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Influence of Forage Type on Ruminal Bacterial Populations and Subsequent In Vitro Fiber Digestion¹

US Department of Agriculture, Clay Center, NE 68933

ABSTRACT

Adaptation of the rumen fibrolytic bacteria to legume, C₃ grass, and C₄, grass forages was examined in a 3 x 3 Latin square. Fistulated steers were fed alfalfa, smooth brome-grass, and switch-grass hays for 6 wk at 1.8% of body weight. Rumen samples were collected weekly after an overnight fast. Bacterial counts were conducted on rumen samples and all rumen samples were used in an in vitro fiber digestion study with three stages of maturity each for alfalfa, smooth brome-grass, and switch-grass as the substrates. Consumption of alfalfa hay resulted in the highest total viable counts of rumen bacteria but a lower proportion of fibrolytic counts than seen on the grass diets. Use of filter paper as the isolation substrate gave higher fibrolytic counts than seen with NDF of the forage fed as the isolation substrate. Fifty percent or more of the fibrolytic bacteria were Bacteriodes succinogenes, and the switch-grass diet resulted in higher proportions of this organism in the fibrolytic population than seen for alfalfa and smooth bromegrass hays. The rumen inoculum from animals fed alfalfa degraded the fiber fractions of all substrate forages best. Improved in vitro digestibility of a forage was not observed due to feeding the same forage to the donor animals. Volatile fatty acid concentrations and proportions in the in vitro fermentations were related more to forage substrate than diet source. The results indicate that adaptation of the rumen population to diet forage composition occurred, but in vitro digestilibity was unrelated to fibrolytic bacterial numbers or proportions.

¹ Mention of a trade name, proprietary products or specific equipment does not constitute a guarantee of the product by the USDA and does not imply its approval to the exclusion of other products that may also be suitable.

² USDA-ARS Roman L. Hruska US Meat Animal Research Center.

³ Present address: USDA-ARS US Dairy Forage Research Center and Department of Animal Science, University of Minnesota, St. Paul 55108.