In five herds of purebred Holstein dairy cows, altogether 220 bulk milk samples were collected with the following average parameters: rennet coagulation time (RCT) 265.2 sec., titratable acidity 6.40 °SH, active acidity 6.72 pH, specific density 1.0281 kg.l−1, fat content 3.81 g.100g−1, protein content 3.28 g.100g−1, lactose content 4.72 g.100g−1, and content of solids non-fat content 8.89 g.100g−1; average values of milk batch volume per herd and diurnal air temperature were 6,072 kg and 8.02 °C, respectively. It was found out that the variable “season“ (spring, summer, autumn or winter) significantly influenced nearly all parameters under study (the only exceptions were values of titratable acidity and milk batch volume). As compared with all other seasons, the significantly shortest RCT was recorded in summer. Further it was also found out that the variable “herd” influenced all parameters with the exception of average diurnal air temperature. Regardless to differences existing among individual herds, the shortest average RCT was recorded always in summer.

CHLáDEK, G., ČEJNA,V.: The relationship between freezing point of milk and milk components and its changes during lactation in Czech Pied and Holstein cows. Acta univ. agric. et silvic. Mendel. Brun., 2005, LIII, No. 5, pp. 63-70 The freezing point of milk (FPM) is an instant indicator of violated technological quality of raw milk, especially of dilution. FPM can also vary due to numerous effects associated with changes in milk composition and milk characteristics. Beside the effect of season, phase of lactation, breed, milk yield, subclinical mastitis etc. the impacts of nutrition and dietary or metabolic disorders are the most significant and the most frequent (GAJDŮŠEK, 2003). FPM is a relatively stable physical characteristic and due to osmotically active elements it ranges from -0.510 to -0.535 °C (HANUŠ et al., 2003b). Recently ŠUSTOVá (2001) studied the freezing point of milk in pool samples; she observed seasonal changes in FPM of mixed milk and the effect of different diets on FPM values. KOLOŠTA (2003) looked into the effect of grazing season on FPM. HANUŠ et al. (2003a) analysed possible effects of handling of milk components on FPM. The aim of this work was to describe the relationship between FPM and milk components and the impact of breed, number and phase of lactation on FPM. We analysed 328 milk samples in total, out of which 137 samples were of Czech Pied cows and 191 samples of Holstein cows. The effect of number and phase of lactation was evaluated for both breeds together. The greatest coefficients of correlation in total were found between FPM and lactose content (r = 0.600) and solids non fat (r = 0.523). Lower coefficients of correlation were found between FPM and milk fat content (r = 0.235), milk protein content (r = 0.260) and urea concentration (r = 0.256). These coefficients were considerably lower in Holstein cows than in Czech Pied cows. The coefficients of correlation between FPM and number and phase of lactation and somatic cells count were insignificant. The total mean value of FPM was -0.534 °C. Breed statistically significantly (P<0.01) affected FPM (+0.006 °C in C breed) and milk fat content (+0.19 % in H breed). Breed highly significantly (P<0.001) affected daily milk yield (+4.9 kg milk in H), milk protein content (+0.27 % in C) and solids non fat (+0.37 % in C). On the contrary, breed had no significant effect on lactose content, urea concentration and somatic cells count. Variability of FPM was greater in Czech Pied cows (5.9 %) than in Holstein cows (0.9 %). Number of lactation had no significant effect on FPM (maximum difference between lactations was 0.008 °C). Phase of lactation had no significant effect on FPM either. Our study revealed the fact that FPM was most of all affected by lactose content and solids non fat. However, the decrease of lactose content was compensated by a tendency of mammary glad to keep Ročník LIII 7 Číslo 5, 2005 63