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The secret language of heat stress: What your dry cows are telling you

Bethany Dado-Senn

20 July 2022

Dry, non-lactating dairy cows are more thermo-tolerant relative to their lactating counterparts, as they are not putting high levels of energy into making milk.


While dry cows of the past were traditionally not considered for heat stress abatement on-farm, the conversation around cooling dry cows is becoming louder as farmers and industry professionals notice negative impacts of heat stress on dam and calf performance. Research has shown that dry, pregnant cows or nulliparous pregnant heifers exposed to heat stress will make less milk in their next lactation, and their gestated calves will be born earlier and smaller with reduced milk yields at maturity across multiple lactations.


Therefore, it is critical to properly identify heat stress in non-lactating cattle to quickly provide cooling. As with almost any cowside concern, your best tool to monitor dry cow heat stress is to start speaking their language: Observe cow behavior, measure vital responses, and check their surrounding environment.


Watch cow behavior

Your first line of heat stress defense is employing your “cow senses” to watch for changes in dry cow behavior. This can be accomplished through visual monitoring or use of various cow monitoring technologies. Dry cows and pregnant heifers under heat stress will have lower-than-usual feed intakes, rumination and chewing times. They will also stand for more hours of the day to promote heat loss, leading to issues in cow comfort. Under more severe heat stress, you may also notice cool-seeking behaviors like bunching under natural or artificial shade, crowding near ventilation or standing near or in water sources. Only in extreme cases will you notice dry cow behaviors like open-mouth panting or standing in water troughs. It is ideal to intervene with heat stress abatement before cows reach this level.


Track cow and environmental benchmarks

Another relatively simple tool is counting respiration rates. Count flank movements for roughly 10% of your dry cow pen and, if respiration rates exceed 60 breaths per minute, the cows are likely experiencing some level of heat stress. As numbers rise above that target threshold, heat stress becomes more severe.


While measuring individual rectal temperatures is impractical, if your farm uses monitoring systems that can be placed inside the dry cow (such as boluses), you can track core body temperature to both detect heat stress and disease onset. Benchmarks for body temperature will vary depending on cow size and disease status but, in general, look for temperatures between 102.4ºF to 102.6ºF for initial heat stress onset. Rises in respiration rates will occur before rises in core body temperature and serve as an initial indicator of dry cow discomfort and poor welfare, whereas increased body temperature is the true gauge of physiological heat stress and potential productive consequences. Notably, these dry cow respiration rate and rectal temperature benchmarks are nearly identical to lactating cow targets.


If you keep a close eye on the weather forecast along with your cattle, you may also be interested in environmental heat stress thresholds for dry cows. Vital responses in dry cows begin to sharply increase when the environmental temperature-humidity index (THI) reaches 75 to 77 or when ambient temperature reaches around 75ºF to 80ºF (depending on the level of relative humidity). Not surprisingly, these dry cow thresholds are markedly higher than the benchmarks for lactating cows (THI between 68 to 72) due to the difference in milk outputs. An important caveat to these thresholds is that most are established in warmer, southern parts of the U.S. Midwestern or northern U.S. regions might have different cutoffs due to more temperate, variable weather. Precise monitoring of your farm’s barn or outdoor environments can be accomplished through tools ranging from simple outdoor thermometers to site-specific temperature, humidity, THI and air speed meters.


As the summer sun continues to blaze, the task of cooling cattle can seem daunting, especially for dry cows that lack the classic heat stress tell of lower milk in the bulk tank. However, by adopting some of these simple behavioral, vital and environmental tools to speak the heat stress language, farmers can rapidly detect dry cow heat stress and quickly intervene to rescue dam and offspring productivity for generations to come.


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