Effect of physically effective fiber on animal growth performance, carcass characteristics, digestibility, and rumination in finishing beef cattle.

The objective of this experiment was to evaluate the effects of altering dietary peNDF concentration by modifying forage particle size on digestibility, rumination, and performance of finishing beef steers. Forty crossbred beef steers (initial BW = 387 ± 38.2 kg) were blocked by BW and assigned to 10 pens with 4 steers per pen. Pens were then randomly assigned to one of two dietary treatments (n = 5 pens per treatment; 4 steers per pen). Diets were formulated identically, differing only in forage particle size. Prairie hay was processed through a commercial tub grinder equipped with a 17.8- or 10.2-cm screen, resulting in mean particle sizes of 11.54 and 7.68 mm for long-cut and short-cut, respectively. Treatments were based on dry-rolled corn and included 8% long-cut or 8% short-cut prairie hay (DM basis). Particle distribution and peNDF were estimated using the Penn State Particle Separator. Data were analyzed using mixed models. Treatment was included as a fixed effect and block as a random effect for pen-level variables, with pen serving as the experimental unit. For variables measured on individual animals, animal nested within pen was included as a random effect, and day was included as a repeated measure with animal nested within pen as the subject. Decreasing forage particle size reduced dietary peNDF from 14.23% to 12.33% of DM (P = 0.014). Neutral detergent fiber digestibility was reduced in short-cut TMR (P < 0.001), whereas DM and starch digestibility were unaffected (P ≥ 0.62). Feedlot performance and carcass characteristics did not differ between treatments (P ≥ 0.18). Fecal pH differed between treatments at specific sampling days, resulting in a treatment × day interaction (P = 0.001). Serum amyloid A concentrations tended to be greater for long-cut TMR (P = 0.09). Rumination time did not differ between treatments (P = 0.53). These results suggest that a 1.90 percentage-point decrease in dietary peNDF via forage particle size did not affect growth performance, rumination behavior, or carcass characteristics in finishing beef cattle, but may reduce NDF digestibility.