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Structural Factors Affecting Leaf Degradation of Old World Bluestem and Weeping Love Grass¹

US Department of Agriculture, Athens, GA 30613 El Reno, OK 73036

Abstract

Samples of old world bluestem [Bothriochloa caucasica (Trin.) C. E. Hubb.] cv. Caucasian and weeping love grass [Eragrostis curvula (Schrad.) Nees] cv. Morpa were examined for chemical composition, anatomical structure, in vitro dry matter digestibility (IVDMD) and cell wall degradation of leaf blades by scanning electron microscopy in two experiments. In the first experiment, 10-wk-old samples of bluestem and love grass grown without irrigation had the following respective compositional and digestibility characteristics: crude protein, 7.2 and 8.3%; neutral detergent fiber, 60.1 and 76.3%; lignin, 3.1 and 4.3% and IVDMD, 47.3 and 43%. Rate of dry matter loss was slower, but extent of loss was greater with rumen fluid than with pepsin-cellulase treatment. Microscopic evaluation of cell wall degradation at 6, 12, 24 and 48 h supported the digestion pattern noted for IVDMD and the pepsin-cellulase technique. Mesophyll and portions of the epidermis were the major tissues degraded in both grasses. In the second experiment, 4-wk-old samples of irrigated and nonirrigated bluestem and love grass were compared for factors related to quality. Nonirrigated forages had lower protein and higher fiber and lignin contents than irrigated grass, and the difference in quality factors due to irrigation was greater in love grass than in bluestem. The IVDMD (48 h) for bluestem was 66.1% (irrigated) and 54.9% (nonirrigated) and for love grass 64.8% (irrigated) and 45.4% (nonirrigated). No anatomical variations were found within treatments in bluestems, but blades of nonirrigated love grass had more lignified cells when compared with irrigated forage. Microscopic study showed that specific tissues, including the parenchyma bundle sheaths, were a structural feature that reduced digestibility of nonirrigated forage, but that irrigation resulted in forage in which parenchyma bundle sheaths were more easily degraded. These results provide fundamental information on structural factors that limit digestibility of these warm-season grasses adapted to drought conditions. Further, this study shows that irrigation can improve digestibility by rumen microorganisms and indicates the particular role played by cell wall types.

¹ The authors thank William E. Lonkerd, South-western Forage and Livestock Research Station, USDA, El Reno, OK, for providing rainfall and temperature data.

² Richard B. Russell Agricultural Research Center, ARS, USDA, Athens GA 30613.

³ SW Forage and Livestock Research Station, ARS, USDA, EL Reno, OK 73036.

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