IN-VITRO DIGESTION AND TEXTURAL STRENGTH OF RIND AND PITH OF NORMAL AND BROWN MIDRIB STEMS

Intact sections of pith and rind from normal (N) and near-isogenic bro wn midrib (bmr) mutants of TxB623 sorghum (Sorghum bicolor L. Moench.) , GA 337 sudangrass (Sorghum bicolor L. Moench.), and pearl millet (Pe nnisetum glaucum (L.) R. Br.) were analyzed for loss of soluble compon ents, in vitro dry matter digestion after incubation (24 h for pith an d 72-120 h for rind) with rumen microorganisms, and textural strength using an Instron universal measuring instrument before and after in vi tro digestion. Piths from all sudangrass and sorghum plants from the v egetative harvest were extensively degraded (about 900 g kg-1), with o nly 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 i n sudangrass ranged from about 940 g kg-1 for vegetative stems to abou t 600 g kg-1 for mature, upper stems. Similarly, sorghum piths decreas ed 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 less-than-or-eq ual-to 0.05) or tended to be greater (P less-than-or-equal-to 0.10) in digestibility than N rinds. For example, sudangrass digestibilities f or N and bmr rinds, respectively, were 736 g kg-1 and 901 g kg-1 for v egetative plants and 606 g kg-1 and 808 g kg-1 for mature, upper plant s. Shear stress (newtons per area) of piths were about 10-fold less th an that of rinds when all plants were considered. Differences were sma ll and inconsistent for undegraded piths and rinds within N/bmr compar isons for sudangrass and sorghum. Extended disruption by rumen microor ganisms of pith and rind sections of sudangrass and sorghum prevented testing for shear stress of most digested samples. In pearl millet, ri nds in either bmr mutant were about 40% weaker than those in N plants. In this study, shear stresses of pith and rind were not consistently different within N/bmr comparisons, and overall results on textural st rength did not relate well to variations observed in digestibility for N and counterpart bmr stems.