Experimental designs historically used for quantitative trait loci (QTL) mapping in cattle have captured a limited number of parental chromosomes and therefore have only detected the few heterozygous QTL within these parents. Consequently, most genome scans have found only a small number of QTL to influence any one trait and genetic improvement in cattle by marker-assisted selection (MAS) has been hindered by this limited information. By using a twenty-nine generation mapping population (N=1,769) that represents the major commercial bloodlines within American Angus, we have captured the majority of the chromosomes represented within the breed. Our experimental design allows the flexibility of using several different analytical approaches which exploit both the full pedigree information and within family variation. This design maximizes the potential for identifying all of the QTL of large effect that segregate within Angus cattle. Our analyses revealed that a large number of QTL of moderate to large effect influence all of the economically important growth and carcass traits, even for traits under strong selection. Despite considerable selection pressure by Angus producers on economically important traits over the past 30 years there remains a wide range in the allele frequency of trait-enhancing QTL. Thus genetic improvement programs that use information at only one, or a few QTL will have little value in beef cattle. Strategies must quickly be devised to simultaneously test for multiple QTL for MAS to be economically viable. The identification of multiple QTL underlying variation in carcass, growth and reproductive traits in this study will assist in the development of QTL tests. Ideally, QTL information will be integrated with available phenotypic data for the estimation of EPDs in cattle to allow producers to select for genetically superior animals.

The first objective of this study was to identify genomic regions on BTA4 and BTA14 which harbor QTL affecting weight and carcass traits in a commercial Angus population. The mapping population consisted of 1,510 purebred commercial Angus steers grouped into 36 half-sib families with not less than 30 progeny per sire. Forty-seven genetic markers were used to scan two bovine chromosomes for QTL using QTL Express. There is strong evidence for a pleiotropic QTL affecting hot carcass weight and ribeye area, a QTL influencing yield grade and back fat and two marbling QTL on BTA14. On BTA4 there were four birth weight, four marbling, four weaning weight, two hot carcass weight, two ribeye and one yield grade QTL. The second objective of this study was to test published SNPs within Leptin, thyroglobulin and diacylglycerol O-acyltransferase 1 for their effects on growth and carcass traits in the commercial Angus population. The ANOVA analysis results indicate that these SNPs were associated with several of the carcass traits, however, linkage analysis revealed that none of these genes underlie bovine growth and carcass QTL in Angus cattle.

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The combined work of eminent scientists in the field, this compilation contains the latest information on ruminant nutrition from the eighth annual International Ruminant Reproduction Symposium. With discussions on how to improve reproduction by applying nutrition and physiology, this collection spotlights the recent advances regarding the ruminant genome and includes chapters about specific animals, including the dairy buffalo, camel, and reindeer.