Relationship of phenotypic variation in sorghum to nutritive value of crop residues

Improvements in dry matter digestibility of crop residues contribute to greater productivity of livestock consuming dual-purpose sorghums (Sorghum bicolor L. Moench). Delayed leaf senescence (stay-green), brown-midrib (bmr), leaf disease resistance, and high yield are among sorghum phenotypic traits that could enhance nutritive value of crop residues. The objective was to evaluate nutritive value and hydrocyanic acid potential (HCN-p) of leaf or stem components of crop residues of sorghum lines and hybrids representing differences in phenotypic traits. Four replications of eight or more lines and hybrids made up a randomized complete block design during 2 years in central Texas. Dry matter yield, dry weight loss after extraction in water and incubation in rumen, and fiber concentrations of leaves and stems were measured after grain harvest under field conditions. In addition, HCN-p of axillary branches and mature leaves was quantified. Sample weight loss in water extractions without incubation in rumen was negatively correlated with fiber concentrations, which were measured as the sample weights remaining after extraction in neutral (NDF) and acidic (ADF) detergent solutions. Greater concentrations of water-extractable dry matter and lower concentrations of NDF and ADF were observed for stay-green than for bmr or lines that senesced under drought. Yet, the sum of water-extractable dry matter plus ensuing weight loss during 7 days in rumen, expressed as yield ha−1 of leaf and stem digestible dry matter, was not consistently greater for stay-green than drought-susceptible lines. In addition, the yield of the sum of water-extractable and 7-day rumen losses of stay-green lines was comparable to or less than the bmr line. Under conditions in which leaf senescence varied little, analyses of nutritive value indicated water-extractable dry matter was enhanced and fiber concentrations were reduced in crop residues of stay-green compared to senescent lines.