Effects of increasing ruminally degraded nitrogen and abomasal casein infusion on net portal flux of nutrients in yearling heifers consuming a high-grain diet

Seven Meat Animal Research Center (MARC) III heifers (410 +/- 25 kg) fitted with hepatic portal, mesenteric venous, carotid catheters, and an abomasal cannula were used in a 7 x 5 incomplete Latin square design experiment. Th e objective was to evaluate the effects of increasing levels of ruminally d egradable N (RDN) with or without the addition of abomasally infused casein on portal-drained visceral (PDV) flux of nutrients. Treatments consisted o f dietary CP percentage levels of 9.5 (control), control plus .72% dietary urea (11.5U), control plus 1.44% dietary urea (13.5U), control plus abomasa lly infused casein (250 g/d; 11.5C), or control plus .72% dietary urea and abomasally infused casein (250 g/d; 13.5UC). All diets contained (DM basis) 80% ground corn, 15% corn silage, and 5% dry supplement and were provided for ad libitum consumption. Nitrogen intake increased (linear, P < .001) as CP increased from 9.5 to 13.5%. Portal-drained visceral release of ammonia N increased (linear, P < .10) as RDN increased, and was greater (P < .05) when protein was fed compared with heifers fed control (P < .10). Urea N re moval by PDV was not affected (P > .10) by level of RDN but was greatest wh en 11.5C was fed and least when 13.5UC was fed. Net or-amino N (AAN) releas e by PDV was greatest when 13.5UC was fed (309 mmol/h), least when 9.5% CP was fed (112 mmol/h), and intermediate for the other groups (205 to 252 mmo l/h). These data suggest that removal of N by the PDV may promote microbial protein synthesis when dietary RDN is low. When RDN needs have been met an d amino acids are deficient for the host, escape protein should be fed to i ncrease amino acid absorption.