Eating and Digestion
Last revised April 14, 2017.
Feral horses grazing.1
Grass Loves Horses
Horses are grazers. “Graze” comes from Middle English grasen, from Old English grasian, and that from græs, grass, so a grazer is a grass eater. Horses are grazers, as are cows, sheep, bison, buffalo, deer, elk, wildebeest, zebras, and kangaroos.
Special saliva. When a mammal or a plant-eating insect eats dinner, it creates saliva, some of which it leaves on the grazed grass. For over 40 years, scientists have known that grasshopper grazing increased the growth of the grass they ate.2 In 1980, Dyer applied a component of mouse saliva — epidermal growth factor (EGF) — to sorghum seedlings, and found this significantly increased the speed at which shoots and roots grew, and found that such growth was dose-dependent: more saliva meant more growth.3 EGF is not only found in grasshopper saliva. It is also found in mammalian saliva (including yours) and in spitballs.
Growth is one way that grasses benefit from being grazed.4 Increased longevity is another.5 With a longer life, any setbacks — such as reduced seed production — are paid for. Grasses paid for this saliva by being fairly palatable (at least compared to most land plants), encouraging grazing.
Tearing better than cutting. I believe that plants such as grass would prefer to be torn, rather than cut. A study of lettuce found that lettuce that had been torn by hand had a lower respiration rate and deterioration rate than lettuce cut with a knife. The researchers in this study hypothesized that the difference was that with tearing, the tear could follow the naturally weaker areas of the lettuce, resulting in less tissue damage.6 I believe that the same principle applies to horses loving grass: by pulling the grass with their lips, rather than slicing through it with their teeth, they give the grass an opportunity to break-away along lines of least resistance — which could well be lines of least damage. It is possible that grass evolved these break-away lines as they discovered grazers evolving to eat them.
Tearing may be better than cutting for another reason: When your horse tears a blade of grass, he is likely to break more cells open than if you cut that blade with scissors, and exposes a longer damaged edge. But his saliva now has a chance to coat that longer surface, and because it promotes grass regrowth, the plant will bounce back from this damage more quickly than if he had used a pair of scissors to harvest his dinner. Cutting your lawn with a dull blade will likely cause more damage than using a sharp blade, but because your mower doesn’t secrete horse saliva, you’ll likely cause more harm than good. I believe that the tearing approach taken by all grazing animals, along with their magic saliva, is just what the grass doctor ordered.
Grazing height. The mutualism that developed long ago between grazers and the grazed is evident in other adaptations of grass. Of special interest is the basal meristem (the part at the top tip of the root). This is a region of very actively dividing cells in grass, which you can find in the diagram below.
Morphology of a grass, showing a closeup of the basal meristem. When horses graze, they tear the grass above this part.7
- Grasses have developed a basal meristem that is too close to the ground for most grazers to damage, so they are not brutalized by grazing. The meristem allows the grass leaf blade to regrow quickly. Your horse stops eating above the basal meristem, and you should set your lawnmower blade above the meristem. (The basal meristem is at ground level, so only if your mower “scalps” are you causing major damage.)
- Grasses increase their numbers both through seed production and vegetative reproduction, making them more tolerant of their grazers.
- Grasses have a short stature which favors grazing and which comes to the rescue when there is a shortage of precipitation.
- Grasses have other properties that favor grazing: high shoot density, deciduous shoots, below-ground nutrient reserves, and rapid growth.
As a regional climate becomes more arid, the shift favors loss of forest, their replacement with grasses, and the spread of large grazers, such as horses and cattle. Michael Coughenour8 has reviewed how basal meristems, small stature, high shoot density, deciduous shoots (high turnover), below-ground nutrient reserves, and rapid growth allow grasses to evade or to tolerate both semiarid conditions and grazing.9 Both grazing and dry conditions put the same evolutionary pressures10 on grasses. Horses evolved to flourish in the same environments that grasses like.
Grasses know about horses. They’ve evolved to get along with horses, and horses have evolved to get along with them. Fifty million years ago, their ancestors were 50 pound deer-like creatures of swampy, prehistoric woodlands. As they grew larger and faster, they ventured out into more open meadows, and now love the plains that grasses love.11
It is handy for horses that grass grows faster after it has been eaten. The herd’s territory need not be very large. The herd can wander about it, rotating through each grazing area, knowing that when they return in a few days, the grass will have recovered and be ready for additional grazing. But there is a delicate balance. If an area of grasses is fenced and kept free of grazing mammals and insects, it will not prosper. In fact, there is experimental evidence that many grasses cannot survive in the absence of grazers.12 Grasses seem to have co-evolved along with their grazers.
On the other hand, if the same area is filled with too many horses or cows or elk or sheep… As McNaughton writes, “Of course, growth may be inhibited by excessive defoliation and an optimum defoliation level is anticipated.13” And grass does not appreciate being stepped on repeatedly, or wallowed in, or being doused with urine or feces, as is common near the pasture gate. Heavily grazed grass has a better chance of keeping pace with the grazing if there is sufficient available moisture for its growth.14 It might be a good idea to install an irrigation system in heavily grazed pastures, irrigating when the horses are out of the pasture.
Neither undergrazing nor overgrazing are best. Buffalo co-evolved with prairie grasses, and this likely occurred with other migrating grazing animals, such as caribou and wildebeest. Their cyclic migrations ensured that the grass would be grazed twice a year. At the farm, proper pasture management would have many pastures and a paddock. On muddy days, horses would be kept in the paddock, but on other days, horses would rotate into each for a day, every seven or so days. Seven days is certainly all that grass needs to return to its pre-grazing height if there is adequate moisture.
Reacting to Injury
Anyone mowing a lawn knows that the grass will be ready next week for another mow, but because our mowers don’t apply epidermal growth factor as they cut, the grass must manage without it.
While there have been many studies, discussed previously, that grass does better when grazed, there are also hints that grass might not enjoy being grazed. When grass is cut with a mower, that wonderful smell we get is produced by volatile organic compounds (VOCs)15. One important VOC is ethylene, a hormone whose production is stimulated by wounding a plant. The ancient Egyptians discovered that if they gashed figs, they would ripen faster, an effect of ethylene. Ethylene and other VOCs play a role in healing the wound through their antibiotic properties and ability to inhibit the invasion of bacteria into damaged tissues.16 In plants that do not desire to be grazed — which is most of them — the compounds also signal “Danger!” to other parts of the plant and to other plants, giving them a chance to assemble a chemical defense that makes them unpleasant tasting.
