Location: Dale Bumpers Small Farms Research CenterTitle: Utilization of year round data in the estimation of genetic parameters for internal parasite resistance traits in Dorper sheep
|NGERE, LAURETTA - Orise Fellow|
|HERRING, A - Texas A&M University|
|SANDERS, J - Texas A&M University|
|CRAIG, THOMAS - Texas A&M University|
|VAN WYK, JAN - University Of Pretoria|
|RILEY, DAVID - Texas A&M University|
Submitted to: Small Ruminant Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2017
Publication Date: 5/1/2017
Citation: Ngere, L., Burke, J.M., Herring, A.D., Sanders, J.O., Craig, T.M., Van Wyk, J.A., Riley, D.G. 2017. Utilization of year round data in the estimation of genetic parameters for internal parasite resistance traits in Dorper sheep. Small Ruminant Research. 151:5-10.
Interpretive Summary: Selection for low fecal egg counts (FEC) and FAMACHA scores and high hematocrits (measures of parasitism) can be used to genetically enhance resistance to internal parasites in lambs, thereby reducing mortality and the need for dewormer. The primary season of Haemonchus contortus or barberpole worm infection is during warm summer months, and heritability of parasite measures has been determined during this time, but little is known about selection for parasites during cooler months. Scientists from Texas A&M University, University of Pretoria in South Africa,the Oakridge Institute for Science and Education, and the Agricultural Research Service - Booneville, AR determined that inclusion of cool season records may not affect estimates of genetic parameters for these traits. This information is important to sheep producers, scientists, veterinarians, and extension specialists aiming to minimize parasite problems in sheep worldwide.
Technical Abstract: The objective of this study was to evaluate the effect on the estimates of heritability and permanent environmental effects as a proportion of phenotypic variance when year round records are used. Records from 1,008 Dorper sheep in a private South African flock comprised 17,711 FAMACHA scores, 3,758 fecal worm egg counts (FWEC; practically only Haemonchus contortus), and 4,209 hematocrit values that were collected from 1997 to 2000. Animal models were used to conduct single-trait analyses. Data were analyzed in two sets: 1) warm season records only and 2) year round records. Treatment (with anthelmintic) status was investigated as a 2-level fixed effect in both sets; additionally records of treated sheep were removed for another analysis of both data sets. In analyses of warm season records, estimates of heritability and permanent environmental variance as a proportion of phenotypic variance for FAMACHA score were 0.32 ± 0.03 and 0.03 ± 0.02, respectively, when treatment status was modeled, and 0.40 ± 0.02 and 0 respectively when treated records were excluded from the analysis. Heritability estimates for hematocrit value was 0.23 ± 0.06 (treatment status modeled) and 0.28 ± 0.04 (treated records excluded), while permanent environmental variance as a proportion of phenotypic variance was respectively 0.12 ± 0.05 and 0.09 ± 0.06. Fecal worm egg count heritability estimates were 0.11 ± 0.04 (treatment status modeled) and 0.17 ± 0.05 (untreated records only). Permanent environmental variance for FWEC was 0 when treated records were excluded and 0.03 ± 0.04 when treatment status was included in the analyses. In analyses of year round records, estimates of heritability and permanent environmental variance as a proportion of phenotypic variance for FAMACHA score were 0.32 ± 0.03 and 0.03 ± 0.02 respectively (treatment status modeled) and 0.36 ± 0.03 and 0.02 ± 0.02 (treated records excluded). Hematocrit value heritability and permanent environmental variance were, respectively, 0.19 ± 0.05 and 0.18 ± 0.05 in the analyses of untreated records, and 0.18 ± 0.04 and 0.15 ± 0.04, when treatment status was modeled. Heritability estimates for FWEC were 0.05 ± 0.03 for untreated records only, and 0.04 ± 0.02 when treated records were included. Permanent environmental variance was 0.07 ± 0.02 (treated records included) and 0.07 ± 0.03 (treated records excluded) for FWEC. Collection and inclusion of cool season (that is, outside of the regular worm proliferation season) records in analyses may not substantially change estimates of genetic parameters for these traits.