Editor's note: The following article was originally published on the Pennsylvania State University Extension website, available here.
In the time period shortly before calving, large amounts of calcium are removed from the blood and are utilized in the mammary gland to be part of the colostrum. Calcium in colostrum may be eight to ten times greater than in the blood supply.
The rapid drop and the decreased mass of the calcium pool prior to parturition, and the failure of calcium absorption to increase fast enough after the onset of lactation, can predispose animals to milk fever or hypocalcemia.
There are other probable causes that have been associated with inducing milk fever. They include excessive bone formation due to elevated levels of gonadal hormones and rations containing excessive dietary levels of cations, especially potassium. In addition, other metabolic disorders can lead to clinical and subclinical hypocalcemia (i.e. ruminal stasis, displaced abomasum, retained placenta, prolapsed uterus, metritis, and ketosis). Figure 1 lists additional factors and situations.
Figure 1. Conditions associated with milk fever.
|Low calcium intake, especially for dry cows
(< .40% in total ration dry matter (TRDM))
|Heavy corn silage feeding; high moisture corn feeding;
inadequate supplementation; low grain intake (dry cows); low forage–high grain feeding.
|Low phophorus intake
(< .28% TRDM)
|Inadequate supplementation; high forage–low grain (i.e. pasturing dry cows).|
|Excessive calcium intake
(between .70% and 1.00% TRDM)
|High legume intake by dry cows; over supplementation with calcium.|
|Excessive phosphorus intake
(> .40% TRDM)
|Over supplementation; excessive grain feeding.|
|Excessive vitamin D intake
(> 100,000 units per head daily)
|Over supplementation can lead to calcification of tissues and result in heart failure.|
|Low magnesium intake
(< .20% in TRDM)
|Failure to balance low magnesium forages, i.e. corn silage, grasses, and small grains.|
|High potassium intake as it affects anion-cation balance (> 1.2% in TDRM)||Forages high in potassium content - over 1.5% on a dry matter basis.|
|Reduced mineral absorption; rumen pH over 6.8-7.2; higher incidence with increasing age (lack of vitamin D, alimentary tract stasis, lack of motility, constipation||High legume ration; high pH water over 8.5; under 3 to 5 pounds of grain intake; underfeeding forage or effective fiber; excessive protein intake.|
|Selenium or vitamin E deficiency
(< .10 ppm) (< 250 units per head daily)
|White muscle disease; lack of supplementation.|
|Toxemia||Coliform mastitis, other toxin-forming organisms; lower gastrointestinal tract stasis; reproductive tract infections.|
|Nerve or muscle damage||Injury at calving; damage from going down or lying on limbs for a prolonged time period.|
Symptoms and Problem Situations
Stages of milk fever
Milk fever is divided into three stages based on clinical signs. Stage I milk fever often goes unobserved because of its short duration (< 1 hour). Signs observed during this stage include loss of appetite, excitability, nervousness, hypersensitivity, weakness, weight shifting, and shuffling of the hind feet.
The clinical signs of stage II milk fever can last from 1 to 12 hours. The affected animal may turn its head into its flank or may extend its head. The animal appears dull and listless; she has cold ears and a dry nose; she exhibits incoordination when walking; and muscles trembling and quivering are evident. Other signs observed during stage II are an inactive digestive tract and constipation. A decrease in body temperature is common, usually ranging from 96°F to 100°F. The heart rate will be rapid exceeding 100 beats per minute.
Stage III milk fever is characterized by the animal's inability to stand and a progressive loss of consciousness leading to a coma. Heart sounds become nearly inaudible and the heart rate increases to 120 beats per minute or more. Cows in stage III will not survive for more than a few hours without treatment.
Milk fever is considered a herd problem when over 10% to 15% of the cows are afflicted on an annual basis. The higher value may apply to herds where many cows are freshening that have a history of getting milk fever, i.e. older cows being more susceptible.
A problem situation can be when a high proportion of cows in a sizable group of freshenings is affected. An example of this would be when five out of the last eight freshening cows are diagnosed with milk fever.
Forms of Milk Fever
Typical milk fever
An acute form affecting cows usually within a few days after parturition, but it sometimes occurs in late lactation or the dry period. Typical milk fevers respond well to treatment.
Refractory or atypical milk fever
An acute form with little or no response to treatment. The cow may remain alert, eat, and milk but cannot regain her feet. She may become a creeping downer cow with flexed pasterns and posterior paralysis. Rupture of the large muscle or group of muscles in one or both hind legs may complicate the problem. Similar fracture or dislocation of a hind joint may have occurred when the cow went down initially or in struggling to rise.
Tremors or sub-acute
Cows are easily excited with muscle twitching and tremors occurring. Usually, several cows are involved. Many of these animals may be in late lactation, dry, or recently fresh. Often, there is a magnesium deficiency involved as well.