We hypothesized that an increased uptake of ammonia from the gut of dairy cows would increase the use of amino acids for urea synthesis and decrease the availability of amino acids for milk protein production, leading to a reduction in milk protein output. To test this hypothesis, four multiparous Holstein-Friesian dairy cows offered a constant diet based on grass silage were continuously infused with ammonium acetate (ammonia) or acetic acid (control) into the duodenum for 5 d in each period of a changeover design experiment. Silage intake, rumen ammonia, pH and volatile fatty acid concentrations, and feces N excretion and whole body N balance were all unaffected by treatment, although urinary N excretion increased. Similarly, there was no effect of treatment on milk yields or on milk constituent yields. We concluded that ammonia absorption represents a loss of protein to the animal, but that it is unlikely to result in reduced amino acid availability for productive purposes such as lactation.
- ammonia, dairy cows, milk production, nitrogen balance