Heat Stress and Forage Feeding Approaches – Part 2

Rick Grant
William H. Miner Agricultural Research Institute


Feeding Management during Heat Stress

Feeding more of the ration at night during heat stress is a common recommendation, but recent research suggests it may be counterproductive. When a higher percentage of feed is offered at night during heat stress, cows tend to eat quickly – essentially slug feed – according to recent research from Penn State University. When a substantial portion of the daily feed is offered at night, cows have a greater rate of intake and insulin response. Perhaps cows want to eat quickly so they can lie down because resting is the normal dominant behavior at night. Actually, some nutritionists recommend greater dietary forage content at night to slow the eating rate which ought to lessen the risk of sub-acute acidosis.

Multiple nighttime feedings may be better, but are likely not practical. Aharoni et al. (2005) found that the energy expenditure of nighttime fed cows was lower than daytime-fed cows and their efficiency of energy use for milk production was greater. Presumably, cows produced less metabolic heat during the daytime and more during the cooler evening hours. Daytime-fed cows were fed at 30% of their daily allocation at 6:15 am, 20% at 10:00 am, 25% at 3:30 pm, and 25% at 7:00 pm. In contrast, the night-fed cows received 20% of their daily feed at 6:15 am, 30% at 3:30 pm, 25% at 7:00 pm, and 25% at 9:00 pm. We need to learn more about timing of feed delivery to heat-stressed cows.

Perhaps the best advice for now can be found in recent Italian research that showed that cows fed primarily in the morning – although most common – was actually least suitable for cow comfort during heat stress. Compared with cows fed in the evening or either in the morning and evening, cows fed in the morning only were more heat stressed and had higher rectal temperatures and breathing rates. This makes sense. Feeding early in the morning or evening avoids stacking digestive and ambient heat loads on the cow.

Forage and Nonforage Sources of Fiber

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 and still achieve efficient fiber fermentation.

Recent Asian research with cows under extreme heat stress conditions (THI of 86-92 during day, 74-81 at night) found that intake was greatest during the day for cows fed diets containing soybean hull NDF in place of forage NDF. Additionally, meal size and meal length were longer for the diets containing soybean hull NDF. Interestingly, these cows also spent more time lying than cows consuming more forage NDF. Reducing the filling effect of the diet contributed to reduced heat load due to altered feeding and lying behavior. Overall, the cows produced more fat-corrected milk, lost less body weight and had lower rectal temperatures.

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.

Rumen Fermentation Check List to Combat Heat Stress

To promote fermentation of forage and nonforage fiber sources, and minimize digestive heat load, here are recommended nutrition and management approaches:

  • Don’t feed the entire ration during the daytime so that digestive and ambient heat load coincides. But, feeding late into the evening may interfere with the cow’s normal resting patterns and may also reduce the cow’s ability to dissipate accumulated heat load during the evening hours.
  • Cool cows so that they lie down more and ruminate longer thereby contributing to better rumen conditions.
  • Heat stress causes low-pH rumen conditions; so do not compound this problem with a diet that is too low in physically effective NDF and high in starch.
  • Feed forages with high NDF digestibility. If forage NDF is higher than desired, consider partial replacement with highly fermentable NDF from nonforage fiber sources. This change will reduce the uNDF and slow-digesting NDF content of the ration and should reduce heat production.
  • Use feed additives that reliably enhance rumen fiber digestibility.
  • Remember that higher NDF, lower digestibility forages take longer to eat – do your cows sufficient access to the bunk?
  • Don’t feed to an empty bunk and minimize overcrowding during heat stress.
  • Make sure feed is pushed up and evenly distributed since cows may congregate at fans and cooler portions of the bunk.

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. And remember that high-producing cows experience heat stress with THI as low as 65 – ambient conditions that may even seem to cool to us.

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.