The effect of genetic, lactation, nutrition, seasonal and other factors on the variability of milk, fat and protein production and fat to protein content ratio was investigated in a herd of adult dairy cows reared in a milk record system. The herd was kept on a farm at Oponice, belonging to a School Agricultural Enterprise (2,505 ha) of the University of Agriculture at Nitra. Dairy cows were crossbreds of the Slovak Pied breed with the Black-Pied Holstein-Friesian Lowland breed (n=59) and of the Slovak Pied breed with the Red-Pied breed of Canadian origin (n = 190). Basal feed ration (BFR) in the winter season: quality class I and II maize silage of the average dry matter content 24.3 % and quality class Il beet top silage of the dry matter content 22.1 %. BFR in the summer season: green fodder obtained from a two-crop system practices in a green belt. Supplemental feeds: DOP, and/or DOG concentrate mixtures for dairy cows, balancing granular mixture containing 35 % chopped straw, 30 % wheat meal, 28 % lucerne dry products and 7 % molasses, malt combs, maize cob silage, etc. (Tab. I). Ten types of feed rations consisting of 60 to 80 % of bulk feeds and 20 to 40 % concentrates in dry matter were fed to dairy cows. Energy and nitrogen balance was mostly at a standard level with a small surplus in the summer season. Minerals were supplied through mineral supplements. Vitamins A and D were administered in the period of dairy cow physiological loading. No statistically significant differences were recorded between the sets of dairy cows in average dally milk output (16.0 kg and 15.5 kg per lactation, resp.), fat content (4.08 % and 4.05 %, resp.) and protein content (3.47 % and 3.44 %, resp.). The fat to protein content ratio (F/P) was considerably variable in the particular calendar months (1.06 to 1.24 and 1.09 to 1.24, resp.) - Tabs. II and Ill. Dairy cows fed BFR containing maize- silage and beet top silage produced milk with very high and high fat contents, with intermediate and lower protein contents and with very variable and variable F/B ratios (1.20 to 1.24). The effect of season on protein content was negative, but this did not apply to fat content in March and April (it might be explained from the lower quality of crude protein). Dairy cows fed BFR produced milk with high and intermediate fat and protein contents and with the F/P ratio 1.13 to 1.18 particularly in the summer season in comparison with the winter season (for five or eight months). Agabriel et al. (1991) report that milk with this F/P ratio has the higher protein content and better technological quality for cheese-making. A low fat content and high protein content with a narrow F/P ratio 1.06 to 1.10 were found in milk in summer and in autumn for five months, and/or one month. The higher proportion of lupine, green lucerne and concentrates in feed rations in June to August prevented a seasonal decrease in fat and protein content in milk. The feed ration formula in September resulted in a more marked decrease in fat content and a less marked decrease in protein content in milk. The wide F/P content ratio over 305-day lactation (1.21) was recorded as a result of the decrease in protein content (3.30 and 3.36 %) in milk due to a great reduction in concentrate ration in the third part of lactation (Tab. IV). The calving time does not represent an independent factor of changes in milk production, fat and protein content. Calving in spring and summer increased milk output most probably as a result of green fodder administration, fat and protein contents in winter and summer were increased by the higher quality of bulk feeds (Tab. V). Permanent milk recording in dairy cows, checks of feed quality and feed ration formulae enabled to analyze the great variability of milk composition and F/P ratio in individual dairy cows and for the different calendar months. The value of the F/P ratio 1.20 to 1.24 seems to be determined by the level of protein content in milk, the value of the F/P ratio 1.13 to 1.18 determines the level of fat content in milk, and the value of the F/P ratio 1.06 to 1.10 changes in dependence on the level of both components in milk. Interactions of the investigated factors were responsible for great changes in the F/P ratio (1.06 to 1.24) in milk in the particular calendar months; nutrition factors were found to play a decisive role. Different kinds and proportions of feeds with specific characteristics used in feed rations influenced the fermentation quality in rumen and digestive and metabolic mechanisms. This contributed to the production of milk of a variable composition on the one hand, but with high fat and protein contents on the other. The results of this investigation were obtained in favourable conditions with respect to the herd of crossbred dairy cows, their health condition, feed kind and quality, nutrition level, environment, method and utilization.