This makes great sense. As Carol Yoon argues, “When a plant is wounded, its body immediately kicks into protection mode. It releases a bouquet of volatile chemicals, which in some cases have been shown to induce neighboring plants to preemptively step up their own chemical defenses and in other cases to lure in predators of the beasts that may be causing the damage to the plants. Inside the plant, repair systems are engaged and defenses are mounted, the molecular details of which scientists are still working out, but which involve signaling molecules coursing through the body to rally the cellular troops, even the enlisting of the genome itself, which begins churning out defense-related proteins … If you think about it, though, why would we expect any organism to lie down and die for our dinner? Organisms have evolved to do everything in their power to avoid not being. How long would any lineage be likely to last if its members effectively didn’t care if you killed them?17”
The differences in what these plants produce defines who will eat them. Browsers (such as deer and goats) will smell a plant, take a bite, and move on. On that first bite, it is likely that the plant has not yet gotten any warning about trouble in the neighborhood. After the bite, the remainder scrambles to produce compounds that will warn the rest of the plant (and nearby plants) to produce chemicals that give it an unpleasant taste. But our clever browser will only take another bite or two before moving on, thwarting this plant’s self-defense efforts. You can see this effect with the way that insect browsers damage leaves: toward the end of summer, some plants will be missing a few bites from every single leaf, but leaves will not be damaged beyond utility — they’ll still be able to produce sugars for the plant through photosynthesis.
I do not believe that grazers and browsers use the same shearing techniques when eating grass. Grazers love grass, while browsers prefer leaves, soft shoots, and fruits of high-growing woody plants such as shrubs.18 If you study your horse, you’ll see him collecting a mouthful of grass with his lips, then tearing it off with a small quick twist of his head. The approach of a browser to his dinner seems to involve more biting, and less lip use. Biting makes sense on a shrub branch, and the browser probably uses this same technique on grass. Grazers seem to have lips and cheeks that are better developed than those of browsers. Horses always chew with their mouths closed; deer seem to chew with their mouth open.
During hard times, a browser must relax his standards, and eat what is distasteful. So when you walk in the woods, you’ll see mountain laurel and rhododendron with no lower leaves, and maybe no lower branches. The deer have already gotten that low-hanging fruit. Next spring, the plant will try again, and send out some new leaves and twigs, drawing on its stored reserves. Next winter, the browser will continue their pruning, and the cycle will continue until the leaves are finally above the browser’s reach.
Grazers (such as cows and horses) don’t wander very fast, because they are dining on grasses which don’t seem to create a bad taste response when a leaf is wounded. You may find a herd of deer in a pasture, but you’ll not find a herd of horses dining on mountain laurel in the woods.
Why doesn’t the plant taste bad at first bite? Presumably because the chemicals required to make the plant taste bad require a costly amount of energy to create or maintain, or perhaps they interfere with other processes in the plant, so that it is better to only create them when needed.
So if newly cut grass smells so good, and that smell comes from compounds that were created because of the wounds received and that were designed to warn other parts of the plant and other plants, why doesn’t cut grass that smells good taste bad? My suspicion is that grass flipped a gene off, so that the volatile organic compounds are created as in other plants, but the grass ignores them.
What’s for Dinner?
In the preceding text, I’ve talked only about grass. But if a horse had his way, there would be much more on the menu. Horses are generalist herbivores, meaning that they’ll happily eat a wide variety of plants. Feral horses are found worldwide in many different habitats, from desert to savannah.19 If grasses are available, they may be a horse’s first choice, but shrubs, leaves, stems, bark, and roots all find their way into a horse’s dinner in the wild.20 When it is not grass-growing season, horses favor any species that provide shelter and nutrients, such as gorse, holly, and deciduous woodland.21
Horses Love to Eat
So do I22. In the horse’s case, this need to feed comes from the limited capabilities of its digestive system and the limited usable energy in its food supply. Grass and other plants are low energy, so an animal must either process a lot of it, or process it very efficiently, to make do on this energy source. (When horses are not crowded in a pasture, the available sugars in the grass may exceed what Mr. Horse finds in the wild or that he was designed for, and weight gain and laminitis may result.23 Horses always eat out of desire, not always need.)
Elsie’s amazing feats: Ruminants like cattle have four “stomachs” or holding tanks that process their dinner: rumen, reticulum, omasum, and abomasum. Each holds a cow’s dinner in various states of decomposition. The rumen is the largest of these, and holds grass or hay that has been poorly chewed, along with billions of bacteria, protozoa, molds and yeasts. During her constant rumination, the rumen moves this grass around and the microorganisms in the rumen, who are symbionts, dine to their heart’s content. The combination of microorganisms gives the cow versatility in diet, because it is able to digest many feeds, including grass, hay, corn, grains, corn stalks, silage, and urea. The rumen is able to move the grass that is most in need of further chewing to the front, and formed into a “bolus” of food, which the cow can regurgitate. She can now chew her cud in safety and comfort — maybe lying under a tree or standing with friends — like a ball player or cowboy might chew tobacco (but without the health risks). She may do 20 chews on one side, then 20 on the other before swallowing it again. The cow then swallows this cud again, and this wad is passed to the back of the rumen. In the meantime, the microorganisms have been busy digesting the plant fiber and nitrogen and producing volatile fatty acids, essential amino acids, and vitamins. Every minute the rumen contracts, mixing the contents to stimulate fermentation, avoid stagnation, and expel fermentation gases. Some byproducts are absorbed by the walls of the rumen, and give the cow 60-80% of the energy she needs. Other byproducts include methane, produced by the bacteria who must live without much oxygen. Cattle belch frequently, helping with climate change. The remainder of the digestive system in the cow is busy with absorbing fluid and nutrients, including the proteins of our fermentation heroes, who get digested for their efforts.
Amazing feats of Harvey, Bugs, and Br’er: A rabbit doubles the length of his digestive system by sending some of its dinner through it twice. After food goes down the hopper (get it? Har har har), a rabbit’s stomach mixes the food with acid and enzymes, and digestion begins. The small intestine adds more enzymes to extract nutrients from the fiber, and the extracted nutrients are captured by the intestinal lining and absorbed in the blood stream. Then a rabbit’s colon sorts what has been partly digested into two groups: indigestible fiber that is now useless and the good stuff. Indigestible fiber is transformed into hard round droppings which are “passed” and abandoned. Digestible fiber, on the other hand, heads off to the cecum (also spelled caecum), where bacteria ferment it and capture stored nutrients. The rabbit then passes some of the contents of the cecum, enveloped in mucus, as cecal droppings which the rabbit eats.24 Most of what a rabbit eats goes through his system twice.
