetic value = 1). The value of heterosis here is quite small, 5% of genetic standard deviation in an F1 animal. For comparison, the value of heterosis for a trait such as yearling weight could be as high as 200% of a genetic standard deviation. At (p = 0.3), dominance variation is small, and the advantage of mate selection strategies which consider within-breed dominance is small. When dominance variation is large (p = 0.7) the advantage of the mate selection strategies which consider within breed dominance in breeding decisions is large.

The relative performance of the breeding strategies changes if heterosis is large. Table 1 shows the relative performance of the breeding strategies when heterosis was 72.5% of the genetic standard deviation.

The large advantage of mate selection strategies over truncation selection and mating strategies seen in Table 1 comes through use of parents which contribute to progeny merit with optimally balanced heterosis and additive merit. Exploiting individual dominance effects has little advantage with these parameters.

Significance to the Industry

Mate selection strategies allow producers to make matings which balance additive merit, heterosis, and within and between breed dominance to produce future progeny with maximum genetic merit. These strategies are particularly useful if heterosis or within breed dominance variance is large.

Acknowledgments

Funding from Beef Improvement Ontario and the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

Table 1. Average total progeny merit as a proportion of genetic standard deviation for four mating strategies at p = 0.7, with a large value of heterosis (72.5% of genetic standard deviation).

Figure 1. Average total progeny merit as a proportion of genetic standard deviation for four mating strategies at different values of variance components.