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

Authors: NOTTER, DAVID - Virginia Tech; NGERE, LAURETTA - Oak Ridge Institute For Science And Education (ORISE); Burke, Joan; MILLER, JAMES - Louisiana State University; MORGAN, JAMES - Katahdin Hair Sheep International

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2018
Publication Date: 5/4/2018
Citation: Notter, D.R., Ngere, L., Burke, J.M., Miller, J.E., Morgan, J.L. 2018. 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. Journal of Animal Science. 96(5):1579-1589. https://doi.org/10.1093/jas/sky100.
DOI: https://doi.org/10.1093/jas/sky100

Interpretive Summary: Selection for parasite resistance in sheep allows farmers to minimize the need for parasite control and dewormers. However, sometimes selection for one trait (ie.,parasite resistance) leads to poorer performance in another, such as reproduction traits. Scientists from the Oakridge Institute for Science and Education, the Agricultural Research Service - Booneville, AR, Virginia Tech, Louisiana State University and Katahdin Hair Sheep International determined that fecal egg counts (FEC) around the time of lambing are a heritable trait, and can be selected for to decrease reliance on dewormers at a time when ewes are susceptible to parasites and would yield high loads on pasture transferring infection to offspring. Small antagonisms exist between FEC traits and reproduction traits, but should not stop farmers from selection for resistance. This information is important to sheep producers, scientists, veterinarians, and extension specialists aiming to improve genetic parameters and parasite resistance in sheep.

Technical Abstract: This study estimated genetic parameters for ewe reproductive traits [number of lambs born (NLB) and weaned (NLW) per ewe lambing] and peri-parturient (PPR) fecal egg counts (FEC) at lambing (PPR0) and 30 d postpartum (PPR30), and their genetic relationships with lamb BW and FEC in Katahdin sheep. The study used records of NLB and NLW for 23,060 lambings by 9,295 Katahdin ewes, 1,230 PPR0 records from 750 ewes, 1,070 PPR30 records from 611 ewes, BW records at birth, weaning, and (or) post-weaning from 12,869 lambs, and FEC records at weaning and (or) post-weaning for 4,676 lambs. Genetic parameters were estimated for all traits using univariate and bivariate animal models in ASReml and included estimates of direct additive, permanent environmental, and residual (co)variances. Fixed effects included effects of ewe management group and ewe age for all traits, ewe birth type for PPR0, and ewe rearing type for PPR30 (all P < 0.05). The number of days between lambing and measurement was also fitted as a continuous effect for PPR0 and PPR30. Heritability estimates 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, 0.23 ± 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. Direct additive correlations between ewe reproductive traits and PPR were low (0.11 to 0.14; SE = 0.13 to 0.19). Estimates of direct additive correlations between lamb BW and subsequent ewe reproductive performance ranged from 0.07 to 0.20 with SE of 0.12 to 0.18. The genetic association of PPR with lamb BW ranged from -0.03 to 0.29 with SE of 0.15 to 0.22. Moderate (0.27 to 0.40) and high (0.56 to 0.77) genetic correlations existed between ewe reproductive traits and lamb FEC and between PPR and lamb FEC, respectively. Correlations between maternal additive effects on BW and direct additive effects on PPR were low (-0.08 to 0.10), while those between maternal additive effects on BW and direct additive effects on ewe reproductive traits were variable (-0.36 to 0.11).