Author
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ENGELS, FERDINAND - WAGENINGEN UNIVERSITY |
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Jung, Hans Joachim |
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Submitted to: Animal Feed Science and Technology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/3/2005 Publication Date: 6/1/2005 Citation: Engels, F.M., Jung, H.G. 2005. Alfalfa stem tissues: impact of lignification and cell length on ruminal degradation of large particles. Animal Feed Science And Technology. 120:309-321. Interpretive Summary: Alfalfa is the most widely grown forage crop in the U.S. and is a mainstay of the dairy industry for feeding cows. While the leaves of alfalfa are rich in protein and highly digestible, the stems of alfalfa account for at least half of the crop's mass and stems are poorly digested. Improving the feeding value of alfalfa through plant breeding requires a clear understanding of what characteristics of alfalfa limit stem digestibility. Because alfalfa varies dramatically among germplasm sources for stem length, a study was conducted to determine if stem length influences the digestibility of alfalfa stems. A unique experimental system was developed using stem pieces cut out of different length stems, coated with wax to limit bacterial access to one end, and examined under a microscope after partial digestion by bacteria found in the stomach of cows. Observation of these stem pieces showed that longer stems allowed bacteria to degrade more deeply into the stem pieces than seen for shorter stems. The reason longer stems were more digestible was because the longer stems had longer individual cells in the stem. This allowed the bacteria to move more quickly through the stem. If other alfalfa germplasm also has this same trend for longer stems being more digestible, then plant breeders can use stem length as a quick and inexpensive method to identify the plants which will be of greatest value for breeding more digestible alfalfa. Technical Abstract: Experiments were conducted with alfalfa to determine how deeply rumen microorganisms can degrade various stem tissues. The seventh internode from the base of the stem was collected from alfalfa clone 718 after 4 wk of regrowth. Internode length and diameter were measured, and 2-cm stem pieces were excised from the internodes. Stem pieces were incubated with rumen fluid in vitro for 24 h. Bee's wax was used to coat the stem pieces to prevent microbial access other than at one end of the stem pieces. After exposure to the rumen microorganisms, stem pieces were serially cross-sectioned starting at the exposed surface. Sections were examined by light microscopy to determine which tissues had been degraded and to what depth into the stem piece degradation had occurred. Non-lignified alfalfa stem tissues (chlorenchyma, collenchyma, cambium, and primary xylem parenchyma) were degraded to great depth (3700 to 8200 micron) in stem pieces, but degradation of lignified tissues (phloem fibers and xylem fibers) was much more limited (150 to 1360 micron). Depth of degradation was greater in stem pieces derived from long internodes compared to short internodes. It was found that mean cell length increased by approximately 50% with a doubling of internode length for all tissues examined. Many cell layers of non-lignified tissues were degraded whereas only the exposed cell layer of lignified tissues exposed at the cut end of the internode pieces was susceptible to degradation. Depth of degradation for non-lignified tissues was attributed to a combination of cell wall degradation, cell length, and the presence of intercellular spaces in chlorenchyma tissue. The lignified wall established a complete barrier to degradation of cells below those mechanically ruptured. |
