The influence of restricted feeding on site and extent of digestion and flow of nitrogenous compounds to the duodenum in steers

The effects of restricted feeding on site and extent of digestion, flow of N-containing compounds to the duodenum, and ruminal metabolism were determined. In Trial 1, corn silage-based diets were fed at one of three intakes to three ruminally and duodenally fistulated steers (420 +/- 4.4 kg BW) in a repeated 3 x 3 Latin square experiment. Feeding levels were 100% of ad libitum intake, 90% of ad libitum intake, and 80% of ad libitum intake. Site of digestion was not affected by restricted feeding; however, extent of DM, NDF, and apparent N digestion were increased (P < .03) 2.8, 4.1, and 6.2 percentage units for each 1-kg restriction in DM intake. Restricted feeding increased (P < .06) ruminal pH and ammonia concentrations, decreased (P < .05) ruminal liquid dilution rate, and had no major effects on bacterial concentrations. In Trial 2, all-concentrate diets based on whole shelled corn were fed at 100, 90, or 80% of ad libitum intake to six ruminally and duodenally fistulated steers (440 +/- 4.1 kg BW) in a replicated 3 x 3 Latin square experiment. Restricted feeding increased (P < .06) apparent ruminal DM digestion; however, true ruminal OM digestibility was not affected. Extent of apparent N digestion increased 3.0 percentage units for each 1-kg restriction in DMI. Ruminal pH was not affected by restricted feeding; however, ruminal ammonia concentrations increased (P < .06) and ruminal liquid dilution rate decreased (P < .04) with restricted feeding. In both Trials, total N flow to the duodenum was approximately 100% of N intake for the 80% of ad libitum intake. Flow of N exceeded N intake in a linear (P < .01) manner as DMI increased. Decreases in duodenal N flow when intake was restricted resulted from decreases (P < .01) in bacterial N flowing to the duodenum, and also in decreases (P < .01) in non-ammonia, non-bacterial N flow when whole shelled corn-based diets were fed. We concluded that restrictions in digestible energy intake are less than the DM restriction when corn silage-based diets are fed but are equal to the DM restriction when whole shelled corn-based diets are fed. Also, restricted feeding strategies reduce the flow of N to the duodenum from both bacterial and feed origins.