|Jung, Hans Joachim|
Submitted to: Netherlands Journal of Agricultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2001
Publication Date: N/A
Citation: Interpretive Summary: Only a portion of the nutrients in forage crops are actually digested by dairy cows. The undigested portion of the feed is excreted as manure and is a disposal problem. Forages are more completely digested if cows are eating only small amounts of feed, but typically dairy cows consume very large amounts of feed which reduces digestibility. Manure production would be reduced if alfalfa were made more digestible at the high levels of feed consumption typical of dairy cows. We identified several tissues in alfalfa that had different digestibilities. Certain tissues were very rapidly and completely digested, however, these tissues were only a small portion of the alfalfa plant. The most abundant tissues were very poorly digested. The rapidly digested tissues are those that will be digested by dairy cows eating large amounts of feed. This information will aid feed quality improvement research for alfalfa because it identifies the specific tissues sthat should be increased in alfalfa and which tissues should be reduced in abundance.
Technical Abstract: Alfalfa stem internodes of advanced maturity were used to examine the variability among tissues for rate and extent of cell-wall degradation by rumen microorganisms. Thin sections were incubated with rumen fluid for 0, 2, 4, 8, 16, 24, 48, 72, and 96 hr. The degradation of tissue cell walls was evaluated for each fermentation time interval against a nondegraded control section by light microscopy. Cell-wall thickness of alfalfa stem tissues was measured using scanning electron microscopy for both control and fermented sections. Rate and extent of cell-wall degradation were calculated from these measurements. Non-lignified epidermis, collenchyma, chlorenchyma, cambium, and primary xylem parenchyma were rapidly and completely degraded within the first 8 hr of fermentation. Rates of degradation ranged from 0.04 microns/hr for thin-walled primary xylem parenchyma tissue to 0.11 microns/hr for thick-walled collenchyma tissue. The non-lignified secondary wall of the primary phloem fibers required 24 hr for complete degradation. Cell walls of some lignified tissues (pith parenchyma and secondary xylem fibers) were only partially degradable even after 96 hr of fermentation. The primary and secondary xylem vessels appeared to be completely nondegradable. The observed rates of cell-wall degradation for nonlignified alfalfa stem tissues were two to five times faster than previously estimated for nonlignified grass mesophyll tissue. However, extent of degradation for the lignified tissues of alfalfa stems were less than reported for lignified grass stem sclerenchyma. These differences in cell-wall degradation characteristics among tissues within alfalfa and compared to grasses are related to cell-wall lignification and polysaccharide composition of individual tissues.