Potassium (K) is a cation, or positively charged mineral, required for many physiological functions: acid-base balance, osmotic balance, muscle contraction and carbo-hydrate and protein metabolism.
The daily K requirement of dairy cows is about 1% of the diet dry matter (DM). This concentration is easy to achieve in dry or lactating cow diets, as most forages are high in K.
In fact, most forages are 20% to 50% higher in K than the requirement, making a K deficiency quite unlikely. A more likely problem is feeding too much K.
The effects of feeding excess K to dry cows have been known for several years. But until recently, no one suspected a deficiency in lactating cows, except maybe during periods of heat stress.
For dry cows, the problem is that most diets contain too much K. High intakes of K near the time of calving compromise calcium metabolism, leading to milk fever and
other metabolic problems. The nutrition strategy to prevent these problems has been to feed low K forages (diets) and/or negate the effects of high K by feeding a negative dietary cation-anion difference (DCAD) diet three or more weeks before calving.
Finding low K forages and feeds for dry cow rations has generally been difficult. Therefore, most nutritionists resort to feeding high amounts of chloride and sulfur (both anionic minerals) in diets to achieve a very low or negative DCAD ration precalving.
Whether you’re feeding anionic salts or not, keeping K levels below 1.2% of the dry matter makes good sense in dry cow rations.
Diets with 1.8% K may be needed in the first couple of weeks after calving to achieve a positive K balance
Because most lactation diets are in excess of the 1% K requirement, it has been assumed that supplemental K is not needed in diets except perhaps under heat stress. Current research indicates this assumption may be correct for mid- and late-lactation cows, but not for early-lactation cows.
A summary of K balance studies by Joe Harrison of Washington State University (WSU) found that cows less than 75 days in milk were almost always excreting more K (an average of 66 grams per day) through milk, urine and feces than the amount consumed. This negative K balance may not be readily apparent in lost milk production or lower DM intake.
Detecting a K deficiency might be difficult, but German researchers indicate that it might show up through a displaced abomasum. How much K is required in the diet of early-lactation cows? Recent research from Virginia Tech and WSU indicate that a diet with less than 1.4% K (DM basis) in early
lactation is inadequate.
The Virginia Tech research showed that cows fed diets of 1.37% K or less were in negative K balance from calving through 20 weeks of lactation. Based on the DM intake from the Virginia Tech study and the negative balances reported in that and other studies, diets with 1.8% K may be needed in the first couple of weeks after calving to achieve a positive K balance.
Jim Linn is a dairy nutrition consultant and retired Extension nutrition specialist at the University of Minnesota–St. Paul. Contact him at firstname.lastname@example.org.