Evaluation of methods for estimating the amino acid supply to the duodenum of microbial, endogenous and undegraded feed protein on maize silage diets fed to dairy cows

The effect of stage of maturity of maize silage on protein metabolism in the gastrointestinal tract of cows was evaluated in an extended 3 × 3 Latin square experiment. Three multiparous lactating Danish Holstein–Friesian cows fitted with ruminal, duodenal and ileal cannulae were offered maize silage based diets supplemented with soybean meal and a grass-clover hay. The three treatments differed in the dry matter (DM) content of the maize silage, being 257, 350 or 403 g/kg. Rumen bacteria were harvested from mixed samples of rumen particles and fluid by sequential centrifugations. Protozoa were harvested by centrifugation after clearance of rumen fluid by flocculation. Synthesis of microbial amino acid nitrogen (AAN) was estimated by the internal markers diaminopimelic acid (DAPA), RNA and total and individual purine bases. Undegraded feed AAN was estimated by the in situ nylon bag method. Further, the origin of duodenal N flow was estimated by use of the amino acid profile (AAP) method. Average DMI was 15.9 kg/day. Average N and AAN intakes were 0.457 and 0.329 kg/day, respectively. The duodenal flow of N tended to increase from 0.484 to 0.581 kg/day with increased maturity of maize (P=0.08). Effective protein degradability (EPD) for the maize silage, the soybean meal and the grass-clover hay was 0.84, 0.71 and 0.57, respectively. Apparent differences were found in the bacterial and protozoal amino acid (AA) profiles (g AA/kg total AA), especially for Lys and Ala. Purine profiles (mmol purine/mol total purine) differed for three out of five purines between bacteria and protozoa. The internal RNA marker estimated a microbial AAN net synthesis of 0.165–0.204 kg/day. Using the AAP method to estimate the distribution of duodenal protein resulted in the fractions: microbial 0.56, endogenous 0.23 and undegraded feed AAN 0.21. It was not possible to make reasonable estimates using the purine profiles. Average microbial net synthesis of AAN estimated by AAP method (0.198 kg/day) and RNA marker method (0.184 kg/day), respectively, was comparable. A duodenal AAN flow of 0.112 kg/day of undegraded feed protein estimated by in situ degradation was 1.5 times higher than the 0.074 kg/day estimated from the AAP method. A substantial amount (0.081 kg/day) of endogenous AAN was estimated from the AAP method. The AAP method seemed reliable in this experiment for estimating the supply of protein to duodenum from a number of sources.