Ruminants are particularly affected by heat stress for several reasons. First, the heat from rumen fermentation is a plus during colder weather because it operates as an “internal furnace” for ruminants.
But during hotter weather, lactating cows in particular do not need this heat from rumen fermentation. And, in fact, they may have an increased energy requirement in trying to contend with this heat stress. Second, cows trying to reduce this heat of rumen fermentation will reduce their dry matter intake (DMI). High-fiber, low-quality roughage sources produce more heat from rumen fermentation than lower-fiber, higher-quality diets.
Thus, somewhat lower-fiber diets may be fed to dairy cows using higher-quality forage in order to minimize heat of rumen fermentation. But care must then be taken to avoid marginal acidosis with lower-fiber/forage diets. Dairy cattle have a zone of thermoneutrality between 41ºF and 77ºF (5ºC and 25ºC). Above 77ºF (25ºC), the body must modify physiology and behavior to keep their core body temperature near normal.
While heat stress has been known for years to be a problem, especially for lactating dairy cows, more has been recently learned about the mechanisms involved, that heat stress occurs at a lower temperature-humidity index (THI) than previously thought and how to alleviate it.
Heat stress affects several aspects of the dairy industry and has been estimated to cost the U.S. dairy industry $2 billion annually. Heat stress has been found to directly and/or indirectly affect feed intake, cow body temperature, maintenance requirements and metabolic processes, feed efficiency, milk yield, reproductive efficiency, cow behavior and disease incidence. Reduced DMI and lower milk production are the most commonly noted effects of heat stress in lactating dairy cows.
More recently, it has been noted that heat stress will increase a cow’s standing time as it tries to dissipate heat over its entire body surface, or decrease resting time which reduces milk production, and increases risk of lameness. Prolonged heat stress also increases core body temperature and increases estrus cycle length, but decreases estrus length and can increase embryo mortality. Lameness also contributes to lower reproduction due to decreased DMI and less activity.
Typically, a THI over 72 [75ºF (23.9ºC) with 65% relative humidity (RH) to 90ºF (32.2ºC) with 0% RH] was established as the lower threshold of heat stress. But with increased milk production per cow since initial development of the THI, a 22-pound-per-day increase in milk production will decrease the threshold for heat stress by 9ºF (5ºC). A recent re-evaluation of the THI has been modified due to improved milk production. The THI heat stress threshold was lowered to 68 [72ºF (22.2ºC) with 45% RH to 80ºF (26.7ºC) with 0% RH].
When cows experience heat stress, DMI decreases. At the same time, maintenance requirements are increased due to activation of the thermoregulatory system. This can increase maintenance requirements by 7% to 25%. This decreased DMI can account for about 36% of decreased milk production due to shifts in post-absorptive metabolism and nutrient partitioning. Under heat stress, cows also have lower non-esterified fatty acids (NEFA) concentrations and a higher rate of peripheral glucose utilization. This reduced DMI precedes by several days reduced milk production.
Heat abatement involves a number of actions: providing shade, air movement, misters and fans, feeding more earlier and later in day, use of high-quality forages to minimize heat of rumen fermentation and avoiding feeding fat sources that can reduce DMI or contribute high levels of fatty acids such as linoleic or palmitic. When DMI is reduced as in heat stress, using a mostly saturated free fatty acid supplement with a combination of stearic and palmitic can increase energy intake by increasing energy density as long as DMI is not reduced.
A study near Shanghai, China during the summer illustrated these factors (Table 1). There were 16 Holstein cows per treatment used with 2.2 parity average. The study began at 184 days in milk for 10 weeks, and total mixed rations (TMRs) were fed containing 41% forage and 0%, 1.5% and 3% of a fat supplement with saturated free fatty acids (FFA).
First, there was no decrease in DMI with either level of FFA supplementation. Increased net energy of lactation (NEL) intake went more into solids-corrected milk (SCM) production and components with the FFA supplementation. Rectal temperatures were also reduced during the hottest part of the day for treatments containing FFA in rations.
Granted this was in summertime hot and humid Shanghai, China, but what is too commonly overlooked is how prevalent heat stress occurs over the U.S. Length of heat stress is much more extended in the southern U.S., but even Northern states have a number of days or weeks of heat stress during summer – and some fall and spring days too where THI is over 72 [75ºF (23.9ºC) with 65% RH]. And now we know that an even lower THI of 68 is heat stress for lactating dairy cows.
In conclusion, when THI reaches 68 and greater, use heat abatement actions such as providing shade, air movement, misters and fans, feeding more earlier and later in day, use of high-quality forages to minimize heat of rumen fermentation and avoid feeding fat sources that can reduce DMI or contribute high levels of fatty acids such as linoleic or palmitic. The most effective fat supplementation is avoiding DMI decrease and using a combination of mostly saturated free fatty acids such as stearic and palmitic, as shown in the China study.
References omitted but are available upon request by sending an email to the editor.
