In a series of trials among Dohne Merino on Wauldby Farm in the Eastern Cape's Stutterheim district, Greta Snyman (Grootfontein Agricultural Development Institute) and Alan Fisher (Queenstown Provincial Veterinary Laboratory) sought to establish protocols for selection for resistance against barber's pole worm (Haemonchus contortus).
This pest is a roundworm that grows up to 30mm long and sucks blood from the lining of the sheep's stomach, causing anaemia. Symptoms include swelling under the jaw (bottle jaw), weight loss and reduced wool growth and tensile strength. In lactating ewes, milk production can decline. More severe infections can prove fatal.
Female worms have red-and-white stripes, hence the common name.
Data on faecal egg counts (FEC), Famacha score (FAM) and body condition score (BCS) were collected annually on all lambs born from 2011.
Selection in the Wauldby flock was aimed at increasing resistance to H. contortus, while maintaining reproductive performance, body weight, wool weight and fibre diameter, and improving wool quality traits. Selection for the production traits was done on the basis of selection indices and BLUP of breeding values for the mentioned traits measured at 14 months of age. Selection for resistance to H. contortus was based on a selection index incorporating FEC, FAM and BCS (see panel).
The data collected over the years were used to estimate heritabilities and genetic correlations among the traits. FAM had a high genetic correlation with FEC. Fisher and Snyman found that identifying animals that required anthelminthic treatment according to FAM would ensure that only truly susceptible animals were identified and destined to be culled. Resilient as well as resistant animals would not be targeted.
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