The duration of time that beef cattle are fed a high-grain diet affects feed sorting behavior both before and after acute ruminal acidosis1,2

The objective of this study was to determine how duration of time that cattle are fed a high-grain diet affects feed sorting, both before and after an episode of acute ruminal acidosis. Sixteen Angus heifers (261 ± 6.1 kg; BW ± SEM) were assigned to 1 of 4 blocks and fed a backgrounding (BG) diet (60% forage, DM basis). Within block, heifers were randomly assigned to 1 of 2 treatments differing in days fed a high-grain (HG; 9% forage, DM basis, fed ad libitum) diet before a ruminal acidosis challenge: 34 d for long adapted (LA) and 8 d for short adapted (SA). Ruminal acidosis was induced by restricting feed to 50% of DMI as a proportion of BW (determined individually for each heifer) for 24 h followed by an intraruminal infusion of ground barley at 10% of DMI as a proportion of BW measured before feed restriction. Feed and orts were sampled during the BG period, the first 26 d on the HG diet (only for LA cattle), the 8-d baseline (BASE) period, on the day of the ruminal acidosis challenge (CH), and during 2 consecutive 8-d recovery periods (REC1 and REC2) for each heifer and subjected to particle size analysis: 19-mm (long), 8-mm (medium), and 1.18-mm (short) screens and a pan (fine). On the BG diet, sorting for medium particles tended to be greater (104.2 vs. 102.1%; P = 0.07) for LA heifers than SA heifers, while sorting against short particles was greater (98.2 vs. 100.0%; P = 0.05) for LA heifers. During the first 26 d on the HG diet, LA cattle sorted for (P < 0.001) long (118.8%), medium (117.8%), and short (104.1%) particles and sorted against (P < 0.001) fine particles (45.3%). This sorting pattern was consistent for LA heifers during BASE period, CH day, and recovery periods, across which SA heifers exhibited less sorting (P ≤ 0.1). Greater duration of pH < 5.5 during the BASE period was associated with greater sorting for long particles (R(2) = 0.75, P = 0.01) in LA heifers and for long (R(2) = 0.49, P = 0.05) and medium (R(2) = 0.88, P < 0.001) particles in SA heifers. Long-adapted heifers linearly increased the extent of sorting for long (P = 0.007) and medium (P < 0.001) particles and against fine particles (P = 0.05) during the days following the challenge to a greater extent than SA heifers. Overall, the results demonstrate that longer-term exposure of beef heifers to a HG diet, which caused persistent low rumen pH, influenced feed sorting of heifers, both before and after an induced bout of acute ruminal acidosis, in a manner that would help attenuate the effects of acidosis.