Feeding behavior and ruminal acidosis in beef cattle offered a total mixed ration or dietary components separately

Eighty continental crossbred beef heifers (414.9 +/- 37.9 kg of BW), 16 of which were ruminally cannulated, were used in a 52-d experiment with a generalized randomized block design, to assess if self-selection of dietary ingredients modulates ruminal pH and improves rumen function of feedlot finishing cattle. Treatments were total mixed ration [TMR; 85% barley grain (BG), 10% corn silage (CS), 5% supplement]; or free-choice (self-selection; FC) diets of barley grain and corn silage (BGCS), barley grain and wheat distillers grain (BGDG), or corn silage and wheat distillers grain (CSDG). Heifers were housed in groups of 10 in 8 pens equipped with the GrowSafe System (Airdrie, AB, Canada) enabling feed intake and feeding behavior to be continuously monitored. Two cannulated heifers were randomly assigned to each pen and equipped with indwelling pH probes for continuous measurement of ruminal pH during 4 periods (d 1 to 4, d 7 to 14, d 21 to 28, and d 42 to 49). Rumen fluid samples were collected from cannulated heifers on d 7 and 42 before feed delivery, and on d 4 and 49 at 2 h post-feed delivery for determination of VFA. Heifers fed the TMR had shorter (P - 0.01) and smaller (P - 0.03) meals than those fed the FC diets. Cattle fed BGCS and BGDG increased (P < 0.01) intake of BG over time by up to 80 and 70%, respectively. Increased consumption of BG arose from an increase (P < 0.01) in eating rate over the same (P > 0.10) feeding time, which was accompanied by an increase (P < 0.05) in eating rate but a decrease (P < 0.05) in feeding time of either CS or DG. Even with increased BG consumption, ruminal pH and VFA profiles were not different (P > 0.10) among FC diets or compared with the TMR. Cattle fed FC CSDG consumed DG at 60% of dietary DM over the trial, resulting in greater (P < 0.05) mean ruminal pH and acetate-to-propionate ratio and less (P < 0.05) area under the curve than those given the other FC diets or the TMR. Finishing feedlot cattle fed FC diets containing BG self-regulate intake of diets that have a similar composition, intake level, and ruminal fermentation profile to those fed a TMR.