Heat Stress and Forage Feeding Approaches

Rick Grant
William H. Miner Agricultural Research Institute

Summer’s heat and humidity are just around the corner. We need to be prepared with effective heat abatement systems that will cool our cows and allow them to respond to the diets fed. Unless a cow is properly cooled, adjusting dietary ingredients and feeding highly digestible forages will not result in the expected responses in rumen fermentation, feed intake, or milk production. According to recent research from Iowa State, the temperature-humidity index (THI) should be 68 (or even as low as 65) for high-producing cows. Our modern dairy cows have been selected based on heat-producing processes such as milk synthesis and consequently heat abatement has become ever more critical.

There is considerable information available on cow-cooling systems, so this article focuses on important aspects of forage quality and cow behavior that mitigate the negative consequences of heat stress.

Heat stress predisposes the cow to rumen acidosis

Classic data from Missouri show that heat stress conditions may reduce rumen pH to levels well below 6.0 (Figure 1). During heat stress, there is less bicarbonate in the saliva to buffer and maintain a healthy rumen pH (Baumgard et al., 2014). Heat-stressed cows ruminate 20 to 25% less, drool (so the saliva and its buffers don’t enter the rumen), pant, and are more likely to sort their diet or slug-feed. Consequently, heat stress is not a good time to feed to a clean bunk which may also encourage cows to eat faster. Overcrowding at the bunk or stall will also reduce rumination by as much as 1 to 2 hours daily.

All of these negative behavioral changes during hot, humid weather contribute to lower rumen pH and predictable negative consequences for rumen VFA profiles, rumen biohydrogenation, and milk production. Figure 1 shows that the extent of reduction in rumen pH associated with hot, humid conditions is virtually identical to reducing dietary forage content from 65 to 35% of dry matter! Under these low-pH conditions, we can expect inefficient rumen fiber fermentation and lower microbial protein output – both of which result in lower milk and milk component production. A traditional dietary recommendation during heat stress has been to feed less forage and more concentrates in an attempt to lessen the heat load of digestion. However, if we feed a diet that is borderline in fiber and high in starch, that will only exacerbate an already compromised rumen pH and microbial fermentation.


Figure 1. Influence of temperature and humidity or dietary roughage content on rumen pH (Mishra et al., 1970).

Forage quality and chewing response during heat stress 

Research shows that the ration fiber content has a large impact on cow chewing response. Arizona dairy scientists in the early 1990s found that, as ration ADF content increased from 19 to 25% of DM, cows spent greater time eating, had longer meal lengths, and increased their sorting activity. In fact, 10 to 30% of the energy provided by feed can be used for chewing. For the heat-stressed cow, feeding forage that is lower in NDF and higher in NDF digestibility is crucial because it allows the cow to process the feed in less time and expend less energy in chewing because of the greater fragility of the NDF.

More recently, we conducted a study that evaluated a range of forage content and NDF digestibility for corn silage-based diets. Table 2 shows the basics of the diets and the cow’s chewing responses. The production responses will be discussed in a follow-up article, but here is the point regarding heat stress and cow management: the difference in eating time between the lower forage diet containing BMR corn silage and the higher forage diet containing conventional corn silage was nearly one hour per day. What effect will this range in eating time have on the cow’s time budget? Particularly under heat stress, feeding a higher forage, lower NDF digestibility diet may have negative consequences. That hour spent standing at the bunk eating subtracts an hour from some other activity (such as resting) and certainly forces the cow to stand more – potentially placing more stress on her feed during a time when the risk of lameness is already elevated due to the heat stress.

Likewise, the low forage diet may be too “hot” and in fact we noted signs of rumen acidosis for cows fed this diet. The best diet may well be the higher forage that contains the highly digestible NDF. It only required about 20 minutes more to consume, but it also stimulated adequate rumination to maintain efficient rumen function.

Table 2. Diet composition and cow chewing response to forage content and fiber digestibility.


What if you have poor forage-fiber digestibility?

