Inbreeding Depression and Purging for Meat Performance Traits in German Sheep Breeds

SIMPLE SUMMARY: In meat sheep and sheep bred for other purposes, meat quality is one of the factors evaluated in a breeding programme. In this study, we included 25 sheep breeds from Germany with a sufficient amount of performance and pedigree data. The heritability, genetic and residual correlations were estimated for daily weight gain, meatiness score and ultrasound measurements for the muscle and fat thickness of the sheep breeds. We employed animal models to analyse inbreeding depression by estimating linear slopes of inbreeding coefficients on meat performance traits. For daily weight gain, inbreeding depression was significant for all sheep breeds. When considering single breeds, significant inbreeding depression was only found in two breeds. The meatiness scores did not show significant inbreeding depression across breeds, nor in single breeds, but only in one breed. The ultrasound measurements for muscle and fat thickness were not significant for inbreeding depression either across or within breeds. Purging effects did not prove to be significant either across or within breeds. Our data show that inbreeding depression and purging can be present in traits under strong selection. ABSTRACT: This study provides estimates on genetic parameters, inbreeding depression and purging for meat performance measures from 25 German sheep breeds. All German meat, merino sheep breeds and breeds of other breeding directions with a sufficient number of pedigree and performance data were included in this study. Phenotypic traits retrieved from the national database OviCap were evaluated: daily weight gain, meatiness score and ultrasound measurements for muscle and fat thickness. We employed animal models to estimate heritability, variance and covariance components for these meat performance traits as well as inbreeding depression and purging. The heritabilities, on average, reached estimates of 0.55, 0.34, 0.53 and 0.61 for daily weight gain, meatiness score and ultrasound measurements for muscle and fat thickness, respectively. We estimated the linear regression slopes for the individual rate of inbreeding, new and ancestral inbreeding, as well as the inbreeding coefficient and its interaction with the inbreeding coefficient of Ballou, employing animal models with non-genetic effects and the additive genetic effect of the animal. Across all breeds, inbreeding was only significant for daily weight gain, whereas for all other traits, estimates were not significant. Within sheep breeds, we found significant inbreeding depression for daily weight gain in German Mutton Merino and German Blackheaded Mutton as well as for the meatiness score in German Whiteheaded Mutton. Significant effects for purging, based on ancestral inbreeding and the interaction effect of the classical inbreeding coefficient with the inbreeding coefficient of Ballou, were not obvious either across or within any sheep breed. A 1% increase in inbreeding significantly decreased the phenotypic trait median of daily weight gain across all sheep breeds by 0.50% and 0.70% of phenotypic and genetic standard deviation, respectively. Purging effects due to ancestral inbreeding were not significant in any breed or across breeds. The results of this study may indicate that inbreeding depression may be more harmful in traits under stronger selection than in traits that exert low selection pressure. The results of this study demonstrate the different effects that result in meat performance traits due to inbreeding. With increasing rates of inbreeding and critical effective population sizes, selection intensity for breeding objectives has to be critically reviewed for each sheep breed. Inbreeding depression and purging should be evaluated in order to prevent a decrease in trait means due to inbreeding and to determine whether detrimental alleles are eliminated.