INDIGESTIBLE MARKERS: METHODOLOGY AND APPLICATIONS IN RUMINANT NUTRITION

Scope of Study. The research of this dissertation concerns the validation and use of indigestible markers for measuring flow of material in the digestive tract of steers. Five research areas were explored in five different trials. First, the potential of five water soluble markers for measuring rumen liquid volume and dilution rate was evaluated. Liquid marker techniques were used to examine the influence of roughage: concentrate ratio upon composition of ruminal contents and outflow. Second, ytterbium chloride was examined as a particulate marker. Binding affinity and binding capacity of various feeds for ytterbium, effect of ytterbium labelling upon feedstuff digestibility and rate of ytterbium release from feedstuffs under disequilibria were estimated in vitro. Third, ytterbium chloride was used as a particulate phase marker to estimate ruminal and total digestive tract passage rates of feeds differing in size, density and dietary concentration. Fourth, a new marker technique was derived and tested which permits estimation of (1) rumen liquid volume and dilution rate with a single digesta sample and (2) direct ruminal escape of dietary feedstuffs and liquids. Fifth, digesta flow markers were used to study associative interactions between whole shelled corn grain and two roughage sources.;Findings and Conclusion. Water soluble markers (WSM) tested (polyethyleneglycol, Cr-EDTA, Fe-EDTA, Yb-EDTA) were distributed similarly between liquid free in the rumen and liquid contained within ruminal solids with two exceptions. These were (1) whole shelled corn grain excluded marker while imbibing liquid and (2) aqueous PEG concentration is reduced in the presence of cottonseed hulls. Ruminal dilution rate and volume estimates obtained using the different WSM were similar and equally accurate for steers fed both a high concentrate and a high roughage ration. However, liquid volume, measured by manual removal of ruminal contents, was 4% lower than volume estimated by the WSM procedure. In the second trial, all feedstuffs tested exhibited a capacity to bind Yb, though the quantity bound varied considerably among feedstuffs. In general, feeds high in crude fiber or in protein bound higher (76-205 (mu)M Yb/g feed) quantities of Yb at saturation while feeds high in starch content bound lower amounts (5-37 (mu)M Yb/g feed). Each feedstuff exhibited plots suggestive of at least two types of binding for Yb. The weaker sites had molar association constants ranging from 4.5 to 9.2 x 10('5) while stronger sites had association constants of 1.2 to 3.8 x 10('7). Dialysis of Yb tagged feedstuffs against water (dilution rate 47%/hr) detached Yb from binding sites at rates ranging from .09% to .57% per hour. Yb labelling reduced digestibility of feedstuffs. Altered feedstuff digestibility and the potential for Yb migration questions the validity of Yb data obtained after extensive time periods of digestion. In the third trial with steers fed either concentrate or roughage rations. Flow rates of Yb labelled particulate matter from greatest to least was SBM > WSC = CSH > PH (P .10) with both rations. Grinding hay and corn grain increased passage rate of the ground particles from the rumen by 6 to 14%. In the fourth trial, the dual marker-single sample techniques were found to have promise. Estimates of volume and dilution rate obtained with single sampling did not differ (P = .47) from conventional multiple sample methodology. Ruminal escape of liquids was estimated to be 5.7%. In the fifth trial, digestibility by steers of starch from whole shelled corn grain was found to differ with roughage source, roughage level and feed intake level. Responses to added roughage were not explained by ruminal passage or ruminal digestion rates of whole shelled corn. Results suggest that the increase in starch digestion seen with cottonseed hull addition is due to mastication and reduction in particle size of the grain.