GENETIC AND PHENOTYPIC RELATIONSHIPS AMONG ANTI-TRICHOSTRONGYLUS COLUBRIFORMIS ANTIBODY LEVEL, FECAL EGG COUNT AND BODY-WEIGHT TRAITS IN GRAZING ROMNEY SHEEP

Serum levels of an antibody (Ab) and immunoglobulin G(1) (IgG(1)) 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 IgG, 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 log(c)Ab, log(c) IgG(1) and log(c) (FEC + 100) at one site, and -0.89, -0.94 and 1.89 phenotypic standard deviations at the other. Repeatabilities and heritabilities of log(c)Ab, log(c) IgG(1), and log(c) (EEC + 100) were estimated. For data collected in March, the genetic correlations between log(c)Ab or log IgG(1) and log(c) (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 log(c)Ab or log(c) IgG(1) and the average of log(c) (FEC+100) in January, March and May. The results suggest that ram selection 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.