Normal‐12 and Brown Midrib‐12 Sorghum. II. Chemical Variations and Digestibility1

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Abstract
Marginally digested tissues in the brown midrib‐12 (bmr) sorghum (Sorghum bicolor (L.) Moench.) mutant are more readily degraded than similar tissues in the normal (N) line. Reduced lignin concentrations have been reported to be a factor affecting digestibility of bmr genotypes, but lignin type could also influence digestibility. The objectives of this work were to (i) examine N‐12 and bmr‐12 plants for variations in chemical composition, including phenolic acids and phenolic structure, using solid‐state nuclear magnetic resonance (NMR) spectroscopy; and (ii) evaluate by microscopy the effect of permanganate delignification. Leaf blades of bmr plants were significantly (Ρ ≤ 0.05) lower in neutral detergent fiber (NDF) and permanganate lignin (PML), and higher in in vitro dry matter digestibility; leaf sheaths contained significantly less PML. Solid‐state NMR spectroscopy suggested that bmr leaf blades had lower amounts of bound syringyl moieties. The predominant alkali‐soluble (1.0 M NaOH) phenolic acids in both plant types were ρ‐coumaric and ferulic acids, and ρ‐coumaric acid was significantly lower (Ρ ≤ 0.05) in concentration in bmr leaves. Treatment of intact blade sections with permanganate to oxidize lignin compounds resulted in a reduced staining intensity for lignin identified by both the acid phloroglucinol and chlorine‐sulfite tests in all but midrib sclerenchyma; lignin staining intensity did not differ between plant types. Degradation of delignified midribs by rumen microbes indicated that the tissues were more degradable in bmr than in N plant types. Results suggest that reduced concentrations of phenolic acids (especially ρ‐coumaric acid) and reduced levels of etherated syringyl moieties in conjunction with reduced concentrations of lignin are factors in the improved quality of bmr‐12 sorghum.