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Feedlot Bloat Studies. III. Rumen Fatty Acids and Metabolic Activity of Rumen Bacteria of Animals Placed on a Bloat-Producing Diet^{1, 2,}

University of Maryland and and USDA^,4

Abstract

The concentration of volatile fatty acids increased from approximately 110 to 150 uM per ml. of rumen fluid when the animals were changed from a hay-silage diet to a bloat-producing diet. The ratio of short chain rumen volatile fatty acids changed with each diet fed and with time in the case of the bloat-producing diet. Acetic acid decreased from 55 to 46 molar percent and propionic acid increased from 21 to 29 molar percent of the total volatile fatty acids in the rumen at 4 and 39 days, respectively, on the bloat-producing diet (average of 4 animals). The trend then reversed and acetic acid increased and propionic acid decreased to 60 and 20 molar percent, respectively, after the animals had been on the bloat-producing diet for 74 days. Reducing substances were never found in the rumen liquor, whereas lactic acid was nearly always found in the rumen samples taken 4.5 hours after feeding the bloat-producing diet.

The dissimilation of glucose and cellobiose by rumen microorganisms removed from the rumen 4.5 hours after feeding showed that the proportion of the fatty acids produced changed with the diet. Following the changeover to the bloat-producing diet, the proportions of volatile fatty acids from the dissimilations were altered in an accentuated manner in the same direction as the rumen fluid. This indicates that as the individual fatty acid production in the rumen changes, the relative concentration in the rumen changes in the same direction and that relative concentration in the proportions of rumen volatile fatty acids may represent relatively large changes in their production. There were striking similarities in the volatile fatty acid proportions which resulted from the cell suspension dissimilations of glucose and cellobiose.

The proportions of the fatty acids produced by in vitro dissimilations changed with each diet, and during the 74 days when the animals were fed the bloat-producing diet, were changing constantly. Valeric acid was apparently synthesized from carbohydrate in unusually large quantities by rumen bacteria.

¹ Scientific Article No. A661, Contribution No. 2860 of the Maryland Agricultural Experiment Station. Taken from a thesis presented by Don R. Jacobson to the Graduate School, University of Maryland, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Dairy Husbandry, University of Maryland.

² This work was supported in part by Animal Disease and Parasite Research Division, A.R.S., U.S.D.A.

³ Animal Husbandry Research Division, A.R.S., U.S.D.A., Beltsville, Md.

⁴ Dairy and Microbiology Departments, Maryland Agricultural Experiment Station, College Park.

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