The Unlucky horse: The horse was not so lucky to be born a rabbit or a cow. Its efficiency derives from nearly the same plumbing that humans have (though instead of a useless appendix, the horse has a useful cecum), and it is not efficient. A higher proportion of the food value of its intake winds up on the ground behind him that it does for cow or rabbit. And this inefficiency is unfortunate, because he has chosen to specialize in a diet that is low in energy. (Offer him a cheeseburger, and see what he says.) His only option is to devote his life to eating. Free ranging horses graze and browse 13-18 hours a day,25 and pastured horses about 15-18 hours each day.26 (See the time budget material in the section on “The Natural Horse”.) Horses don’t sleep through the night — they will graze intermittently throughout the day and night. The bulk of the grazing, though, occurs around dawn and dusk — possibly an adaptation from long ago to the daytime heat and/or flies. Horses will rarely fast longer than four hours, unless the weather or flies are bad.27
A horse does a lot of chewing — maybe 60,000 grinds each day, though he’ll do less if fed grain or confined to a stall. Coarse-textured feed gets more chewing, and ponies and small horses need to chew more than large horses. During chewing, the horse sweeps his jaws from side to side. Out in a typical pasture, all this sweeping removes any sharp edges and hooks from the outside edge of his teeth, but if he is getting lots of grain, he’ll chew less, and sharp edges will develop, which interfere with further sweeping. Your horse’s dentist or vet needs to keep these under control with twice yearly visits, because they reduce chewing efficiency and appetite, and affect his temperament.
When a horse chews, saliva is added to the mix — between 20-80 liters (5-20 gallons) of saliva a day. In the saliva, grass finds bicarbonate to buffer and to protect amino acids in the very acidic stomach. Saliva also adds amylase, which gets to work on digesting carbohydrates before the horse even swallows.
Once chewed, the horse’s dinner, along with saliva, heads down his 5 foot long esophagus as a bolus — a small rounded blob of warm grass and saliva. But bad things can happen if your horse has not chewed it sufficiently. Belching is not on the list of horse tricks, and food stuck in his esophagus will cause trouble. Ensure that your horse chews adequately instead of bolting his dinner in his stall. Put a brick in his grain bucket, provide a slow-feed system, add chaff to his feed, provide coarse hay, or reduce the amount of grain he gets in one feeding.
Digesta — ingested stuff moving through central processing — first reaches a football-sized stomach that only holds two or three gallons. I know some good old boys who can top that. In relation to his body size, the horse’s stomach is the smallest of all domestic animals. The job of the stomach is to mix the digesta with gastric acid to break it down and pepsinogen to begin protein digestion, and to regulate the rate at which digesta is sent to the small intestine. The acid in the stomach makes life rough on any living thing that enters it, such as a grasshopper. By the time food has reached the final parts of the stomach, the pH has dropped to about 5.4 (halting fermentation) and as the food continues on down, the pH drops to around 2.6, killing any fermentable lacto-bacteria that have made it this far.
The stomach operates like a cement mixer, churning the porridge and letting the chemicals do their job. Liquid may stay in the stomach only 15-30 minutes,28 but hay may linger there for half a day, with 3-4 hours being the average length of time. Eating moves things through the system: if he continues to eat, food might stay in his stomach as little as 15 minutes; if he has fasted, the stomach might retain its contents for a day.
Because the stomach is so small, Mr. Horse needs to eat often. If you are a stall boarder and your horse is indoors for more than five hours, you are eating in to his eating time. Stomach acid is produced non-stop in a horse, so in any horse confined to a stall where it can’t graze, or who is otherwise prevented from eating, stomach acid will cause problems. Excess stomach acid has nothing to do but digest the walls of the stomach. Ulceration of the stomach wall affects four out of five thoroughbreds — presumably because most are confined before they are sent off to become sausages. Ensure that horses get plenty of roughage, an opportunity to graze, and small frequent meals.
From the stomach, digesta passes to the small intestine — all 70 feet of it. Here secretions continue the digestion of protein, simple carbohydrates, and fat. Nutrients such as amino acids, glucose, vitamins, minerals and fatty acids are absorbed here as soon as they are ready. Digesta lingers here for as little as 30 minutes to as much as 8 hours. The small intestine can digest carbohydrates, including starches and sugars, but is helpless against fiber.
After leaving the small intestine, digesta reaches the cecum — a fermentation vat that holds about 8 gallons in it 4 foot length. The cecum is located right at the spot where you attach your girth, and cinching your girth compresses the cecum. The horse relies on a big load of microorganisms in its cecum and large colon, making the cecum the horse’s counterpart of the cow’s rumen. Enzymes from these microbes ferment fiber into volatile fatty acids, including acetic acid, propionic acid, butyric acid, and lactic acid. As byproducts, this fermentation creates heat and methane, making the cecum a real gas tank.
After the cecum, the digesta makes its way through 10-12 feet of large colon and another 10-12 feet of small colon. Some of the microorganisms that have worked so hard meet their end in this 23 gallon tank. The colon absorbs the fatty acids , amino acids, B-vitamins and Vitamin K that the microbes have created, and most of the liquids are absorbed for re-use. Your horse’s dinner comes out from this 90 feet of torture in 1 to 3 days, and looks quite a bit different than when it started.
Comparing our animals, it looks like the foregut (esophogus, stomach, and small intestine) is much like a human’s or rabbit’s. The hindgut (the cecum and colons, which comprise about 62% of the digestive system, are about 23 feet in length, and can hold about 38 gallons) are something like the cow’s system, fermenting and absorbing whatever has made it this far, but without producing a cud to chew. Enzymes having starred in the efforts of the foregut, while the action in the hindgut is mostly microbial. Even with 90 feet of digestive system, the horse is not as efficient as cow or rabbit.
Horses rarely run in the wild. They conserve energy when possible, as all animals do. But for them, conserving energy is important because grass is such a poor energy source. As a result, if you try to turn a horse into a performance horse (endurance, fox hunting, ranch work, polo, carriage pulling, etc.), he will come up short on the energy he could get from 19 hours of grazing. A stall and your work with him may shorten his grazing time even more. You can offset this deficiency by giving him a bucket of grain or commercial feed, but this causes different problems. He’ll eat when it is presented, and perhaps put too much into his stomach. Then he won’t eat when his bucket is gone, and his stomach acid will build up while he looks for something to do. Meanwhile, in the hindgut, a surge of high energy digesta may overwhelm (with stomach acid) the beneficial bacteria that are at working fermenting what is coming along. Good bacteria get replaced by bad bacteria, and the loss of good bacteria allows the hindgut to become even more acidic.
