Selection programs for fleece rot resistance in Merino sheep

Abstract

Midside wool samples can be measured for predictive colour (PC colour, acetone extract colour after incubation), incubated colour (IYZ colour, yellowness reflectance of wool after incubation; IY colour, brightness), average fibre diameter (FD), FD variability (FDCV, coefficient of variation; FDSD, standard deviation), potassium (K) content of water extract, and pH of water extract. These characteristics were assessed for their potential as indirect selection criteria for fleece rot (FR) resistance on midside wool samples from 2095 Merino hoggets representing the progeny from 84 sires in 7 sire evaluation schemes. Indirect selection can be superior to direct selection, in terms of increasing FR resistance, when FR incidence is low. The indirect traits with the highest phenotypic correlations with hogget FR liability, calculated from a 2-category (incidence) model, were clean fleece weight (CFW), FDCV, and FD. Based on combined estimates of the heritability of the indirect trait and its genetic correlation with FR score (0-5), the traits that showed the highest relative potential (RP, the estimated gain from selecting on the indirect trait relative to the gain from direct selection at the same selection intensity) were FD, pH, FDSD, and CFW. The RP value, however, takes no account of the responses in other economic traits or the costs of selection. As FR score increased, there was a linear increase in mean CFW and FDCV and a decrease in FD. Changes in other trait means with FR score followed inconsistent pat terns. By calculating selection indices, it was shown that the use of indirect criteria in ram selection increased the response in breeding objective in a self-replacing flock by $0.01-0 15/ ewe lifetime. year (based on selecting 5% of hogget rams/year). These increases were restricted by the undesirable correlations of FR score with the main economic traits, i.e. sheep with high CFW and low FD were predicted to breed sheep more susceptible to FR. Selection programs based on CFW, FD, FDSD, and FDCV with or without FR score were the most economic single-stage selection options studied. There was a lower but positive estimated net financial benefit, after 10 years of selection, when all ram hoggets were measured for CFW, FD, FDSD, FDCV, and IYZ colour and assessed for FR score in a single-stage selection program. As expected, 2-stage selection was more cost-effective than single-stage selection. The most economic 2-stage option studied was to measure CFW, FD, FDSD, and FDCV and assess FR score on all hoggets at stage 1 then measure the best 7-10% (on Index) for IYZ colour. The best 5% of all hoggets were then selected at stage 2 for joining on the basis of an index that included CFW, FD, FDSD, FDCV, HFS, and IYZ colour.