Effects of postrumen starch infusion on milk production and energy metabolism in dairy cows

Two experiments were conducted to determine effects of postrumen starch inf usion on milk production and energy and nitrogen utilization in lactating d airy cows. In experiment 1, four cows in early lactation fed grass silage a nd concentrates were continuously infused into the duodenum with water or 7 00, 1400, or 2100 g of purified maize starch daily for 10 to 12 d in a 4 x 4 Latin square design with 2-wk periods. Starch infusion increased milk yie ld linearly and decreased milk fat concentration in a quadratic manner such that increases in fat-corrected milk and calculated milk energy yield were minimal except at the highest rate of infusion. Changes in milk energy out put suggest that even at the highest infusion rate metabolizable energy sup plied by infused starch was used for tissue energy or oxidized. In experime nt 2 energy and nitrogen balance were measured in four cows in late lactati on fed a mixture of dehydrated lucerne, grass silage, and concentrates duri ng the last 6 d of 2-wk abomasal infusions of 1200 g of purified wheat star ch daily or water in a balanced switchback design with 5-wk periods. Measur ements of fecal starch concentration indicated nearly all the starch infuse d was digested, but decreased fecal pH and apparent nitrogen digestion sugg ested an increase in hindgut starch fermentation. Starch infusion decreased urine nitrogen output in part because of increased tissue nitrogen retenti on but had no effect on milk nitrogen output. In absolute terms, numerical decreases in feed energy intake and energy digestion reduced the recovery o f starch energy infused as digestible and metabolizable energy, but in term s of changes in total energy supply with starch infusion, 79% was recovered as metabolizable energy. Starch infusion had no effects on heat or milk en ergy but increased net energy for lactation due to a numerical increase in tissue energy, implying that in late-lactation cows, starch digested postru minally was used with high efficiency for tissue energy retention as protei n and fat.