Inter- and intra-individual differences in serum copper and zinc concentrations after feeding different amounts of copper and zinc over two lactations.

The influence of different copper and zinc contents in rations on blood serum concentrations was tested in a feeding trial at the Institute of Animal Nutrition of the Federal Research Institute for Animal Health (FLI). A total of 60 German Holstein breed cows (38 first lactating and 22 second lactating cows) were tested. All animals received a diet based on maize- and grass silage ad libitum. The animals were divided into two groups with 30 cows each. Group A received concentrates supplemented with copper and zinc as recommended, whereas Group B was offered a concentrate with roughly double the amount of copper and zinc. Blood samples were taken several times during the lactation. The concentrations of copper and zinc in the serum were measured and correlations between trace element intake and their serum concentrations were calculated. The mean dry matter intake was significantly higher for cows in the second lactation (19.1 kg/day) than for cows in the first lactation (16.4 kg/day), whereas the DM-intake was constant in the different feeding groups (17.3 and 17.4 kg/day). The correlation between feed intake in the first and the second lactation was r = 0.76. The copper concentration in the serum was not influenced by the different feedings (11.9 and 12.5 micromol/l), but high variations were found between the cows. The influence of the lactation number was not significant (12.1 and 12.4 micromol/l). In contrast to copper, the zinc concentration in the serum was significantly higher for Group B (14.0 micromol/l) than for Group A (12.2 micromol/l), respectively, for second lactating cows (13.8 micromol/l) in relation to 12.7 micromol/l for first lactating cows. The individual variation was also high as for copper. Neither the correlation between copper intake and copper serum concentration nor the correlation between zinc intake and zinc serum concentration was significant.