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.