We all know that cows require adequate fiber for proper rumination, rumen function and VFA production. Cows require a physically effective NDF content of 21 to 24% of dietary dry matter, or about 75% of total NDF from forages. Adequate fiber is essential to help prevent rumen acidosis and high quality forages are critical. Highly digestible forage NDF will help to reduce the overall heat load on the cow and provide the proper amounts and ratios of VFA to maintain milk and milk component synthesis. When high quality forage is not available, some research has also indicated that substituting byproduct NDF for the lower digestibility forage NDF can improve milk production during heat stress (i.e. soybean hulls, beep pulp, and similar nonforage sources of fiber). Forage NDF can be reduced from 75% to 60% of total NDF under heat stress with efficient fiber fermentation. Research at Nebraska has shown that a critical component of successfully feeding higher nonforage sources of fiber in place of forage fiber is maintaining a well formed rumen digesta mat. This mat serves to slow passage of the nonforage fiber sources and will increase rumen NDF digestion by as much as 40% resulting in greater milk and milk component output.  The bottom line is we need to provide forage that is highly digestible. Adding things to the diet that will promote a rumen environment to improve forage fiber digestion like feed additives and buffers should also be considered.

Bottom line: feed highly fermentable fiber during heat stress

The bottom line is that feeding highly fermentable NDF – from forage and nonforage sources of fiber – will maintain rumen function, VFA profiles that promote milk component output, and avoid altered pathways of rumen biohydrogenation that depress milk fat.

We keep learning more about heat stress effects on cow behavior, feed intake, and health. This new information is useful, but we can’t neglect the basics of heat stress abatement. Ration adjustments (such as increasing digestible NDF) will not be fully effective unless cows are cooled effectively.


Baumgard, L. H., M. K. Abuajamieh, S. K. Stoakes, M. V. Sanz-Fernandez, J. S. Johnson, and R. P. Rhoads. 2014. Feeding and managing cows to minimize heat stress. Pages 61-74 in Proc. Tri-State Dairy Nutr. Conf. April 14-16, Fort Wayne, IN.

Mishra, M., F. A. Martz, R. W. Stanley, H. D. Johnson, J. R. Campbell, and E. Hilderbrand. 1970. Effect of diet and ambient temperature humidity on ruminal pH, oxidation reduction potential, ammonia and lactic acid in lactating cows. J. Anim. Sci. 30:1023–1028.


Cows fed Amaferm while experiencing heat stress:

  • Consumed more feed
  • Higher water intakes
  • Lower body temperature
  • Produced more milk – 5.5% increase during Heat Stress

Maintaining good rumen function is critical all the time but especially during periods of heat stress.  Let’s take a look at Amaferm’s mode-of-action during heat stress.

Amaferm gives a one/two punch to increasing feed intake by stimulating both the bacteria AND the rumen anerobic fungi.

  • First, Amaferm stimulates both of the major lactate fermenting bacteria, so it naturally assists with the stabilization of rumen pH providing a more favorable environment for fiber digestion.
  • Second, Amaferm uniquely stimulates the rumen fungi.  These fungi have the unique ability to physically break forage fiber down, increasing the secondary bacterial access to the forage particle.  This increased access results in an increase in both rate and extent of digestion.

Increasing water intake during heat stress is important for several reasons.  Drinking cool water helps to lower ruminal temperature and thus cool the cow.  Water can also have a buffering effect in the rumen.  Amaferm research has proven that not only was there an increase in dry matter intake but also increased water intake during heat stress.

Processing/fermenting less digestible components of the plant creates heat. Without Amaferm, these components need to have a longer period of rumination to break them down which requires extra energy from the cow.  With Amaferm, the fungi can break these particles down more rapidly to get them passed from the rumen.  Better efficiency = less heat.

Amaferm showed an increase of 5.5% in milk production during times of heat stress across all lactation periods – early, mid and late.


Bottom line is that Amaferm cows are able to increase digestion along with ruminal removal of the more undigestible components of the diet so the are able to eat more, drink more and maintain performance during heat stress.

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