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Macromineral nutrition by heat stress interactions in dairy cattle: review and original research.

This paper examines whether or not responses of dairy cattle to various dietary macromineral elements differed in hot weather compared with thermoneutral conditions. The consequences of heat stress and the interrelationships of macromineral elements on feed intake, digestive function, use of dietary buffers, mineral element uptake from the portal-drained viscera, perturbation of acid-base physiology and related mineral element nutrition, effects of dietary macromineral element concentrations on lactational performance as affected by season, and influence of mineral elements in drinking water are reviewed, and new research results are presented. Dietary mineral buffers aided in alleviation of the decline in DMI and milk yield induced by heat stress. New research results showed that portal plasma flow declined with heat stress or by restriction of DMI in a thermoneutral environment and that uptake of P from the portal-drained viscera by lactating cows was reduced 50% by heat stress compared with that of cows with the same DMI in a thermoneutral environment. Accelerated respiration rate caused respiratory alkalosis and apparently compensated metabolic acidosis, changing the demand for Na and K during heat stress. There is need to characterize more accurately the nyctohemeral pattern of acid-base physiology in the heat-stressed lactating dairy cow and to relate it to macromineral element needs. A large data set (n = 1444 cow period means) was used to compare milk yield and DMI responses to varying dietary concentrations of P, Na, K, Cl, Ca, Mg, and cation-anion difference in summer compared with those in winter. Interactions of dietary concentrations of Cl, Ca, and Mg with season on DMI and K, Ca, and Mg with season on 4% FCM yield were detected. Interactions of Na by Cl, Na by P, and Cl by P with season on DMI and of Cl by P with season on 4% FCM yield were detected. High concentrations of sulfate and chloride in drinking water jeopardized productivity of cows during hot weather.