Comparative Effects of the Sorghum bmr-6 and bmr-12 Genes: II. Grain Yield, Stover Yield, and Stover Quality in Grain Sorghum

Therefore, enhancing the quality of grain sorghum stover may offer an opportunity to add value to the overall Nearly 3 million hectares of grain sorghum [Sorghum bicolor (L). system when post grain harvest stover residue is used Moench] are harvested in the USA each year. It may be possible to add value to crop and animal systems by enhancing the digestibility as a forage base for livestock. of the stover residue by the use of brown midrib (bmr) genes if grain Chemical and genetic approaches have been emyields can be maintained. The objectives of this study were to evaluate ployed to improve forage fiber digestibility by reducing the effect of bmr-6 and bmr-12 genes on grain yield of sorghum and the amount of lignin or the extent of lignin cross linked to evaluate the effect of the bmr genes on stover yield and quality in with cell wall carbohydrates. Brown midrib forage genothese genetic backgrounds: ‘Wheatland’, ‘Redlan’, RTx430, Tx623, types usually contain less lignin and may have altered Tx630, Tx631, and the hybrid AWheatland RTx430. Plant height, lignin chemical composition (Bucholtz et al., 1980; Chermaturity, grain yield and test weight, stover neutral detergent fiber ney et al., 1991; Vogel and Jung, 2001). Activities of two (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), and separate enzymes involved in lignin synthesis are reduced in vitro NDF digestibility (IVNDFD) were measured in split-plot as the result of the bmr mutations bmr-6 (reduced cinnaexperiments replicated four times in each of four environments with lines being whole-plots and genotypes being subplots. Brown midrib myl alcohol dehydrogenase downregulation) (Bucholtz genes reduced grain yield and residue yield in the lines; however, et al., 1980) and the allelic (Bittinger et al., 1981) bmr-12 yield reduction was not observed in the bmr-12 AWheatland and bmr-18 (reduced caffeic acid O-methyl transferase RTx430 hybrid. The bmr-12 near-isolines generally had lowest stover activity) (Bout and Vermerris, 2003). To date, genetic lignin content and highest fiber digestibility, bmr-6 was intermediate, control of the lignification process through use of bmr and wild-type counterparts had highest lignin content and lowest fiber genes has offered the most direct and productive apdigestibility. When all data are considered, the bmr-12 gene appears proach to reducing lignin concentration and increasing superior to the bmr-6 gene in terms of potentially adding value to the digestibility of sorghums (Gerhardt et al., 1994). stover of grain sorghum for use in crop/animal systems. The variable The bmr phenotype is generally associated with reexpression of bmr-12 and bmr-6 in different lines indicates that selecduced vigor and yield. Previous research in maize has tion of compatible genetic backgrounds will be critical in determining the realized impact on value. demonstrated reductions of both the grain and stover yield (Lee and Brewbaker, 1984; Miller et al., 1983) of bmr maize compared with wild-type maize. Our research with near-isogenic forage sorghum demonstrated deM (Zea mays L.) crop residues often serve as creased average total DM yield in bmr forage lines (Olian economical feed for ruminant animals (Klopver et al., 2005). However, in that study significant line fenstein et al., 1987). Nearly 3 million hectares of grain gene interactions were detected, with bmr-6 or bmr-12 sorghum are harvested in the USA each year (USDA near-isolines being equivalent in total biomass yield to NASS, 2004), yet little research effort has been invested their wild-type counterpart in three of the four grain in the enhancement of grazing value of grain sorghum sorghum genetic backgrounds studied. stover residue. There is less digestible organic matter In a previous forage sorghum study we reported infound in sorghum stover than maize stover (Irlbeck et creased fiber digestibility associated with mature bmr-6 al., 1991). The DM digestibility of sorghum stover, as and bmr-12 forage sorghum (Oliver et al., 2005). The well as the crude protein content, is low when compared effects of bmr genes on grain yield of grain sorghum, with maize stover (Osafo et al., 1997). Subsequently, and the effect on quality of post grain harvest stover when given the choice, producers choose to graze maize residue are unknown. It may be possible to add value stalks. However, in some parts of the U.S. environmento total crop/animal systems by enhancing the digestital conditions are not conducive to growing maize. bility of the stover residue if grain yields can be mainA.L. Oliver and T.J. Klopfenstein, Dep. of Animal Science, Univ. of tained at acceptable levels. Therefore, the objectives of Nebraska-Lincoln, Lincoln, NE 68583-0908; J.F. Pedersen, USDAthis study were to evaluate the impact of the bmr-6 and ARS, NPA Wheat, Sorghum and Forage Research, Dep. of Agronbmr-12 genes on the grain yield of grain sorghum and to omy, Univ. of Nebraska-Lincoln, Lincoln, NE 68583-0937; R.J. Grant, evaluate impact of the bmr genes on the yield and qualW.H. Miner Agric. Res. Institute, Chazy, NY 12921; H. D. Jose, Dep. ity of the post grain harvest stover residue. of Agricultural Economics, Univ. of Nebraska-Lincoln, Lincoln, NE 68583-0922. Joint contribution of the USDA-ARS and the Univ. of Nebraska Agric. Exp. Stn. as Paper no. 171865, Journal Series, NeMATERIALS AND METHODS braska Agric. Exp. Stn. Received 15 Nov. 2004. *Corresponding author ([email protected]). Except where methods differed from Oliver et al. (2005), only a brief description is provided for the reader’s convePublished in Crop Sci. 45:2240–2245 (2005). Crop Breeding, Genetics & Cytology doi:10.2135/cropsci2004.0660 Abbreviations: ADF, acid detergent fiber; ADL, acid detergent lignin; bmr, brown midrib; DM, dry matter; IVNDFD, in vitro neutral deter© Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA gent fiber digestibility; NDF, neutral detergent fiber. 2240 Published online September 23, 2005