Influence of chosen factors on milk nutritional value

The aim of the work was to evaluate the influence of chosen factors, i.e. cow's geno-type, year seasons (feeding system), age (following lactation) and lactation phase on the nutritional value of milk. The milk chemical composition was analyzed in six genetic groups of cows (in total, 2227 samples) maintained in the Lubelszczyzna Region and Bieszczady Mountains. It was found out that milk obtained from the Simmental cows had the most beneficial chemical composition. Those cows' milk also showed the highest protein-fat ratio (0.89%). In bovine milk from the autumn-winter season higher fat (4.51%) and protein concentration (3.68%), including casein (2.69%), was observed. However, in milk from spring-summer season more beneficial protein and fat proportion (0.85%) was noticed. With the progressing lactation, the concentration of fat (from 4.27% to 4.53%), protein (from 3.47% to 3.78%) casein (from 2.50% to 2.82%) and dry matter (from 13.22 to 13.67%) increased. Moreover, the protein-fat ratio increased (from 0.82 to 0.84) as well. The evaluation of the considered genetic-environmental interactions showed substantial variations (P ≤ 0.01) in the breed and production season array for all analyzed milk characteristices, besides the casein content. In the arrangement of simultaneous influence of the four factors, i.e. breed and production season and lactation phase and the following lactation, no essential interactions were stated, apart from the lactose content (P ≤ 0.01).