Strategies and cost–benefit of selecting for a polled sheep nucleus by using DNA testing
T. Granleese A B D , S. A. Clark A B , N. Duijvesteijn A B , P. E. Bradley C and J. H. J. van der Werf A BA Cooperative Research Centre for Sheep Industry Innovation, University of New England, Armidale, NSW 2351, Australia.
B School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia.
C Meat and Livestock Australia, University of New England, Armidale, NSW 2351, Australia.
D Corresponding author. Email: tgranle2@une.edu.au
Animal Production Science 59(8) 1428-1437 https://doi.org/10.1071/AN17720
Submitted: 18 October 2017 Accepted: 30 August 2018 Published: 26 September 2018
Abstract
The present study assessed the effectiveness and cost–benefit of several genotyping strategies for breeding poll Merino sheep in a closed nucleus with different initial allele frequencies and assuming a single-gene responsible for the horn or poll phenotype. We assumed that selection was based on phenotypes or genotypes for a single gene conferring polledness via a complete-dominance model. Under such a model, a complete fixation of the ‘polled allele’ (P) requires genotyping of the ewe-selection candidates. Testing a higher proportion of female candidates resulted in a faster fixation of the P-allele. Fixation ranged from 1 year of selection with a high starting P-allele frequency of 0.9, to 7 years for low starting P-allele frequencies of 0.3. When premiums of AU$50 or AU$100 were paid for rams with a PP genotype, breeding for PP genotypes was not profitable when the starting P-allele frequency was below 0.7. If the starting allele frequency was above 0.7, net profitability was positive over 10 years when premiums of AU$200 were paid for known PP-genotype rams. While fixing the P-allele, genetic gain for production traits was slowed down in the first 5 years of selection by up to 23% and 3% for initial P allele-frequencies of 0.3 and 0.9 respectively. Lost genetic gain due to fixing the P-allele, which can never be recovered in a closed nucleus, incurred 200–800% higher costs than the DNA testing costs. Rates of genetic gain recovered to pre-P-allele selection level rates of genetic gain once the P-allele was fixed. Testing a maximum of 25% ewe-selection candidates was the least expensive strategy across all starting allele frequencies and premiums. To avoid large losses of genetic gain in a closed nucleus with low P-allele starting frequencies, opening the nucleus should be considered to increase starting P-allele frequencies and also to potentially increase rates of genetic gain to offset the economic loss caused by P-selection.
Additional keywords: breeding program design, genetic gain.
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