Feed low-potassium forages to close-up cows Dairy cows need good levels of potassium in their diets - except during the three weeks just before calving.

The reason is milk fever.

In the last 10 years, dairy scientists have learned much about the causes of this metabolic disorder, which affects about 6% of freshening cows. They have focused on reducing dietary potassium (K) as the best route to reduce the incidence of milk fever, and that means selecting forages low in potassium.

"It's difficult to do that," says Jim Linn, extension dairy nutritionist at the University of Minnesota. "The only forage that comes close to being low enough in potassium is corn silage. But forages vary in potassium content. While those with low potassium are not easily found, dairymen should look for them."

Lower in K than most other forages, corn silage also is lower in protein and fiber, but higher in energy.

"You'll need to supplement corn silage with other feeds, particularly a good fiber source," says Linn.

David Beede, a Michigan State University dairy nutritionist, agrees with Linn that it's easier to prescribe a low-K diet for close-up cows than it is to make one.

"Conceivably, dairy producers could dedicate some ground to production of close-up dry-cow forage," he says. Lowering the level of potassium fertilization can produce lower-K forage. But that's "not very predictable," he says. So he, too, recommends a shift to more corn silage.

"A good rule of thumb is for about two-thirds of the forage portion of a close-up ration to come from corn silage with the other third coming from alfalfa or grass forage," Beede says.

Corn silage typically contains 1-1.3% K. Alfalfa runs higher, averaging 2.75% and ranging up to nearly 4%. "Alfalfa forage, on average, has the highest potassium content of typical dairy feeds," says Beede. To reduce milk fever, rations should contain less than 1.5% total K, he adds.

Linn and Beede both recognize two problem areas.

"In the last 15 years, dairymen have applied more manure to their fields and potassium levels have increased," says Beede.

"Manuring is the major contributor to high potassium levels in forage," Linn says.

The second situation involves alfalfa. For high yields and low winterkill, alfalfa likes lots of potassium fertilizer, and dairy producers have applied it. Forage from fields where alfalfa is dying out might be a good choice for dry cow feed.

"Whatever they choose to do, dairy producers should test their feeds to know what potassium levels are," says Linn. "That means using a good wet chemistry lab test, not an NIR."

Dairy producers might follow these strategies to reduce K:

- Shift away from alfalfa toward corn silage for dry cow feed.

- Use forages from fields where K is depleted or buy forage from areas where K levels are lower. Western prairie states are locations Beede suggests.

- Harvest alfalfa for dry cows in full bloom. It has less K than less mature alfalfa.

- Feed second-cutting hay rather than first. Potassium is less available to plants when soil is dry, so the second cutting often contains less K than the first.

Milk fever is caused by low calcium in the blood, says Beede. As calving time nears, cows need to absorb more calcium from their ration and mobilize calcium from their bones. Colostrum contains 1.5 to 2.0 times the calcium level of normal milk.

The "pump" that causes calcium absorption and mobilization doesn't work effectively when blood pH is high, says Beede. Blood alkalinity is caused by too high a level of cations, and potassium and sodium are the leading causes.

Adding calcium directly to the dry cow diet hasn't worked because cows don't take up calcium very well from their feed. Adding anions from anionic salts such as calcium chloride and ammonium sulfate hasn't worked well because they aren't very palatable and may reduce feed intake. Some dairymen do manage them successfully.

In research trials, feed treated with hydrochloric acid as a source of anions worked effectively, Beede reports. But it takes proper ration formulation and good feeding management.

"We do not recommend that dairy producers add HCl on-farm; this needs to be done by a feed manufacturer using proper safety guidelines," he says.

That's why the focus has turned to reducing potassium in cow diets the last two or three weeks before freshening.