Substantial genetic gains in reducing breech flystrike and in improving productivity traits are achievable in Merino sheep by using index selection

Abstract

Context. The availability of effective indirect rather than direct selection criteria for genetically reducing breech flystrike is crucial for the Australian wool industry, as expression of breech flystrike is routinely suppressed by management interventions to minimise the risks to sheep health, welfare and productivity. Understanding how much genetic gain can be made in reducing breech strike, while also selecting for other key traits is important in choosing breeding objectives, particularly for Merino genetic improvement programs. Aims. We predicted genetic gains from within-flock selection to reduce breech flystrike incidence (FS) and improve productivity under a range of scenarios that all used indirect selection criteria in the genetic evaluation of FS. Methods. Three breeding objectives were modelled, by adding FS as a trait to the breeding objective for the Dual Purpose Plus (DP+), Fibre Production Plus (FP+) and Merino Production Plus (MP+) indexes available from MERINOSELECT. A large range of economic values were examined for FS, from 0 to-$240/strike.sheep per year in $20 increments. For all scenarios, full records of productivity traits and pedigree information were assumed to be available for use as selection criteria, as well as records of breech wrinkle, dag and breech cover scores. Predictions were conducted for scenarios assuming (a) moderate heritability for FS (b) low heritability for FS, and (c) low heritability for FS, but higher heritability for dag score than assumed for scenarios (a) and (b). Key results. After 10 years of selection, under assumption (a) predicted genetic gains in FS ranged from 0, up to maximum reductions of 0.19, 0.21, and 0.20/strike.sheep per year from use of the FP+FS, MP+FS and DP+FS indexes, respectively. Under assumption (b) predictions of genetic gain for FS were considerably lower and ranged from 0 to maximum reductions of 0.04, 0.06 and 0.06/strike.sheep per year based on use of FP+FS, MP+FS and DP+FS indexes, respectively. Under assumption (c), predicted genetic gains in FS were very similar to those under assumption (b). When 70% of the maximum gains in FS studied were targeted, from 70% to 93% of genetic gains in the overall index (excluding FS) were predicted to be retained when FS had moderate heritability, and from 85% to 95% when FS had low heritability. Conclusions and implications. There is a practical range of economic values for FS

The current status and outlook for insecticide, acaricide and anthelmintic resistances across the Australian ruminant livestock industries: assessing the threat these resistances pose to the livestock sector

“…Breeding for resistance to parasites has long been recognised as the most effective long-term solution for those parasitic diseases of livestock for which significant levels of heritable resistance to infection have been identified. The potential for breeding to provide resistance to infection has been described for the cattle tick, 64,65 the sheep blowfly, [66][67][68] intestinal worms of sheep [69][70][71] goats 72 and cattle, 73 sheep louse, 74 sheep liver fluke, 75 and buffalo fly. 76 However, there are a number of issues that act against the widespread adoption of breeding for parasite resistance in livestock, including the slow rate of progress in increasing resistance within a flock/herd, the lack of direct phenotypic measures of resistance in some cases (e.g.…”

Control of sheep flystrike: what's been tried in the past and where to from here

“…Brien et al 147 extended these studies by including all three of the main indirect criteria for which ASBVs are available in MERINOSELECT (breech wrinkle, dag and breech cover) and examined the rates of gain in flystrike resistance that could be made by adding flystrike as a trait to three MERINOSELECT indices: Dual Purpose Plus (DP+), Fibre Production Plus (FP+) and the Merino Production Plus (MP+). They showed that substantial genetic gains in flystrike resistance could be made without unrealistically compromising rates of genetic improvement in the other production traits.…”

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