Potassium in Dairy Cow Diets

Although potassium (K) is an essential nutrient for dairy cattle, increasingly high levels in forages have made cattle more susceptible to metabolic diseases such as grass tetany, udder edema and milk fever.

Potassium requirements of ruminants vary depending on the class of animal. Feedlot cattle, range cattle and growing heifers require about 0.6% K in their diet. Cattle undergoing significant stress, whether it be production, health or environmental (high temperatures), lose significant amounts of K from their body and the diet must be formulated to compensate for this loss. Lactating dairy cows require 1.2% potassium on a dry matter basis. Heat stressed dairy cows require about 1.5% K. Receiving diets for calves arriving in a lot should be formulated for 1.3% K.

Forages generally contain 1%-3% K. Grains contain about 0.5-0.8% K. The K level in forages is extremely variable and is influenced by species/variety, plant maturity, soil type and fertilization. Corn silage contains less K than do legumes. Grasses such as timothy, brome and reed canary grass tend to be lower in K than orchard grass which is a strong accumulator of K. Potassium levels in plants decrease with increasing plant maturity. Potassium is water soluble and will be leached out if plant material is rained on. Clay soils accumulate more K and will pass this on to the plant. Heavy fertilization with either inorganic fertilizers or manure is one of the main causes of high K levels in forages.

It is now believed that high potassium intake prior to calving is the major factor influencing susceptibility towards milk fever. Although the recommendation for years has been to prevent milk fever by reducing calcium intake in the close-up dry period, it is important to note that these strategies also had the effect of lowering K intake. Research published in 1997 shows that feeding up to 1.5% Ca had no effect on milk fever incidence. Increasing K from 1.1% to 2.1% increased the incidence of milk fever from 10% to 50%. A further increase to 3.1% had no additional effect.

Potassium, along with sodium, are cations. Sulphur and chloride are anions. A diet high in cations increases the DCAD (dietary cation-anion difference) and blood pH. This leads to a decrease in Ca absorption from the gut and in bone mobilization. This can result in milk fever.

What to Do with High K Forages:

1.- All dry cow forages should be analyzed by wet chemistry for K.
2.- Feed dry cows a forage with less than 1.5% potassium.
3.- If K is between 2-2.5% , consider feeding anionic salts. Ideally, feeding anionic salts in a close-up dry cow ration will result in a negative DCAD. The lower blood pH will result in increased Ca absorption from the gut and increased bone mobilization.
4.- If K is over 2.5%, it will be almost impossible to lower the DCAD sufficiently with anionic salts. A low K forage must be fed.
5.- The best long term solution is to grow a low potassium "dry cow" hay. Avoid legumes and orchard grass which tend to accumulate potassium.
6.- Start planning now! Choose land which has not been heavily fertilized - it may take several years to decrease soil K. Avoid clay soils and avoid the application of potash fertilizer and/or manure on the designated land.

Karen Dupchak
Animal Nutritionist, Animal Industry Branch
Manitoba Agriculture & Food
204-545 University Crescent
Winnipeg, Manitoba, Canada R3T 5S6

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