I don’t believe that horses are intentionally skinny, but rather skinny out of necessity: they just can’t do any better. I believe that most organisms go through life hungry all the time, which motivates their constant search for food. Carnivores go from feast to famine, but grazing animals probably never feel that they have had enough to eat. Humans and our dogs and cats are exceptions to this. As predators, we are designed to binge eat, and digest high protein sources. With grass or hay for a diet, the horse cannot extract as much energy per day as we can with a few cheeseburgers. This accounts for why the horse seems to eat all the time, and why they don’t get fat. (You might think your horse is fat, but look around you: if a horse had as much fat as your favorite author does, you’d need a new saddle.)
Some products are more effective than others as foods for the horse. Fats such as vegetable oil are well digested, and the horse is able to extract almost all of the energy it contains. Beet pulp — the fibrous residue after sugar has been extracted from sugar beets — has high levels of fiber, but after being soaked in water is very easily digested, so it produces a good amount of digestible energy per pound eaten. Lower in food value are grains and fats which can be digested in the small intestine, and the lowest food value comes from hay, which must be handled by the large intestine and quickly becomes manure, heat, and gas.
I recommend that once your horse has acquired a taste for beet pulp, use it at rest stops in endurance rides, between chukkas, and at other opportunities for your horse to catch up. Not only is it easily digested, but he’ll get extra fluid as he eats it.
Gastric ulcers are the the most common problem of the foregut. Ulcers are the result of prolonged contact of the upper parts of the stomach with the acids from the lower part.
Colic — abdominal pain — is the most common in the hindgut. Colonic ulcers are common, especially in performance horses.
Colic has multiple causes. It can happen with abrupt changes in hay or grain, or from large amounts of grain, or from abrupt changes in activity, or large amounts of stall time, or lack of water, or poor parasite control — anything that suddenly changes what is going on in Mr. Horse’s 90 foot chemistry lab. It has multiple locations in the gut, several treatments, and one dreadful potential outcome: it is the leading cause of death in domestic horses.
A female feral horse is exhibiting elimination by urination. Urination behavior in horses is sexually dimorphic: female horses urinate in a posterior direction and male horses urinate in an anterior direction.29
Feral stallions create fecal middens or stud piles, and return to them again and again to defecate. Such middens might have a role in communication or establishing ownership of an area.30 In a band of feral horses with several stallions, if two stallions contribute to a stud pile, the dominant stallion will work to be the last to contribute to it,31 covering the scent of the subordinate and proving that love stinks. They may alternate in this overmarking up to eight times in succession.32
Male feral horses also express harem tending by maintaining fecal middens, or stud piles, in which feces are aggregated at strategically located positions within the band’s home range.33
Pooping may be a stall vice, at least until we teach our horses to clean up after themselves.
How Does a Horse Graze?
Ruminants, such as cattle, oxen, sheep, goats, deer and elk, do not have upper incisors. So they cannot “cut” grass between their upper and lower incisors. Instead, their lips can seize the plant, their tongue pressed against their palate can secure it, and they can pull, tearing it. Horses have both upper and lower incisors, and so their grazing would seem to be easier.
With a horse, the process of choosing a spot to graze likely involves this sequence:
- Scan the nearby area, with eyes, for favorite plants. Plant color may matter: horses can see blue and green (but not red, and there will be no red plants in the pasture). Horses may initially select their forage visually, based on color, size or shape.
- Approach the area, and when a few feet away, move forward slowly, with nose to ground. This will allow the horse to double check what he thinks he has seen, identify the species a few feet away, and approach to where the smell is strongest. This close up, the intended target is within the horse’s blind spot, so he needs to entirely trust the inputs from his nose, whiskers, lips, and taste buds.
- As he comes in for a snack, he uses proprioception to judge the distance from lips to ground, and uses whiskers —if you haven’t clipped them off — for feedback on his proximity to the plant.
- He now uses lips to double check the selection, and grasps it if desired.
- After seizing the grass with his lips, he closes his incisors and lips on the grass to hold it firmly. He now jerks his head to the side, tearing the grass.34 The head jerk will tear the grass and give him his prize.
- Once it is in his mouth, he uses his tongue to press it against his palate. Now the taste buds on his tongue evaluate how sweet, salty, sour, and bitter the mouthful is.35 The process helps him associate the connection between a plant’s appearance, its scent, its texture, and its taste.
- Chew. This releases some more aromatic compounds which may detected by the nasal passage.
For a horse to distinguish between two plants based on their scent, it is fortunate that his nose is just an inch from his incisors and lips. And it is fortunate that his nose works so well. A horse can identify plants from their scent alone. As we learn in the chapter on olfaction, a horse can identify 1,816,285,375,084,304,096,155,409,990,400 different scents. That is probably more than the number of species of grass, though it didn’t seem like it in Botany 101.
Humans approaching a cheeseburger use their incisors like scissors, to cut through it, separating a bite from the rest of the burger. A horse doesn’t eat cheeseburgers or grass this way. He grasps it firmly and tears it. As I suggested, the difference between cutting and tearing may be important to the plant.
To drink, horses simply place their lips in the water and suck, keeping their teeth together. Deer, it seems, make modest biting motions in the water, and may be swallowing more like a cat. I think grass would prefer to be eaten by a grazer than a browser. If you are considering dinner guests, your horse will show the best etiquette.
What motivates a horse to move on to a new spot to graze?
The horse’s nutritional options vary across its pasture or territory. To deal with this fact, horses are like other herbivores: they have use a “patch feeding strategy” in which they regularly return to preferred plant communities, and sample them.36 Horses may use scent, shape, color, texture and flavor to positively identify what they are eating, and will quickly learn to connect these qualities so, for instance, a horse can recall flavor or texture from a scent.
Horses seem to prefer the youngest, greenest parts of plants, which are highest in nutritional value. 37 The idea that a horse would choose a plant for its nutritional value is supported by research that has found that horses can learn to associate a food’s sensory characteristics with the energy or nutrients it obtains after eating it.38 If what it eats makes it feel sick — like locoweed — it learns to avoid it,39 especially if the food is novel.40
The height of the grass may be involved. When grass is short, cattle significantly reduce their grazing,41 and prefer to graze where they can have the most rapid intake rates — where the forage density is highest.42 It seems safe to assume that the same principles apply to horses: when an area has been grazed, the horse chooses to move on “to greener pastures.”
