Heritability of and genetic correlations among faecal egg counts and productivity traits in romney sheep

A total of 2611 spring-born Romney ewe lambs, the progeny of 60 sires over 3 years, were studied on a Hawke's Bay farm to estimate the heritability of faecal egg count (FEC), and correlations between FEC and production traits. Weaning weight, an autumn weight, the weight gain between these 2 times, and yearling fleece weight were recorded in each of 3 years. A dag score was obtained in the final 2 years. An anthelmintic drench was administered to 1987-born lambs in mid January after which further treatment was withheld until April. In the 2 subsequent years, drench treatment was administered in the first week of December and further treatment withheld until the mean FEC in a small monitor group reached a target value of 1000-1500 eggs/g. Once the target value was reached, a faecal sample was obtained from all animals for an FEC, the dag score and autumn liveweight were taken, and routine drenching was then resumed. Variance components were estimated using a multivariate restricted maximum likelihood (REML) procedure, with a sire model. Heritabilities were low for the liveweights and gain traits (0.08 ± 0.04 to 0.13 ± 0.04), and moderate for fleece weight (0.41 ± 0.09), dag score (0.24 ± 0.08), and both FEC and logeFEC + 1) (0.34 ± 0.08 and 0.27 ± 0.07 respectively). Phenotypic correlations between logeFEC + 1) and all production traits were negative, but very low (−0.01 to −0.05). The corresponding genetic correlations were also negative and ranged from −0.05 to −0.36. Broadly similar correlations occurred between productivity traits and untransformed FEC. Phenotypic and genetic correlations between FEC or logeFEC + 1) and dag score were positive, and thus favourable. Overall, the results demonstrate that breeding for low nematode egg output in lambs (and thus by implication improved resistance to nematode establishment) is feasible in a commercial ram breeding enterprise. However, to achieve any substantial improvement in growth rates or dag scores of lambs under untreated exposure to worm infection, an independent culling approach incorporating these traits is probably the simplest method at present. © 1992 Taylor & Francis Group, LLC.