title: Estimates of genetic parameters for breech strike and potential indirect indicators in sheep.
abstract: OBJECTIVE
  To determine flystrike incidence, and estimate genetic parameters and potential indicator traits to reduce breech strike susceptibility in Romney sheep in New Zealand.
  
  
  METHODS
  Seventeen Romney-based flocks, on 11 farms from throughout New Zealand, were enrolled in a case-control study in 2009/10 and 2010/11. Farmers observed lambs with flystrike and recorded dag score, breech bareness score and flystrike location for each lamb. Flystrike location was classed as breech, body, belly, shoulders, head or feet. Paternity of lambs was determined from tissue samples collected from lambs and all potential sires on farms. Control lambs without flystrike were selected by matching birth year, flock and sex. Due to the majority of strike occurring in the breech, genetic parameters for breech strike were explored. Dag score and breech bareness were investigated as indirect indicators of breech strike. Heritabilities and genetic and phenotypic correlations were estimated for breech strike, dag score and breech bareness using an animal model.
  
  
  RESULTS
  For the 2009/10 season, 484 cases of flystrike were recorded with mean incidence rate per farm of 1.76 (min 0.47, max 2.95)%. For the 2010/11 season, 352 cases were recorded with a mean incidence rate per farm of 2.54 (min 0.43, max 8.18)%. Over both years 694/792 (88%) cases of flystrike occurred on the breech. Heritability on the observed scale for breech strike was 0.32 (SE 0.10). Heritabilities for dag score and breech bareness were 0.23 (SE 0.09) and 0.35 (SE 0.11), respectively. Breech strike had a high positive genetic correlation with dag score (0.71) and a low negative genetic correlation with breech bareness (-0.17). Breech strike had a high phenotypic correlation with dag score (0.62) and negative phenotypic correlation with breech bareness (-0.06).
  
  
  CONCLUSIONS
  The high genetic and phenotypic correlations between breech strike and dag score makes dag score a viable option for indirect selection for breech strike resistance in Romney sheep in New Zealand. The heritability, genetic and phenotypic correlations require validation in other dual-purpose breeds, before breeding values for breech strike can be implemented, for use throughout the New Zealand sheep industry.
authors.affiliation: a AgResearch , Invermay Agricultural Centre , Private Bag 50034, Mosgiel 9053 , New Zealand.
authors.forename: N K
authors.initials: NK
authors.lastname: Pickering
authors.forename: H T
authors.initials: HT
authors.lastname: Blair
authors.forename: R E
authors.initials: RE
authors.lastname: Hickson
authors.forename: P L
authors.initials: PL
authors.lastname: Johnson
authors.forename: K G
authors.initials: KG
authors.lastname: Dodds
authors.forename: J C
authors.initials: JC
authors.lastname: McEwan
mesh_terms: D000818:Animals; D005260:Female; D020022:Genetic Predisposition to Disease; D008297:Male; D009198:Myiasis / Q000662:veterinary*; D009520:New Zealand / Q000453:epidemiology; D012756:Sheep; D012757:Sheep Diseases / Q000453:epidemiology / Q000235:genetics / Q000469:parasitology*
keywords: Romney breed; Sheep; breech strike; dag score; flystrike; heritability; myiasis