Source: Manitoba Agriculture Food, and Rural Initiatives
Milk fever is one of the more expensive metabolic diseases faced by dairy producers. 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
- 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.
- 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.
- 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.
- 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 competent nutritionist.
For further information contact:
Farm Production Extension, Animal Nutritionist
Manitoba Agriculture Food, and Rural Initiatives
204-545 University Crescent
Winnipeg, MB R3T 5S6