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The hot weather this past week generated several questions on how much potassium (K) should be in the diet of a lactating dairy cow. One reason for the question is many Midwestern diets are devoid or low in alfalfa right now which is a good source of K, averaging 2.5 percent of the DM (range two to three percent). In most years, alfalfa would be about 50 percent of the forage DM in Midwest dairy diets and supply 40 to 50 percent of the total K needed in the diet. As a result, a deficiency of K in a lactating dairy cow diet was almost never encountered in Midwestern lactation diets. With the loss of alfalfa this winter, many producers are now feeding more corn silage and/or byproduct feeds which contain only about half as much K as alfalfa. This has lead to K levels in diets closer to one percent (DM basis), the minimum requirement amount before supplementation.
Potassium is a cation or positively charged mineral required for many physiological functions (acid-base balance, osmotic balance, muscle contraction, carbohydrate and protein metabolism). Milk is an excellent source of K at 0.15 percent and about 20 percent higher than calcium. As a result, K, not calcium, is the mineral required in the highest amount in lactating cow diets. Heat stress will increase the K requirement as cows lose K through sweating. The 2001 Dairy NRC list K requirements of lactating cows at about 1.1 percent of the diet DM and 0.55 percent for dry cows.
Because most lactation diets are in excess of the 1.1 percent K requirement, it has been assumed supplemental K is not needed in diets except maybe 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 from Washington State found cows less than 75 days in milk were almost always excreting more K (average of 66 g/day) through milk, urine and feces than the amount consumed. However, this negative K balance may not be readily apparent in lost milk production or lower DM intakes. Detecting a K deficiency or shortage in lactating cows may be difficult, but German researchers indicate one symptom may be displaced abomasums. They found lower activity and intensity of abomasal muscle contractions when muscle strips were incubated in a low or deficient K buffer solution. Another way a low K diet may be exhibiting itself is through alterations in milk fatty acids, namely increased trans-fatty acids in milk. Research from Washington State and Clemson has shown a low K diet alters the fatty acid biohydrogenation process in the rumen yielding less saturated stearic acid (18:0) in milk and more trans forms of unsaturated oleic acid (18:1).
How much K is required in the diet of early lactation cows? Recent research from Virginia Tech and Washington State indicate a diet with less that 1.4 percent K (DM basis) in early lactation is inadequate. The Virginia Tech research showed cows fed diets of 1.37 percent 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 theirs and other studies, diets with 1.8 percent K may be needed in the first couple of weeks after calving to achieve a positive K balance. As DM intake increases with cows progressing in lactation, diets of 1.5 percent or more K should be adequate to achieve a positive K balance. Feeding close to 1.8 percent K in the diet throughout lactation may be a good idea, when unsaturated fats such as vegetable oils and even tallow are included in the diet and during summer heat stress.
When supplementing K in diets, one has to be cognizant of the effects K has on other minerals. Potassium reduces magnesium absorption and therefore, as K increases in the diet, magnesium should also. A minimum of 0.30 percent magnesium (DM basis) should be in the diet when K is less than two percent of the diet DM. Some research suggest, magnesium should be increased another 0.02 to 0.05 percent with K dietary levels above two percent. Because sodium has complimentary physiological effects to K, a ratio of about 1 sodium to 4 K has been suggested in some research, especially during heat stress. As both K and sodium are cations (+), increasing them in diets also increases the DCAD (dietary cation - anion difference) of the diet which has been shown to help buffer the rumen, increasing milk production and milk fat test.
For dry cows, the problem is 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 metabolic problems has been to feed low K forages (diets) and/or negate the effects of high K by feeding a negative DCAD diet three or more weeks before calving. Finding low enough K forages and feeds for dry cow rations has generally been a problem and therefore, most nutritionists resort to feeding high chloride and sulfur (both anionic minerals) amounts in diets to achieve a very low or negative DCAD ration pre-calving. Recent research from the University of Minnesota has shown what previous studies have in that a negative DCAD diet (-15 meq/100g DM) pre-calving helps prevent metabolic problems and that a negative DCAD diet can be fed for as long as 42 days pre-calving rather than the traditional 21 days without any problems. Whether feeding anionic salts or not, keeping K levels below 1.2 percent of the dry matter makes good sense in dry cow rations.
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