In vitro digestion and textural strength of rind and pith of normal and brown midrib stems

Abstract

Intact sections of pith and rind from normal (N) and near-isogenic brown midrib (bmr) mutants of TxB623 sorghum ( Sorghum bicolor L. Moench.), GA 337 sudangrass ( Sorghum bicolor L. Moench.), and pearl millet ( Pennisetum glaucum (L.) R. Br.) were analyzed for loss of soluble components, in vitro dry matter digestion after incubation (24 h for pith and 72–120 h for rind) with rumen microorganisms, and textural strength using an Instron universal measuring instrument before and after in vitro digestion. Piths from all sudangrass and sorghum plants from the vegetative harvest were extensively degraded (about 900 g kg −1), with only vascular bundles remaining. In contrast to those of pearl millet, N and counterpart bmr piths from either sudangrass or sorghum were not different ( P > 0.05) in digestibility at 24 h. Pith digestibilities is sudangrass ranged from about 940 g kg −1 for vegetative stems to about 600 g kg −1 for mature, upper stems. Similarly, sorghum piths decreased from 840 g kg −1 in vegetative plants to about 690 g kg −1 in mature, upper stems. In all plants, bmr rinds were greater ( P ≤ 0.05) or tended to be greater ( P ≤ 0.10) in digestibility than N rinds. For example, sudangrass digestibilities for N and bmr rinds, respectively, were 736 g kg −1 and 901 g kg −1 for vegetative plants and 606 g kg −1 and 808 g kg −1 for mature, upper plants. Shear stress (newtons per area) of piths were about 10-fold less than that of rinds when all plants were considered. Differences were small and inconsistent for undegraded piths and rinds within N/bmr comparisons for sudangrass and sorghum. Extended disruption by rumen microorganisms of pith and rind sections of sudangrass and sorghum prevented testing for shear stress of most digested samples. In pearl millet, rinds in either bmr mutant were about 40% weaker than those in N plants. In study, shear stresses of pith and rind were not consistently different within N/bmr comparisons, and overall results on textural strength did not relate well to variations observed in digestibility for N and counterpart bmr stems.