Horses will not find all grasses equally palatable. In grasses that herbivores prefer, water soluble carbohydrates are higher than in non-preferred grasses.43 Horses are very sensitive to the presence of soluble carbohydrates in what they eat, and prefer food high in these carbohydrates.44 They are also usually short on salt, and actively look for foods rich in it.45 While your horse likely craves carrots and peppermints, horses — like many other species — differ in their preference for moderate vs. high concentrations of sucrose.46
Plants that are highest in certain organic acids may also be preferred.47 One study found that both malate and citrate will increase salivation and intensify sweet flavors in the diets of “monogastric” animals such as horses.48 So if a grass contains these acids, it can both taste sweeter to your horse, increase his chances of grazing, and increase the chance that his saliva will be left on the edge of the plant, stimulating regrowth.
Herbivores are all likely to have plant preferences, and are likely to determine the species of grass from some distance. In one study, sheep were provided with a nursery of spaced plants, that included 85% crested wheat grass (Agropyron cristatum), 14% mallow, and 4% “Spredor 2” alfalfa (Medicago sativa). After the ewes were introduced to the pasture, they roamed the area, investigating and sampling what was available. According to the study authors, “Within hours, ewes recognized the presence of highly sought alfalfa plants randomly scattered across a pasture area (44 x 44 yards). The sheep relished the alfalfa, and within hours, several of the lead ewes were observed stretching their necks and scanning for other alfalfa plants. Once sighted, the sheep walked and sometimes ran to eagerly graze the alfalfa plants.49” So the ewes looked for more alfalfa after realizing that it was present in the pasture. We know that our horses love alfalfa hay.
The aroma of the forage also matters. In a study of cattle, the acceptability of a lower preference fescue was increased when sprayed with juice from a highly preferred Italian rye grass. But the acceptability of the rye grass was reduced when sprayed with juice from the fescue.50 Such aromatic differences are not detectable by humans,51 perhaps because 7 out of 10 of the genes we would need for the job are dysfunctional.52 (That is, we have the genes, but most don’t work. Our distant ancestors might have been able to smell the differences.)
Horses know about grasses.
Finally, horses dislike monotonous diets. Free-ranging horses eat more than 50 varieties of plants,53 and domestic horses show the same need for dietary diversity.54 Horses locked in a stable may become sated with most any food, and look for variety.55 Your horse would likely prefer a green pasture of many species of grass than one that contained just a single species. Variety may be the spice of life.56 To reduce their boredom when in their stall, my horse and mule have a rack of hay and a bucket of forage. In a second bucket they have a supply of electrolytes, in two more buckets they have water, and finally they have a bucket at the door of their stalls for their grain snacks. Five buckets per horse should be plenty of variety, but if you want to add to their dietary adventures, try a Jolly stall snack.
Hay is a grass, such as alfalfa, which has been cut and dried. Grasses differ in their “total nonstructural carbohydrates” depending on the time of day: during the day, photosynthesis increases carbohydrates; at night, photosynthesis stops, respiration continues, and the carbohydrates are reduced. Cattle, sheep, and goats (and presumably horses) can differentiate between forages cut in the morning versus the afternoon.57 Hay cut late in the day contains more carbohydrates than hay cut early in the morning, and dairy cows produce more milk when fed such hay.58 Your horse will likely prefer the afternoon cut.
Contrafreeloading is a voluntary behavior in which an animal chooses to work for food, rather than eat what it has been provided. If you were a mouse, gerbil, rat, bird, fish, cat, monkey, raccoon, or chimpanzee, would you prefer to work for your food, or have it handed to you? If you are like any of the animals in studies of contrafreeloading, you’d rather hunt for it.59 Gerbils, for instance, would rather press a foot pedal to get food than just lift it out of a dish. Raccoons in my back yard will put their hand in my hand, and scrape cashews out onto the ground, where they now have to search for them. Or they will reach into a bucket of dry dog food, lift out a handful, and dump it on the ground, where they now must search for the individual pieces of kibble.
A recent book about house cats, “The Lion in the Living Room”, has some interesting things to say about Felis catus. In house cats, various gastrointestinal, dermatological, and neurological ailments all trace to indoor living and lethargy.60 The solution to such ailments, it seems, may be “food puzzles”, or toys which must be operated to extract food. There is evidence that such puzzles increase activity and reduce problem behaviors in dogs and produce emotional wellbeing in cats.61 Various forms of enrichment appear useful in reducing stress and contributing to weight loss in house cats.
The widespread presence of contrafreeloading tendencies in fish, birds, and mammals suggest that horses would likely rather graze than eat hay from a feeder, given a choice of equally desirable forages. We should take a look at offering a horse hay in a bin alongside hay in a net, to determine how much they enjoy working for their supper.62
Heavy duty hay net, with 2 1/4” mesh. This one can be hung or tossed into the field.63
1 image source: Ransom, Jason I., and Brian S. Cade. “Quantifying Equid Behavior–A Research Ethogram for Free-Roaming Feral Horses.” U.S. Geological Survey Techniques and Methods 2-A9, 23 p. (2009) http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1018&context=usgspubs
2 Dyer, M. I., and U. G. Bokhari. “Plant-animal interactions: studies of the effects of grasshopper grazing on blue grama grass.” Ecology 57, no. 4 (1976): 762-772. But not all researchers agree that herbivory benefits plants. Consider this paper: Belsky, A. J. “Does herbivory benefit plants? A review of the evidence.” American Naturalist (1986): 870-892.
3 Dyer, M. 1. 1980. Mammalian epidermal growth factor promotes plant growth. – Proc. Natl Acad. Sci. USA 77: 4836-4837.
4 Dyer, M. I. and Bokhari, U. G., 1976: Plant-animal interactions: studies of the effects of grasshopper grazing on blue grama grass. Ecology, 57: 762-772.; Reardon, Patrick O., C. L. Leinweber, and L. B. Merrill. “Response of sideoats grama to animal saliva and thiamine.” Rangeland Ecology & Management Archives 27, no. 5 (1974): 400-401.; Reardon, P. O. and Merrill, L. B., 1978: Response of sideoats grama grown in different soils to addition of thiamine and bovine saliva. Abstracts of the First International Rangeland Congress, Denver, Colorado, p. 37.
5 Owen, Denis F., and Richard G. Wiegert. “Mutualism between grasses and grazers: an evolutionary hypothesis.” Oikos (1981): 376-378; McNaughton, S. J. 1979. Grazing as an optimization process: grass-ungulate relationships in the Serengeti. – Am. Nat. 113: 691-703.
