Composition and susceptibility to rumen microbial degradation of nonmesophyll cell walls isolated from caucasian bluestem [Bothriochloa caucasica (Trin)] leaf tissue

This experiment was conducted to isolate sufficient quantities of nonmesophyll (NM) cell wall (CW) from caucasian bluestem [Bothriochloa caucasica (Trin.) C. E. Hubbard] leaf tissue such that NM CW composition and in vitro degradability could be studied as the plant matured. The mesophyll (M) fraction was also collected and subjected to similar analyses. Fresh leaf material was harvested at the vegetative (V), jointing, and early and late reproductive (LR) stages of development. Leaf tissue was ball milled (ca. 15 g of wet tissue/L of water) for 24 h to macerate the tissue, and NM cells were collected on a 75-micromole sieve. Cells not retained by the 75-micromole screen were collected as the M fraction. Cell wall of NM, M, and whole leaf (WL) tissue was isolated by ethanol (80% v/v) and chloroform extraction followed by treatment with an alpha-amylase. The ratio of M to NM CW tissue varied from 1.45:1 to 0.87:1 (V and LR stages, respectively). Crude protein content (% N X 6.25) was lower (P < 0.05) in NM than WL. Klason and acid detergent lignin concentrations were highest (P < 0.05) for M and similar (P > 0.05) between NM and WL, with no effect due to maturity. The CW and NM contained a higher concentration of glucose (P < 0.01) and total monosaccharides (P < 0.05) compared to M and WL, although the concentration of glucose and total monosaccharides did not change as the grass matured. In vitro CW digestibility was greatest (P < 0.01) for NM and lowest (P < 0.01) for M compared to WL and was not affected by maturity. In vitro digestibility of arabinose, xylose, and glucose did not differ among tissue types, and xylose digestibility decreased linearly (P < 0.01) as the grass matured. On the basis of these data, we concluded that isolation of a NM CW fraction was possible, although it did not appear to concentrate factors suspected to limit CW degradation by ruminal microbes.