Ja. Johnson et al., INFLUENCE OF SEASON ON DIETARY-COMPOSITION, INTAKE, AND DIGESTION BY BEEF STEERS GRAZING MIXED-GRASS PRAIRIE IN THE NORTHERN GREAT-PLAINS, Journal of animal science, 76(6), 1998, pp. 1682-1690
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.