6 Martinez, Ines, Gaston Ares, and Patricia Lema. “Influence of cut and packaging film on sensory quality of fresh cut butterhead lettuce (Lactuca sativa)” Journal of food quality 31, no. 1 (2008): 48-66.
7 Image source: Manske, Llewellyn L., Amy K. Kraus, Thomas C. Jirik. “Manipulating Grass Plant Growth Can Enhance Forage Production” Dickinson Research Extension Center. https://www.ag.ndsu.edu/archive/dickinso/grassland/news/news1.htm
8 Coughenour, Michael B. “Graminoid responses to grazing by large herbivores: adaptations, exaptations, and interacting processes.” Annals of the Missouri Botanical Garden (1985): 852-863.
9 Coughenour, M. B. 1985. Graminoid responses to grazing by large herbivores: adaptations, exaptations, and interacting processes. Ann. Mo. Bot. Gard. 72:852-863
10 Sometimes called “convergent selection pressure”.
11 “50 Million Years of Horse Evolution. The Evolution of Horses, from Eohippus to the American Zebra” May 27, 2016 http://dinosaurs.about.com/od/otherprehistoriclife/a/horses.htm
12 Owen, Denis F., and Richard G. Wiegert. “Mutualism between grasses and grazers: an evolutionary hypothesis.” Oikos (1981): 376-378.
13 McNaughton, S. J. “Grazing as an optimization process: grass-ungulate relationships in the Serengeti.” American naturalist (1979): 691-703.
14 Schönbach, Philipp, Hongwei Wan, Martin Gierus, Yongfei Bai, Katrin Müller, Lijun Lin, Andreas Susenbeth, and Friedhelm Taube. “Grassland responses to grazing: effects of grazing intensity and management system in an Inner Mongolian steppe ecosystem.” Plant and Soil 340, no. 1-2 (2011): 103-115.
15 For a review of this topic, see Davison, Brian, A. Brunner, C. Ammann, C. Spirig, M. Jocher, and A. Neftel. “Cut-induced VOC emissions from agricultural grasslands.” Plant Biology 9, no. S 01 (2007): e60-e68.
16 Croft, Kevan PC, Friedrich Juttner, and Alan J. Slusarenko. “Volatile products of the lipoxygenase pathway evolved from Phaseolus vulgaris (L.) leaves inoculated with Pseudomonas syringae pv phaseolicola.” Plant Physiology 101.1 (1993): 13-24.
17 Yoon, C. K. “No Face, but Plants Like Life Too.” The New York Times (2011).
18 Ortega, Isaac M., Sergio Soltero-Gardea, Fred C. Bryant, and D. Lynn Drawe. “Evaluating grazing strategies for cattle: deer and cattle food partitioning.” Journal of Range Management (1997): 622-630.
19 Boyd, L.E. & Keiper, R.R. (2005). Behavioural ecology of feral horses. In The domestic horse. The evolution, development and management of its behaviour (eds D.S. Mills & S. McDonnell), pp. 55-82. Cambridge University Press.
20 Duncan, P. (1992) Horses and Grasses. The Nutritional Ecology of Equids and their Impact on the Camargue Springer-Verlag, New York.; Fleurance, G., Duncan, P., & Mallevaud, B. (2001) Daily intake and the selection of feeding sites by horses in heterogeneous wet grasslands. Animal Research, 50, 149-156.; Gill, E.L. (1987) Factors affecting body condition of new forest ponies, Ph.D. Thesis, University of Southampton.; Hansen, R.M. (1976) Foods of Free-Roaming Horses in Southern New Mexico. Journal of Range Management, 29, 347.; Mayes, E. & Duncan, P. (1986) Temporal patterns of feeding behaviour in free-ranging horses. Behaviour, 96, 105-129.; Menard, C., Duncan, P., Fleurance, G., Georges, J., & Lila, M. (2002) Comparative foraging and nutrition of horses and cattle in European wetlands. Journal of Applied Ecology, 39, 120-133.; Putman, R.J. (1986) Grazing in temperate ecosystems: Large herbivores and the ecology of the new forest Croom Helm Ltd.; Tyler, S.J. (1972) The behaviour and social organization of the New Forest ponies. Animal Behaviour Monographs, 5, 85-196
21 Gill, E.L. (1987) Factors affecting body condition of new forest ponies, Ph.D. Thesis, University of Southampton.; Putman, R.J. (1986) Grazing in temperate ecosystems: Large herbivores and the ecology of the new forest Croom Helm Ltd.; Tyler, S.J. (1972) The behaviour and social organization of the New Forest ponies. Animal Behaviour Monographs, 5, 85-196
22 Information for this section was derived from a variety of sources, including Cubitt, Tania. “The horse’s digestive system”. Jan 1, 2010. http://www.hygain.com.au/horses-digestive-system/; Hardy, Emma and Patrick Warczak, Jr. “What’s Really Wrong with my Horse?” The Equine Chronicle June/July 2010. http://www.succeed-equine.com/education/tools-resources/articles/whats-really-wrong-with-my-horse/; Gray, Lydia “An Overview of Horse Digestion” https://www.smartpakequine.com/content/horse-digestion; Mills, D., and S. Redgate. “Behaviour of horses.” The ethology of domestic animals: an introductory text, modular texts Ed. 2 (2009): 137-150.; Pratt-Phillips, Shannon “The Equine Digestive System”. Oct 22, 2016 http://www.thehorse.com/articles/32922/the-equine-digestive-system; Waldridge, Bryan. “Gastrointestinal Tract Basics: The Horse’s Foregut” EquiNews March 28, 2011. http://www.equinews.com/article/gastrointestinal-tract-basics-horse%E2%80%99s-foregut; “Feeding Behaviour: Foraging and feeding behaviour” Assessment of Equine Behaviour Author and date unknown. https://assessmentofequinebehaviour.wordpress.com/feeding-behaviour/
23 Lockyer, C. (2005) How to reduce the risk of nutritionally associated laminitis. In The 1st BEVA & WALTHAM Nutrition Symposia (eds P.A. Harris, T.S. Mair, J.D. Slater & R.E. Green). Equine Veterinary Journal Ltd, Newmarket, Harrogate, UK.
