UREA CONCENTRATIONS IN MILK AND THEIR REL ATIONSHIP TO THE CRUDE PROTEIN AND ENERGY RATIO IN FEED RATIONS

It is important to know the dietary nutrient ratio to provide for nutr ient intake in high-yielding dairy cows. High or on the contrary low i ntake of crude protein has adverse effects on the performance, health state and reproduction of dairy cows. Ide et al. (1966) demonstrated u rea concentrations in blood to depend on protein and energy intake. Re fsdal (1983), Oltner and Wiktorsson (1983) report on high values of co rrelation coefficients between urea concentrations in blood plasma and milk. Seven types of feed rations were successively administered to a group of 22 dairy cows (Tab. I); at the same time the intake of dry m atter, digestible crude protein and starch units and milk yield were d etermined. Protein content, milk fat content and urea concentration we re determined in milk. Tab. II shows the average intake of nutrients a nd protein content in milk and urea concentration in milk serum for th e different experiments. Variability of the evaluated parameters for t he whole group is reviewed in Tab. III, and the values of correlation coefficients are shown in Tab. IV. This table indicates the values of correlation coefficients between urea concentrations in milk and nitro gen efficiency ratio (NER) (r = 0.446), intake of digestible crude pro tein (r = 0.048) and starch units (r = -0.180). Fig. 1 shows a relatio nship between NER and urea concentrations, and the distribution of the values of measurements. The average value of urea concentration in th e whole group was 3.435 mmol/l. With the increasing performance of dai ry cows the urea concentration in milk serum rose by 0.026 mmol/l per 1 kg FCM milk. An evaluation of the experimental results shows that th e urea concentration in milk within the range of 3.691 to 4.109 mmol/l indicates a balanced feed ration. The urea concentration higher than 4.109 mmol/l is related to an increased NER value (dietary crude prote in surplus) while the urea concentration lower than 3.691 indicates a low NER value (dietary energy surplus).