Incorporation of Carbon‐14 into the Indigestible Fraction of Timothy Herbage1

Abstract

A better understanding of the nature of herbage‐quality deterioration of forages as they mature may aid in making crop management and crop improvement decisions. The purpose of this study was to follow the incorporation of carbon‐14 into the indigestible fraction of timothy (Phleum pratense L.) herbage at initiation of stem elongation, ear‐emergence, and anthesis. Plants grown in cans outdoors were exposed to 14C‐labeled CO2 and then harvested at intervals of 1, 3, 8, 32, and 168 hours following initial exposure at each growth stage. Herbage was divided into leaf blades, leaf sheaths, and stems at each sampling time. In vitro dry matter digestibility of each tissue fraction was determined, and the residue from each fraction was analyzed for 14C concentration. Rates of decrease in digestibility were greatest between ear‐emergence and ear‐emergence plus 168 hour growth stages for all tissue fractions. Proportions of whole tissue 14C remaining in the undigested residue of leaf sheaths and stems also were greater at ear‐emergence than at anthesis. Results suggest that rate of incorporation of 14C into indigestible fractions of timothy herbage is greater near ear‐emergence than near initiation of stem elongation or anthesis. Results also suggest that radioisotope studies can be effectively used to obtain a better understanding of plant tissue chronology, cell wall polysaccharide formation, and forage digestibility.