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.

This study estimated genetic parameters for ewe reproductive traits [number of lambs born (NLB) and weaned (NLW) per ewe lambing] and fecal egg counts (FEC) during the peri-parturient rise (PPR) for use in genetic evaluation of Katahdin sheep. Data included NLB and NLW for 23,060 lambings by 9,295 Katahdin ewes, 1,230 PPR at lambing (PPR0) for 750 ewes, 1,070 PPR at approximately 30 d postpartum (PPR30) for 611 ewes, BW at birth, weaning, and (or) post-weaning for 12,869 lambs, and FEC at weaning and (or) post-weaning for 4,676 lambs. Direct additive, permanent environmental, and residual (co)variances were estimated in univariate and bivariate animal models. Fixed effects included effects of ewe management group and ewe age for all traits, and, for PPR, a continuous effect of days between lambing and measurement. Effects of litter size on PPR0 and number of lambs suckled on PPR30 were included in univariate models but excluded from bivariate models for PPR and NLB or NLW. Heritability estimates in univariate models for NLB, NLW, PPR0, and PPR30 were 0.09 ± 0.01, 0.06 ± 0.01, 0.35 ± 0.06, and 0.24 ± 0.07, respectively. Estimates of permanent environmental variance as a proportion of total phenotypic variance were 0.02 ± 0.01 for NLB, 0.03 ± 0.01 for NLW, 0.05 ± 0.06 for PPR0, and 0.13 ± 0.07 for PPR30. Direct additive, phenotypic, permanent environmental, and residual correlations between NLB and NLW were 0.88 ± 0.03, 0.74 ± 0.004, 0.54 ± 0.15, 0.74 ± 0.003, respectively; corresponding correlations between PPR0 and PPR30 were 0.96 ± 0.07, 0.46 ± 0.03, 0.98 ± 0.50, 0.18 ± 0.05, respectively. The additive genetic correlation (rd) between ewe reproductive traits and PPR ranged from 0.12 to 0.18. Estimates of rd between lamb BW and subsequent ewe NLB and NLW ranged from 0.07 to 0.20, and those between PPR and lamb BW ranged from -0.03 to 0.29. The rd between ewe reproductive traits and lamb FEC ranged from 0.27 to 0.40, and those between PPR and lamb FEC ranged from 0.56 to 0.77. Correlations between maternal additive effects on BW and direct additive effects on PPR were low (-0.08 to 0.10), and those between maternal additive effects on BW and direct additive effects on ewe reproductive traits were variable (-0.36 to 0.11). We conclude that FEC in growing lambs and peri-parturient ewes are controlled by similar genes and that modest, but manageable, genetic antagonisms may exist between FEC and ewe productivity.

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