Heat Stress and Forage Feeding Approaches – Part 1

Rick Grant
William H. Miner Agricultural Research Institute

Summer’s heat and humidity are fast approaching. As always, we need to be prepared with effective heat abatement systems. 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. 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. Recent research indicates that cows become heat stressed at a temperature-humidity index (THI) of only 65 to 68.

Heat Stress Makes Cows Stand and Ruminate Less

As air temperature increases from the mid-70s to about 100oF, eating decreases by 46%, ruminating decreases by 22%, standing increases by 34%, and drinking increases by 30%. Higher producing cows (>70 lb/cow/day) are typically more sensitive to heat stress than lower producing cows, especially for resting, ruminating, and standing activity. Unabated heat stress can easily reduce intake by 10-15% or more and seriously compromise forage digestion.

Body temperature mediates the cow’s standing and lying response to heat stress. Cornell researchers found that during heat stress conditions, core body temperature appears to control whether the cow lies down or stands. The cow will stand up once her temperature reaches ~102.0oF and won’t lie back down again until her core body temperature reaches ~100.9oF. Research published in 2015 by Arizona and Missouri researchers confirmed that cows with elevated core body temperatures stood longer in an effort to dissipate heat and rested less. The bottom line is that cooled cows lie down longer, ruminate more, and have rumen conditions more conducive to efficient fermentation.

Heat Production and Forage Digestion

The cow’s total heat production consists of the heat increment from digestive fermentation and nutrient metabolism plus heat from basal metabolism and activity. For the lactating dairy cow, the heat increment comprises about 67% of the heat generated. Heat generated by fermentation is substantial and it varies with diet – especially the forage component. This heat load is an energetic burden to the heat-stressed cow because otherwise productive energy must be used to cool the cow’s body temperature.

With heat stress, we must focus on high NDF digestibility of forage and nonforage feeds to reduce the heat increment per unit of net energy supplied to the cow. Over the range in diets that might commonly be fed to dairy cows, from high to low heat increment, we can easily observe changes in metabolic heat loads that equate to 3 to 6oF – which means the cow consuming the more digestible diet has far less heat load to dissipate (Chandler, 1994). Basically, forages that contain more undigested NDF (uNDF) plus relatively more slow-digesting than fast-digesting NDF, will result in greater heat production during fermentation.

Heat Stress and the Rumen Environment

There is no doubt that successfully feeding our modern high-producing dairy cow requires efficient rumen fermentation of forages. But, the variable feed intake patterns associated with bouts of heat stress predispose cows to sub-acute rumen acidosis. Basically, the rumen conditions of a heat-stressed cow do not allow efficient fermentation: less rumination, greater drooling, lower bicarbonate delivery to the rumen, slug feeding, more sorting, and lower/more variable feed intake. Bottom line: heat stress predisposes the cow to rumen acidosis and poor conditions for fiber fermentation.

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 the fines from their diet and 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 may 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 heat stress is virtually identical to reducing dietary forage content from 65 to 35% of dry matter!

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

Heat Stress pH Graph

Under these low rumen 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, it will only exacerbate an already compromised rumen pH and microbial fermentation.

Forage Quality and Chewing Response during Heat Stress

Research shows that dietary 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.

In our research at Miner Institute we have measured the difference in eating time between a lower forage diet (~53% forage) containing brown midrib corn silage and a higher forage diet (~65% forage) containing conventional corn silage to be about one hour per day. What effect will this one-hour greater eating time have on the cow’s time budget? Particularly under heat stress, feeding 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 feet during a time when the risk of lameness is already elevated due to the heat stress.

We need to avoid low forage diets that may be too “hot” and result in symptoms of rumen acidosis. Likewise, avoid diets too high in fiber that take too long for the cow to process. The best diet may well contain moderate forage levels (55-65% forage) that contain highly digestible forage-NDF. This type of diet only requires about 20 minutes more to consume than a low-forage diet, but it also stimulates adequate rumination to maintain efficient rumen function.

Selected References

Aharoni, Y., A. Brosh, and Y. Harari. 2005. Night feeding for high-yielding dairy cows in hot weather: effects of intake, milk yield and energy expenditure. Livest. Prod. Sci. 92:207-219.

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.

Calamari, L., F. Petrera, L. Stefanini, and F. Abeni. Effects of different feeding time and frequency on metabolic conditions and milk production in heat-stressed dairy cows. Int. J. Biometeorol. 57:785-796.

Chandler, P. 1994. Is heat increment of feeds an asset or a liability to milk production. Pages 13-17. Feedstuffs Bottom Line.

Kanjanapruthipong, J., W. Junlapho, and K. Karnjanasirm. 2015. Feeding and lying behavior of heat-stressed early lactation cows fed low fiber diets containing roughage and nonforage fiber sources. J. Dairy Sci. 98:1110-1118.

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.


Amaferm is research proven to increase milk production during heat stress by 5.4%.


  • Amaferm increases NDFD by 4-11 points
  • Amaferm increases rumen pH
  • Amaferm lowers body temperature

Dr. Grant points out that next to heat abatement, managing NDF digestibility is key for heat stress management.  Two of the main reasons NDF digestion is an issue during heat stress are:

  • Undigested NDF limits intake and generates heat
  • Lowered rumen pH from sorting equals less buffering ability, which negatively affects fiber digestion

Amaferm increases NDF digestibility across forage qualities and forage types:

NDF graph

Stabilizing pH during heat stress can be a challenge for many dairies.  In the below graph by Westvig, you can see that the pH difference was significant at 9 hours after feeding – with the Amaferm treatment staying within the 5.8 – 6.2 normal range and the control falling below.

pH Graph

Dr. Grant also advises that 10-30% of energy could be used for chewing to break down particle size.  The unique ability of Amaferm to stimulate the rumen anaerobic fungi is not only key in increasing forage digestion, but also physically breaking hte forage particles down in size and helping to keep that energy expense for milk instead of digestion.

Amaferm is the best tool to reach for during times of heat stress.

To contact us about adding Amaferm to your summer rations CLICK HERE.

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