Diet Composition and Dry Matter Intake of Beef Steers Grazing Tall Fescue and Alfalfa

Alkanes are a noninvasive method to estimate dry matter intake (DMI) of grazing herbivores. Other cuticular wax components such as long chain fatty alcohols (LCOH) have been used for estimation of diet composition of ruminants eating mixed diets. This study estimated diet composition using n-alkanes and LCOH and estimated DMI using naturally occurring and dosed n-alkanes. Beef steers (Bos taurus) (16 mo. old, 358 +/- 9 kg) grazing vegetative adjacent monocultures of tall fescue (Festuca arundinacea Schreb.) and alfalfa (Medicago sativa L. subsp. sativa) were used in this study, which also evaluated diet preference. The LCOH (C-26, C-28, and C-30) added additional characterization of the forages, but diet composition estimates were not different (P >= 0.22) than when estimated using four n-alkanes (C-27, C-29, C-31, and C-33). Diet composition estimation indicated that steers consumed similar (P = 0.13) diets of 79 and 70% alfalfa in Year 1 and Year 2, respectively, corresponding to previous work showing a partial preference for legumes. Dry matter intake in Year 2 was lower (P = 0.0002, 4.7 kg d(-1)) than Year 1 (9.2 kg d(-1)), likely due to hot weather in Year 2. This study suggests that if n-alkane profiles of the forages being grazed are distinct, the additional analysis needed to determine LCOH concentrations may not be necessary. Analyzing preliminary forage and fecal samples for n-alkanes to estimate diet composition could reduce labor and expenses by eliminating additional laboratory analyses.