Experimental periods were 14 d; each period consisted
of 9 d for adaptation to treatment diets and 5 d for
sample collection. Steers were housed continuously in
metabolism crates to allow for total collection of urine
and feces. Steers were allowed ad libitum access to water
and were fed equal amounts of the total mixed ration
twice daily at 0500 and 1700 h. The amount of feed
offered was near ad libitum intake, as determined for
each individual steer before the experiment. Five grams
of Cr2O3 was manually mixed into diets at each feeding
(10 g/d) beginning on d 7 and continuing through the
end of the period to serve as an indigestible marker of
nutrient flow to the duodenum.
A clean urine collection vessel containing 900 mL of
10% (wt/wt) H2SO4 was placed under each steer at
0530 h for daily collection of urine from d 10 through 13
of each period. Feces were collected from d 10 through
13 of each period in pans located behind the steers as
part of the metabolism crates. Blood (10 mL) was collected
by jugular venipuncture into heparinized (143
USP units of heparin) Vacutainer tubes (Becton Dickinson,
Franklin Lakes, NJ) 4 h after feeding (0900 h)
on d 10. Samples were placed in ice water immediately
after collection and centrifuged at 1,200 × g at 4°C for
15 min within 1 h of collection. Plasma was isolated
and frozen for later analysis of plasma urea-N (PUN),
glucose, and creatinine.
On d 10, a temporary indwelling catheter was placed
into an ear vein for infusion of 15N15N-urea. Indwelling
jugular catheters were used to deliver the continuous
infusions in 2 steers during period 2 and in 3 steers during
period 3 because of an inability to place ear catheters.
Sterile saline solution (0.9% NaCl) was infused
continuously after catheters were placed until 0530 h on
d 11 of each period. Continuous infusion of the 15N15Nurea
solution (4.16 mL/h) began at that time and continued
through the end of each period. The infusion of
the 15N15N-urea solution delivered 0.48 mmol of urea