Dry period length of dairy cows : milk composition and quality

A dry period of dairy cows is historically seen as a period during which the cow can restore its body condition and regenerate its mammary epithelium in order to be high yielding in the successive lactation. Recent work has indicated that high yielding cows generally experience a severe negative energy balance in early lactation. Dry period reduction is a strategy to improve the energy balance of dairy cows in early lactation. This thesis aimed at evaluating the influence of dry period length on milk composition and milk quality. Milk composition parameters indicate the processing quality of milk for the dairy plant, and may also reflect the physiological condition and energy status of the cow. In this work, omission of the dry period was related to a reduced β-casein fraction in early lactation milk. Applying a short (4 weeks) instead of a conventional (8 weeks) dry period resulted in increased plasmin activity in milk, but did not affect the β-casein fraction. Increased plasmin activity in relation to a shortened dry period was particularly found in milk of cows of third or higher parity, that generally had relatively high somatic cell count. At low somatic cell counts, dry period reduction or omission only tended to result in an increased plasmin activity due a higher casein concentration in milk. From increased concentrations of a number of low abundant proteins in colostrum of cows with a short dry period it was hypothesized that a short dry period was related to increased proliferation of mammary epithelial cells during the first days in lactation. Although the casein composition of milk was related to both plasmin activity in milk and the metabolic status of cows, which are both influenced by dry period length, quantitative differences in casein composition of cows with different dry period length were small. It was concluded that shortening or omitting the dry period of cows with good mammary health obtains milk with a higher protein content with little differences in protein composition.