Introduction

One of the greatest challenges to dairy producers in North America is heat stress, whether chronic or short-term. Even in dairy production areas where there is less humidity, evening and night cooling can still be limited during the hottest part of the season. In more humid zones, such as the upper Midwest, elevated temperatures may be less frequent, but humidity is always high and effective heat stress, or heat index, can register in the severe range for extended periods. The long daylight hours of summer also cause seasonal hormonal reductions in milk fat in lactating cow, representing another serious economic challenge during the warm season. Finally, forage quality and availability can also be reduced during the summer in some areas of the US. This makes maintaining optimal rumen function the primary key to nutritional management of heat stress. There are numerous, excellent reviews on the physiological effects of heat stress and the environmental, formulation, and management strategies that can be used to reduce its effects in dairy cattle. This paper will focus on approaches that may improve feed intake and rumen digestion, and why these approaches may have added economic benefit during heat stress.

Why focus on the rumen?

Chronic heat stress has strong inflammatory health effects on the cow and can greatly impair rumen function. Both beef and dairy cows appear to be more susceptible to acidosis during heat stress. High ruminal acid loads cause further reductions in feed intake, strain whole-body acid-base (electrolyte) balance, and contribute to low milk fat syndromes. There is rarely a single root cause for rumen acidosis during heat stress. As such, during the warm season, many formulation changes could be considered, including the use of the highest quality forages, closer management of carbohydrates, the use of buffers, elevated potassium and positive DCAD. In addition, there are many feed management strategies that are beneficial to keep feed fresher and keep cows interested in eating, including reducing crowding, changing feed delivery schedules, and procedures to limit sorting.

Rumen buffers and their electrolyte contribution

Buffers have regularly been included in the dairy ration. The lactating dairy ration is arguably starch dense and the resulting rumen fermentation challenges the cow’s ability to neutralize and clear the volume of rumen acids produced. The main goal of mineral buffers is to supplement the cow’s natural buffer system to mitigate possible excess rumen acid. In the 80s and 90s, research began to show the additional value of the “electrolyte function” of sodium (Na) and potassium (K) buffers to cow health, especially during warm weather. Only Na and K buffers provide net electrolyte value, Na and K salts do not. We can assess the strength of this electrolyte contribution of buffers using a ration software calculated number called positive DCAD (see internet for dietary cation-anion difference).

Positive DCAD becomes a focus for many nutritionists when heat stress becomes a more serious threat. For reference, a “normal” +DCAD level for a lactating cow would run in about the 20 to 25 meq/100 g of DM range, as calculated by ration software. During heat stress, many nutritionists increase the calculated -DCAD value to 30 to 40 meq/100 g of DM. In practical terms, this means that the amount of mineral buffer fed is being greatly increased.

Potassium buffers have greater impact

The cow’s basic potassium requirement is about 1% of DM. Forages typically supply most of that, except with heavy corn silage rations or where forages are grown in low K soils. Unlike Na, K is a high turnover electrolyte and is not actively retained by the kidney, as is Na. Potassium’s usage and turnover seem to increase in proportion to the stress on the cow. Conservatively, challenges such as heat stress can increase the cow’s K needs by more than 50%. Research and field application of buffers show that increasing either K or greater +DCAD benefits cow health and milk production, especially during heat stress. It seems logical that maintaining a higher ration K will increase the physiological impact of a higher total +DCAD, but published research does not make that completely clear.

Because of the obvious logic behind elevating electrolytes during heat stress, most nutritionists would likely prefer most of the ration +DCAD coming from K. However, the higher cost of K buffers bring question to formulating K into diets, as K buffers are three to four times more costly than Na buffers. The added cost of K buffers can be partially offset by the added benefits of higher K on butterfat. Higher K helps limit milk fat depression by altering fat biohydrogenation in the rumen. Higher levels of rumen K inhibit rumen bacteria that produce trans-10 fatty acids, a known cause of milk fat depression. Including 50 or more grams of +DCAD K, may increase milk fat by as much as 0.2 points. This positive effect is likely to be exaggerated during heat stress. Obviously, the amount of K fed as part of the total +DCAD must be carefully calibrated against the current price of milk and especially the price of butterfat.

Potassium carbonate or feed-grade “Potcarb” is the only available source of buffered K. Some nutritionists are wary of common Potcarb because it is hygroscopic and chemically reactive with many common dairy ingredients. Manufacturing SOPs for mineral/protein blends are often geared to address Potcarb’s risks through dilution and mixer addition protocols. In the summer, ambient humidity levels are higher and the handling risks with Potcarb become magnified. As such, it is highly recommended to utilize a stabilized potassium carbonate, such as Barentz Animal Nutrition’s K-Carb Plus™. In higher humidity regions, such as the upper Midwest and Canada, mineral blenders use only stabilized forms of potassium carbonate year-round.

Ionophores

Although there are few published data to support that ionophores, such as Rumens in, are comparatively more effective during heat stress, their benefits during heat stress are well-recognized in the field. Dairy cattle are more prone to acidosis during heat stress and ionophores do seem to mitigate rumen acid production and help elevate rumen pH. In addition, monensin reduces rumen protein degradation which helps moderate the negative effects of high degradable protein rations during the summer.

Yeast and microbial additives

Among the microbial additives that appear to be beneficial during heat stress, yeast is perhaps the best accepted by U.S. dairy nutritionists. Not surprisingly, virtually all suppliers of yeasts and yeast cultures in North America make a claim for the benefits of yeast for greater lactation performance during heat stress. Both live and killed culture forms of yeast have been shown to improve rumen function and lactation performance of dairy cattle under heat stress. Many nutritionists see yeast as a stress or “feed intake” tool and often increase their usage of yeast during the summer. Chronic heat stress in lactating dairy cattle can also lead to measurable immunosuppression. Yeast cell walls and other immunostimulatory materials appear to have beneficial effects on the long-term health of stressed cows. Barentz Animal Nutrition has several options for both live yeast and yeast cell walls for use in dairy rations.

Essential oils and phytogenic materials

Essential oils and related phytogenic plant materials are now commonly used in North America. There are several commercially available products that use variations of essential oils, often along with other more traditional nutritional additives. There is evidence that some essential oil combinations have many of the same benefits in the rumen as are observed with ionophores (mentioned above). Often it is the more exotic effects of essential oils on vasodilation and “animal well-being” that captures the imagination of nutritionists and producers looking for nutritional heat stress solutions. Capsaicin (hot pepper extract) is one example of these types of applications that are promoted for their ability to reduce core body temperature in growing and lactating. Many phytogenic compounds also have immunostimulatory properties and are promoted for their effects on both gut health and general immunity.

Conclusion

Arguably, the degree of heat stress sets the level of production that can be achieved by the lactating cow. That is, lactation performance will generally not be increased until the animal is made more comfortable. This means that management practices for heat abatement always carry more direct economic impact than manipulating the ration. However, there are a number of additives and other nutritional changes that canachieve favorable ROI, if used judiciously during heat stress.