Digestion and absorption of nutrients by beef heifers fed a high concentrate diet

A replicated, 3 X 3 Latin square design was used to evaluate effects of calculated metabolizable energy (ME) intake by beef heifers on apparent digestibility (AD), retention and net absorption (portal-arterial plasma concentration times portal plasma flow measured by dye dilution) of various nutrients. Six Hereford X Angus heifers (295 +/- 4 kg) with catheters in portal and mesenteric veins and iliac artery were fed once daily a high concentrate, pelleted diet to provide 84, 157 or 225 kcal ME/kg0.75. Linear (P less than 0.05) responses to increased ME intake included AD of dry matter (84.9-76.7%); AD of digestible energy (DE) (83.4-74.3%); AD of nitrogen (N) (70.5-62.1%); N retained (-6.3 to 25.6 g/day); portal blood flow (444-782 liters/hour); and net absorption of volatile fatty acids (VFA) (311-795 mmol/hour), L-lactate (62.5-118.4 mmol/hour), and urea-N (-19.9 to -51.4 mmol/hour). Net ammonia-N absorption increased linearly (P less than 0.10) from 35.6 to 43.8 mmol/hour as ME intake increased. Net glucose absorption was negative at low intake, but tended to increase to positive values as intake increased. Moles of acetate (46.3), propionate (29.3) or 4-carbon VFA (6.3) per 100 mol of net organic acid (VFA plus L-lactate) absorbed were similar among intake levels. Intake level did not affect proportions of DE intake (0.43) or calculated ME intake (0.46) accounted for by net organic acid absorption.