Submitted to: Canadian Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2003
Publication Date: 4/1/2004
Citation: Blummel, M., Grings, E.E., Haferkamp, M.R. 2004. Fermentation profiles and degradability measurements in extrusa diet samples collected from brome-suppressed and undisturbed pastures and their relationship to weight gain of steers. Canadian Journal of Animal Science 84(1):105-111. Interpretive Summary: A reliable prediction of animal performance by in vitro techniques could assist forage and range scientists in preliminary investigation of various research hypotheses within a smaller, less costly and more closely controlled experimental framework than direct animal experimentation. Forage is largely utilized ruminally, and routine in vitro procedures that estimate the extent of ruminal feed degradation exist. However, these measurements alone may not be sensitive enough for prediction of animal response, and additional information such as rate of forage degradation may contribute to a more comprehensive understanding of forage quality and, possibly, to a better prediction of animal performance. Diet quality of free-ranging domestic ruminants is often assessed by extrusa samples collected from esophageally fistulated animals. Due to salivary contamination of extrusa samples with nitrogen, in vitro procedures might need use different nitrogen levels in the incubation medium than those recommended for feed or clipped forage samples. It was our aim to investigate in vitro ruminal fermentation characteristics of extrusa samples and compare this information with the seasonal weight gain of grazing steers. Extrusa samples were analyzed for gas production profiles in nitrogen-supplemented and unsupplemented incubation medium and for gravimetric in vitro organic matter degradability. Best prediction of animal weight gains from in vitro gas production measures was obtained from 24-hour gas volume recordings. Best choice of model to describe gas production kinetics depended on grazing period and nitrogen level.
Technical Abstract: The effects of chemical suppression of annual bromes (Bromus japonicus Thunb. and Bromus tectorum L.) on Northern Great Plains (NGP) rangeland on the nutritive quality of extrusa diet samples (EDS) collected monthly were investigated, and EDS quality estimates were compared with weight gain of grazing steers from mid-May until mid-September. In addition to crude protein (CP), EDS were analyzed for gas production profiles in nitrogen (N) supplemented and unsupplemented incubation medium and for gravimetric in vitro organic matter degradability (IVOMD). Brome-suppression tended (P = 0.07) to increase CP content but effects on gas production kinetics and IVOMD were dependent on N-level in the incubation medium. In N-unsupplemented incubations, asymptotic gas production was significantly lower and rates of gas production were significantly greater in EDS originating from brome-suppressed compared to undisturbed pasture. No such differences were found for N-supplemented incubations. Over the whole grazing period, weight gains of steers grazing brome-suppressed pastures were about 16% greater (P = 0.007) than gains from control pastures. Using cross-validation procedures, R2 for the comparison of predicted and measured gains were 0.90 (P < 0.0001), 0.96 (P < 0.0001) and 0.90 (P < 0.0001) using CP, IVOMD (N-low) and IVOMD (N-rich) as the predicting variable, respectively. Best predictions using in vitro gas production measurements were obtained from 24 h gas volume recording (R2 = 0.93, P < 0.0001). Best-fit model (sigmoidal vs. exponential) depended on grazing period and N-level, and the sigmoidal Gompertz model best described most gas production profiles. Highest R2 for predicted and measured gain achieved by equation variables was 0.91.