EVALUATION OF ARTIFICIAL CHALLENGE WITH INFECTIVE H. CONTORTUS LARVAE
AS SELECTION CRITERION FOR GENETIC RESISTANCE
AGAINST NEMATODES IN SOUTH AFRICAN SHEEP BREEDS
M. A. Snyman1, M. van Heerden1 & A. Karstens2
1 Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900
2 Middelburg Provincial Veterinary Laboratory, Private Bag X528, Middelburg (EC), 5900
Internal parasite infestation negatively influences the ability of animals to produce and reproduce to their full capability. Despite various control measures, internal parasites remain one of the largest animal health problems in the small stock industry. To make matters worse, resistance of parasites to available anthelmintics increased drastically during the past decade. The possibility of selection for animals resistant to internal parasites has been investigated and selection programmes based on faecal egg counts (FEC) have been implemented in Australia and New Zealand. However, repeatability of FEC under various conditions has been shown to be highly variable. Furthermore, many of the stud breeders in South Africa are located in the drier, western parts of the country, with a lower parasite infestation. As no breeding programme will have parasite resistance as its sole objective, the genetic relationship of resistance with other production and reproduction traits should also be considered. The objectives of this study were to identify effective selection criteria for internal parasite resistance in different sheep breeds under intensive as well as extensive conditions and to estimate correlations among these criteria and other production traits. Animals from the Fine wool Merino stud at Cradock Experimental Station (CFM), Afrino (CAF) and Dorper flocks at the Carnarvon Experimental Station and the Merino stud at Grootfontein ADI (GMS) were used in this study. Since March 1998, lambs in these flocks were infected with third stage Haemonchus contortus larvae at 6-8 months of age. Faecal egg counts (FEC), haematocrit (HEMA) and FAMACHA8-scores (FAM) were done 28 days after infection. Haemoglobin type of each lamb was also determined. Data on artificial challenge faecal egg counts, haematocrit and FAMACHA8-score collected from 1998 till 2001 on the CFM (n=1118), CAF (n=878) and the GMS (n=752) were analysed with mixed model procedures (Harvey, 1993) to obtain heritability estimates and genetic and phenotypic correlations amongst these traits. Correlations of these traits with birth weight, 42-day body weight and weaning weight were also estimated. Heritability for FEC ranged from 0.07"0.06 for the Grootfontein Merino stud, to 0.32"0.11 for the fine wool Merino stud. Variable heritabilities were also estimated for HEMA (0.05"0.15 to 0.32"0.11) and FAM (0.13"0.06 to 0.23"0.10) for the different breeds. Phenotypic correlations between FEC and HEMA ranged from -0.23 to -0.33 for the various breeds, indicating animals with higher FEC=s had lower heamatocrit values. Genetic correlations estimated for the two Merino studs were much higher (-0.76"0.18 and -0.41"0.49). Phenotypic correlations between FEC and FAM ranged from -0.08 to -0.23, while variable genetic correlations were estimated. High genetic (0.71"0.17 to 0.91"0.16), as well as phenotypic correlations (0.50 to 0.63) were obtained between HEMA and FAM for the different breeds. Generally, no noteworthy phenotypic correlations were estimated between FEC, HEMA and FAM on the one hand, and body weight on the other. The corresponding genetic correlations were very variable between breeds and even for the different body weights. No relationship between haemoglobin type and HEMA or FAM could be discerned, while lambs with haemoglobin type BB had lower FEC=s. The number of data records are still too few to draw any reliable conclusions with regard to genetic parameters. More accurate parameters will be estimated as soon as more data become available.