Genetic and phenotypic aspects of foot lesion scores in sheep of different breeds and ages

Abstract

Footrot is a costly endemic disease of sheep. This study investigates the potential to decrease its prevalence through selective breeding for decreased lesion score. Pedigreed mule and Scottish Blackface (SBF) ewes were scored for lesions on each hoof on a 0 to 4 scale for up to 2 (SBF ewes) or 4 (mules) times over 2 years. One score was obtained for SBF lambs. An animal was deemed to have lesions (severe lesions) if at least one hoof had a score of at least 1 (2). The prevalence of lesions was 34% in lambs, 17% in SBF ewes and 51% in mules. The heritability of lesions (severe lesions) analysed as repeated measurements of the same trait in a threshold model was 0.19 (0.26) in SBF ewes and 0.12 (0.19) in mules. Estimates for the sum and maximum of scores as well as the number of feet affected were much lower, as were estimates for permanent animal effects (i.e. non-genetic effects associated with an animal). When successive scores on the same animal were analysed as correlated traits, heritability estimates for most traits tended to be higher, except for severe footrot in mules where estimates varied greatly over time. The phenotypic correlations between successive scores in SBF ewes were close to 0, genetic correlations were moderately positive (0.18 to 0.55). Correlations in mules were generally of a similar size, but some genetic correlations were higher (up to 0.92). There was a clear trend for heritabilities for lesions and severe lesions to increase with higher prevalence of lesions, even when analysed in a threshold model. Heritability estimates for traits that combine scores over several events in mules, identifying the more persistently affected animals, ranged from 0.12 to 0.23 with the highest estimates for the average number of feet that were (severely) affected in animals scored for a minimum at two events. The heritability of all lesion traits in lambs was estimated as 0. It is concluded that selection for lower lesions is possible in ewes but not lambs, and that a simple binary score at an animal level is at least as effective as a comprehensive score at hoof level. Given the low repeatability of lesion scores, repeated measures over time will improve effectiveness of selection. Selection across environments (flocks, seasons) with different prevalences of lesions scores will need to take account of variation in the heritability.

Breeding objectives for sheep in Ireland: A bio-economic approach

“…Lambing difficulty trait genetic standard deviation was derived assuming maternal variance proportions (m 2 ) of 0.22. Genetic parameters for the remainder of the traits were estimated from known phenotypic variances and heritability estimates from the literature (Conington et al, 2001;Safari et al, 2005;Nieuwhof et al, 2008b).…”

A survey of management practices and flock performance and their association with flock size and ewe breed type on Irish sheep farms

“…A multitude of sheep breeds are used across Irish sheep systems, ranging from maternal to terminal to hill breeds (McHugh & Diskin 2010). The effect of ewe breed type on performance indicators such as lamb performance (Dawson & Carson 2002 b ; Casas et al 2004; Fogarty et al 2005), lambing difficulty (Dwyer et al 1996; Dawson & Carson 2002 a ; Speijers et al 2010), prolificacy rate (Hanrahan 1994, 2001; Dawson & Carson 2002 a ) and health traits (Good et al 2006; Nieuwhof et al 2008) has been studied extensively. However, the association between ewe breed type management practices and flock performance has not been investigated to date.…”

Implications of Host Genetic Variation on the Risk and Prevalence of Infectious Diseases Transmitted Through the Environment

“…The impact of host heterogeneity (in terms of both the source of variation and the underlying genetic architecture) was assessed by four different epidemic characteristics: first, we calculated the probability of the disease to exist in the population at different time points (calculated as the proportion of replicates with at least one infected individual Survival time for infectious bacteria in the environment 1, 1.5, or 12.0 days b N Population size 1000 a a Arbitrary values. The value for T S was chosen to yield footrot prevalences similar to those reported in field studies (e.g., Nieuwhof et al 2008) for unit shedding rates k D and k C , respectively. For more information on the value for T C , see text.…”