Genetic and phenotypic relationships among anti-Trichostrongylus colubriformis antibody level, faecal egg count and body weight traits in grazing Romney sheep

Serum levels of an antibody (Ab) and immunoglobulin G1 (IgG1) to the larval (L3) stage of the internal parasite, Trichostrongylus colubriformis, were determined in 3 years in Romney lambs. Animals from two pairs of selection flocks, bred for increased or reduced resistance to internal parasites, were compared for Ab or IgG1 levels, and a progeny test flock was recorded to provide data for genetic parameters (n = 64 sires). Lambs were exposed to a natural parasite challenge on pasture, following an anthelmintic drench treatment at weaning. Blood and faecal samples from 4- to 6-month-old lambs (January samples) were taken when the mean FEC of a monitor group reached 800 to 1500 eggs/g. Lambs were again drenched with anthelmintic and a further cycle of the protocol was carried out (March samples). Selection flock differences in March (High flock minus Low flock means) averaged -0.73, -0.93 and 1.27 phenotypic standard deviations for logeAb, loge IgG1 and loge (FEC+ 100) at one site, and -0.89, -0.94 and 1.89 phenotypic standard deviations at the other. Repeatabilities and heritabilities of logeAb, loge IgG1, and loge (FEC+ 100) were estimated. For data collected in March, the genetic correlations between logeAb or log IgG1 and loge (FEC+ 100) were -0.56 ± 0.18 and -0.35 ± 0.19 respectively, whilst they were -0.48 ± 0.17 and -0.47 ± 0.15 between logeAb or loge IgG1 and the average of loge (FEC + 100) in January, March and May. The results suggest that ramselection based on serum antibody levels in March would result in 51-67% of the genetic gain in reducing FEC that could be achieved by using FEC directly. © 1995.