Effects of grass silages with two levels of free amino acids on degradation of amino acids and fixation of nitrogen in bacterial protein in bovine ruminal fluid using the rumen simulation technique (Rusitec)

• Grass silages may contain different levels of free amino acids (FAA).
• High FAA levels increased ruminal production of branched-chain volatile fatty acids.
• This increased production indicated increased ruminal degradation of amino acids.

The objective of the present study was to evaluate if grass silages with high contents of free amino acids (FAA) lead to enhanced amino acid catabolism compared with grass silages with low contents of FAA when fermented in a rumen simulation system (Rusitec). Eight grass silages with contents of FAA lower than 300 mmol/kg dry matter (DM; GS-LOW) and eight grass silages with contents higher than 300 mmol/kg DM (GS-HIGH) were tested in vitro for their effects on selected variables of nitrogen metabolism. Ammonia concentration, production of the branched-chain volatile fatty acids (BCVFA) isobutyric and isovaleric acid, concentration of FAA in fermenter fluid, and the content of bacterial protein in fermenter fluid were measured daily from days 1 to 28 (ammonia, BCVFA), 3 and 6 to 28 (bacterial protein), and days 7, 8, 11 to 13, 16 to 21, and 26 to 28 (FAA), respectively. Subsequent to an initial 9-day adaptation period during which hay (10.5 g DM) and concentrate (3.0 g DM) were added to fermenters, grass silages (10.5 g DM) were individually incubated in the fermenters for a period of 10 days (experimental period). The experimental period was followed by the recovery period during which the additions of hay and concentrate were equal to those of the adaptation period. Addition of GS-HIGH increased the concentration of ammonia (P<0.05) and production of isobutyric (P<0.05) and isovaleric (P<0.05) acid in contrast to GS-LOW. The concentration of FAA, as well as the content of bacterial protein in fermenter fluid, showed no differences after addition of GS-LOW or GS-HIGH (P>0.05). The calculated efficiency of nitrogen fixation in bacterial protein was expressed as units of bacterial protein per g nitrogen input with the silages and was lower or tended to be lower, respectively, with GS-HIGH on 5 days of the experimental period (P<0.1). Increased concentration of ammonia and production of BCVFA indicate that amino acid catabolism was increased by additions of GS-HIGH compared to GS-LOW whereas efficiency of nitrogen fixation in bacterial protein seemed to be decreased. The observed effects may be due to higher metabolic activity of amino acid-fermenting bacteria.