Objectives were to determine net release or uptake of alpha-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 alpha-amino N across portal-drained viscera were decreased. Net mammary uptake of a-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 alpha-amino N uptake by the mammary gland.
Objectives were to determine net release or uptake of a-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 alpha-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 alpha-amino N. Flaking versus rolling of corn improved N utilization in dairy cows by increasing urea cycling to the gut and uptake of a-amino N by the mammary gland. Higher mammary uptake of alpha-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.
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