Warm Weather Health Tips

Dr. Eleanor Jenson
USU Extension Veterinarian

Optimal environmental temperatures for cows range from 25-65 degrees F. When environmental temperatures exceed this range, cows are prone to heat stress. Heat stress can result in decreased milk production and increased somatic cell counts (SCC). The use of fans in free stalls or other lounging areas and the use of misters over feed bunks will reduce heat stress and improve cow comfort. These techniques should be considered if your cows have been uncomfortable this summer.

Environmental pathogens and external parasite numbers also increase during hot weather. Exposure of teat ends to increased pathogen numbers may precipitate subclinical and clinical mastitis cases (elevated SCC). External parasites such as horn flies are also more prevalent in hot weather. Fly bites may damage teat ends, making them more susceptible to colonization of bacteria. Research has shown that Staphylococcus aureus, a pathogen that frequently causes mastitis, can be spread between animals by horn flies. A preplanned fly control program, properly implemented, is a good economic investment.

In addition to affecting the health of the cow, mastitis results in economic losses from reduced milk quality and quantity. Therefore, identifying cows with mastitis is an important management tool. While clinical mastitis may be readily diagnosed by the abnormal appearance of the milk, the diagnosis of subclinical mastitis requires additional diagnostic tools. The DHI report gives SCC information monthly on individual cows that can be very helpful in monitoring udder health.

Another such tool that can be used is the California Mastitis Test (CMT). To perform the CMT, milk is stripped from each quarter into a plastic paddle containing four shallow cups. The cups are labeled A, B, C, and D to identify the specific quarter from which the milk was collected. A reagent is added to the milk in each cup, and the paddle is rotated with a circular motion to mix the solution. It is important to �read� the test within 10 seconds of adding the reagent to the milk. The following chart (published by the Department of Veterinary Medicine, Ohio State University) shows the scoring and interpretation of the CMT test:

Negative (N) - - Mixture remains liquid, uniform in color, with no evidence of thickening.

Trace (T) - - The slight thickening that forms is seen best by tipping the paddle back and forth and observing the mixture as it flows over the bottom of the cup. Trace reactions tend to disappear with continued rotation of the paddle.

Weak Positive (1) - - A distinct thickening of the liquid forms, but there is no tendency toward a gel formation. With some milk, the thickening may disappear after prolonged rotation of the paddle (20 seconds or more).

Distinct Positive (2) - - Mixture thickens immediately, and a gel formation is suggested. As the mixture is swirled, it tends to move in toward the center, exposing the bottom of the outer edge of the cup. When the motion is stopped, the mixture levels and covers the bottom of the cup.

Strong Positive (3) - - A gel is formed, which causes the surface of the mixture to become elevated like a partially fried egg. There is usually a central peak that remains projecting above the main mass, even after the rotation of the paddle is stopped.

Each CMT score can be correlated with an average SCC:

CMT Score Somatic Cells
(per milliliter)
N (negative) 100,000
T (trace) 300,000
1 900,000
2 2,700,000
3 8,100,000

The CMT score and the SCC are good indicators of subclinical mastitis in dairy cows. Management factors, including biting fly control and minimizing the effects of hot weather, should be addressed to reduce high SCC and minimize the number of clinical and subclinical mastitis cases. ©