24 Tamsin. “How the Rabbit Digestive System Works” The Rabbit House http://www.therabbithouse.com/diet/rabbit-digestive-system.asp
25 Arnold, G.W. (1984) Comparison of the time budgets and circadian patterns of maintenance activities in sheep, cattle and horses grouped together. Applied Animal Behaviour Science, 13, 19-30.; Duncan, P. (1980) Time-budgets of camargue horses. Time-budgets of adult horses and weaned sub-adults. Behaviour, 72, 26-48.; Duncan, P. (1992) Horses and Grasses. The Nutritional Ecology of Equids and their Impact on the Camargue Springer-Verlag, New York.; Francis-Smith, K., Carson, R.G., & Wood-Gush, D.G.M. (1982) A Grazing Recorder for Horses – Its Design and Use. Applied Animal Ethology, 8, 413-424.; Mayes, E. & Duncan, P. (1986) Temporal patterns of feeding behaviour in free-ranging horses. Behaviour, 96, 105-129.; Menard, C., Duncan, P., Fleurance, G., Georges, J., & Lila, M. (2002) Comparative foraging and nutrition of horses and cattle in European wetlands. Journal of Applied Ecology, 39, 120-133.; Salter, R.E. & Hudson, R.J. (1979) Feeding ecology of feral horses in western Alberta. Journal of Range Management, 32, 221-225.
26 Arnold, G.W. (1984) Comparison of the time budgets and circadian patterns of maintenance activities in sheep, cattle and horses grouped together. Applied Animal Behaviour Science, 13, 19-30.; Francis-Smith, K., Carson, R.G., & Wood-Gush, D.G.M. (1982) A Grazing Recorder for Horses – Its Design and Use. Applied Animal Ethology, 8, 413-424.
27 Davidson, N. & Harris, P. (2002). Nutrition and Welfare. In The Welfare of Horses (ed N.K. Waran). Kluwer Academic Publishers, The Netherlands.; Ellis, A.D. & Hill, J. (2005) Nutritional physiology of the horse Nottingham University Press, Nottingham.; Gill, E.L. (1987) Factors affecting body condition of new forest ponies, Ph.D. Thesis, University of Southampton.; Mayes, E. & Duncan, P. (1986) Temporal patterns of feeding behaviour in free-ranging horses. Behaviour, 96, 105-129.
28 Harris and Arkell find that digesta is likely to move on to the small intestine within 20 minutes of being swallowed: Harris, P.A. & Arkell, K.A. (2005) How understanding the digestive process can help minimise digestive disturbances due to diet and feeding practices. In The 1st BEVA & WALTHAM Nutrition Symposia ‘Equine Nutrition for all’ (eds P.A. Harris, T.S. Mair, J.D. Slater & R.E. Green), pp. 9-14, Harrogate, England.
29 image source: Ransom, Jason I., and Brian S. Cade. “Quantifying Equid Behavior–A Research Ethogram for Free-Roaming Feral Horses.” U.S. Geological Survey Techniques and Methods 2-A9, 23 p. (2009) http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1018&context=usgspubs
30 Feist, J.D., and McCullough, D.R., 1976, Behavior patterns and communication in feral horses: Zeitschrift für Tierpsychologie, v. 41, n. 4, p. 337–371.; Rubenstein, D.I, and Hack, M.A., 1992, Horse signals—The sounds and scents of fury: Evolutionary Ecology, v. 6, p. 254–260.
31 Feist, James D., and Dale R. McCullough. “Behavior patterns and communication in feral horses.” Ethology 41, no. 4 (1976): 337-371.
32 Miller, Richard. “Male aggression, dominance and breeding behavior in Red Desert feral horses.” Ethology 57, no. 3‐4 (1981): 340-351.
33 image source: Ransom, Jason I., and Brian S. Cade. “Quantifying Equid Behavior–A Research Ethogram for Free-Roaming Feral Horses.” U.S. Geological Survey Techniques and Methods 2-A9, 23 p. (2009) http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1018&context=usgspubs
34 Mayland, Henry F., and Glenn E. Shewmaker. “Plant attributes that affect livestock selection and intake.” (1999).
35 Grovum, W.L. and H.W. Chapman. 1988. Factors affecting the voluntary intake of food by sheep: 4. the effect of additives representing the primary tastes on sham intakes by oesophageal-fistulated sheep. British J. Nutr. 59:63-72.
36 Fleurance, G., Duncan, P., & Mallevaud, B. (2001) Daily intake and the selection of feeding sites by horses in heterogeneous wet grasslands. Animal Research, 50, 149-156.; Prache, S., Gordon, I.J., & Rook, A.J. (1998) Foraging behaviour and diet selection in domestic herbivores. Ann. Zootech., 47, 335-345.
37 Duncan, P. (1992) Horses and Grasses. The Nutritional Ecology of Equids and their Impact on the Camargue Springer-Verlag, New York.; Menard, C., Duncan, P., Fleurance, G., Georges, J., & Lila, M. (2002) Comparative foraging and nutrition of horses and cattle in European wetlands. Journal of Applied Ecology, 39, 120-133.
38 Cairns, M.C., Cooper, J.J., Davidson, H.P.B., & Mills, D.S. (2002) Association in horses of orosensory characteristics of foods with their post-ingestive consequences. Animal Science, 75, 257-265.; Laut, J.E., Houpt, K.A., Hintz, H.F., & Houpt, T.R. (1985) The effects of caloric dilution on meal patterns and food intake of ponies. Physiology and Behavior, 35, 549-554.; Redgate, S.E., Hall, S., Cooper, J.J., Eady, P., & Harris, P.A. (2006) Post-ingestive feedback on diet selection in horses (Equus caballus); dietary experience changes feeding preferences. In Proceedings of the International Society for Applied Ethology, August 8th-12th, Bristol, UK.
39 Pfister, J.A., Stegelmeier, B.L., Cheney, C.D., Ralphs, M.H., & Gardner, D.R. (2002) Conditioning taste aversions to locoweed (Oxytropis sericea) in horses. Journal of Animal Science, 80, 79-83.
40 Houpt, K.A., Zahorik, D.M., & Swartzman-Andert, J.A. (1990) Taste Aversion Learning in Horses. Journal of Animal Science, 68, 2340-2344.
41 Ungar, E.D., A. Genizi and M.W. Demment. 1991. Bite dimensions and herbage intake by cattle grazing short hand-constructed swards. Agron. J. 83:973-978.
