Ingestive behavior includes feeding as well as drinking. Drinking is rela-tively infrequent. However, feeding and resting occupy most of a horse’s day. As one increases, the other typically decreases proportionally. Feed-ing predominates when horses are on pasture or open range and must actively seek sufficient forage to satisfy their needs.
Horses are adaptable to a variety of foods and ingestion schedules. They can tolerate rather desolate conditions with a scarcity of food and water;
yet, horses show a preference for grasses and grass-like plant materials as well as for a nearby water source. Unlike ruminants, their cecal digestion, high intake, and rapid food passage enable horses to adequately maintain themselves on a high fiber, low protein diet (Janis 1976). When preferred foods, such as grasses and legumes, are no longer available, their diet may include roots, herbs, shrubs, woody plants, or aquatic plants; oftentimes, a variety of foods are consumed in one day. Seasonal variation in ingestive behavior occurs in most locations. Thus during an annual cycle male and female horses vary their feeding times, but only slightly. For example, the 24-hour feeding time of weaned, free-ranging Camargue horses varied by less than 10 percent in relation to age, gender, and reproductive state (Dun-can 1992b).
In horses, the lips and tongue are especially agile and accomplish manip-ulation of food and placing it in the mouth. Attached food items are snipped free by the upper and lower incisor teeth and a quick yank of the head. Once food is within the mouth, mastication is accomplished by the grinding action of upper and lower cheek teeth (the molariform premolars plus molars).
Foals commence nursing within an hour or two of birth. Nursing declines over the next few months as time spent grazing increases. In the review that
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follows, nursing will be considered separately from other feeding and drinking behaviors.
Feeding
While grazing or browsing, horses manipulate plant materials with their dexterous upper lip. The preferred plant items are isolated from adjacent materials using the upper lip, then the bundle is passed between the upper and lower incisor teeth where biting, assisted by a jerk of the head poste-riorly, snips off the leaves or other parts, and with the aid of the tongue the materials are ingested into the mouth for chewing (see Figure 6.1).
Other bites may be taken before a bout of chewing commences. Hay, grains, and concentrated feeds are ingested with the combined action of tongue and lips followed by chewing.
Grinding of the food with the well-suited cheek teeth occurs at a rate of 1 to 1.7 times per second (Okuda et al. 1980). The macerated material is then swallowed as one or more boluses of food passing along the esopha-gus to the stomach of 7–14 liter capacity. Commonly, a horse shifts its neck from side to side as it grazes slowly forward, stepping to make additional plants accessible. Selective feeding is typical, yet individuals vary in their selectivity (Marinier and Alexander 1991; 1992).
The rate of feeding varies with the situation. On a Himalayan alpine meadow, Negi et al. (1993) found horses averaged 51 bites/min and 99 mil-ligram dry weight per bite; total intake was 3.25 kg dry matter per day (40 percent was forbes). Duren et al. (1989) observed the ingestive behavior of exercised versus unexercised yearling horses grazing orchardgrass (Dactylis glomerata). The unexercised yearlings ate at the rate of 14.8 bites/min with intake at 861 mg/bite, whereas the exercised individuals (during the first 20 minutes following exercise) ate at the rate of 12.6 bites/min with intake at 865 mg/bite. Thus, taking fewer but larger bites characterized the yearlings who had experienced exercise prior to feeding.
In most cases, the neck must be lowered to place the mouth close to the food material. The body axis is often kept parallel to the direction of wind, and a vigilance is maintained using the eyes and ears. While chewing, the neck and head are momentarily raised and observations of the surround-ings are made before the grazing pattern then continues.
It is not unusual for an entire herd to graze at the same time and in the same direction, maintaining an individual distance of at least one meter
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between each member of the group (Figure 9.1). Even in stables, social fac-tors can influence feeding. For example, Sweeting et al. (1985) found ponies in individual stalls spent considerable time feeding when they could maintain visual contact with their neighbors; however, when solid partitions prevented visual contact, the ponies reduced their feeding time (especially in the afternoon) and spent more time standing alert.
Sometimes horses must dig for food by pawing with a foreleg. In win-ter, pawing is especially utilized in deep snow where numerous strokes may be used to clear a crater and expose plants. In a snow depth of 40–50 cm, Salter and Hudson (1979) found horses pawed an average of 9.7 times per bout (9.1 bouts per 5 minutes) compared to 5.4 strokes per bout (1.4 bouts per 5 minutes) when the snow depth was 10 cm. In shallow snow, horses push away the snow with their muzzle without the need for pawing. In arid habitats where food supplies have dwindled, horses dig up roots using pawing movements.
When feeding on submerged aquatic vegetation, a horse may need to immerse its muzzle well below the water surface. Some stabled horses learn to moisten dry forage by hay dunking—the dunking of roughage into their water supply before chewing (cf. McDonnell et al. 1999). One mare I observed in detail lifted hay routinely to a shelf near an automatic waterer and would proceed to dip mouthfuls of hay at the average rate of 5.1 per minute between bouts of chewing. If the waterer was turned off, or if the roughage was fresh cut or previously moistened, the mare would not do the dunking routine before swallowing (Waring 1974).
The time of day as well as the total time spent feeding are dependent on the quality and quantity of food available to horses, plus such factors as exercise, lactation, weather, and insect pests (e.g., see Martin-Rosset et al.
