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

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. ______________________________________________________________________________________

Environmental and genetic factors affecting faecal worm egg counts in Merinos divergently selected for reproduction

“…Extreme individual variation of FWEC is commonly reported in literature. Studies by Cloete et al (2007), Snyman (2007) and Matebesi-Ranthimo et al (2014) reported an individual variation for FWEC ranging from 0 to 13700, 0 to 87800 and 0 to 32700 epg wet faeces, respectively. In a study by Khusro et al (2004), means for FWEC were 736 for hoggets and 779 for yearling Merino sheep, while the individual variation for FWEC ranged from 0 to 50000 epg in both age groups.…”

“…present as major species and Haemonchus contortus as a minor species (Reinecke et al, 1987). Individual faecal samples were analysed at Stellenbosch Provincial Veterinary Laboratory, using the McMaster technique, with a sensitivity of 100 eggs per gram (epg) of wet faeces (Van Schalkwyk et al, 1994;Matebesi-Ranthimo et al, 2014).…”

“…Previous studies have reviewed genetic factors that affect FWEC at international level (Khusro et al, 2004;Safari et al, 2005;Morris, 2011;Pickering et al, 2012). However, in South Africa, these genetic factors have been limited to the works of Nieuwoudt et al (2002), Snyman (2007), Cloete et al (2007) and Matebesi-Ranthimo et al (2014), all reporting data of flocks under institutional control. Therefore, the issue of genetic and environmental factors affecting FWEC still needs to be investigated to ensure that these results are robust across environments.…”

“…However, for breeding to be implemented, genetic variation of a trait of interest should be present. Literature has reported a variation in parasite resistance, measured indirectly by faecal worm egg count (FWEC), ranging from 0.00 to 0.51 (Greeff et al, 1995;Cloete et al, 2007;Matebesi-Ranthimo et al, 2014). Selecting animals with the lowest FWEC would thus improve parasite resistance in sheep production.…”

“…Gray (1991) identified FWEC as a viable means of indirectly measuring parasite resistance. As a result, FWEC has been used widely to measure parasite resistance, following artificial or natural infestation (Mandonnet et al, 2001;Cloete et al, 2007;Matebesi-Ranthimo et al, 2014). Previous studies have reviewed genetic factors that affect FWEC at international level (Khusro et al, 2004;Safari et al, 2005;Morris, 2011;Pickering et al, 2012).…”