Alfalfa hay (30% dry matter) was ensiled as untreated (AS) or treated with formic acid (ASF, 24 mL kg-1 DM) or formic acid+urea (ASFU, 24 mL and 4 g kg-1 DM, respectively). Disappearance of protein and dry matter from the samples was determined using the in situ mobile bag procedure. In lactation trail, 12 Holstein cows received diets containing 28.5% AS or ASF or ASFU, 30% corn silage, 7% alfalfa hay and 34.5% concentrate for 8 weeks. The chemically processed applied caused a significant decrease in ruminal and an increase in post-ruminal dry matter disappearances of the silages evaluated (p<0.05). Dry matter intake and yields of milk did not influenced by the experimental diets. Milk urea nitrogen concentration of the cows received the diet containing ASFU were significantly lower than the others (p<0.05). Blood plasma concentration of glucose was increased when the diet containing of ASF was fed to the animals (p<0.05) compared with the others.

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