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Structure of Leaves and Stems of Arrowleaf and Crimson Clovers as Related to In Vitro Digestibility¹

Samples of ‘Amclo’ arrowleaf clover (Trifolium vesiculosum Savi) and ‘Dixie’ crimson clover (T. incarnatum L.) were studied to determine structural factors that limit digestibility. Clovers were harvested at prebloom and full bloom stages, and in a second study, arrowleaf clover was harvested at prebloom, early bloom, full bloom, and 95% seed set. The clover plants were separated into individual morphological parts and analyzed for dry weight and in vitro dry matter digestibility (IVDMD), and examined microscopically for anatomical characteristics and specific tissue digestibility. In all samples, the proportion of stem to leaves increased with age. In the first study, the IVDMD of leaves declined by about 5% from prebloom to full bloom, but the IVDMD of stems decreased by about 20% in both clovers. However, arrowleaf clover was consistently higher than crimson in digestibility. The second study further indicated a loss in digestibility of arrowleaf stems at each state, but not in leaves until the seed set stage. Examination of cross-sections of plant parts showed anatomies typical of herbaceous dicotyledons. Studies of specific tissue degradation with scanning electron microscopy showed that leaflets and petioles at various ages for both clovers were digested in a similar, rapid manner and only the lignified vascular tissue and cuticle resisted digestion. Mesophyll cells were less rapidly degraded in older samples. The primary difference in digestibility resided in the stem fraction. The pith was totally degraded, although less rapidly in older samples, but the cuticle, vascular bundles, and lignified interfascicular parenchyma resisted degradation. This latter region was wider in the lower vs. upper internodes as well as in older vs. younger stems, and appeared to be the major anatomical characteristic of these clovers related to variations in digestibility. These results help to explain the decline in digestibility of clovers with increased maturity.

Key Words: Light microscopy • Scanning electron microscopy • Plant anatomy • Plant morphology • Trifolium vesiculosum Savi • Trifolium incarnatum L. • Amclo arrowleaf clover • Dixie crimson clover • Digestibility • Nutritive value

² Microbiologist, Field Crops Research Unit, Richard B. Russell Agricultural Research Center, AR-SEA-USDA, and plant physiologist, Southern Piedmont Conservation Research Center, ARS, USDA, Watkinsville, GA 30677.

Received for publication April 23, 1981.