FACTORS AFFECTING THE PREVALENCE AND SEVERITY OF FOOTROT IN A MERINO FLOCK SELECTED FOR RESISTANCE TO HAEMONCHUS-CONTORTUS

An outbreak of virulent footrot occurred in three lines of sheep selec ted for different levels of resistance to Haemonchus contortus, provid ing an opportunity to examine the factors affecting footrot severity. The selection lines did not differ significantly in average footrot sc ore, but various other factors were found to have an effect. In breedi ng ewes, the average score was 10.2 (maximum possible score 20) and th e prevalence of footrot was 96%. There was a significant age effect, w ith average score increasing from 9.1 +/- 0.8 in 2-year-old ewes, to 1 3.4 +/- 0.9 in 7-year-old ewes. Ewes that lost their lambs tended to h ave higher scores than those that either failed to lamb or successfull y reared their litter. Significant differences in footrot score were c aused by the lambing paddock in which the ewes had been, during the pe riod seven weeks to one week before infection. Among lambs, aged about 20 weeks, the average footrot score was lower (5.0), as was the preva lence (75%). The score increased by an average of 0.09 +/- 0.03 per da y of age and the prevalence increased by an average of 0.6% +/- 0.3% p er day of age. Lambs born and reared as twins had significantly lower scores than singles, and progeny of maiden dams had lower scores than progeny of older ewes. There was no significant difference because of the sex of the lamb. The paternal half-sib estimate of heritability of footrot score was 0.20 +/- 0.14 and of the genetic correlation betwee n footrot score and worm egg count was essentially zero (0.02 +/- 0.20 ), but these estimates were probably biased by the confounding of sire s with lambing paddocks. Nevertheless, the results strongly suggest th at there are no adverse genetic associations between resistance to foo trot and resistance to internal parasites.