Abstract
Gastrointestinal nematodes result in severe economic and production losses to the sheep industry. An increase in resistance of the nematodes to chemicals used for control, as well as a demand of consumers for meat products free from chemicals, has fostered research on alternative control strategies. Breeding for resistance to nematodes offers an alternative to control parasitism but its effectiveness depends on genetic variation in faecal worm egg count (FWEC), an indirect measure of parasite resistance. A historic dataset of FWEC from four Merino lines subjected to natural parasite challenge was used to estimate genetic parameters for FWEC in three seasons (autumn, winter and spring) using a repeated records animal model, followed by a three-trait animal model analysis treating FWEC in different seasons as separate traits. The effects of selection line, birth year, sex, the sex x birth year interaction, season and the season x year interaction were significant when using 4994 records recorded from 1997 to 2000 (p < 0.001). The heritability of log-transformed FWEC amounted to 0.09 ± 0.02, with no contribution from the animal permanent environmental variance to the between animal variation across seasons. Three-trait heritability estimates for log-transformed FWEC amounted to 0.07 ± 0.05 in autumn, 0.13 ± 0.05 in winter and 0.19 ± 0.05 in spring. These results suggest sufficient genetic variation in FWEC to support selection for lower log-transformed FWEC. However, the best time to record data for selection is after the break of the season in winter and in spring, when sheep are stimulated by a greater intake of infective larvae from the pasture after the first rains. Genetic correlations among FWEC in the respective seasons were moderate to high, ranging from 0.55 to 0.89. Phenotypic correlations, on the other hand, were significant but lower in magnitude, ranging from 0.09 to 0.16. These results provide useful information for developing strategies for the genetic improvement of ovine resistance to gastrointestinal nematodes under Mediterranean conditions in South Africa by using FWEC as an indicator trait.
Highlights
The profitability of sheep production relies, in part, on the efficient application of proper strategies to reduce nematode infestations under challenge conditions
The distribution of the data was improved by transformation to natural logs resulting in coefficients of variation (CV) that were below 20%
Log-transformed faecal worm egg count (FWEC) means in autumn declined quite markedly from 1997 to 2000; those recorded in winter were higher in 1997 and 2000 and Autumn Winter Spring
Summary
The profitability of sheep production relies, in part, on the efficient application of proper strategies to reduce nematode infestations under challenge conditions. The major control strategy for nematodes is the use of chemical drenches to treat and prevent production and economic losses associated with infestation. Consumer perception is in favour of the usage of minimum chemicals during the meat production process. These developments force sheep producers and researchers to consider alternative strategies relying less on chemicals, like an integrated pest management programme, including selection for parasite resistance to control parasitism over the long term (Khusro et al 2004). No comparable results on response to selection for a lower FWEC have been published for South African conditions
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Journal of the South African Veterinary Association
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
