|Jung, Hans Joachim|
Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 3, 2006
Publication Date: June 1, 2006
Repository URL: http://www.ars.usda.gov/sp2UserFiles/Place/36401000/2006/Jung178450.pdf
Citation: Jung, H.G., Casler, M.D. 2006. Maize stem tissues: impact of development on cell wall degradability. Crop Science. 46:1801-1809. Interpretive Summary: Corn silage is one of the most important feeds for dairy and beef production. Grain represents half of corn silage and is highly digestible; however, stover, the other half of corn silage, is of very limited digestibility. Like all grasses, corn stover declines in digestibility as the corn plant matures because the plants accumulate fiber and lignin, a chemical part of fiber that limits digestibility. Improving the digestibility of corn stover requires a better understanding of where, when, and how lignin is deposited in the plant tissues of corn stover and how this addition of lignin reduces digestibility. A study was conducted on the development of a specific corn stem segment to describe the pattern of lignin addition and the resulting changes in fiber digestibility. Microscopic and chemical measurements of development and digestibility were combined to more accurately define the impact of lignin. It was observed that while corn stem segments were growing in size that lignin did not inhibit fiber digestibility because there was little lignin in the fiber; however, the amount of cross-linking of lignin to carbohydrates in the fiber did increase and caused a reduction in digestibility to begin. Once the corn stem segment stopped growing in size, its digestibility began to decline rapidly as lignin content of the fiber increased, but now the amount of cross-linking had no impact. This information should be used by corn biotechnologists to target reduced cross-linking during the growth period of corn stems and reduce lignin accumulation during the maturation phase as the most effective route to increasing overall corn silage feeding value for livestock.
Technical Abstract: Degradability of grasses declines as cell wall concentration and lignification increase during maturation. The role of tissue development in causing the degradability decline was examined in maize stem internodes harvested at 10 developmental stages from early elongation through physiological maturity. The fourth elongated, above-ground internode was collected from three maize hybrids grown in a two-year, replicated field trial at St. Paul, MN. Thin sections from alcohol preserved internodes and ground samples were incubated in vitro for 24- and 96-h with rumen microbes. Tissue degradation was examined microscopically and degradability of cell wall polysaccharides from ground internodes was determined. All tissues in elongating internodes were completely degradable except for protoxylem vessels, which was the only lignified tissue in these immature internodes. After elongation, degradability of all tissues declined markedly except for phloem which never lignified. Tissues with thick, lignified secondary walls (sclerenchyma and rind-region parenchyma) required longer incubation times for degradation to be observed. Degradation of lignified tissues remained incomplete, leaving a thin-walled residue. All cell wall polysaccharide components were highly degradable in immature internodes, but degradability declined after elongation ceased and reached a minimum by harvest eight. The decline in degradability was greatest for glucose and xylose residues. Total cell wall polysaccharide degradation was related to Klason lignin and ferulate cross-linking concentrations in a complex manner. The primary wall may have more lignin/arabinoxylan cross-linking by ferulates, providing more effective protection from degradation than occurs in the secondary wall.