Splanchnic and mammary nitrogen metabolism by dairy cows fed differently-processed sorghum and corn grain

Abstract

Objectives were to determine net uptake or release of α-amino N (AAN), ammonia N (NH₃N) and urea N (UN) across portal-drained viscera (PDV), liver, splanchnic (PDV + liver) and mammary tissues of lactating cows fed differently processed sorghum or corn. Cows were fitted with catheters in blood vessels (artery and portal, hepatic and mammary vein), and received in a switchback design TMR containing 40% dry-rolled (DR; 643 g/L or 50 lb/bu) or steam-flaked (SF; 360 g/L or 28 lb/bu) sorghum (Exp. 1), and steam-rolled (SR; 489 g/L or 38 lb/bu) or SF corn (Exp. 2). Neither daily intake of DM and nutrients nor milk yield and efficiency of milk production were altered by feeding processed sorghum or corn. Milk fat yield was lowered (P=.03) by SF vs. DR sorghum; SF vs SR of corn tended to increase (P=.07) concentration but not yield of milk protein. Blood flows in portal (1988 ± 109 L/h) and hepatic vein (2368 ± 215 L/h) were not altered by diets. Steam-flaked vs. DR sorghum tended (P=.18) to increase UN cycling (162 vs. 95 g/d), and reduced (P≤.09) absorption of AAN (223 vs. 269 g/d) and NH3N (257 vs. 318 g/d) by PDV. Flaking vs. rolling of sorghum increased (P≤.07) estimated uptake (83 vs. 67 g/d) and extraction ratio (14.5 vs. 11.1%) of AAN by mammary tissues. Net uptake and release of AAN and NH3N across splanchnic tissues of cows were not altered by corn processing; however, SF vs. SR corn increased (P=.08) UN cycling to PDV by 143% (212 vs. 87 g/d). Steam-flaked vs SR corn increased (P≤.02) estimated uptake (88 vs. 61 g/d) and extraction ratio (14.7 vs. 11.0%) of AAN by mammary tissues. It is concluded that feeding of steam-flaked sorghum or corn compared to dry-rolled sorghum or steam-rolled corn improves N utilization of dairy cows by increasing urea transfer to the gut and uptake of amino acid by the mammary gland.