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Lower milk production and more transition cow problems were a common occurrence on many Midwestern dairy farms this fall. Often times the specific reason(s) for these problems cannot be determined so we blame it on summer heat stress. The decrease in milk production of cows during heat stress is readily apparent and can be easily documented, but what effects heat stress has on dry cows and their subsequent lactation is lesser known and has not been well documented until a recent research paper (I. M. Thompson and G. E. Dahl, 2012. Professional Animal Scientist 28:628-631) from Florida. The authors examined DHI records from 2,613 cows of first or greater lactation over a three-year period (2007 - 2010) to determine what effects seasonality of the dry period has on milk production, cow heath and reproduction.
Thompson and Dahl looked at two seasons for dry period and calving. They defined the months of December, January and February as the cool season and the months of June, July and August as the hot season. For each season during the three-year period, a Temperature Humidity Index (THI) was calculated. The average cool THI was 55.1 and the hot was 76.1 over the three-year period. Contrast this to a THI for the St. Cloud area this past year of 28.4 for December, January and February and a 68.5 during June, July and August. Unfortunately, we are getting a lot closer to Florida's hot season than Florida is getting to our cool season.


These records are from a large commercial dairy farm in Florida and provide a larger database than previous studies in documenting heat stress. Previous research has indicated heat stress compromises mammary gland development, which is regeneration of milk secretory cells, during the dry period. In an earlier study by Tao et al. 2011 (J Dairy Science 94:5976-5986) from the same Florida research group showed cooling dry cows during heat stress increased milk production an average of 11 pounds per day during the first 40 weeks of lactation compared to heat stressed cows. What also is interesting from the Tao et al. study is cooled cows and heat stressed cows had similar daily dry matter intakes during the subsequent lactation and therefore, heat stressed cows had a lower feed efficiency during lactation. Heat stress with no cooling decreased dry matter intake before calving, but not after. The compromising of mammary gland development and decreased dry matter intake of heat stressed cows before calving are likely related to the reduced colostrum production commonly observed in early fall also.
The reproductive data from Thompson and Dahl shows heat stress during the dry period has a detrimental effect on reproduction in the next lactation even though cows are no longer exposed to heat stress at the time of breeding. Heat stress most likely effects follicle development resulting in weaker or fewer dominate follicles developed 40 to 50 days following heat stress.
We try to find a nutrition solution to regain the lost milk production and decreased reproductive performance following heat stress, but changing the physiological status of a cow through nutrition is very difficult if not impossible to do. Heat abatement in dry cow and close up facilities is the best way to mitigate the effects of summer heat stress. The snow and cold may make dairy farming challenging, but our cows prefer it and reward us for living in the greater north country. Here is another thought related to this time of year. Cold and snow may also be good for cow longevity. Has Santa Claus ever had to replace any of his faithful reindeer? I know of no new replacements since Rudolph was added many years ago.
Merry Christmas and happy New Year to all.[[In-content Ad]]

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