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Interaction of Ruminal Bacteria and Fungi with Southern Forages¹

U.S. Department of Agriculture², Athens, GA 30613

Abstract

The ruminal microbial population has enormous potential for fiber digestion. Microorganisms responsible for the greatest attack on plant cell walls are anaerobic bacteria and fungi, with the latter better able to attack lignocellulosic tissues. Warm-season grasses have unique characteristics that result in resistance to microbial degradation and low forage quality. The plant anatomy in warm-season leaves, e.g., low proportion of mesophyll and high proportion of parenchyma bundle sheath, could result in slow digestion and physical disruption of these forages and contribute to reduced nutritive value. Further, indigenous characteristics in cell walls appear to slow microbial degradation of tissues in warm-season grasses compared with similar tissues in cool-season species, and these characteristics may alter the influence of anatomical factors. Low molecular weight, saponifiable phenolic compounds (e.g., p-coumaric acid) possibly play a significant role in limiting cell wall digestibility, and warm-season grasses contain more of these compounds and specifically more p-coumaric acid. Improvements in leaf digestibility have taken place apparently by alterations in cell wall chemistry without concomitant changes in plant anatomy, as far as can be observed with the light microscope. Alterations in stem anatomy could reduce structural limitations to plant breakdown. Stimulation or modification of ruminal microorganisms to take advantage of those best able to attack and weaken structural limitations, e.g., ruminal fungi, may offer possibilities to improve the utilization of warm-season forages.

¹ Presented at the Pasture and Forages Symp. entitled "Current Concepts and Future Opportunities in Forage Utilization in the Southeastern United States" at the 77th Annu. Meet. of the Amer. Soc. of Anim. Sci., August 14, 1985, Univ. of Georgia, Athens.

² Plant Structure and Composition Res. Unit, Richard B. Russell Agr. Res. Center, ARS, P.O. Box 5677.