Author
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Riley, David |
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VAN WYK, J - UNIV.PETORIA SOUTH AFRICA |
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Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 5/8/2008 Publication Date: N/A Citation: N/A Interpretive Summary: Worm infestation in sheep causes large loss of productivity and associated income for sheep producers worldwide. Assessment of worm infestation has relied on expensive laboratory tests of blood or feces; recently, a cheap, practical method was developed to identify infected sheep in the field, using FAMACHA© scores, which are assessments of anemia as indicated by the color of mucous membranes of the eye of animals. Assessment of the responses to worm infestation usually requires that a substantial part of a flock experience infestation, and the production losses that accompany infestation. Knowledge of the genetic control over FAMACHA© score was desirable in order to permit selection of worm-resistant sheep as parents. This study determined that there is substantial genetic control of FAMACHA© score that would permit such selection, and essentially would distinguish the best parents in the same way and rank as the more expensive methods involving blood and fecal assessments. We also found that FAMACHA© scores are especially helpful in identifying those resistant animals in times of seasonal high worm populations, more so than in times of low worm populations. Finally, we determined that resistant parents could be identified using FAMACHA© scores at one month prior to the peak worm infestation in a given year. This is of enormous practical importance, as it means that it is not necessary to permit a large-scale worm infestation (and the associated production losses) in a flock in order to identify the most resistant/resilient sheep to select to be parents. Technical Abstract: The objectives of the present study were to estimate, for sheep, genetic parameters for FAMACHA© score and related traits at different levels of worm challenge and to assess the effect of different methods for modelling records from treated lambs on estimates of genetic parameters. Data were collected over consecutive Haemonchus seasons from a total of l,671 Merino lambs using the FAMACHA© clinical evaluation system, and anaemic individuals were treated as needed, until flock health necessitated mass treatment at the peak of the worm season. Records of each sampling occasion were classified into low, moderate, or peak levels of worm challenge. Animal models were run separately for traits within each data set. Alternative analyses were conducted in which records of treated lambs were 1) included without adjustment, 2) included along with a fixed effect representing treatment status of the lamb for each record, and 3) included after application of a penalty. Estimates of heritability for individual FAMACHA© data sets ranged from 0.06 ± 0.04 to 0.24 ± 0.05, the highest estimates being obtained for peak worm challenge data. Estimates of genetic correlation for FAMACHA© with other traits varied, but were always near negative unity for FAMACHA© score with haematocrit value. When data of treated lambs were penalized, higher estimates of heritability were obtained than when not penalized, hence this may be an effective method of including such information. Of special importance is that the estimate of genetic correlation for FAMACHA© score in moderate worm challenge with that in peak worm challenge was almost unity. This suggests that estimation of breeding values for this trait using data from moderate worm challenge may be as effective as that from peak challenge. |
