Clinical footscald and footrot in a New Zealand Romney flock: phenotypic and genetic parameters

Abstract

Phenotypic and genetic parameters for clinical footscald and footrot were investigated in a pedigree flock of New Zealand Romney sheep inspected during outbreaks over six years. At inspections, each sheep's overall status with regard to footscald and footrot was summarized on a combined severity scale (Rating). Sheep were also categorized in terms of two binomial variables specifying the presence or absence of footscald or footrot (FI) and the presence or absence of footrot only (FR). A trinomial variable specified no infection, infection limited to footscald, or footrot with or without footscald (FSR), all variables being derived from the Rating value. Notwithstanding marked annual variations in prevalence of FI, F1 prevalence increased with age among ewes, and a higher prevalence of FI among male than female lambs was noted. Birth rank, birth date and dam's age had no significant effect on Rating in lambs. Repeatability of ewe Rating over successive annual inspections was low. Heritabilities of Rating, FI, FR and FSR, estimated by half-sib analyses were 0.14, 0.28, 0.17 and 0.25, respectively. Heritability of Rating estimated from dam-offspring regression was 0.12. Production traits did not appear to be genetically associated with liability to footrot. Selection of the top 5% of progenytested sires for breeding could be expected to reduce the prevalence of FI by approximately 45% per generation.

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

“…The resulting estimate of 0.35 does not support the suggestion that the footrot results may be an artefact the data structure. The zero heritability contrasts with results for footrot presented by Skerman et al (1988) and Raadsma et al (1994) who found low to medium heritabilities in lambs, although the lambs in these two studies were older, i.e. 8 to 10 months at the start of trial, than in (2) 2006 (1) 2006 (2) 2005 ( the current study where they were an average of 5 months at time of scoring.…”

“…Although differences in heritability estimates for lesion scores were found across the breeds, these estimates do not differ greatly from each other or from previously published values for resistance to footrot. In SBF, the heritability for foot lesions was estimated to be 0.19 and in mules 0.12, this is comparable to 0.16 and 0.31 estimated by Raadsma et al (1994) in Merino lambs and 0.28 for Romney (Skerman et al, 1988, trait defined as footrot or scald), especially when taking into account the effect of prevalence. The respective figures for severe lesions are SBF 0.26, mules 0.19, Merino (1), 0.21, Merino (2) 0.16 and Romney 0.17 (footrot only).…”

“…However, a simple biological reason may be hypothesised: certain genes that affect resistance to footrot, and hence the development of lesions, are possibly not expressed at low infection pressures, hence at a low prevalence. This, if correct, would also explain the difference in estimated heritabilities in lambs between the current study and that by Skerman et al (1988) and Raadsma et al (1994), in which animals were deliberately infected and the resulting prevalences were as high as 57% (Skerman et al, 1988) and over 50% and 80%, respectively, in two trials pre-vaccination (calculated from data used in Raadsma et al, 1994).…”

“…In a lamb population with a prevalence of footrot ranging from 1% to 34% in females and 31% to 57% in males, Skerman et al (1988) calculated a heritability of 0.17 on the underlying scale for the binary trait (i.e. presence or absence of footrot) in Romney lambs of 8 to 9 months of age.…”

The effect of footrot on weight gain in sheep

“…Genetic variation in resistance to footrot has been demonstrated (Skerman et al, 1988;Raadsma et al, 1994;Conington et al, 2007), with the heritability depending on the definition of footrot (number of classes, observed or underlying scale) and reaching values up to 0.3. While Raadsma et al (1994) showed an increased risk of footrot with heavier live weights, they did not estimate the effect of footrot on live weight gain.…”