Forage intake and digesta kinetics of lactating beef cattle differing in feed efficiency while grazing Idaho rangelands

The objective was to determine if 2-yr old cows differing in residual feed intake (RFI) would differ in forage intake and digesta kinetics. Previously classified 2-yr-old lactating Angus × Hereford cows (12 efficient, [EFF]; 12 inefficient, [INE]) were given a pulse dose of an alkane marker and outfitted with grazing collars. Fecal samples were then collected repeatedly over 4 d in June and August. Daily digestible organic matter (DOM) and crude protein (CP) were estimated from fecal near infrared spectroscopy. Data were analyzed by fitting a non-linear digesta kinetics model to individual cows. A mixed model with RFI treatment, period, and their interaction as fixed effects, and cow within treatment as a repeated random effect, was fitted to the combined kinetics data. There were no treatment differences (P > 0.05) between EFF and INE cows for the variables measured, but period differences were present (P < 0.05). Fecal output tended to increase from spring to summer (P = 0.08) and for INE cattle from spring to summer (P = 0.06) but EFF cattle did not differ from spring to summer (P = 0.59). Total marker residence time in the gastrointestinal tract (RTG) was 38.3 and 40.5 ± 1.2 h for INE and EFF cows, respectively in June, and 44.0 and 42.9 ± 1.2 h for INE and EFF cows in August, increasing (P < 0.001) for INE and tending to increase for EFF (P = 0.097). Period RTG was 39.4 vs. 43.4 ± 0.8 h for June vs. August (P < 0.01). Dry matter intake (DMI) in June was 15.6 ± 1.15 kg for INE cows and 17.6 ± 1.10 kg for EFF cows (P = 0.22). August DMI was 17.0 and 16.3 ± 1.10 kg for INE and EFF cows, respectively (P = 0.67). The DOM was 58.9% vs. 53.8% ± 0.36% for June vs. August (P < 0.0001) and CP was 15.0% ± 0.23% vs. 14.2% ± 0.22% (P < 0.05). The CP in the diet declined from spring to summer for EFF cattle (P < 0.05), but not for INE cattle (P = 0.22). The compartmental mass of undigested dry matter (fill) was 6.6 vs. 8.6 ± 0.42 kg for June vs. August (P < 0.01). Bite rates increased from spring to summer (P < 0.05) and for EFF cattle (P < 0.05), but not INE cattle (P = 0.52). Grazing time tended to increase from spring to summer (P = 0.06), and for INE cattle from spring to summer (P = 0.07), but not for EFF cattle (P = 0.41). Reasonable, though slightly inflated, estimates of DMI with the pulse dose procedure were obtained, but differences in DMI for RFI in a feedlot setting did not translate to a range setting.