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