It has been suggested that horses crib-bite to relieve the effects of excessive acidity in the stomach.
Horses secrete acid into the stomach continuously, but only produce saliva when they are chewing. One possibility is that horses start cribbing in a bid to counter the effects of increased acid levels in the stomach.
In a paper published in The Journal of Equine Veterinary Science, Beth Moeller and her research colleagues at Auburn University, in Alabama, described how they compared the amount of saliva produced by crib-biters and normal horses.
To collect saliva, they used a weighed cellulose sponge, which they held under the front of the tongue, where the ducts from the submandibular salivary glands empty.
They held the sponge in place for 30 seconds, then weighed it again. The difference in weight gave them a measure of the amount of saliva produced. Crib-biters were then encouraged to crib 10-15 times, after which a second sample was obtained. A second sample was taken from the normal horses five minutes after the first.
The researchers also measured the acidity (pH) of the saliva and found no difference between the cribbing and non-cribbing horses. So cribbing did not alter the composition of the saliva. But did it increase the quantity of saliva produced?
On average, the first samples from normal horses contained more saliva than those from the cribbers. The researchers point out that this lends weight to the theory that crib-biters produce less saliva than normal and rely on cribbing to make up the difference.
Overall, the second samples contained slightly less saliva. This was thought to be due to a drying effect of the first sample collection.
The horses that were allowed to crib did not show a reduction in saliva collected. However, when the same horses were prevented from cribbing, the second sample again showed a reduction in saliva recovered.
The researchers concluded that "the drop in saliva weight primarily occurred in the non-cribbing horses, indicating that cribbing may stimulate saliva flow and compensate for saliva lost to the initial sampling".
As the non-cribbing horses were not fed between the two sampling times they had no opportunity to replace saliva collected on the first occasion, because saliva is only produced when the horse chews.
After cribbing, however, horses did produce more saliva than non-cribbers - but the difference was small. In fact, the researchers note that it seemed insufficient to have a significant buffering or flushing effect on the stomach contents.
"Although the saliva weights found in this study seemed too small to have a significant effect, the high frequency of cribbing behavior may translate these small amounts into a meaningful biologic difference for the cribbing horse.
"These data do not support the hypotheses that cribbing produces enough excess saliva to buffer the stomach or to flush the gastrointestinal tract. However, because cribbing does increase saliva production, gastrointestinal irritation could be a motivating cause for cribbing."
More information 7.3.09
Estimation of saliva production in crib-biting and normal horses.
B A Moeller, C A McCall, S J Silverman, W H McElhenney.
J Equine Vet Sci (2008) 28, 85 - 90