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1 Department of Animal Sciences, University of Arizona, Tucson 85721
2 University Animal Care, University of Arizona
3 Department of Veterinary Sciences, University of Arizona
4 Department of Animal and Poultry Sciences, Virginia Polytechnic Institute, Blacksburg, 24061
5 Department of Animal Science, Hebrew University, Rehovot, Israel 76100
Objectives were to determine net release or uptake of 
-amino N, ammonia N, and urea N across portal-drained viscera, liver, splanchnic, and mammary tissues of lactating Holstein cows (n = 6; 109 ± 9 d in milk) fed alfalfa hay-based total mixed rations (TMR) containing 40% steam-rolled or steam-flaked corn grain. The TMR were offered at 12-h intervals in a crossover design. Blood samples were obtained from indwelling catheters in portal, hepatic, and mammary veins and mesenteric or costo abdominal arteries, every 2 h for each cow and diet. Steam-flaked compared with steam-rolled corn greatly increased in vitro starch hydrolysis (56 vs. 34%). Daily intake of dry matter (18.4 ± 0.4 kg/d), starch, N, and net energy for lactation by cows were not altered by processing corn; neither were daily yield of milk (29.1 ± 0.7 kg/d), fat-corrected milk, nor fat-corrected milk per dry matter intake. Steam-flaking tended to increase percent milk protein (2.97 vs. 2.82%; P = 0.07), but not yield, and decrease percent lactose (4.83 vs. 4.94) but not yield. Portal and hepatic blood flows were not affected by diet, nor were net absorption of -amino N and ammonia N. Steam-flaking compared with steam-rolling increased urea N cycling to portal-drained viscera (212 vs. 87 g/d) by 140%, estimated mammary uptake and extraction ratio of -amino N. Flaking versus rolling of corn improved N utilization in dairy cows by increasing urea cycling to the gut and uptake of -amino N by the mammary gland. Higher mammary uptake of -amino N (78 vs. 50 g/d) by dairy cows fed steam-flaked corn tended to increase milk protein content and may explain the previously observed effects of cows fed steam-flaked versus steam-rolled corn.
Key Words: nitrogen metabolism • dairy cows • flaked corn
Submitted on October 20, 2000
Accepted on August 14, 2001
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H. Tagari, K. Webb Jr., B. Theurer, T. Huber, D. DeYoung, P. Cuneo, J. E. P. Santos, J. Simas, M. Sadik, A. Alio, et al. Portal Drained Visceral Flux, Hepatic Metabolism, and Mammary Uptake of Free and Peptide-Bound Amino Acids and Milk Amino Acid Output in Dairy Cows Fed Diets Containing Corn Grain Steam Flaked at 360 or Steam Rolled at 490 g/L J Dairy Sci, February 1, 2004; 87(2): 413 - 430. [Abstract] [Full Text] [PDF]
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