Typically, when the topic of heat stress comes up, northern New York is not the first region that pops into most people’s minds.

Although it is true we do not tend to suffer from prolonged, extreme heat here in the north country, it is also true that it does not take much heat for high-producing dairy cows to begin experiencing the physiological distress that can ultimately lead to compromised production, immune function and reproduction.

With funding from the farmer-driven Northern New York Agricultural Development Program (NNYADP), the research team at Miner Institute spent the summer of 2017 assessing the impact of episodic heat stress on the behavioral and productive response of lactating dairy cows under varying management and heat abatement systems.

Despite last summer being a relatively mild summer, with virtually no extreme heat events, our northern New York cows still experienced heat stress and displayed pronounced behavioral and production responses.

With the cooperation of four willing farmers, we were able to conduct our data collection on four dairies in Franklin and Clinton counties from the start of July through the end of September 2017. The study included both freestall and tiestall farms, stalls with sand bedding and sawdust on rubber mats, and different stocking densities.

The heat abatement used within the different farms included natural ventilation, fans over the stalls, naturally assisted ventilation and forced ventilation. On each farm, 30 early- to mid-lactation, high-producing Holstein or Jersey cows were identified as our focal group.

These cows were selected based upon the expectation that they would remain within the same lactating group for the duration of the summer and that they did not have a pronounced lameness problem, with a locomotion score less than a 3 on a five-point scale.

In an attempt to increase the amount of surface area for heat to dissipate through, a dairy cow will spend more time standing when she is overheated compared to when she is at a comfortable temperature.

To measure this, all 30 focal cows on each farm were equipped with a hind leg-mounted data logger that continuously measured and logged the positioning of the leg, therefore allowing us to differentiate between the amount of time the cow spent standing versus lying.

In addition, we mounted a livestock heat stress monitor within the pen that calculated and recorded the temperature humidity index (THI) continuously throughout the duration of the summer.

At the finish of the study, we isolated the four coolest days (average THI was 60.9) and the four hottest days (average THI was 72.3) of the summer, and evaluated the change in lying behavior. We found that regardless of the type of heat abatement system in place, the cows on all four farms experienced a pronounced decrease in lying time from the cool days to the hot days, as illustrated in Figure 1.

The most dramatic change in lying behavior occurred on the farm using solely natural ventilation as their heat abatement system, with a full two-and-a-half-hour, or 20 percent, decrease in lying time and one and a half more lying bouts per day.

The decrease in lying time means cows are spending more time up on their feet, whether it be standing or perching in stalls, standing at the waterer or idly standing in the alleys. A study done in 2007 by the School of Veterinary Medicine at the University of Madison – Wisconsin found that increased amount of time standing is associated with greater overall herd lameness.

With this in mind, it should come as no surprise that the percentage of cows lame at the end of the study increased for the two farms with the greatest decrease in lying time, one farm using natural ventilation and the other farm using fans over the stalls in addition to forced-ventilation fans.

It is not a secret that lameness in dairy herds is associated with decreased milk production. Increased lameness compounded with the decreased dry matter intake that heat-stressed cows have been shown to exhibit made us expect to see an impact in milk production for the herds we worked with.

By using the farm’s payment testing records or taking our own daily samples and analyzing them at the Miner Institute milk lab, we were able to compare production parameters to THI.

Although we did not pick up an association between THI and the pounds of milk shipped, we did find that, for the farm with natural ventilation only, about 32 percent of the variation in their milk protein concentration could be explained by THI. As THI increased, milk protein concentration decreased. This could be explained by a decrease in dry matter intake, and therefore a decrease in protein consumption or microbial protein production.

However, since we did not measure intake and did not see a decrease in pounds of milk shipped, we cannot say for sure that is the explanation. In addition, there are a number of physiological changes that dairy cows experience during bouts of heat stress that influence the amount of protein available for milk protein synthesis.

It is clear that dairy cows in northern New York are negatively impacted by heat stress, even during a mild summer when the few bouts of heat stress we did experience were infrequent and short-lived.

Although every farm, regardless of the heat abatement system, experienced a decrease in lying time that was associated with increasing THI, cows on farms with minimal heat abatement also experienced an increase in the prevalence of lameness and a decrease in milk protein concentration, resulting in an expected negative impact on farm profitability.

Through continued funding from the farmer-driven NNYADP, this study has continued in 2018. This summer has been hotter and more humid, with continuous stretches of heat stress. Stay tuned for an update.