|Jung, Hans Joachim|
Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2001
Publication Date: 3/1/2002
Citation: JUNG, H.G., ENGELS, F. ALFALFA STEM TISSUES: CELL-WALL DEPOSITION, COMPOSITION, AND DEGRADABILITY. CROP SCIENCE. 2002. V. 42. P. 524-534.
Interpretive Summary: Dairy cows require high fiber feeds such as alfalfa in their diet in order to remain healthy. Unfortunately, the fiber in alfalfa is poorly digested and does not provide as much energy for milk production as do grains. Therefore, dairy farmers feed large amounts of grain to achieve economical milk production, but this often results in cow sickness. If the digestibility of alfalfa fiber were improved, farmers could feed more alfalfa and less grain as a way to protect dairy cow health without reducing milk production and profitability. Our study described the changes that occur in fiber amount, chemical composition, and digestibility in stems of alfalfa plants as they mature. It was demonstrated that alfalfa adds a specific tissue type, similar to the wood in trees, as it matures. This woody tissue is almost completely undigestible. The increase in biomass yield of alfalfa as it matures is predominately due to the addition nof this undigestible, woody tissue. Our results indicate that improving alfalfa fiber digestibility will require that plant breeders and biotechnologists target their efforts at modifying alfalfa specifically for reductions in the amount of this woody tissue or altering the chemical composition of the woody tissue to make the fiber more digestible. Knowledge gained from our research will help guide these future efforts to improve alfalfa fiber digestibility.
Technical Abstract: Declining cell-wall degradability of alfalfa (Medicago sativa L.) stems with maturation limits the nutritional value of alfalfa for ruminant livestock. This study characterized changes in cell-wall concentration, composition, and degradability by rumen microbes resulting from alfalfa stem tissue proliferation and development during maturation. Internode 7 from the shoot base of 3 alfalfa clones was sampled after 12, 17, 21, 31, and 87 d of regrowth in 1996 and 21 and 31 d in 1997. Cross sections were examined by light microscopy for tissue development, and after 48 h in vitro degradation. Cell-wall concentration and composition of the internodes were determined by the Uppsala dietary fiber method, and cell- wall degradability by rumen microbes was measured after 12 and 96 h. All stem tissues were pectin-rich and nonlignified at the two youngest maturities, except for primary xylem vessels which had lignified and thickened walls, and internode 7 was actively elongating. Primary xylem wa the only tissue not degraded from immature stems. The 21-d old internodes had completed elongation and begun xylem proliferation. Xylem lignified immediately, and secondary wall thickening and lignification of primary phloem and pith parenchyma began when elongation ended. As internode tissues lignified, their cell walls became undegradable. With maturation, polysaccharide composition shifted from predominantly pectin toward cellulose because xylem cell walls were cellulose-rich and low in pectin. Degradability of pectin remained high regardless of maturity stage, but cellulose and hemicellulose degradabilities declined as xylem proliferated. Degradability of alfalfa stems would be improved if the amount and rate of xylem lignification were reduced.