Genetic parameters for fecal egg counts and their relationship with body weights in Katahdin lambs

Abstract

Reliance on anthelmintic drugs to control internal parasites in sheep is no longer sustainable because of the development of resistance to these drugs in parasite populations. Genetic selection may offer an alternative long-term solution, as differences in parasite resistance exist both within and among sheep breeds. However, selection for parasite resistance may have correlated effects on other production traits. The objectives of this study were to estimate genetic parameters for weaning (WFEC) and postweaning (PWFEC) fecal egg counts (FEC) and assess their relationship with birth (BWT), weaning (WWT), and postweaning (PWWT) BW in Katahdin lambs. The study used WFEC (n = 2,537), PWFEC (n = 3.421), BWT (n = 12,869), WWT (n = 10,961), and PWWT (n = 7,812) from 12,869 lambs measured between 2003 and 2015 in 13 flocks enrolled in the U.S. National Sheep Improvement Program. Animal and sire models were fitted to the data using the ASReml statistical package. Records were corrected for fixed effects of dam age, joint effect of type of birth and rearing, and management group (defined by joint effects of flock, sex, and birth year and season); lamb age in days at each measurement time was fitted as a covariate. Maternal additive and maternal permanent environmental effects were not significant (P > 0.05), but litter effects influenced (P < 0.01) both WFEC and PWFEC. Heritability estimates ranged from 0.18 to 0.26 for WFEC and 0.23 to 0.46 for PWFEC, depending on the model used. Heritability estimates from sire models were higher than estimates from animal models. Direct additive, litter, residual, and phenotypic correlations between WFEC and PWFEC were 0.82, 0.25, 0.15, and 0.29, respectively. Bivariate analyses revealed low to moderate correlations between BW and FEC. Moderate heritabilities for FEC in this study indicated that genetic progress for this trait can be achieved in Katahdin lambs and that selection for low FEC should have little or no effect on BW.

Genetic parameters for ewe reproductive performance and peri-parturient fecal egg counts and their genetic relationships with lamb body weights and fecal egg counts in Katahdin sheep

“…Heritability estimates for FEC in peri-parturient ewes were 0.35 ± 0.06 at lambing and 0.24 ± 0.07 at 30 d postpartum and somewhat higher than heritability estimates for FEC of 0.19 to 0.24 in growing Katahdin lambs (Ngere et al, 2018). These results suggest that genetic improvement in parasite resistance in Katahdin sheep is possible for both growing lambs and peri-parturient ewes.…”

“…Bivariate models for lamb FEC and ewe NLB and NLW included only direct additive covariances between the pairs of traits. Univariate models for lamb FEC (Ngere et al, 2018) revealed no significant maternal additive or dam permanent environmental effects on lamb FEC, and these effects therefore did not appear in bivariate models involving lamb FEC. Covariances between residual effects on lamb FEC and future permanent environmental and residual effects on ewe NLB and NLW were also assumed equal to 0.…”

“…Records of reproductive traits were restricted to ewes that were between 1 and 10 yr old at lambing; PPR records came from ewes that lambed at 1 to 7 yr of age. The study also used birth (BWT), weaning (WWT), and (or) post-weaning (PWWT) BW from 12,869 lambs from the 100 flocks (Ngere et al, 2017) and lamb FEC records at weaning (WFEC) and (or) post-weaning (PWFEC) for 4,676 lambs from 13 of these flocks (Ngere et al, 2018). Means ± SD for lamb ages at weaning and post-weaning were 66 ± 10 and 121 ± 21 d, respectively.…”

“…Models for lamb BW (Ngere et al, 2017) included fixed effects of management group, dam age in years, and either litter size (for BWT) or (for WWT and PWWT) a joint effect of the number of lamb born and reared (i.e., present at 14 d postpartum), a continuous linear effect of age at weighing for WWT and PWWT, a random additive animal effects, and random additive maternal, permanent environmental, and temporary environmental (i.e., litter) effects of the dam. Models for lamb FEC (Ngere et al, 2018) included the same fixed effects fitted for lamb BW, a continuous linear effect of age at measurement, and random additive animal and litter effects. Additive maternal and dam permanent environmental effects on lamb FEC were tested in preliminary models but were not significant and were removed from the final models.…”

Genome‐wide association study to identify genetic loci associated with gastrointestinal nematode resistance in Katahdin sheep

“…Some mature Katahdin ewes have been shown to have GIN resistance traits similar to St Croix upon natural infection (Burke & Miller ). Recent work estimated the heritability ( h 2 ) of FEC in Katahdin lambs at 60 ( h 2 = 0.18–0.26) and 120 ( h 2 = 0.23–0.46) days of age (Ngere et al ), and periparturient ewes at lambing ( h 2 = 0.29–0.41) and 30 days postpartum ( h 2 = 0.17–0.31) (Notter et al ). These generally moderate heritabilities for FEC suggest that parasite resistance can be improved through genetic selection.…”