Net Portal Fluxes of Nitrogen Metabolites in Holstein Steers Fed Diets Containing Different Dietary Ratios of Whole-crop Corn Silage and Alfalfa Hay

The objectives of the present study were to investigate the effects of different dietary ratios of whole-crop corn silage and alfalfa hay on nitrogen (N) digestion. duodenal flow and metabolism across the portal-drained viscera (PDV) of growing beef steers, and to elucidate their relationships. Four steers (236 +/- 7 kg BW) fitted with duodenal cannulae and chronic indwelling catheters into the portal and mesenteric veins and abdominal aorta were used in a 4x4 Latin square design, Animals were fed (at 12-h intervals) the 4 diets consisting of whole-crop corn silage (C) and alfalfa hay (A) in 80:20 (C8A2) 60:40 (C6A4) 40:60 (C4A6) and 20:80 (C2A8) ratios of which dietary crude protein (CP) was 10.5, 12.0. 13.5 and 15.0% of dry matter (DM), respectively Feeding level was restricted to 95% of ad libitum intake to measure N digestion. blood flow and net flux of N across the PDV. Digestibility of DM and neutral detergent fiber and digestible energy intake linearly increased as the ratio of alfalfa hay increased. The N intake, duodenal flow and intestinal disappearance increased linearly with increasing alfalfa hay. Arterial and portal concentrations of alpha-amino N showed a quadratic response to increasing levels of alfalfa hay and were the highest in steers fled the C6A4 diet. The net PDV release of alpha-amino N and ammonia N increased linearly with increasing alfalfa hay. but urea N uptake by PDV did not differ among diets. As a percentage of apparently digested N in the total gut. net PDV release of alpha-amino N linearly decreased from 66 to 48% with increasing alfalfa hay. Conversely. net PDV recovery of alpha-amino N to intestinal N disappearance varied with increasing alfalfa hay accounting for 49, 50, 58 and 61% on C8A2. C6A4. C4A6 and C2A8 diets, respectively. Net PDV uptake of urea N. relative to apparently digested N, linearly decreased from 81 to 25% as alfalfa hay increased from 20 to 80% of DM intake. Considering PDV uptake of urea N, microbial efficiency and conversion of total tract digested N to PDV alpha-amino N net supply, a diet consisting of 80% whole-crop corn silage and 20% alfalfa hay (10.5% CP) was the best. while considering the quantities of intestinal N disappearance and alpha-amino N absorption, a diet of 20% whole-crop corn silage and 80% alfalfa hay (15% CP) would be preferred. The proportion of alpha-amino N recovered by PDV relative to the intestinal N disappearance may vary, with energy intake level of mixed forage diets.