LAUSR

Abstract In the present study, the influence of mountain hay diets with elevated proportions of herbs and phenolic contents on nitrogen utilization and milk fatty acid profile of dairy cows was investigated. Two grass hay-based diets, either low or high in nitrogen (N) content (GN − , GN + ) and similar in fiber content and lignification, and two diets consisting of hays with high proportion of herbs (H) with either low or high phenolic (P) content (HP − , HP + ) were fed iso-energetically and without concentrate. All diets contained a Lolium perenne dominated basal hay and experimental hay in proportions of 0.45:0.55, except for diet GN + (0.80:0.20) with excessive N. Feed intake, milk yield and total amount of feces and urine were recorded and sampled from 24 multiparous mid-lactation cows (eight Brown Swiss, 16 Holstein) producing on average 33 kg/day of energy-corrected milk. The experiment was performed in three runs with two cows per diet. Data was analyzed by a general linear model considering diet and run as effects. Intake was highest in GN + and HP + and lower with GN − and HP − . No condensed tannins (CT) were detected in GN − and GN + . Intake of phenolic compounds (g/day per cow) was high in HP + (402) lower with HP − (302) and lowest with GN − and GN + (ca. 190). The intake of CT was higher in HP + (115 g/day) compared to HP − (31 g/day). Yield of milk and energy-corrected milk as well as gross milk constituents were not affected by diet. Apparent total tract nutrient digestibility was higher for the grass-based diets (GN − , GN + ) than for the diets with high herbal proportion (HP − , HP + ). With GN + , absolute urinary N losses and those in proportion of total excreta N were higher than in the other diets and was lowest with HP + . Utilization of N was lower with GN + and HP + compared to HP − . The milk fat of cows fed HP + had higher proportions of polyunsaturated fatty acids compared to that of GN − . The transfer rate of C18:3 n -3 from feed to milk was highest for the two herbal hay diets. The secretion of C18:1 t 11 in relation to the amount of C18:2 n -6 + C18:3 n -3 ingested was highest for diet HP − . In conclusion, mountain hay rich in herbs was found to be a dietary means to lower the N emission potential of the manure by lower urinary N excretion demonstrating that inclusion of herbs into grasslands may be beneficial.