The Measurement of Nutritive Quality in a Bluegrass Pasture Using In Vivo and In Vitro Techniques

Abstract

A series of trials was carried out to determine the influence of growth phase (first vs. regrowth), stage of maturity, nitrogen fertilization (60 lb. N vs. 240 lb. N) and cutting management (continuous growth vs. interval clipping) on the digestibility of Kentucky bluegrass clipped and fed in the fresh form to sheep. Concurrent laboratory trials were performed to determine the feasibility of predicting digestibility coefficients by means of in vitro techniques, and to examine the influence of method of preparation (as fresh, freeze-dried or oven-dried grass) of the sample and the source of rumen fluid inoculum (from grass or hay-fed sheep) on the in vivo-in vitro relationship. Significant differences in both animal and in vitro digestibility coefficients were obtained due to pasture treatment effects, and there were also significant differences in in vitro digestibility caused by method of preparation of the grass and sources of inoculum. The correlations between in vivo dry matter and cellulose digestibility and the in vitro dry matter or cellulose digestibility of either fresh, freeze-dried or oven-dried grass, using either grass or hay inocula, were found to be highly significant. This indicates that any combination of preparation and inoculum might justifiably be used in the prediction of animal digestibility coefficients of dry matter or energy. However, analysis of the data suggested that several of the variable factors examined might significantly affect the in vivo-in vitro digestibility relationship. Protein digestibility in vivo was found to be as highly related to protein content of the forage as to any measure of in vitro protein digestion. Analysis of volatile fatty acid distribution in the rumen or in in vitro fermentation mixtures indicated that (1) molar proportions of acids in the rumen of grass-fed sheep differed from those in sheep maintained on hay, containing relatively less acetic and more propionic acid, (2) acetic acid increased and propionic acid decreased in the rumen contents with increasing maturity of the herbage, (3) vegetative growth in August and September produced relatively less propionic acid than a comparable growth in May, (4) there was no apparent effect of level of nitrogen fertilization on the distribution of acids in the rumen, (5) molar proportions of volatile fatty acids produced at 36 hr. in vitro were similar to those found in the rumen fluid of sheep fed the same substrate, although there was a tendency for higher levels of propionic acid to be present in vitro, and (6) method of preparation of the sample did not affect in vitro distribution of volatile fatty acids; however, source of inoculum was a significant factor.