Date of Graduation


Document Type


Degree Type



Davis College of Agriculture, Natural Resources and Design


Animal and Nutritional Sciences

Committee Chair

Eugene E. D. Felton.


Residual feed intake (RFI) class differences in gut function and grazing intake were evaluated using 14 crossbred steer progeny of first parity dams with known RFI phenotypes and a commercially available Angus sire. Steers were housed in a drylot facility and fed a forage based complete feed ad libitum, a forage diet at maintenance (1.5% BW as fed), and a concentrate diet at maintenance in Exp. 1, 4, and 5 respectively. Voluntary DMI and solid and liquid passage rate were evaluated while grazing (Exp. 2 and 3, respectively). Plasma concentrations of albumin (ALB), urea nitrogen (UN), cholesterol (CHOL), glucose (GLU), triglycerides (TG), and VFA were assessed immediately pre- and at 3 h intervals for 12 h post-feeding as indicators of nutrient absorption in Exp. 1, 4, and 5 respectively. Rumen pH, ammonia (NH4), and VFA were evaluated 12 h post-feeding. Steers were classified as either positive (POS; n = 8) or negative ( NEG; n = 6) according to dam RFI (mean 0.46 kg TND/d; range -4.27 to 5.70 kg TDN/d). Rumen pH, ammonia, and VFA's did not differ based on RFI classification (P > 0.05) in any of the experiments. Plasma metabolite and VFA concentrations did not differ between RFI classes in Experiment 1 (P > 0.05); however, UN was greater in POS (12.80 vs. 12.14 mg/dL; P = 0.02) steers when fed a forage diet at maintenance (Exp. 4) and greater in NEG (16.74 vs. 15.42 mg/dL; P = 0.01) steers when fed a concentrate diet at maintenance (Exp 5). Plasma valerate and butyrate (P = 0.02 and 0.02, respectively) were greater in NEG steers in Exp. 1 and 5, respectively. Evaluation of RFI group differences by collection time revealed a significant (P = 0.04) interaction of RFI and Time for TG and plasma isobutyric acid in Exp. 1. Concentrations of TG were greater (P = 0.04) in NEG steers at 3 hours after feeding but did not differ at any other time points. Plasma isobutyric acid concentrations rarely differed from zero but were greater in POS steers 6 hours post-feeding. The opposite was true at 12 hours post-feeding. No other interactions between RFI and collection time were significant. Negative RFI steers consumed numerically 8.9% less DM per day while grazing than POS steers but did not differ in voluntary DMI (P = 0.74). Liquid passage rate was greater in NEG (7.16 vs. 6.32 % marker/hour; P = 0.04) steers but solid passage rate was not different (P = 0.88). These data indicate that RFI classification may be associated with variation in the regulation and absorptive capacity of the gastrointestinal tract.