More than half of the barley grown in the USA is used for livestock feed, with the remaining stocks diverted for human food and malting purposes. The use of barley grain as a major source of cattle feed has been criticized because of its rapid digestion in the rumen, which can result in digestive disorders in cattle. In sacco dry matter digestibility (ISDMD) and particle size (PS) after dry rolling have been found to play a role in the feedlot performance of barley as a feed grain. Reducing the rate of ISDMD is predicted to result in significantly improved animal health and average daily gain. A recombinant inbred line population derived from a cross between a high ISDMD, two-rowed barley cultivar (Valier) and a six-rowed Swiss landrace line (PI370970) exhibiting far slower ISDMD has been developed for studying the underlying genetic locations and mechanisms of these traits. To detect associated quantitative trait loci (QTLs), we collected and analyzed data from irrigated and rain-fed environments. A significant negative correlation was observed between ISDMD and PS. High heritability estimates for ISDMD and PS suggest that early selection for these traits during breeding would be achievable. Four QTLs were identified on chromosomes 2H, 6H, and 7H, explaining 73-85% of ISDMD phenotypic variation, while three QTLs on 2H and 7H were associated with variation in PS and explained 58-77% of its variation. A major QTL on chromosome 2H tightly linked to the morphology-modifying gene vrs1 was found to dramatically control 35-62% of the phenotypic variation of ISDMD and 26-53% of that of PS. The impact of the vrs1 locus on ISDMD was validated in two populations representing different genetic backgrounds. Our results suggest that it may also be advantageous to simultaneously overlap these QTLs around the vrs1 locus.
