Effect of dietary calcium and sodium on potassium requirement for lactating dairy cows.

In two experiments we studied the effect of dietary calcium and sodium on the potassium allowance of .7 to .8% for lactating cows. In Experiment 1, 12 first-lactation cows in midlactation were fed three diets containing .52, .77, and 1.04% potassium with four dietary calcium concentrations .50, .69, .87, and 1.03%. Increasing dietary potassium from .52 to 1.04 increased intake of dry feed as a function of body weight. With rations containing 17% coarsely ground fiber, increasing dietary calcium up to 1.03% increased intake and milk production 1.3 kg per day and 1.6 kg per day linearly. In Experiment 2, 16 cows in midlactation were fed rations with two potassium (.42 and .84%) and with two sodium concentrations (.31 and .52%). Increasing dietary potassium increased dry matter intake 1.2 kg per day while dietary sodium had no effect on intake. Potassium had no effect, but increasing sodium decreased fat-corrected milk 1.7 kg per day. Of potassium, sodium, magnesium, and calcium in serum, potassium was the element most consistently affected in both experiments. Increasing dietary potassium to .77 and .84% increased serum potassium. Dietary calcium and sodium had little effect on potassium required for lactating cows.