Ruminal Fiber Degradation Kinetics within and among Warm-Season Annual Grasses as Affected by the Brown Midrib Mutation

SIMPLE SUMMARY: Warm-season annual grasses alternative to corn are an attractive option for feeding dairy cattle due to decreased seeding costs and tolerance to drought stress. Warm-season annual grasses include corn, sorghum, and pearl millet. Similar to corn, some varieties of sorghum and pearl millet can contain the brown midrib (BMR) mutation in their genome, which may result in a greater degradability of neutral detergent fiber (NDF). In this study, we evaluated whether forages containing the BMR mutation exhibit the highest NDF digestibility regardless of forage type. For this evaluation, we determined the degradability of NDF by placing forage samples in porous bags within the rumen of lactating dairy cows and measuring how much NDF disappeared over time. Results from this study show that, although forages containing the BMR mutation may show greater NDF degradability than non-BMR forages of the same grass type, forages containing the BMR mutation do not always have the greatest fiber degradability when compared to conventional varieties of other grasses. ABSTRACT: The objective of this study was to compare the nutritional composition and the neutral detergent fiber (NDF) degradation kinetics of brown midrib (BMR) and non-BMR genotypes within and across warm-season annual grasses. Four commercial varieties (two non-BMR and two BMR) of corn, sorghum, and pearl millet were planted in plots. Forage samples were incubated in the rumen of three rumen-cannulated cows for 0, 3, 6, 12, 24, 48, 96, and 240 h. On an NDF basis, all forage types showed lower acid detergent lignin (ADL) concentrations for BMR genotypes, but the magnitude of the difference differed among forage types. The concentration of undegraded NDF (uNDF; NDF basis) differed among forage types and between genotypes. Corn had the least, pearl millet had the intermediate, and sorghum had the greatest concentration of uNDF. Non-BMR genotypes had greater concentrations of uNDF than BMR genotypes. No interaction existed between forage type and genotype for the concentration of uNDF. In conclusion, although BMR forages may show lower ADL concentrations in the cell wall and greater NDF degradability than non-BMR forages of the same forage type, BMR forages do not always have the least ADL concentration or the greatest NDF degradability when comparing different forage types.