Fermentation profiles and degradability measurements in extrusa diet samples collected from brome-suppressed and undisturbed pastures and their relationship to weight gain of steers

Abstract

The effects of suppression of annual bromes (Bromus japonicus Thunb. and Bromus tectorum L.) by atrazine application on the nutritive quality of extrusa diet samples (EDS) collected from the esophagus were investigated, and EDS quality estimates were compared with weight gain of grazing steers. Analysis on EDS included crude protein (CP), in vitro organic matter degradability (IVOMD), and gas production profiles in N supplemented and unsupplemented incubation media. Brome-suppression tended (P = 0.07) to increase CP content but effects on gas production kinetics and IVOMD were dependent on incubation medium N-level. In N-unsupplemented incubations, asymptotic gas production was less and rates of gas production were greater in EDS from brome-suppressed compared to undisturbed pasture. No such differences were found for N-supplemented incubations. Weight gains of steers grazing brome-suppressed pastures were 16% greater (P = 0.007) than from control pastures. The 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. Key words: Forage quality, gas production, weight gain, beef steers