Heat stress in dairy herds can be a serious problem in Japan and Illinois. Milk production can decline 30 percent or more, milk fat and protein decreases, reproduction efficiency declines, and metabolic disorders can be life threatening. Several nutritional challenges and changes occur when cows are under heat stress which can impact the feeding program.

1. Feed intake can decline 20 to 30 percent (related to heat increment increases).
2. Blood flow is reduced to the digestive tract as cows attempt to dissipate heat.
3. Digestibility increases slightly due to greater retention time in the digestive tract.
4. Maintenance requirements increase to regulate heat by over 30 percent compared to cows in thermal neutrality (Table 1).
5. Cows will eat during cooler times of the day resulting in variable intake patterns.
6. Farmers decrease forage intake in an attempt to increase energy intake and reduce heat increment.
7. Wet feeds can heat up in feed bunks due to secondary fermentation, lack of shade, and low quality silage resulting in less palatable rations and lower intake.
8. Rumen acidosis is a greater risk because of variable feed intake, less forage being consumed, slugging of grain, and feed selection.
9. Adding fat as oilseeds (such as soybeans or cottonseeds) can increase the amount of unsaturated fatty acids with can reduce fiber digestibility.

These factors (energy intake, rumen digestion, and rumen acidosis) must be managed. Buffers can be one management solution.

Role of buffers

The dairy cow has a complex acid-base regulatory system with the rumen varying in pH from 5.5 to 6.8. If the rumen pH is not optimal (Figure 1), microbial yield and efficiency drops, dry matter intake declines, and metabolic disorders can increase. Buffers are a combination of weak acid and it salt which resists changes in pH or hydrogen ion concentration. An ideal rumen buffer should tie up hydrogen ions (equivalence point or pKa) near the desired rumen pH. An alkalinizing or neutralizing product increases the pH in the rumen fluid. Several products can be incorporated into buffer packs (Table 2). Recommended levels must be fed to have impact in the rumen.

• Sodium bicarbonate (bicarb) is the standard buffering agent with a pka at 6.25. Bicarb has been extensive studied, buffers at a pH 6.25, increases rumen osmolarity, and shifts rumen VFA (volatile fatty acids) patterns.
• Sodium sesquicarbonate contains a mixture of sodium bicarbonate (buffer) and sodium carbonate (alkalinizer). The pH of a one percent solution is 9.9. Research results indicate it is an effective buffering agent.
• Magnesium oxide is a source of magnesium (54 percent by weight) and functions as an alkalinizer. It can increase uptake of milk fat precursors by the mammary gland. Solubility and particle size can affect rumen action.
• Calcium carbonate has little if any buffering action in the rumen. It can increase fecal pH when high starch diets are consumed by increasing starch digestion in the lower digestive tract.
• Sodium bentonite is a clay mineral used as a pellet binding agent. It can swell in the rumen, shift rates of passage, and adsorb minerals and ammonia. It does not have buffering capacity.
• Potassium carbonate is an effective buffering agent which also provides added potassium needed under heat stress conditions. Research results are limited, but favorable. Cost is typically higher than bicarb for the level of buffering capacity.

Mineral relationships

Higher levels of sodium (from .18 to 0.5 percent) in the total ration dry matter is beneficial under that stress (Table 3). Sodium was excreted in urine while potassium losses occurred due to sweating. Florida workers suggest a ratio of 3 part potassium to 1 part sodium (for example, 1.2 to 1.5 percent potassium and .4 to .5 percent sodium). Magnesium should also be increased from .25 to .35 percent of the ration dry matter with higher levels of potassium. Higher cationic rations (from +180 to +350 meq per kg) increased dry matter intake based on the equation: (sodium + potassium) - chlorine on a meq basis

Strategies with buffers

1. Add .75% of the total ration dry matter as sodium bicarbonate or sodium sesquicarbonate ( 25 kg D.M. X .0075 = 187 grams/cow/day). Adjust intake based on dry matter consumption.
2. Supplement a buffer under the following conditions, especially with heat stress occurring:
   1. Total ration ADF below 19%
   2. Total ration NDF below 28%
   3. Forage NDF below 21%
   4. Feeding more than 3 kg of grain per meal
   5. Feeding more than 2% of cow's body weight as forage
   6. When a buffer pack is added, monitor dry matter intake to assess effectiveness (an increase one kg/cow/day is desirable).
3. Offering sodium bicarbonate free choice allows the cow that crave buffer can be indicator of acidosis (no controlled research has been conducted to support this application). To satisfy craving for dirt, offer sodium bentonite and salt as a source of sodium and chloride free choice, but separate. In all case, the recommended amounts of buffer and salt should be force fed through the TMR or grain mixture.