Freezing point of milk in a herd of high yielding dairy cows
Factors affecting the freezing point of milk in a herd of 200 Black and White cows with the average milk yield of 8 386 kg in the lactation and 8 328 kg in the standard lactation were examinated. Over the period of one year (2002) and based upon 1 773 individual monthly collected milk samples with the average contents of 3.91% fat, 3.26% protein, 4.54% lactose, 33.4 mg/100 ml urea and 331000 somatic cells per ml in milk were determined. The average freezing point of milk (n = 1 680) was
... 1 680) was estimated to be –0.527 ºC, with a range from -0.562 ºC to -0.423 ºC. In 210 (12.5%) samples was higher than -0.515 ºC. The lowest freezing point (-0.532 ºC) was found in the samples collected in the first month after calving and highest (-0.522 ºC) in the samples of 12th month of lactation. The differences between the freezing point of milk after the first and the second calving (-0.530 ºC; P < 0.05) and those after the fifth calving (-0.523 ºC; P < 0.05) were also significant. The samples collected in month from January to April (-0.538 ºC to -0.532 ºC) were significantly lower in comparison to samples collected in May and June (-0.517 ºC and -0.519 ºC). The following statistically significant correlation coefficients between cows' properties, milk composition and the freezing point of milk were found: month of lactation r = 0.233 (P < 0.001); lactation number r = 0.196 (P < 0.001); age of cows (years) r = 0.231 (P < 0.001); month of the year r = 0.0253 (P < 0.001); milk yield per milking day r = -0.106 (P < 0.001); fat corrected milk content (FCM) per milking day r = -0.234 (P < 0.001); lactose % r = -0.530 (P < 0.001); fat % r = -0.351 (P < 0.001); protein % r = 0.058 (P < 0.05); urea mg/100 mL r = 0.091 (P < 0.001) and somatic cell count r = 0.154 (P < 0.001). The sum of effects (month of the year, lactation lenght and fat, protein and lactose content of milk) was found to account for about 70% variability of the total depression of milk freezing point (R2 = 0.698).