The effect of protein nutrition on production, metabolic transformation of nitrogen and ecological load in dairy farming

The aim of the study is to evaluate the relation of protein nutrition to transformation of nitrogen (N) from feed to milk and its urinary excretion in dairy cows under the farm conditions. In breeding conditions of dairy farms in the groups of dairy cows in the 1st phase of lactation, with low content of CP (139 ±11.8 g/kg), optimum (161.8 ±6.3 g/kg) and high content of CP (179.7 ±8.5 g/kg) in dry matter of total mix ration (TMR), was confirmed the effect of increased the level of ammonia (NH3) in the rumen (17.4 ±1.7; 19.6 ±2.3; 22.8 ±3.4 mg/100 ml). The same tendency with higher statistical significance (P <0.001) was confirmed to the increased concentration of urea in the blood (22.8 ±3.7; 27.8 ±4.8; 35.1 ±6.6 mg/dl) and to the concentration of urea in the milk (18.2 ±2.6; 24.8 ±3.9; 33.3 ±6.7mg/dl). In the groups with increasing content of CP in the TMR, the ecological load of the environment increases, that was confirmed by a significant increase (P <0.001) of excreted N in the urine (139.4 ±19.8; 189.5 ±30.4; 254.7 ±51.3 g N/day) and an increase (P <0.001) in the emission of ammonia into the environment (68.1 ±6.1; 83.7 ±9.4; 103.8 ±15.9 g/day). The highest efficiency of N utilization at the level of 31.1 ±1.4% was confirmed at a CP content of 150–170 g/kg dry matter of TMR. The evaluation of direct regression dependence between the analysed CP content in TMR and analysed parameters of N transformation in individual farms was confirmed a significant relationship in CP and NH3 content (r = 0.641), CP and urea in the blood (r = 0.689), resp. CP and excretion of N in urine (r = 0.600). Direct relation was confirmed by the analysed NH3 content in the rumen and the urea content in the blood (r = 0.795), and urea in the milk (r = 0.786), as well as the urea content in the blood in relation to the urea in the milk (r = 0.767). The obtained results confirm the suitability of the use of the monitored markers to assess the level of protein nutrition and the environmental load at the farm.