Genetic parameters for indicators of host resistance to parasites from weaning to hogget age in Merino sheep1

Abstract

Fecal egg count (FEC) has been widely used as an indicator of host resistance to gastrointestinal parasites in sheep and has been shown to be a heritable trait. Two other possible indicators of parasites, dag score (DS; accumulated fecal material) and fecal consistency score (FCS), were investigated in this study, along with BW. All four traits were studied to see how heritability and genetic correlations varied with age from weaning (4 mo) to hogget age (approximately 400 d). More than 1,100 lambs, the offspring of 37 rams, were recorded eight times between weaning (3 to 5 mo of age) and hogget age (13 to 18 mo of age) on two farms. Sire models were fitted to the data from each trait at each recording and in a repeatability model involving the whole data set. Overall, the heritabilities were 0.28+/-0.072 (FEC), 0.11+/-0.036 (DS), 0.12+/-0.036 (FCS), and 0.23+/-0.070 (BW). By fitting random regression models to the time-series data, it was possible to see how these heritability values varied as the lambs aged, from weaning to hogget age. The heritability of FEC rose from 0.2 at weaning to 0.65 at 400 d. Dag score had a higher heritability (0.25) in the middle of the age range and a low value at weaning (<0.1) and hogget age (0.16). The heritability of FCS was low, with a value of 0.2 at weaning reducing to 0.05 as the animals aged. Body weight had zero heritability at weaning, which rose to greater than 0.6 at hogget age. Most traits had low genetic correlations between them, the only exception being that between FCS and DS (0.63). Most genetic correlations varied little over the age range with the exception of FEC and BW, which fell from 0 at weaning to -0.63 at hogget age. Whereas FCS and DS may be good indicators of scouring, they are very different from FEC as an indicator of host resistance to gastrointestinal parasites.

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

“…Untransformed FEC data suggested very high variation and a non-normal distribution, which were similar to findings reported by Pollott & Greeff (2004), Yadav et al (2006) and Cloete et al (2007). However, the application of transformations on data improved their properties to a point that the coefficient of variation was 19.77% in the log-transformed data.…”

“…Phenotypic and genetic variation of FEC in the current study, as well as in the literature, suggests that worthwhile genetic progress in South African sheep breeds is feasible. Selection for reduced FEC resulted in increased profit in Australia , which could be achieved in South Africa if sound data collection procedures are followed, and the timing of sampling for FEC is optimized (Greeff & Pollott, 2004). Objective wool traits have been studied in the past and were reviewed recently by Safari et al (2005).…”

“…The model with only the direct additive (σ 2 a ) effect fitted the data best for all the FEC traits in the present study (Table 2). A similar random model was used in Australian Merinos (Pollott & Greeff, 2004). However, Cloete et al (2007) fitted a different random model including both σ pe as well as a litter effect on 14 -17 months of age for GFW, CFW, FD, CY, SDFD and CVFD.…”

“…Data for FEC were transformed to logarithms to the base of 10 (after a base number of 100 was added to each value to account for zero counts) or to cube roots to improve data distribution. These transformations have been used extensively by a number of researchers to normalize FEC data in Australia Karlsson et al, 1995;Doyle & Eady, 2001;Brown & Tier 2003;Pollott & Greeff, 2004), New Zealand (Morris et al, 1997), India (Yadav et al, 2006), South Africa (Cloete et al, 2007) and the United Kingdom (Matika et al, 2010). Untransformed data and data subjected to the transformations listed above resulted in three analyses on FEC.…”

Quantitative genetic architecture of parasite-induced cataract in rainbow trout, Oncorhynchus mykiss

“…This is logical because rapid growth is typically phenotypically and genetically related to increased body condition (Kause et al, 2003). Similar to our study, in sheep, genetic correlations between nematode resistance and growth are mostly favourable (Pollott et al, 2004), but unfavourable genetic trade-offs have been observed especially in young animals (Bisset et al, 1992). In contrast to our study, though, genetic trade-offs have been observed between resistance against mastitis causing pathogens and milk production in cows (Lund et al, 1999).…”