So, that winter we hung buckets of water along the fence every morning and evening at feeding time. It seemed our horses drank very little warm water from the buckets. Instead, they kept going to the stream even when it was partially frozen over. On days that the stream was completely frozen, they would drink from the buckets. We thought they might not like something about the hanging buckets, which were quite a distance from their hay racks.
So, the next year we put a heated plastic stock tank in the pasture near the hay racks. We put in a large heating element so the water stayed warm to the touch. Again, once we started heating the water in the tank, our horses seemed to drink mostly from the stream, as if they really preferred the ice cold stream water to the warmed water in the tank.
This year, the day before Thanksgiving, we had a sudden cold snap. I filled up the stock tank, but forgot to plug in the heating element. In the morning, there was a thin layer of ice on the tank. We were surprised to see that the horses had been drinking from the cold tank, breaking through the icy crust. Anyway, I turned the heater on so it wouldn’t freeze. After two weeks of paying close attention, I’m pretty sure they don’t seem to drink much at all from the tank when the heater is on. They are going back to the icy stream.
My veterinarian and I were talking about this, and she thought you might have been involved with drinking behavior research cited in the magazine article, or that she might have heard you talk about it somewhere. Was that you, or do you know about it? Do you have an explanation why in these circumstances our horses drink the ice cold water from the creek or tank rather than the warm water in the buckets or the water that is warmed in the tank? It would seem they actually prefer cold water over warm water.
—Robert, New York
A: Thank you so much for bringing up this question. I was involved in the research on effects of water temperature on drinking behavior. You are right that there has been some confusion about the results, and it is great to have the opportunity to clarify what we know.
The observations you describe and your tentative conclusions that your horses prefer icy cold water to warm water are exactly consistent with the findings of that study. What we found was that if during cold weather horses have only warm water available, they will drink a greater volume per day than if they have only icy cold water available. But if they have a choice between warm and icy water simultaneously, they drink almost exclusively from the icy, and drink less volume than if they have only warm water available. So in the experiment, we had “single option” comparisons, where each subject had only one bucket, either cold or warm. After a period of time on that system, each subject would be switched over to the opposite temperature, and after a period of acclimation, drinking behavior and volume of water consumed was compared. We also did separate “preference” experiments, in which each subject had two buckets available simultaneously, one icy cold and the other warm.
Unfortunately, if the full scientific report is not read carefully, it’s easy to conclude that “horses prefer warm water,” rather than the more complex findings we reported. I remember several articles and news clips in horse magazines that described the quick, but incorrect conclusion. Over the years I have had many calls with questions such as yours.
My co-investigator for this project was Michaela Kristula, DVM, Section Chief of the Field Service group at New Bolton Center. She had the clinical impression that horses and ponies tend to experience a higher rate of impaction colic during the first deep cold spell of the winter. She, like many others before and since, had wondered whether horses might not drink as much water when it turns icy, which could increase the risk of impaction colic. She wondered if horses avoid icy cold water, especially during the first cold snap of the winter.
We have thought a lot about how to explain these findings, but have no good scientific evidence for an explanation. In human terms, we have wondered whether the basic instinct of the horse is to drink the coolest water available. This behavior might be adaptive in the sense that in nature the cool, running water probably is the freshest and least likely to be contaminated.
Then we would have to explain why they would drink a greater volume if the warm water is all they have. We have wondered if warm water is less satisfying to thirst. Or maybe icy water is less comfortable to ingest in large quantities, or that the icy crusts might deter long drinks. These are really only guesses, and we would have to do much more than the couple of small studies we did to address them properly.
Since that study, Dr. Kristula has recommended that if you want to increase the volume of water your horse drinks in winter, provide warm water and make sure there is not a source of cold water. Even though a horse might choose cold water over warm, it will most likely drink a greater volume if the only water available is warm.
In those studies, we observed some other interesting aspects of drinking behavior of horses. For example, horses in stalls which are fed hay and grain typically do most of their drinking within a few minutes after eating their grain and within an hour or so after they are given hay. That observation has been fairly consistent in other drinking behavior and nutrition studies of horses. We also saw a lot of hay wetting behavior, in which a horse dips each mouthful of hay or places a flake of hay in its water before eating it.
You can read the original research journal reports of the winter study and a follow-up study done in the summer. The journals can be found in most university libraries, and copies of the papers can be viewed and downloaded from our laboratory web site.
Kristula, M.A.; McDonnell, S.M. Drinking water temperature affects con-sumption of water during cold weather in ponies. Applied Animal Behaviour Science 41: 155-160, 1994.
McDonnell, S.M.; Kristula, M.A. No effect of drinking water temperature on consumption of water during hot summer weather in ponies. Applied Animal Behaviour Science 49: 149-163, 1996.