Genetic parameters and relationships of faecal worm egg count with objectively measured wool traits in the Tygerhoek Merino flock

P A Matebesi-Ranthimo,J J Olivier,S W Cloete,J B Van Wyk

doi:10.4314/sajas.v44i2.11

Abstract

The costs of internal parasite control and treatment are potentially very high in grazing sheep. Faecal worm egg count (FEC) has been suggested as a suitable criterion for selection for resistance to nematode infestation in livestock. Genetic parameter estimates for FEC and its relationship with wool traits were assessed in the current study, using data from Merino sheep from a selection experiment maintained at Tygerhoek Research Farm. Data consisted of ~7 100 animals born between 1989 and 2010. Rectal faecal samples were taken from individual sheep at 13 to 16 months of age, after drenching had been withheld for at least 10 weeks, generally in July to September. Nematode eggs were counted with the McMaster technique, with a sensitivity of 100 eggs per gram of wet faeces. The fixed effects of birth type, sex, birth year and sex x birth year interaction were included in the operational model for FEC. Only the effect of additive animal affected the data for FEC. Heritability estimates of FEC ranged from 0.10 for untransformed FEC to 0.16 for Log (FEC + 100). The genetic relationships of Log (FEC + 100) with staple strength (SS), staple length (SL), coefficient of variation of fibre diameter (CVFD) and standard deviation of fibre diameter (SDFD) were favourable. The genetic correlations of FEC with wool weight traits were unfavourable in absolute terms, but not significantly different from zero. Selection for FEC is unlikely to result in unfavourable correlated responses to wool traits in South African Merinos, with the exception of wool weights.Keywords: Internal parasites; fleece weight; fibre traits; staple traits; correlations; resistance

Highlights

IntroductionInternal parasite infestations constitute a major disease problem for domestic livestock, which poses a threat to sustainable production worldwide (Karlsson et al, 1995; Zhang et al, 1997; Nieuwoudt et al, 2002; Perry et al, 2002; Chauhan et al, 2003)
The present study suggested that Faecal worm egg count (FEC) is variable and heritable, and that selection should result in additive gains in the ability of animals to resist nematode infestation in South Africa
Selection for a reduced FEC is unlikely to result in unfavourable correlated responses in objectively measured wool traits in South African Merinos, as suggested by generally favourable genetic relationships of wool traits with FEC

Summary

Introduction

Internal parasite infestations constitute a major disease problem for domestic livestock, which poses a threat to sustainable production worldwide (Karlsson et al, 1995; Zhang et al, 1997; Nieuwoudt et al, 2002; Perry et al, 2002; Chauhan et al, 2003). Resistance of gastro-intestinal parasites to anthelmintics has become more prevalent over the past years (Gray, 1991; Eady, 1995; Karlsson et al, 1995; Greeff & Karlsson, 1999; Nieuwoudt et al, 2002) and is at a very high level that threatens sustainable control and treatment of internal parasites (Karlsson & Greeff, 2006; Snyman et al, 2007; Reynecke et al, 2011). Gary (1991) identified faecal worm egg count (FEC) as a valuable criterion for improving genetic resistance to gastro-intestinal nematodes

Methods

Results

Conclusion