Six duodenally and ileally cannulated
steers were used in 3 sequential studies to measure 1) basal
nutrient flows from a soybean hull-based diet, 2) small
intestinal digestibility of raw cornstarch continuously
infused into the duodenum, and 3) responses of small
intestinal starch digestion to duodenal infusion of 200 or
400 g/d casein. Our objective was to evaluate responses in
small intestinal starch digestion in cattle over time and to
measure responses in small intestinal starch digestion to
increasing amounts of MP. On average, cattle consumed
3.7 kg/d DM, 68 g/d dietary N, and 70 g/d dietary starch.
Starch flow to the duodenum was small (38 g/d), and N
flow was 91 g/d. Small intestinal digestibility of duodenal
N was 57%, and small intestinal digestion of duodenal
starch flow was extensive (92%). Small intestinal starch
digestibility was 34% when 1.5 kg/d raw cornstarch was
continuously infused into the duodenum. Subsequently,
cattle were placed in 1 of 2 replicated Latin squares that
were balanced for carryover effects to determine response
to casein infusions and time required for adaptation.
Duodenal infusion of casein linearly increased (P ≤
0.05) small intestinal starch digestibility, and small
intestinal starch digestion adapted to infusion of casein
in 6 d. Ethanol-soluble starch and unpolymerized glucose
flowing to the ileum increased linearly (P ≤ 0.05) with
increasing infusion of casein. Plasma cholecystokinin
was not affected by casein infusion, but circulating levels
of glucose were increased by casein supplementation
(P ≤ 0.05). Responses in small intestinal starch digestion
in cattle adapted to casein within 6 d, and increases in
duodenal supply of casein up to 400 g/d increased small
intestinal starch digestion in cattle.