Genetic and environmental effects on twin pregnancy and length of reproduction in Thoroughbred mares

Reproductive data of 1910 Thoroughbred mares from seven Polish studs were analyzed using linear animal model. The mares were born between 1929 and 1994. The frequency of multiple pregnancies was 3.5% in this population. Fecundity (defined as ratio of number of progeny to number of pregnancies) and length of reproduction were recorded. Heritability estimates of fecundity and the number of pregnancies were 0.03 and 0.07, respectively. Genetic correlation between these traits was −0.42 whereas phenotypic and environmental correlations were close to zero (−0.05 and −0.04, respectively). In general, genetic, phenotypic and environmental trends for fecundity and the number of pregnancies were non-negative. However, these annual effects are highly fluctuated. It may be attributed to the influence of some sires incorporated into population. Hence, a segregation of single locus can be hypothesised.