As the temperature increases during the next few months, many farms may notice their somatic cell count (SCC) is jumping up and down even though the weather is still mild at about 50° F with no humidity. There is a misconception that the absence of summer heat and humidity eliminates the chance of heat stress. I often remind farmers that yes, heat is the main stressor on cows that leads to higher SCC, yet another stressor is temperature fluctuations. The reason for this is that dairy cows have a thermoneutral zone of approximately 40° F to 60° F (source 1). Any temperature outside of that zone equals stress, especially when we see dips in temperatures in the spring and fall. These times of the year are when many of the farms I work with see that stress is reflected in their SCC. For this reason, mastitis is a year-round disease and is not limited to the summer months.

California Mastitis Test
My first recommendation for dairies is to utilize the California mastitis test (CMT) paddle. It is the quickest and cheapest method of finding cows with mastitis; especially finding sub-clinical cases (ones that do not necessarily show during pre-stripping). My recommendation for farms that have a large number of milk cows that cannot individually be checked is that they examine any fresh cows on a CMT before joining the main milking herd. If the SCC is still lingering too high and the fresh cows are clear, look at the late lactation cows. Fresh and late lactation cows are usually the culprits of high SCC due to the suppression of their immune system either pre or post-calving. They are also more likely to be “stressed” when temperatures change rapidly.

Other great sources for assistance with SCC are your veterinarian for milk cultures and/or consultations, county extension for resources on types of mastitis and where to combat them (Minnesota Extension is the first resource I utilize), and industry specialists you work with (ex. field representatives, state inspectors, nutritionists, etc.) These specialists can help determine the source of the SCC to narrow the number of areas to check. Relating to the thermoneutral zone/heat stress, some operational items to inspect include the following: Are all fans working and set to turn on at the correct temperature? Has all dust been removed? Are the sprinklers working throughout the feed alley? Are the curtains being maintained and working properly? And most importantly, are the cows comfortable?

To determine if cows are comfortable, I look for these 3 main things.
1. Cleanliness from the hock up. I want to see clean bellies, udders, and flanks when I walk through barns or open lots.
2. Is their bedding clean and comfortable? Utilize the knee check for your operation where if you can kneel in the freestall or open area, rest comfortably, and stand back up with little to no wetness on your jeans afterward, the bedding is acceptable.
3. Am I comfortable standing in the barn or can I feel a breeze/mist in an open lot? If you are not comfortable, the cows are not either.

Other issues to consider
Being outside the thermoneutral zone of dairy cattle not only affects the probability of heat stress, but also impact milk yield, reproduction (mainly on estrus cycle and pregnancy rates), and feed intake which relates to lower milk yield. (Source 2) I recommend that my farms establish a standard operating procedure (SOP) that all fresh cows and any suspect cows are examined on a CMT paddle, or a milk sample is sent to be tested for the SCC number, and perhaps a culture is performed. There should be a plan in place to complete the regular maintenance of blowing out fans and fixing any holes or non-working parts BEFORE the summer arrives. The importance of providing comfortable living areas for cattle throughout the operation cannot be dismissed, as if you are not at ease standing in the same area your cows stand in, something is amiss.

Source 1: Mike Hutjens YouTube Video “Consequences of Heat Stress on Dairy Cattle”
Source 2: https://www.ajas.info/journal/view.php?number=24190 “Effects of heat stress on body temperature, milk production, and reproduction in dairy cows: a novel idea for monitoring and evaluation of heat stress — A review” by Jiangjing Liu, Lanqi Li, Xiaoli Chen, Yongqiang Lu, Dong Wang