Comparative Effects of the Sorghum <i>bmr</i>‐6 and <i>bmr</i>‐12 Genes: I. Forage Sorghum Yield and Quality

Brown midrib (bmr) forages usually contain less lignin and exhibit increased digestibility. Recent research has identified the modifications in biochemical pathways resulting from bmr mutations. In sorghum [ Sorghum bicolor (L) Moench.], bmr ‐6 has been linked to a decrease in cinnamyl alcohol dehydrogenase (CAD) activity. The allelic bmr ‐12 and bmr ‐18 genes decrease caffeic acid O ‐methyl transferase (OMT) activity. There has been only limited research comparing bmr genes to each other and wild type in isogenic sorghum. The objective of this study was to determine the impact of bmr ‐6 and bmr ‐12 on forage yield and quality in these genetic backgrounds: ‘Atlas’, ‘Early Hegari‐Sart’, ‘Kansas Collier’, and ‘Rox Orange’. Height, lodging, neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), in vitro NDF digestibility (IVNDFD), and dry matter (DM) yield were measured in a split‐plot experiment replicated four times in each of four environments with lines being whole‐plots and genotypes (wild type, bmr ‐6, and bmr ‐12) being subplots. Brown midrib genes generally had negative agronomic impact, but these were not uniformly expressed across backgrounds. The bmr ‐6 gene generally resulted in a shorter plant and less DM yield, but did not reduce ADL. The bmr ‐12 gene generally resulted in reduced ADL, later maturity, and reduced or equivalent DM yield when compared with the wild type. There is a more digestible NDF fraction in both bmr ‐6 and bmr ‐12 forage sorghums. When all data are considered in aggregate, the bmr ‐12 gene appears superior to the bmr ‐6 gene in terms of less negative impact on agronomic performance and greater positive impact on ADL content and fiber digestibility.