The experiment was designed to investigate the effect of increasing the concentration of Cu2+ in the diet of the lactating dairy cow with a view to stimulating the activity of the mammary gland △9–desaturase enzyme system to increase the C18:1/18:0 ratio in the milk fat.
In a preliminary study, two lactating dairy cows were provided with ground rapeseed/maizebased concentrates containing either a normal or a high concentration of supplementary Cu2+, the animal offered the normal concentration supplement was also provided with an oral CuSO4 solution drench so that both animals received an equal daily supply of Cu2+. Daily blood samples were collected from the cows and total plasma Cu2+ concentration was determined. A blood plasma lipid analysis was also carried out. In a second study, eight lactating dairy cows were provided with a silage and concentrate diet. The lipid portion of the concentrates was provided by either ground or unground rapeseed while the Cu2+ in the mineral supplement of the unground rapeseed-based concentrate was manipulated to provide one of the following three concentrations of Cu2+ in the concentrate (mg/kg DM); <10, 50 or 100. The subsequent milk fat was analysed for fatty acid composition.
Presentation of an increased concentration of Cu2+ to the cow was more efficiently absorbed when provided in the concentrate mix than when provided as a CuSO4 solution in an oral drench. Increased dietary Cu2+ in the concentrates was successful at increasing the concentration of plasma Cu2+ from a normal concentration of 80–100 mg/cm3 to over 200 mg/cm3. An increased concentration of Cu2+ in the blood of the cows in the first experiment resulted in a significant increase (P<0·05) in the C18:1 content of the plasma lipid, possibly through the increased activity of the intestinal △9–desaturase enzyme system.
The composition of the milk, including the fatty acid composition of the milk fat, from the second experiment was unaffected by the concentration of Cu2+ in the diet of the dairy cows within the concentration range of <10 to 35 mg/kg DM per day.