SEASONAL-CHANGES OF PROTEIN SUBSTANCES AN D PROPERTIES OF COWS MILK

69 milk cows on the first lactation, non-selected crossbreds of the Bo hemian Pied and milk breeds calving from January to February, were inv estigated in the first 100 days of lactation in 14-day intervals for c omposition of milk from the morning milking. The content of net protei ns, casein, whey proteins, non-protein nitrogen (NPN) and urea was det ermined. From the values assessed, the percentage of casein from net p roteins and the percentage of urea nitrogen from NPN was calculated. F urther, pH, titrable acidity and the time of milk coagulation by renne t at the temperature of 35-degrees-C were observed. After a one-hour i ncubation of milk treated with rennet at the temperature of 35-degrees -C, the quality of curd was evaluated and the titrable acidity of sepa rate whey assessed. The bulk component of feed ration (KD) was supplie d by maize silage in winter and by maize silage and green fodder in su mmer. The basic feed ration comprised further and equalizing mixture c ontaining dry fodder and concentrates. A production feed ration given. according to milk performance was made out of a complete concentrate m ixture. The transition to a summer feed ration was gradual during one week (beginning on 15th May, the 135th day from the beginning of the y ear). The season and nutrition affected the milk composition and prope rties more than the stage of lactation. That is why the results of ana lyses were evaluated in dependence on the season. In spring, net prote ins and casein in milk decrease and whey proteins slightly increase. D ue to these changes, the percentage of casein from total proteins decr eases. After the transition to green fodder, the percentage of casein from net proteins decreased even more (Tab. 1). NPN content does not c hange markedly during the spring months. After the transition to the s ummer feed ration in spite o f a gradual habituation of cows, a transi tional increase of urea content in milk was found. The share of urea n itrogen from the total NPN content increased from the original 30 % to 65 % on the 145th day from the beginning of the year (Tab. II). The c hanges in milk composition mentioned display also in changed milk prop erties; titrable acidity and milk pH decrease. Titrable acidity of whe y increased slightly due to the increased content of whey proteins. Af ter the transition to the summer feed ration, the decrease in titrable acidity only transitionally stopped and at the same time, a transitio nal increase in pH occurred. The changes in protein representation and milk acidity reflected in a transitional prolongation of the time of milk coagulation by rennet and in a decreased quality of curd (Tab. II I). It can be followed from results obtained that in the spring months , changes in the representation of both proteins and NPN in milk occur even with an optimum feed ration which is due to decreasing quality o f feeds. Milk properties change in this period as well. In spite of a good level of nutrition of milk cows in the agricultural enterprise ob served, the transition to the summer feed ration reflected in the chan ged milk composition and properties.