EFFECTS OF FORAGE CHARACTERISTICS AND VOLUNTARY FEED INTAKE ON RUMINAL PASSAGE OF DIGESTA FRACTIONS IN LACTATING DAIRY COWSByKimberly L. Kammes Passage from the rumen is a dynamic, complicated process that involves numerous animal and feed factors. Ruminal passage rates affect intake, digestibility, and the amount and type of fermentation endproducts and protein available to dairy cows. The success of existing nutrition models is limited by the lack of data for passage rate of individual digesta fractions, and the inability to account for effects of feed intake and forage characteristics of passage rates of specific feed fractions. These limitations can be addressed using the pool and flux method, which allows measurement of rates of passage for fractions within feeds, and an experimental design optimized to assess the interactions of diet characteristics and dry matter intake (DMI). Five experiments were conducted to evaluate the effects of level of feed intake on the response of passage rate of digesta fractions in dairy cows to forage characteristics. All experiments utilized ruminally and duodenally cannulated cows with a wide range of DMI in a crossover design, where DMI was measured during a preliminary period (pDMI), and rates of digestion and passage of feed fractions were determined by the pool and flux method. The use of pDMI, an index of nutrient demand, allowed the evaluation of treatments on animal responses in relation to level of intake and effects of intake level independent of treatments. Level of intake ranged from approximately 20 to 30 kg dry matter per day among cows on the five experiments, and each experiment evaluated individual pairs of forage treatments including: 1) legume particle size, 2) grass particle size, 3) legume maturity, 4) grass maturity, and 5) forage family. Alfalfa and orchardgrass were selected as a representative legume and grass, respectively, and were the sole source of forage used in diets, which were formulated to contain a similar forage neutral detergent fiber concentration among diets within experiment. We hypothesized that the normal variation in diet characteristics related to forage alters both the passage rates of feed fractions and the extent to which passage rates of these fractions are affected by voluntary DMI. In general, it has been accepted that ruminal passage rates, microbial nitrogen flow to the duodenum, and efficiency of microbial synthesis increase with DMI; however, this is likely an oversimplification based on our research. Results from these experiments demonstrated passage rates of individual digesta fractions from the rumen were highly variable at a given level of voluntary DMI across cow periods, depended upon the forage characteristic being evaluated, and were inconsistent among the forage treatments evaluated. Additionally, microbial efficiency was not related to level of intake but was related to other factors, which varied among the experiments and included passage rates of starch and potentially digestible neutral detergent fiber, digestion rate of starch, and amount of true ruminally digested organic matter. Furthermore, there was no evidence that the filling effect of diets affected feed intake differently for cows with high intake compared to cows with low intake for any of the forage treatments evaluated. Finally, this research illustrated the complexity of ruminal passage and emphasized the difficulty involved in accurately predicting ruminal passage and digestibility. Although the effects of DMI on passage rates are not consistent, these experiments provide absolute passage rates of digesta fractions for use in the development of equations to predict ruminal digesta passage and the foundation for additional research in this area. Data obtained from these experiments and others using the pool and flux method can be compiled and used in a meta-analysis, with the potential to discover relationships. The results will improve the accuracy of nutrition models to predict nutrient intake, passage, and utilization in dairy cows.
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