The following answer is provided by Lance Baumgard, associate professor of Animal Science at Iowa State University. It is excerpted from his presentation at the 4-State Dairy Nutrition and Management Conference in Dubuque, Iowa earlier this month.
Q: What nutritional strategies can help mitigate the effects of heat stress?
A: The biological mechanism by which heat stress impacts milk production and dairy cow reproduction can be partially explained by reduced feed intake. Obviously, heat-stressed cows eat less.
However, other factors are also at work, like altered endocrine status, a reduction in rumination and nutrient absorption, and, increased maintenance requirements. All of this results in a net decrease in nutrient/energy that’s available for production. In turn, the decrease in energy results in a reduction in energy balance, regardless of state of lactation. Essentially, a heat-stressed cow enters a bioenergetic state that’s similar (but not as severe) as the negative energy balance seen in early lactation.
In addition to improved heat-abatement strategies, there are several nutritional factors that you can work with to help mitigate these heat-stress effects.
First of all, the heat-stressed cow is prone to rumen acidosis, and many of the last effects of warm weather can probably be traced back to a low rumen pH during the summer months. This many be explained by increased respiration rate, which results in enhanced carbon dioxide exhalation. In order for there to be an effective blood pH buffering system, a cow’s body needs to maintain a 20:1 bicarbonate to carbon dioxide ratio. When a cow hyperventilates in an attempt to cool herself, this decreases blood carbon dioxide and reduces the bicarbonate that can be used to buffer and maintain a healthy rumen.
In addition, the heat-stressed cow ruminates less because of reduced feed intake and increased time respiring, which reduces saliva production. Furthermore, heat-stressed cows drool, and combined with less saliva, this reduces the amount of buffering agents entering the rumen.
Therefore, use extra care when feeding “hot” rations in the summer. Monitor bicarbonate in the ration. Also monitor ration fiber quality. This is important all the time, but even more so during warm temperatures since it has some buffering capacity and stimulates saliva production.
Due to reduced feed intake, dietary protein levels may need to be increased during period of heat stress, too. Research shows that heat-stressed cows fed lower soluble protein levels had increased milk yield and increased dry matter intake. Recent recommendations suggest that the addition of dietary crude protein, specifically rumen un-degradable protein is not helpful.
Keep in mind that blood urea nitrogen is elevated in heat-stressed cows, although it is not clear whether this is from excess rumen ammonia production or from skeletal muscle breakdown. Regardless, excess ammonia must be eliminated, which has an energy cost as it is metabolized to urea and excreted in urine.
More research is needed to understand how heat stress affects dietary protein requirements before nutrition scientists can generate more appropriate recommendations.
What you may not realize is that the jury seems to still be out on the strategy of increasing the amount of dietary fat during heat stress, as well. This has been a widely accepted strategy in the dairy industry in order to reduce basal metabolic heat production, and a seemly rational decision. But, there are surprisingly few experiments designed to evaluate how supplemental dietary fat affects body temperature indices or even milk production parameters.
Most experiments report little or no differences in rectal temperatures and only one paper demonstrated a slight reduction at a specific time of day, but not at the other times. In fact, one report in 2010 indicated that cows fed additional fat actually had increased rectal temperatures. The same study also indicated that additional fat-fed cows had increased respiration rates.
Plus, data also show that the effects of dietary fat on milk composition during heat stress also vary and no clear consensus has been reached.
Overall, results from a limited number of experiments vary, but little or no apparent benefit was typically observed when supplemental dietary fat was included. Reasons for the discrepancies are unclear, but could be due to the type of fats used (saturated vs. unsaturated), rate of inclusion, type of “protection” environmental factors, or other dietary interactions. Again, more research is needed to make intelligent ration-balancing decisions regarding the inclusion of dietary fat.
One nutritional factor that is not questionable is water availability. It is essential for milk production and to help keep cows cool. Make sure waterers offer plenty of clean, fresh water during the summer months. Keeping water tanks clear of feed debris and algae is a simple and cheap strategy to help cows remain cool.