Effect of feeding vitamins and trace minerals to dairy cows after long-term non-supplementation of these nutrients

Dairy cows are usually fed amounts of vitamins and trace minerals that largely exceed their minimum requirements. This study was conducted at the experimental dairy farm of the University of Bologna and was aimed at evaluating the effect of feeding a vitamin and trace mineral mix to dairy cows that had previously not received any supplementation of these nutrients. Thirty Holstein dairy cows were fed for 8 months a total mixed ratio with no vitamin and trace mineral supplementation. After this period, animals were divided into two groups of 15 cows each (homogenous for milk yield, days in milk and age) receiving for 60 days a total mixed ratio (12.2 kg grass hay, 2.8 kg dehydrated alfalfa, 8.4 kg corn meal, 2.2 kg soybean meal, 0.9 kg molasses, 0.2 sodium bicarbonate, 0.1 kg sodium chloride) added or not (control) with a vitamin and trace mineral mix (VTM mix). Control diet met dairy cow minimum requirements for manganese and iron but covered only 50 and 40% of zinc and copper requirements (NRC, 2001), respectively; conversely, all trace mineral requirements were met in cows receiving the VTM mix. During the whole experimental period, compared with data previously collected at the same farm, incidence of health events was not increased by not supplementing cows with vitamins and trace minerals. Feeding the VTM mix did not influence milk yield and composition. Similarly, plasma total protein, albumin, globulins, lysozyme and haptoglobin concentrations were not affected by diet. After 30 days, haemolytic complement activity was higher (50.0 vs. 42.8 C’H50/150 µl; P<0,05) in cows fed VTM mix. Oxidative stress as well as the number of leukocytes in blood did not differ between treatments. Plasma levels of copper and zinc were not influenced by treatment but, after 60 days, plasma iron levels were higher in cows fed VTM mix (28.9 vs. 24.7 µmol/l; P<0.05). These data seem to suggest that feeding high levels of vitamins and trace minerals to lactating dairy cows might have only minor influence on animal productivity and immune status.