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 ad vancing season on dietary composition, forage intake, and digestion. S ix 10-d sample collections were conducted from mid-June to mid-Decembe r 1995. Diet quality data demonstrated a linear(P < .01) decline in CP and in vitro OM digestibility (IVOMD), whereas NDF, ADF, acid deterge nt-insoluble CP (ADICP; percentage of CP), and undegradable intake pro tein (UIP; percentage of CP) increased linearly (P < .01) with advanci ng season. Fecal output and ruminal OM fill (percentage of BW) demonst rated 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 dis appearance (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 rapid ly degraded CP fraction increased (P < .10) linearly, and the rate of in situ CP disappearance (ISCPD) followed a linear and quadratic decre ase (P < .10) with advancing season. In summary, these data indicate d ecreases (P < .10) in dietary CP, IVOMD, ISNDFD, and ISCPD with advanc ing season. In addition, seasonal increases (P < .10) in forage NDF, A DF, ADICP, UIP (percentage of DM) and fecal output were observed and c orrespond to the grazing of increasingly mature forage. These data sug gest 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.