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Rate of Rumen Metabolism of C¹⁴-Labeled Alfalfa Hay as Determined by Specific Activity of Rumen Volatile Fatty Acids, Blood, Urine, and Feces¹

Departments of Dairy Science, Bacteriology, Agronomy, and Crops Research Division USDA, Kansas State University, Manhattan

ABSTRACT

Studies were initiated with C¹⁴-labeled alfalfa hay to determine how the unaltered forage is metabolized in the rumen. Labeled hay was placed in the rumens of three fistulated cows fed unlabeled hay initially and at 12-hr intervals. The specific activity (SA) of rumen volatile fatty acids (VFA) was high in the first sample taken (15 min). Peak activity occurred at 1 hr, then declined steadily until the last sample was taken (75 hr). The SA curve for urine was similar to that of rumen VFA, but the peak SA of urine lagged behind that of rumen VFA. The SA of jugular vein blood increased during the first 24 hr and remained at this level until the last sample was obtained (80 hr). A small degree of activity appeared in the first fecal sample obtained at 6 hr. Fecal SA increased rapidly and reached maximum values between 20 and 38 hr, thereafter declining until collections were terminated (95 hr). In a second experiment, C¹⁴-labeled hay was partitioned into a hot water-soluble extract and an insoluble residue and administered separately to a pair of identical-twin cows. The soluble extract was metabolized about four times faster than the insoluble residue. Apparently, certain constituents of the insoluble residue were metabolized to rumen VFA at a rapid rate. Samples taken 5 min after feeding had high specific activities. The SA peaks for urine, blood, and feces were similar to those observed in Experiment 1. These results indicate that the rumen metabolism of certain components of alfalfa hay begins a few minutes after feeding and the metabolism of others may continue for several hours.

¹ Contribution No. 331, Department of Dairy Science, Contribution No. 417, Department of Bacteriology, Contribution No. 896, Department of Agronomy, Kansas Agricultural Experiment Station, Manhattan, Kansas.

² Present address: Animal Nutrition Section, Punjab College of Veterinary and Animal Sciences, Hissar, India.