Impact of dietary amino acid supply on milk urea and urine nitrogen excretion and energy corrected milk of dairy cows fed hay-based diets

This study aims to investigate the impact of amino acids supply from hay-based diets on lactating dairy cows (LC) N excretion and energy corrected milk ECM (lNEx or hNEx). Twenty herds (184±89 cows; 178±19 DIM; 678±33 BW) were enrolled in the study. Feed intake was measured in each farm during 2 consecutive days by difference between the TMR offered and refused. Urine samples were collected twice from 10% of the LC. Milk yield was recorded, and 2 bulk milk samples were collected at 12h intervals. The BW was measured on 20% of the LC. The urinary creatinine content was determined through the Jaffe colorimetric method and used to calculate urinary output. Urine nitrogen (UN) and Milk Urea Nitrogen (MUN) were determined by Dumas and Milkoscan respectively and summed for NEx calculation. The dietary aminoacid (AA) content was estimated using the internal standard method (Trp), and an external calibration curve (all other AA) after analysis of the samples through RP-UPLC/ESI-MS in the SIR scan mode. The herds were categorized as low or high NI (<600, 601 to 650 and >650 g/d respectively) and, within each intake category, as lNEx or hNEx (respectively the lower and higher 50% of the data). The NEx was reduced by the increase of dietary serine (y = -100.29x +530.46; R2=0.155), threonine (y = -113.63x+529.88; R2=0.148) and tyrosine (y = -70.137x+360.95; R2=0.127), while it was increased by the increment of dietary lysine (y = 66.179x+104.15, R2=0.178). The ECM was increased when dietary arginine increased from 2.5 to 4.0 g/MCal (y=2.0579x+26.819; R2=0.130). The lNEx herds received higher content of Met (0.26±0.03 vs 0.24±0.02% DM; P=0.040) and Ser (0.66±0.07 vs 0.61±0.04% DM; P=0.034), and a trend for a higher content of Ala (0.067±0.04 vs 0.64±0.05% DM; P=0.058), Thr (0.57±0.06 vs 0.54±0.03% DM; P=0.090), and total AA (12.08±0.87 vs 11.40±1.01% DM; P=0.061). These differences disappeared when the amount of amino acids administered were expressed on an energy base (g/Mcal). The NEx from LC fed hay-based diets seems to be reduced by the administration of specific amino acids and the role of dietary energy should be considered in modulating NEx. Research supported by the European Union’s H2020 research and innovation program, G.A. No 777974.