Influence of season on dietary composition, intake, and digestion by beef steers grazing mixed-grass prairie in the northern Great Plains.

We used six beef steers (377 +/- 12.7 kg) to evaluate the effect of advancing season on dietary composition, forage intake, and digestion. Six 10-d sample collections were conducted from mid-June to mid-December 1995. Diet quality data demonstrated a linear (P < .01) decline in CP and in vitro OM digestibility (IVOMD), whereas NDF, ADF, acid detergent-insoluble CP (ADICP; percentage of CP), and undegradable intake protein (UIP; percentage of CP) increased linearly (P < .01) with advancing season. Fecal output and ruminal OM fill (percentage of BW) demonstrated linear and cubic responses (P < .10) across the grazing season. Intake of OM (percentage of BW) followed quadratic and cubic (P < .10) patterns across season. Rate (percentage per hour) of in situ NDF disappearance (ISNDFD) decreased and lag time increased linearly (P < .01) with advancing season. Likewise, quadratic (P < .07) and cubic (P < .06) responses were observed for rate and lag time of ISNDFD. The rapidly degraded CP fraction increased (P < .10) linearly, and the rate of in situ CP disappearance (ISCPD) followed a linear and quadratic decrease (P < .10) with advancing season. In summary, these data indicate decreases (P < .10) in dietary CP, IVOMD, ISNDFD, and ISCPD with advancing season. In addition, seasonal increases (P < .10) in forage NDF, ADF, ADICP, UIP (percentage of DM) and fecal output were observed and correspond to the grazing of increasingly mature forage. These data suggest that mixed-grass range forage consumed by cattle in western North Dakota during November and December is deficient in degradable intake protein (DIP). Investigation of protein supplementation (particularly DIP) during these times is warranted.