Amino acid profile of metabolisable protein in lactating dairy cows is affected by dry matter concentration in grass-clover silage

Our previous study showed that supply of metabolisable protein (MP) to lactating dairy cows increased with increasing dry matter (DM) concentration in grass-clover silage. The aim of this study was to examine how amino acid (AA) profile of MP was affected by silage DM concentration. Eight grass-clover silages with DM concentrations ranging from 283 to 725 g/kg were fed ad libitum to four multi-fistulated dairy cows in an incomplete balanced scheme over five periods. Individual AA were analysed in silages, in microbes isolated from the rumen, and in duodenal and ileal chyme, respectively. Proportion of arginine in silage AA increased, proportions of cysteine and proline in silage AA tended to increase, whereas proportions of alanine, histidine, isoleucine, lysine, methionine, ornithine, serine, threonine and valine in silage AA decreased with increased silage DM concentration. Crude protein (CP) and AA concentrations in DM and AA concentration in CP in microbial matter were not affected by silage DM concentration, but serine proportion in microbial AA increased and valine proportion in microbial AA decreased with increased silage DM concentration. In total duodenal AA profile, histidine proportion decreased, lysine proportion tended to decrease and glutamate proportion tended to increase with increased silage DM concentration, mostly driven by changes in duodenal feed + endogenous AA profile. Small intestinal digestibility and absolute amount digested of all individual AA increased, either numerically or significantly, with increased silage DM concentration. However, lysine proportion decreased, histidine proportion tended to decrease and glutamate proportion tended to increase in digested AA. The higher supply of MP with increased silage DM concentration would be expected to increase milk production, but the unfavourable effect on AA profile will probably diminish this potential increase, as either histidine or lysine often are the first limiting AA in grass based diets.