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

Abstract

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.

The cost of host genetic resistance on body condition: Evidence from divergently selected sheep

“…We performed a supplementary analysis of the correlations between stage‐specific responses to infection, which further indicated that FEC was highly repeatable across stage‐specific infections (correlation between stages >0.5 in general; Appendix S2 and Table S7). Our results were thus globally consistent with the high genetic correlations observed between FEC measures at different ages in other sheep populations (Bishop & Stear, 2001; Brown & Fogarty, 2017; Goldberg et al, 2012; Notter et al, 2018; Pollott et al, 2004). Overall, there is little chance that periparturient limits to selection for parasite resistance stem from a lack of genetic variation at this stage (Bishop & Stear, 2001; Goldberg et al, 2012).…”

“…and Table S7). Our results were thus globally consistent with the high genetic correlations observed between FEC measures at different ages in other sheep populations Brown & Fogarty, 2017;Goldberg et al, 2012;Notter et al, 2018;Pollott et al, 2004). Overall, there is little chance that periparturient limits to selection for parasite resistance stem from a lack of genetic variation at this stage Goldberg et al, 2012).…”

“…In particular, if resource allocation constraints do occur, we do not know to which extent and how they might be circumvented through selection on host resistance. Population studies in sheep have shown strong positive genetic correlations between lamb resistance and ewe resistance (Bishop & Stear, 2001; Brown & Fogarty, 2017; Goldberg et al, 2012; Notter et al, 2018; Pollott et al, 2004). Sheep lines that have been selected for high resistance also exhibit consistent responses to infection across life stages (Morris et al, 1997; Woolaston et al, 1990), although ewes from those resistant lines still undergo a PPRI (Morris et al, 1998; Woolaston, 1992).…”

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.…”

Relationship of Anti-Mullerian Hormone to Reproductive Traits in Katahdin Ewes Bred in Late Spring or Fall

“…Estimated breeding values for NLB as well as NLW in the current study were correlated with mean number of lambs born. The flock also had a higher value correlation between NLB and NLW, implying that there is low voluntary lamb removal (i.e., artificial rearing of lamb or fostering of lambs to other ewes) [13]. However, other EBVs, WWT,…”