Effects of the hot season on milk protein fractions in Holstein cows

Effects of the hot season on milk protein fractions were studied in 40 mid-lactating Holstein cows (141 67 days in milk). The trial was carried out on a commercial dairy herd located in Central Italy (42 o 15' North, 12 o 21' East, 227 m of altitude). Twenty cows were monitored for six weeks from March to April; the other 20 cows were monitored for six weeks from June to August. The two groups were balanced for parity, days in milk (DIM), genetic index for milk production, and frequency of -casein B and -lactoglobulin B variants. The diet for the two groups was a base-ration fed as a total mixed ration, plus concentrate given by self-feeders. During the two experimental periods, values of air temperatures and relative humidity were recorded daily and were used to calculate the temperature-humidity index; average feed intake was recorded daily, and rectal temperatures were measured weekly. Milk yield was recorded at both morning (06:00 h) and evening (18:00 h) milkings. Individual milk samples were taken weekly and analysed for concentration of crude protein content, casein number, and protein fractions. During the summer, cows had higher rectal temperatures but lower DMI and milk yield. Moreover, milk from cows in the summer had lower contents of crude proteins and caseins, lower casein number, and higher milk serum proteins. Among caseins, the -casein fraction did not differ between the two groups. Conversely, milk from summer cows showed lower contents of s -casein and -casein. The results from the present study indicate that the reduction of milk protein content observed in the summer was due to the reduction in the casein content, which was in turn caused by a reduction in s -casein and -casein contents. These changes might explain the alteration in cheesemaking properties of milk commonly observed during the summer.