Combining genetic test information and correlated phenotypic records for breeding value estimation

Impact of reduced marker set estimation of genomic relationship matrices on genomic selection for feed efficiency in Angus cattle

BACKGROUND

Molecular estimates of breeding value are expected to increase selection response due to improvements in the accuracy of selection and a reduction in generation interval, particularly for traits that are difficult or expensive to record or are measured late in life. Several statistical methods for incorporating molecular data into breeding value estimation have been proposed, however, most studies have utilized simulated data in which the generated linkage disequilibrium may not represent the targeted livestock population. A genomic relationship matrix was developed for 698 Angus steers and 1,707 Angus sires using 41,028 single nucleotide polymorphisms and breeding values were estimated using feed efficiency phenotypes (average daily feed intake, residual feed intake, and average daily gain) recorded on the steers. The number of SNPs needed to accurately estimate a genomic relationship matrix was evaluated in this population.

RESULTS

Results were compared to estimates produced from pedigree-based mixed model analysis of 862 Angus steers with 34,864 identified paternal relatives but no female ancestors. Estimates of additive genetic variance and breeding value accuracies were similar for AFI and RFI using the numerator and genomic relationship matrices despite fewer animals in the genomic analysis. Bootstrap analyses indicated that 2,500-10,000 markers are required for robust estimation of genomic relationship matrices in cattle.

CONCLUSIONS

This research shows that breeding values and their accuracies may be estimated for commercially important sires for traits recorded in experimental populations without the need for pedigree data to establish identity by descent between members of the commercial and experimental populations when at least 2,500 SNPs are available for the generation of a genomic relationship matrix.

Theoretical efficiency of multiple-trait quantitative trait loci-assisted selection

The effectiveness of five selection methods for genetic improvement of net merit comprising trait 1 of low heritability (h(2) = 0.1) and trait 2 of high heritability (h(2) = 0.4) was examined: (i) two-trait quantitative trait loci (QTL)-assisted selection; (ii) partial QTL-assisted selection based on trait 1; (iii) partial QTL-assisted selection based on trait 2; (iv) QTL-only selection; and (v) conventional selection index without QTL information. These selection methods were compared under 72 scenarios with different combinations of the relative economic weights, the genetic correlations between traits, the ratio of QTL variance to total genetic variance of the trait, and the ratio of genetic variances between traits. The results suggest that the detection of QTL for multiple-trait QTL-assisted selection is more important when the index traits are negatively correlated than when they are positively correlated. In contrast to literature reports that single-trait marker-assisted selection (MAS) is the most efficient for low heritability traits, this study found that the identified QTL of the low heritability trait contributed negligibly to total response in net merit. This is because multiple-trait QTL-assisted selection is designed to maximize total net merit rather than the genetic response of the individual index trait as in the case of single-trait MAS. Therefore, it is not economical to identify the QTL of the low heritability traits for the improvement of total net merit. The efficient, cost-effective selection strategy is to identify the QTL of the moderate or high heritability traits of the QTL-assisted selection index to facilitate total economic returns. Detection of the QTL of the low h(2) traits for the QTL-assisted index selection is justified when the low h(2) traits have high negative genetic correlation with the other index traits and/or when both economic weights and genetic variances of the low h(2) traits are larger as compared to the other index traits of higher h(2). This study deals with theoretical efficiency of QTL-assisted selection, but the same principle applies to SNP-based genomic selection when the proportion of the genetic variance 'explained by the identified QTLs' in this study is replaced by 'explained by SNPs'.

Genetic evaluation of Angus cattle for carcass marbling using ultrasound and genomic indicators

The objectives were to estimate genetic parameters needed to elucidate the relationships of a molecular breeding value (MBV) for marbling, intramuscular fat (IMF) of yearling bulls measured with ultrasound, and marbling score (MRB) of slaughtered steers, and to assess the utility of MBV and IMF in predicting the breeding value for MRB. Records for MRB (n = 38,296) and IMF (n = 6,594) were from the American Angus Association database used for national cattle evaluation. A total of 1,006 records of MBV were used in this study. (Co)variance components were estimated with ASREML, fitting an animal model with fixed contemporary groups for MRB and IMF similar to those used in the Angus national genetic evaluation. The overall mean was the only fixed effect included in the model for MBV. Heritability estimates for carcass measures were 0.48 +/- 0.03, 0.31 +/- 0.03, and 0.98 +/- 0.05 for MRB, IMF, and MBV, respectively. Genetic correlations of IMF and MBV with MRB were 0.56 +/- 0.09 and 0.38 +/- 0.10, respectively. The genetic correlation between IMF and MBV was 0.80 +/- 0.22. These results indicate the MBV evaluated may yield a greater genetic advance of approximately 20% when used as an indicator trait for genetic prediction of MRB compared with IMF. However, neither of these indicators alone provides sufficient information to produce highly accurate prediction of breeding value for the economically relevant trait MRB. Given that the goal is a highly accurate prediction of true breeding value for MRB, results of this work point to the need to 1) continue progeny testing, and 2) continue increasing the genetic correlation between the MBV and MRB.