Abstract
This study was conducted in order to evaluate the effects of a hot environment on the major mineral content in the milk of 20 Holstein lactating cows kept under natural conditions. Lactating cows were fed concentrate, beet pulp, corn silage, Italian rydgrass silage and hay. The major mineral intake of lactating cows was virtually adequate. Milk yield (23.2kg/day) in a hot environment (mean monthly temperature: 22.5-27.2°C) was larger than that (21.9kg/day) in an optimum environment (mean monthly temperature: 5.1-20.1°C). However Ca, P, Mg, and Na content in milk in a hot environment was less than in an optimum environment, and the ratios of Ca, P, Mg, and Na content in milk in a hot environment compared to an optimum environment were 92.1-95.0%. There were significant negative correlations between Ca, P, and Mg content in milk and milk yield, and a significant positive correlation between Na content in milk and milk yield in an optimum environment, but in a hot envionment there were significant positive correlations between P content in milk and milk yield. The decreases in Ca, P, Mg, and Na content in milk affected by heat stress were larger than those affected by the change in milk yield. The results suggest that the Ca, P, Mg, and Na metabolisms in lactating cows may be affected by heat stress and that their content in milk may decrease in a hot environment.
