Heat stress in summer alters mineral needs in ruminants. The requirements of major minerals (Ca, P, Mg, Na and K) and trace elements (Fe, Zn, Cu, Mn, Se, Mo and Co) in dairy cows under high temperature conditions were evaluated based on balance and feeding trials. Mineral requirements for maintenance in dairy cows increased at high temperatures, since mineral excretion during fasting was enhanced above 27°C. Calcium, P, Mg, and Na contents in milk decreased at high temperatures, while the contents of other elements were not affected by heat stress.
Calcium and P balances in lactating cows were negative when milk production was high under optimum temperature conditions. Major mineral and trace element requirements in lactating cows under optimum temperature conditions which were calculated from 24 balance trials at 18°C were almost similar to the estimates of NRC standard, except for the Ca, Mn and Co requirements. Since the intake and apparent absorption of major mineral and trace elements in lactating cows were depressed by heat stress, it is suggested that the mineral requirements in lactating cows increase under high temperature conditions.