fever is one of the more expensive metabolic diseases faced by dairy
Costs can easily exceed $400/cow in terms of vet and drugs, discarded milk and
lost milk production. Cows with milk fever are also more likely to develop other
metabolic diseases including retained placentas and ketosis. As with all metabolic
diseases occurring in fresh cows, an ounce of prevention is definitely worth a
pound of cure.
It is well recognized that managing calcium intake in the close-up dry period
is very important in minimizing milk fevers. The role of potassium in milk fever
cases has also been growing in importance as potassium levels in forages seem
to be on the rise. High dietary potassium intake can predispose fresh cows to
milk fever even though calcium intake prior to calving is at recommended levels.
What is happening?
Milk fever results when calcium levels in the blood are too low to meet the
requirement for milk production at calving time. Approximately 25 g of calcium
will go into 10 L of colostrum - this is over 5 times the amount circulating
in the blood! Low blood calcium will show up as acute milk fever (hypocalcemia)
or, in less severe cases, as subclinical hypocalcemia with cows exhibiting depressed
appetites and increased incidence of other metabolic disorders.
The traditional way of preventing milk fever has been to limit calcium intake
during the close-up dry period to less than 100 g/cow/day. Dry cows on high
calcium diets have their metabolism geared towards reducing calcium absorption
from the diet and increasing excretion of excess dietary calcium. When the sudden
demand for calcium occurs, the cow cannot adjust her metabolism quickly enough
to meet this increased requirement. When calcium intake is limited prior to
calving, the cow begins to adjust its metabolism by releasing calcium from the
bones and increasing absorption from the intestine in an effort to maintain
circulating blood calcium levels. This metabolic adjustment takes place over
2-3 weeks and leaves the cow better able to meet the large demands of milk production.
Restricting calcium intake is usually a matter of choosing low calcium forages.
Forages, such as grain silages and grasses, have calcium levels of 0.4% and
0.8%, respectively. Alfalfa can have calcium levels in excess of 2%. Because
of the high calcium level, and also because of its high energy level, alfalfa
is not an ideal feed for dry cows.
Another contributing factor towards milk fever is high potassium in forages.
Potassium levels in heavily fertilized forages, particularly those fertilized
with manure, are often over 2% and can even exceed 3%. High potassium diets
cause blood to become alkaline and blood pH to rise. High blood pH reduces calcium
mobilization from the bone and absorption in the intestine and kidney. This
predisposes cows to milk fever.
Steps to Avoid Milk Fevers:
1.- Analyze for calcium,
phosphorus, potassium and magnesium in forages by wet chemistry not by NIR.
This increases the cost of feed analysis by $10 per sample (Norwest Labs)
but is well worth the extra money as it will allow rations to be accurately
formulated for these very important minerals. NIR is not an accurate method
for analyzing minerals.
2.- Limit calcium intake to
less than 100 g/cow/day and phosphorus intake to less than 45g/day for two
to three weeks prior to calving. Avoid feeding forages high in calcium such
as alfalfa. Grass hays, cereal silages and corn silages are recommended.
Work these forages into properly balanced dry cow diets.
3.- Keep potassium levels
as low as possible - ideally less than 1.5%. Potassium levels over 2% in
the forage may predispose cows to milk fever regardless of calcium intake.
Generally speaking, forages low in potassium are low in calcium.
4.- Anionic salts can be an
effective means of reducing the incidence of milk fevers. If the incidence
of milk fevers is over 10%, if there is difficulty in reducing calcium intake
or if potassium levels exceed 2%, consider feeding anionic salts to dry
cows. These compounds (eg. ammonium sulphate, ammonium chloride, magnesium
sulphate) are acidic and lower the pH of the blood. The body responds to
the drop in pH by releasing calcium from the bone and increasing absorption
from the gut in an attempt to neutralize blood pH. This helps to prevent
milk fevers. Anionic salts should only be used in diets formulated by a
Animal Nutritionist, Animal Industry Branch
Manitoba Agriculture & Food
204-545 University Crescent
Winnipeg, Manitoba, Canada R3T 5S6