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Mean fibre diameter measurements from yearling to 5-year-old Australian fine- and medium-wool Merino sheep were analysed using several multivariate models that varied in covariance structure. A pre-structured multivariate model was found to be the most parsimonious model in comparison with the other models fitted such as banded, autoregressive and random regression. In the preferred model, the ages of mean fibre diameter for fine-wool data were genetically partitioned into yearling, 2 years, 3 years and later ages and for medium-wool data into hogget, 2 years and later ages. The estimates of genetic correlations between mean fibre diameter measured at different ages for medium-wool sheep were higher (0.89-1.00) than those for fine-wool Merino (0.75-1.00). Appropriate genetic covariance structure should be defined among the expressions of a trait taken at different ages. For the ages of fibre diameter in Merino sheep, a pre-structured multivariate model was found to be the most parsimonious model. This would enable drawing correct inferences from the data and help devise the selection strategies. [ABSTRACT FROM AUTHOR]

This study explores the interaction between genetic potential for growth in Merino lambs and their birth type (BT) or rearing type (RT). Data on birthweight (BWT), weaning weight (WWT), post-weaning weight (PWWT), scan fat (PFAT) and eye muscle depth (PEMD) were used from 3920 single and 4492 twin-born lambs from 285 sires and 5279 dams. Univariate analysis showed a significant sire × BT interaction accounting for 1.59% and 2.49% of the phenotypic variation for BWT and WWT, respectively, and no significant effect for PWWT, PFAT and PEMD. Sire × RT interaction effects were much smaller and only significant for PEMD. Bivariate analysis indicated that the genetic correlation (rg) between trait expression in lambs born and reared as singles versus those born and reared as twins were high for BWT, WWT, PWWT (0.91 ± 0.02 – 0.96 ± 0.01), whereas rg for PFAT and PEMD were lower (0.81 ± 0.03 and 0.86 ± 0.02). The rg between traits expressed in lambs born and reared as singles versus those born as twins but reared as singles were lower: 0.77 ± 0.08, 0.88 ± 0.03, 0.66 ± 0.06 and 0.61 ± 0.08 for WWT, PWWT, PFAT and PEMD, respectively. A different RT only affected the expression of breeding values for PFAT and PEMD (rg 0.62 ± 0.04 and 0.47 ± 0.03, respectively). This study showed genotype × environment interaction for BWT and WWT (sire × BT interaction) and for PEMD (sire by RT interaction). However, sires' breeding value of a model that accounts for sire × BT interaction provides a very similar ranking of sires compared with a model that ignores it, implying that there is no need to correct for the effect in models for genetic evaluation. This study explored whether breeding values for bodyweight are equally expressed in lambs with different birth or rearing type. The correlation between breeding values for early growth of lamb when expressed in single or twin birth types was less than one whereas different rearing type influenced expression of breeding value at late growth. However, this interaction between genotype and birth or rearing type did not affect ranking of sire's breeding value in these two different environments, implying that there is no need to account for the effect of genetic evaluation. [ABSTRACT FROM AUTHOR]

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. Australian Merino sheep can be bred with horns or without (polled). There is evidence suggesting polled rams are becoming more desirable due to increased welfare standards for both the individual sheep and their human handlers. As such there is indication that ram buyers will pay a premium for polled rams at present. This paper investigates breeding strategies to infuse the poll gene into a flock and the cost–benefit to the ram breeder. [ABSTRACT FROM AUTHOR]