Specific weight of barley grains is determined by traits affecting packing efficiency and by grain density

Background Specific weight influences the market value of barley grain, and in malting barley a high specific weight is thought to result in an increased malt output. However, links between specific weight and malt output have not yet been established. We hypothesised that packing efficiency and grain density will each contribute to specific weight. These traits would have implications for the malting process, highlighting the need for understanding what grain traits contribute to specific weight, before we can predict its effect on malting performance and efficiency. Results We report that specific weight is a product of grain density and packing efficiency, in our study proportionally contributing 48.5% and 36.5% to variation in specific weight, respectively. We report that packing efficiency is determined by grain dimensions, and is negatively correlated with the sum of grain length and depth. Therefore shorter, thinner grains can result in an increased specific weight, which is likely to be detrimental for malting performance. We also demonstrate that among cultivars which have grains with contrasting size traits, the same specific weight can be achieved through differing grain densities. Conclusions Our results demonstrate that both grain dimensions and grain density must be considered jointly to optimise specific weight, and that the relationship between specific weight and malting performance and efficiency needs to be carefully considered with respect to how a high specific weight is achieved.