Responses of lactating cows to dietary sodium source and quantity and potassium quantity during heat stress.

Effects of heat stress and added dietary sodium bicarbonate (0 or 1.0% of dry matter), sodium chloride (0 or .73% of dry matter), and total dietary potassium (1.3 or 1.8% of dry matter) on acid-base status, production, and mineral metabolism of lactating Holstein cows were evaluated. Design was split-plot with 24 cows in shade or no shade environments; dietary treatments were arranged as a 2 X 2 X 2 factorial within each environment. Basal diet (38% corn silage:62% concentrate) contained .18% sodium; sodium bicarbonate and sodium chloride treatments were in addition. All dietary treatments were equal in chloride content. Cows in no shade exhibited signs of respiratory alkalosis during the hot part of the day. Daily feed intake was lower in no shade than shade but milk yield and percent milk fat were not affected by environment. Sodium bicarbonate addition increased actual and 4% fat-corrected milk yields and percent milk fat. Sodium chloride addition increased actual and 4% fat-corrected milk yield when adjusted for amount of feed intake, and 1.8% dietary potassium increased feed intake and actual milk yield. Increasing total dietary sodium from .18 to .55%, from either supplemental source, enhanced 4% fat-corrected milk production, but combination of sources (.88% total sodium) showed no additional benefit over .55%.