42 Laca, E.A., D. Ungar, N. Seligman and M.W. Demment. 1992. Effects of sward height and bulk density of bite dimensions of cattle grazing homogeneous swards. Grass and Forage Sci. 47:91-102.; Distel, R.A., R.A. Laca, T.C. Griggs and M.W. Demment. 1995. Patch selection by cattle: maximization of intake rate in horizontally heterogeneous pastures. Appl. Anim. Behav. Sci. 45:11-21
43 Orr, R.J., P.D. Penning, A. Harvey and R.A. Champion. 1997. Diurnal patterns of intake rate by sheep grazing monocultures of ryegrass or white clover. Appl. Anim. Behav. Sci. 52:65-77.
44 Wolter, R. L’alimentation du cheval. Paris: Editions Vigot Freres. 1975
45 Salter, R. E., and D. J. Pluth. “Determinants of mineral lick utilization by feral horses.” Northwest Sci 54 (1980): 109-118.
46 Breslin, Paul AS, and Alan C. Spector. “Mammalian taste perception.” Current Biology 18, no. 4 (2008): R148-R155.; Faurion, A. “Taste: a time for re-evaluation, reply.” ECRO News Lett 28 (1983): 291-293.; Kare, Morley R., W. C. Pond, and Joseph Campbell. “Observations on the taste reactions in pigs.” Animal Behaviour 13, no. 2 (1965): 265-269.
47 Mayland, Henry F., Scott A. Martin, Julian Lee, and Glenn E. Shewmaker. “Malate, citrate, and amino acids in tall fescue cultivars: Relationship to animal preference.” Agronomy Journal 92, no. 2 (2000): 206-210.
48 Martin, S.A. 1998. Manipulation of ruminal fermentation with organic acids: A review. J. Animal Sci. 76:3123-3145.
49 Rumbaugh, M.D., H.F. Mayland, B.M. Pendery and G.E. Shewmaker. 1993. Utilization of globemallow (Sphaeralcea) taxa by sheep. J. Range Manage. 46:103-109.
50 Scehovic, J. 1985. Palatability and the organoleptic characteristics of the cultivars and hybrids of tall fescue (Festuca arundinacea). p. 317-319. In: Proc. XV Intl. Grasslands. Congress. Kyoto, Japan.; Scehovic, J., C. Poisson, and M. Gillet. 1985. Palatability and organoleptic characteristics of grasses. I. Comparison between ryegrass and tall fescue. Agronomie 5:347-354.
51 Shewmaker, G.E., H.F. Mayland and S.B. Hansen. 1997. Cattle grazing preference among eight endophyte-free tall fescue cultivars. Agron. J. 89:695-701.
52 Rouquier, S., S. Gaviaux, B.J. Trask, V. GrandArpon, G.van den Engh, J.Dernaille and D.Giorgi. 1998. Distribution of olfactory receptor genes in the human genome. Nature Genetics 18:243-250.
53 Lycurus, Oryzopsis, Artemisia Panicum, and Oenotheria Lesquerella. “Foods of free-roaming horses in southern New Mexico.” (1976): 347.; Putman, R. J., RoM Pratt, JoR Ekins, and P. J. Edwards. “Food and feeding behaviour of cattle and ponies in the New Forest, Hampshire.” Journal of Applied Ecology (1987): 369-380.
54 Archer, Marytavy. “Preliminary studies on the palatability of grasses, legumes and herbs to horses.” Veterinary record 89, no. 9 (1971): 236-240.; Archer, Marytavy. “The species preferences of grazing horses.” Grass and Forage Science 28, no. 3 (1973): 123-128.
55 Goodwin, D., H. P. B. Davidson, and P. Harris. “Foraging enrichment for stabled horses: effects on behaviour and selection.” Equine Veterinary Journal 34, no. 7 (2002): 686-692.
56 Or maybe not. I can eat pizza or cheeseburgers or lasagna again and again and again. So consider this study: Choi, Jinhee, B. Kyu Kim, Incheol Choi, and Youjae Yi. “Variety-seeking tendency in choice for others: Interpersonal and intrapersonal causes.” Journal of Consumer Research 32, no. 4 (2006): 590-595.
57 Fisher, Dwight S., Henry F. Mayland, and Joseph C. Burns. “Variation in ruminants’ preference for tall fescue hays cut either at sundown or at sunup.” Journal of animal science 77, no. 3 (1999): 762-768.
58 Kim, D. 1995. Effect of plant maturity, cutting, growth stage, and harvesting time on forage quality. Ph.D. Dissertation. Utah State University. Logan; Mayland, H.F., G.E. Shewmaker, J.C. Burns and D.S. Fisher. 1998. p. 26-30. In: Proc., 1998 California Alfalfa Symposium, 3-4 December 1998, Reno, NV, UC Cooperative Ext., Univ. of California, Davis.
59 According to Steve Osborne, “Behavior has been maintained on both fixed-ratio (Alferink, Crossman, & Cheney, 1973; Atnip & Hothersall, 1973; Carder & Berkowitz, 1970; Davidson, 1971; Hothersall, Huey, & Thatcher, 1973; Tarte & Vernon, 1974) and variable-interval (Bilbrey, Patterson, & Winokur, 1973; Neuringer, 1970; Rachlin & Baum, 1972; Sawisch & Denny, 1973) schedules of reinforcement when free food was concurrently available. Different strains of rats (Atnip & Hothersall, 1973; Hothersall et al. , 1973; Powell, i974), rats with septal and ventromedial hypothalamic lesions (Singh, 1972a), mice (Pallaud, 1971), chickens (Duncan & Hughes, 1972), pigeons (Neuringer, 1969, 1970), crows (PoweIl, 1974), cats (Koffer & Coulson, 1971), gerbils (Lambe & Guy, 1973), Siamese fighting fish (Baenninger, & Mattleman, 1973), and humans (Singh, 1970; Singh & Query, 1971; Tarte, 1972) have been shown to work for reinforcers when equivalent free reinforcers were available.” — Osborne, Steve R. “The free food (contrafreeloading) phenomenon: A review and analysis.” Animal Learning & Behavior 5.3 (1977): 221-235.
60 Tucker, Abigail. “The Lion in the Living Room: How House Cats Tamed Us and Took Over the World.” October 18, 2016. Simon & Schuster. 256 pp.
61 Dantas, L. M., Delgado, M. M., Johnson, I., & Buffington, C. T. (2016). Food puzzles for cats: feeding for physical and emotional wellbeing. Journal of feline medicine and surgery, 1098612X16643753.
62 I could not find any studies of contrafreeloading in horses, but it seems to me that horses prefer grazing over eating from a manger when there is any grass available, and only resort to the manger when the grass has been replaced with weeds and dirt.
63 Image source: http://www.hayburnersequine.com/store/p12/2.25_inch_Mesh_Heavy_Duty_Hay_net.html