1978). Environmental disruptions, such as storms or intruders, can tem-porarily cause horses to discontinue grazing. Social factors also influence feeding patterns; for example, as one horse begins to graze other group members are more inclined to graze (a form of social facilitation). Under free-ranging conditions, feeding tends to occur as meals separated by inter-vals of varied length (Mayes and Duncan 1986). Stabled horses fed limited amounts of concentrates, grain, and hay generally consume their ration in one feeding bout and, thus, are unable to exhibit ingestive behavior for the rest of the day. With food made constantly available, Ralston et al. (1979) found ponies consumed 80 percent of their daily intake in an average of 10 separate meals. Each meal lasted 44±10 minutes and averaged 0.5 kg of a pelleted ration; on this diet, 38 percent of the 24-hour day was spent feeding.
Figure 9.1: Typical feeding activity of horses showing (a) grazing and (b) foraging along surface for such items as acorns.
a.
b.
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The average interval between meals was 84 minutes. Half of the intake was consumed between 0800 and 1700 hours. Houpt et al. (2000) noticed a sig-nificant decrease in time spent eating when pregnant mares were restricted in their water intake.
Horses on pasture tend to graze in cycles with three or more prolonged feeding periods per day. Breaks of up to several minutes may occur within a period of grazing, but long breaks separate one feeding period from the next.
Francis-Smith et al. (1982), using a special electronic device attached the horse’s halter, recorded a continuous bout of grazing lasting 178 minutes in a 760 kg male horse. When the automatic-recording device was used to mon-itor another horse’s grazing behavior for seven 24-hour intervals spaced over several months, 5–7 major grazing periods characterized each day’s feeding pattern with an average total grazing time of 15 hr 41 min per 24-hour day (range = 14 hr 34 min to 16 hr 50 min; 60.7–70.1 percent of 24-hour day).
Observing yearling horses on pasture, Kusunose et al. (1986) found the mean duration of grazing bouts increased as group size increased from 1 to 4.
In general, undisturbed, free-ranging equids feed 59–69 percent (14–16.5 hr) per day (Duncan 1992b). Feeding is scattered throughout the 24-hour period so that the gut remains relatively filled, but as environmental con-ditions dictate certain hours are often emphasized more than others. Salter (1978) found feral adult horses of western Alberta fed about 75 percent of the daylight hours in winter and spring, whereas foals spent 41 percent of their time foraging. Similarly in England during winter, ponies ranging the New Forest spent most of their daylight hours grazing and browsing; but after May, resting time increased as grazing time decreased. Then as flies became abundant in June, the ponies remained in the shade with few feed-ing excursions between 0900 and 1400 hours (Tyler 1969). Both Tyler (1969) and Salter (1978) noted that peak grazing activity in daylight occurred about dawn and again in the late afternoon. One or two prolonged resting periods were typical between the peak feeding times.
In feral horses of Nevada, Berger (1986) found total time spent feeding was greatest in winter and least in summer. On average, stallions grazed 70.5 percent of the daylight period when their home-range quality was poor, but when stallions had access to high-quality food their grazing time was 57.9 percent. Similarly for mares on poor forage, the total daylight feeding time was 68.3 percent for barren mares and 78.1 percent for lactating mares;
on high-quality forage, barren mares fed 58.4 percent and lactating mares fed 65.8 percent of the daylight hours.
For free-ranging Misaki horses of Toi Cape in Japan, Kaseda (1983) found grazing centered on quality grasslands during the growing season;
however, in winter, those sources waned and the horses frequented forests and weedy grasslands. Grazing in winter occupied 71 percent of the 24-hour period, whereas in summer grazing occupied 76.1 percent of each day. Nighttime grazing was common in summer but was relatively low in winter.
Grazing at night can be a common activity for horses. For example, on Assateague Island along the Maryland-Virginia coast, feral ponies during summer nights were found to graze 54.6 percent of the nocturnal period (Keiper and Keenan 1980). Although grazing occurred periodically through the night, there was a tendency for greatest feeding activity early in the evening and again at dawn. On a barrier island of North Carolina, Ruben-stein (1981) found extensive nocturnal foraging occurred, declining only slightly from the daylight rate; the feral horses he observed fed 75 percent of the 24-hour day.
In Poland, Kownacki et al. (1978) seasonally sampled day- and night-time behavior of horses on a forested reserve. They found adult horses for-aged nearly 70 percent of each 24-hour day, with little apparent change in the total grazing time between early summer, fall, and winter. During the winter, supplementary hay was utilized.
In the Camargue region of southern France, Duncan (1980) found a slight tendency for mares to spend more of their time (58.5–63.1 percent) foraging in all seasons than mature stallions (50.8–59.7 percent). Winter forage was scarce. In the growing season, horses fed extensively on emergent marsh vegetation.
Foals do little grazing during their first few weeks and unless utilizing a slope or hummock must spread their forelegs to allow the mouth to reach plants close to the ground. Some foals reach the plants by flexing the forelegs at the knees. As the foal develops, grazing activity increases. Tyler (1969) found a tendency for foals not only to graze more with age but also to graze significantly more (P<0.001) during the late afternoon (Figure 9.2).
Crowell-Davis et al. (1985b) found similar trends; in their study, foals also showed a second period of feeding during early morning. Almost all graz-ing by foals was done while their mothers grazed. On several occasions foals were seen to eat humus. In the Red Desert of Wyoming, Boyd (1980) noticed a two-day-old orphan foal in its effort to survive cropped tips of grasses and brush with little selectivity.
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