Near- and Mid-Infrared Studies of the Cell Wall Structure of Cereal Straw in Relation to its Rumen Degradability

Summary

Mid-infrared reflection studies using a multiple internal reflection technique have unambiguously distinguished inner and outer surfaces of cereal and rape straws, indicating significantly different chemical compositions. These chemical differences are important in controlling the progress of straw degradation in sheep rumen. The reflection technique analyses a surface layer up to 10 µm thick and has shown that for cereal straw the outer layers containing waxy cuticle and amorphous silica undergo little or no degradation over prolonged incubation. The inner layers, carbohydrate-rich and containing very little lignin, undergo extensive degradation of carbohydrate and accumulation of lignin. This accumulation terminated degradation after about 48 h. In contrast, the outer layers of rape straw contain a deposit of calcite and very little cuticular wax. The acidic conditions in the rumen dissolved the calcite, and carbohydrate in both surfaces degrades simultaneously until, as for cereal straw, lignin accumulation intervenes.

The near-infrared diffuse reflectance (NIR) technique has been used to assess degradability of cereal straw directly. NIR spectra of twenty-five cereal straws and cereal straw fractions were correlated with their dry matter degradability determined by 48 hour incubation in nylon bags in the rumen of sheep. One wavelength segment could account for 89% of the variance in 48 hour degradability giving a standard error of estimate of 5% units.

NIR spectra of leaf and stem fractions of cereal straws were appreciably different over the regions 1600-1700 and 2200-2300 nm. These same regions were analytically useful in estimating degradability and are well established for testing silage for ″in vivo"DOMD. Chemically extracted straw lignins show absorptions in these regions which are negatively correlated to degradability.

A similar relationship between degradability and absorbance of the C=C lignin band at about 1500 cm⁻¹ was observed in the mid-IR range, supporting the inverse relationship between lignin content and degradability of cereal straw.