Abstract. The aim of the study was to assess the impact of varied levels of milk protein and urea content on selected milk production traits of Polish Holstein-Friesian cows. The research included 34,397 data sets related to daily milk production of Polish Holstein-Friesian cows. These animals were kept in 15 cattle herds located in the Mazovian and Podlasie voivodeships. In the first stage of the research, the animals were differentiated based on the level of urea in their milk and the percentage of protein. According to the adopted assumptions, the energy and protein balance level of feed rations was determined based on the concentration of urea and protein in the milk. Due to the level of urea, three cow groups were distinguished: <150, 150–250, and >250 mg · L-1. Depending on the concentration of protein in the milk, the following three groups of animals were determined: <3.2%, 3.2–3.6%, and >3.6%. The assessment of the impact of varied energy and protein balance levels in feed rations on selected milk production traits of cows was conducted in subsequent lactation periods, production level and fat to protein (F/P) groups. Four lactation periods were distinguished, which covered successive months of lactation: 1–3, 4–6, 7–10, and 11–18, three production groups, determined based on the following levels of daily milk yield: <20, 20–30, and >30 kg and as well three groups of cows with different F/P ratio in milk were distinguished: <1.2, 1.2–1.6, >1.6. In the next stage of the study, a detailed assessment was conducted on the impact of varied levels of energy and protein balance in feed rations on the analyzed milk production traits. The analyzed population of cows was divided into 9 groups based on the level of urea content (mg · L-1) and protein concentration (%), with varying levels of coverage of the protein and energy requirements of cows. The interdependencies between milk performance traits of cows fed diets with balanced levels of energy and protein and their peers, for whom the nutritional diets were differentiated based on energy-protein balance, were determined using Pearson correlation coefficients and linear regression. When assessing the energy supply of cows, the study showed that too low energy levels in diets lead to a decrease in the chemical composition of milk, while too high levels lead to a decrease in milk yield compared to peers fed optimally balanced diets. Milk from cows with inadequate energy and protein nutrition contained less fat and protein by 0.54% and 0.59%, respectively, compared to milk from cows fed optimally. On the other hand, excessive energy supply was accompanied by an increase in fat (+0.66%) and protein (+0.57%) concentration with a decrease in daily milk yield (-4,1 kg). When evaluating the level of protein coverage for cows, the study showed that an excess of protein in feed led to an increase in urea levels in milk. The level of urea in milk from cows fed with excess protein in their feed (>250 mg · L-1 and <3.2%; >250 mg · L-1 and >3.6%; >250 mg · L-1 and 3.2–3.6%) contained more urea per liter compared to milk from cows fed a balanced diet, with increases of 127, 107, and 109 mg, respectively.
