The effects of somatic cell count and stage of lactation on the yield and quality of Cheddar cheese were investigated. Cheese was manufactured in a pilotscale factory using milk of low bulk milk cell count (BMCC) from herds in early (LE) and late (LL) lactation, and milk of high BMCC from herds in early (HE) and late (HL) lactation. The deleterious effect of an elevated BMCC on product yield and quality in late lactation was clear. Cheese made from LL milk was significantly superior to that made from HL milk for most yield and quality characteristics measured.Stage of lactation also affected cheese yield and quality, as evidenced by the lower recovery of fat and poorer flavour score for cheese from LL milk compared with that manufactured from LE milk. The observed differences could be explained largely by differences in raw milk composition. We conclude that the effect of stage of lactation was magnified by an elevated BMCC, and that many of the problems encountered when processing late season milk could be overcome by containing mastitis at this time.Bulk milk cell count (BMCC) is a measure of the number of somatic cells in bulk milk and is a commonly used and reliable indicator of the incidence of subclinicalmastitis within a dairy herd (Eberhart et al. 1982). Such subclinical mastitis is usually caused by pathogenic bacteria colonizing the teat cistern (Anon. 1971), and the resulting inflammation can have marked effects on gross milk composition. Damage sustained by the mammary epithelium as a result of mastitis leads to the reduced synthesis and secretion of milk components synthesized de novo. At the same time, the tight junctions between the mammary epithelial cells often rupture, allowing the free influx of serum components from the extracellular fluid into the milk (Kitchen, 1981). These components include a range of hydrolytic enzymes which further modify milk composition via the breakdown of casein and fat (Grieve & Kitchen, 1985). Thus, during mastitis there are generally decreases in concentrations of milk fat, lactose and casein, and increases in concentrations of whey proteins (Auldist et al. 1995).The precise effects of BMCC on the yield and quality of dairy products are less well known. Some researchers have observed the poor coagulating properties of t Present address and correspondence: Dairying

Variations in milk composition during lactation were studied in normal and mastitic dairy cows from 2 seasonally calving herds in northern Victoria. Milk yields, somatic cell counts (SCC), and concentrations of lactose, fat, total protein (TP), casein protein (CP), non-casein protein (NCP), sodium, potassium, and calcium varied considerably as lactation progressed. The basic patterns of change over lactation were similar whether cows were healthy or mastitic (SCC >3 x 105 cells/mL milk). Milk from mastitic cows had higher concentrations of TP, NCP, and sodium, but lower concentrations of fat, lactose, CP, and potassium, than milk from healthy cows, although differences were not consistently significant. The activity of the milk protease, plasmin, was positively correlated with log10SCC in both herds.

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Responses to exogenous growth hormone were measured in lactating dairy cows surgically prepared to allow measurement of nutrient exchanges across mammary and hind-limb muscle tissues. Cows were injected daily with either saline or growth hormone, at a dose of 0·1 mg/kg liveweight, over periods of 6 days.During administration of growth hormone milk yield, milk fat content and yields of milk fat protein and lactose increased. Arterial plasma concentrations of glucose and non-esterified fatty acids were increased, uptake of glucose by leg muscle tissue decreased, lactate release from leg muscle tended to increase, mammary uptake of non-esterified fatty acids increased, blood flow to leg muscle tended to increase and blood flow to mammary tissue increased during injection of growth hormone.The results show that growth hormone affects supply to and utilization of key nutrients by tissues, resulting in the supply to the mammary gland of additional precursors for milk synthesis.

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