Concentrations and Flow of Free Amino Acids in Ruminal and Duodenal Liquid of Dairy Cows in Relation to Feed Composition, Time of Feeding and Level of Feed Intake.

The main purpose of this study was to examine sources of variation in free amino acids (FAA) in the rumen and their relative contribution to the total duodenal amino acid supply. Six cannulated lactating dairy cows fed three concentrate mixtures differing in ruminally degradable intake protein (101, 114 and 134 g kg[sup -1] dry matter) at two levels of intake (19.3 vs 9.8 kg dry matter day[sup -1]) were used. The mean sums of the prefeeding concentrations of 15 FAA in ruminal liquid were 589 and 515 μM at a high and low feeding level, respectively. One hour postfeeding the corresponding values were 2212 and 1097 μM. Free amino acid profiles in ruminal liquid were significantly correlated to dietary amino acid (AA) profiles. The contribution to the total duodenal AA supply from the ruminal outflow of FAA was estimated for mean sums of FAA and for Lys, Met, Thr and His. Less than 2% of the total duodenal AA flow was contributed by passage of ruminal FAA. It is concluded that the supply of FAA to the small intestine is insignificant even in high-yielding cows with a high ruminal liquid passage rate. Intraruminal supplementation of mixtures of free Lys, Met and Thr (100, 200, 300 and 400 mmol of each) caused (P < B0.05) a transient increase in the sums of the net concentrations of the 12 other AA in ruminal liquid 1 h after dosage, with values of 205, 546, 641 and 945 μM, respectively. [ABSTRACT FROM AUTHOR]