Saliva production is very important to high-producing dairy cows because it provides a means to buffer the rumen. High-producing cows generally need to be fed a diet with low forage-to-concentrate ratio and high levels of fermentable carbohydrates in order to meet their tremendous energy demand.

This type of ration can be detrimental to ruminal health because it leads to increased volatile fatty acid (VFA) production, decreased pH and acidosis. Lactating dairy cows secrete 50 liters of saliva per day (or more); the amount produced depends almost entirely on chewing (eating and ruminating) of fiber in the diet.

When saliva enters the rumen, the bicarbonate and phosphate ions it contains act as the primary buffering compounds. These compounds will associate with free hydrogen ions in the rumen, which will increase rumen pH. Bicarbonate and phosphate are very strong buffers at higher pH, but when the pH drops too low (approximately 5.5), VFA will become the primary buffering system in the rumen.

Chewing was first suggested as a means of estimating a feed’s effectiveness in stimulating saliva production. In addition, several methods have been suggested to estimate the effectiveness of fiber. Most methods relate a feed’s effectiveness to its ability to stimulate chewing activity in the cow.

Chewing activity is important in dairy cattle because it has been shown to increase the daily secretion of saliva. Increased saliva secretion will lead to increased buffering capacity in the rumen. Resting saliva secretion is fairly constant throughout the day, but saliva secretion is dramatically increased when the cow is eating or ruminating.

Several studies have shown that increasing forage particle size is a means of increasing chewing activity and saliva secretion in cows. Generally, increasing forage particle size will increase eating activity and may increase ruminating activity.

A group of researchers in Canada published an interesting study on the effects of feedstuffs on saliva secretion. They determined a longer forage particle size does not increase the rate of secretion of saliva; rather it decreases eating rate, allowing more saliva to be secreted per unit of dry matter intake.

They also stated that any variation in feedstuffs that decreases eating rate will increase the amount of saliva secreted per unit of dry matter intake. Type of forage could have an impact on eating rate due to different levels of neutral detergent fiber and differences in fragility. Increasing ration dry matter will also tend to decrease eating rate.

Therefore, a practical way to increase chewing activity and rumen buffering is to increase forage particle size. It should be noted that excessive forage particle size could lead to several problems including decreased dry matter intake, increased ration sorting and improper silage fermentation.

So a compromise on forage particle size must be made that balances these many requirements. Recommendations for particle size in dairy rations are available at http://www.das.psu.edu/research-extension/dairy/nutrition/forages/tmr PD

References omitted due to space but are available upon request to editor@progressivedairy.com.

—Excerpts from Penn State Dairy Digest, April 21, 2011