Is it feasible to select humid sub-tropical Merino sheep for faecal egg counts?

• Faecal egg count (FEC) in Merinos raised in humid sub-tropic environments is a highly variable and moderately heritable trait.
• FEC is negatively correlated with hogget body weight, and negligibly correlated to wool production traits, mainly wool weights and mean fibre diameter.
• This indicates selection for low FEC lambs is expected to increase hogget weights and not to compromise wool production traits.

Genetic parameter estimates for faecal egg counts (FEC), weaning and shearing weights and wool traits were evaluated in 720 Australian Merino lambs raised in Southern Brazil. Weaned lambs were naturally exposed to nematode larvae-contaminated pastures with the objective of recording individual gastrointestinal parasite infection responses. Phenotypic and genetic correlations among production traits such as birth, weaning and shearing weights and wool production traits (greasy and clean fleece weights, scouring yield, mean fibre diameter, and staple length) were estimated. Results showed a high variability for FEC counts among individuals, with a heritability estimate of 0.365 ± 0.001, and low phenotypic and genetic correlations between FEC and all productive traits, except for a negative shearing live weight (rG = −0.305 ± 0.002 and rP = −0.140 ± 0.001). These results indicate that selection for low FEC would ensure genetic progress in reducing gastrointestinal parasite infection levels without limiting major sheep production traits for Merino sheep raised in a humid sub-tropical environment.