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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 = 8, 86 ± 8 d in milk) fed alfalfa hay-based total mixed rations containing 40% dry-rolled or steam-flaked sorghum grain. The total mixed rations were offered at 12-h intervals in a crossover design. Blood samples were obtained from indwelling catheters in the portal, hepatic, and mammary veins and mesenteric or costoabdominal arteries, every 2 h for each cow and diet. Steam-flaking increased in vitro rate of starch hydrolysis compared with dry-rolled sorghum (66 vs. 25%). Diet did not alter dry matter intake (18.2 ± 0.3 kg). Daily milk yield (27.6 ± 0.8 kg), efficiency of production, and most milk components did not differ between diets, but fat yield was reduced (0.86 vs. 0.91 kg/d) by steam-flaked sorghum, and lactose concentration was increased (4.99 vs. 4.82%). Blood flows in portal and hepatic veins did not differ between diets. Steam-flaking tended to increase urea N cycling to the gut (162 vs. 95 g/d) compared with dry-rolling of sorghum, whereas net absorption of ammonia N and -amino N across portal-drained viscera were decreased. Net mammary uptake of -amino N increased more than 20% (83 vs. 67 g/d), resulting in a higher mammary extraction ratio (15 vs. 11%) for steam-flaked versus dry-rolled sorghum. Flaking of sorghum improved the efficiency of postabsorptive N metabolism by increasing urea N cycled to the gut and -amino N uptake by the mammary gland.
Key Words: nitrogen metabolism • dairy cows • flaked sorghum
Submitted on October 20, 2000
Accepted on August 14, 2001
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