Fig. 1–2(A,B). Waterers that automatically fill when their water level is lowered. They may have a thermostatically controlled heater to prevent the water from freezing during cold weather. Although many automatic waterers’ basins hold only 1 to 2 gal (4 to 8 L), they refill rapidly. If horses always have an automatic waterer readily available, one is generally enough for a number of horses, even during hot weather, because regardless of water needs, horses drink a relatively small amount at one time. With increased water needs, horses drink more frequently, not longer nor with more than generally 1 or 2 drinks each drinking bout. This may not be the case, however, if the horses have access to water for only specific short periods of time. Feed and other debris should be removed from water containers daily, and they should be thoroughly cleaned frequently. They should be placed away from the feed-bunk to minimize their contamination with feed (A). A double waterer (B) may be placed between two stalls or two paddocks.

When water is readily available, increased water consumption occurs as a result of increased drinking frequency, not increased drinking duration or the number of drinks taken during a drinking bout. For example, there is a direct correlation between drinking frequency and ambient temperature, with a large increase in frequency at temperatures above 85°F (30°C). When water is readily available, most horses drink once for only about 30 seconds or less every few hours. However, if water is not readily available such as if there is a long distance between preferred grazing areas and water, more and longer drinks may be taken during a drinking bout.

Water Deficiency

Inadequate water intake is quite detrimental. With the exception of inspired oxygen, a deficiency of water produces death more rapidly than a deficiency of any other substance. The first noticeable effect of inadequate water intake is decreased dry feed intake, followed by decreased physical activity and ability. Inadequate water intake is also believed to increase the risk of intestinal impactions and colic. Water deprivation for 24, 48, and 72 hours decreased the normal resting horse’s body weight 4%, 6.8%, and 9%, respectively, when the ambient temperature was 63-81°F (17-27°C). At an ambient daytime maximum temperature of 104°F (40°C), body weight decreased 11 to 13% after 60 hours, and 14 to 16% after 72 hours of water deprivation. Signs of dehydration, such as dry membranes and mouth and sunken eyes, are not evident until at least a 6% loss of body weight has occurred. Less than one-half this amount of dehydration is likely to decrease physical performance. Thus, the horse’s physical performance ability is decreased long before a water deficiency induced dehydration can be detected from the horse’s appearance.

Inadequate water intake occurs when water is poorly palatable or accessible. Palatability is best determined by tasting the water and, if there has been a change in the water or its source, comparing its taste to that to which the horse is accustomed. Poor palatability may be due to poor water quality. Water may be poorly accessible for many reasons, such as if electric heaters with wiring problems cause the animal to be shocked when attempting to drink, or if water is frozen over. Ambient temperature-induced variations in water temperature may not alter water intake. Although this situation has not been studied for horses, cattle drink similar amounts of cold or warm water, although individual cattle or horses may have a preference. Cattle, and therefore possibly horses, will consume sufficient snow or crushed ice to meet their water needs if snow or ice is available but water isn’t. However, in doing so, the total amount of water and feed consumed will be reduced.

Water Quality

The single most reliable indication of water quality is the amount of total dissolved solids (TDS) in the water. The amount of TDS, as given in Table 1–2, provides a useful overall indication of the suitability of a particular water source for livestock use. Water high in TDS may be undesirable or unfit for consumption. This occurrence is most prevalent in arid areas, such as the western non-coastal part of the United States. A TDS of 6,500 ppm (parts per million or mg/L) is considered the upper safe limit in water for horses.

The amount of TDS is the sum of the concentrations of all substances dissolved in water. The term “salinity” as applied to fresh water is often used synonymously with TDS. Another term used to described water quality is total alkalinity, but this is not as good an indication of water quality as is TDS. Total alkalinity is the sum of the concentrations of alkali metals, which are primarily sodium and potassium, but may also include lithium, rubidium, cesium, and francium. The hydroxides of these metals are alkaline; i.e., in water they neutralize acids. The total alkalinity of water is always less than its TDS, or salinity, since TDS and salinity include the sum of the concentrations of all substances dissolved in water, and total alkalinity includes only the sum of the concentrations of alkali metals. Salinity and TDS should not be confused with hardness. Highly saline waters may contain low levels of the minerals responsible for hardness. Water “hardness” indicates the tendency of water to precipitate soap or to form a scale on heated surfaces. Hardness is generally expressed as the sum of calcium and magnesium reported in equivalent amounts of calcium carbonate. Other substances, such as strontium, iron, zinc, and manganese, also contribute to hardness.

TABLE 1–2 A Guide to the Suitability of Water for Livestock

^a Total dissolved solids, total soluble salts, or salinity in the water in ppm or mg/L

Sodium, potassium, calcium, magnesium, iron, chloride, and sulfate in water are not toxic, but high concentrations decrease water palatability. In contrast, a number of other substances, which may be present in water, are quite toxic if sufficiently high concentrations are present. Toxic concentrations of water contaminants, excluding pesticides and herbicides, most commonly occur as a result of stagnant or runoff water that contains disease-producing organisms, or from industrial wastes. A list of the recommended upper limits for some potentially toxic substances in drinking water for horses, and those not toxic but which, if present at concentrations above those given, reduce water palatability, is given in Table 1–3. Some potentially toxic substances do not reduce water palatability and, therefore, water intake. Thus, they are potentially even more harmful than those that do decrease palatability. In addition to these contaminants, drinking water containing some bacteria and algae may be harmful.

Some species of blue-green algae, which grow on pond and lake water, may result in poisoning; therefore, water with heavy algae growth should be avoided. Heavy algae growth occurs most commonly during summer and fall in shallow, still water rich in organic nutrients. These nutrients may be increased, and thus algae growth promoted, by runoff of nitrogen or phosphate from slurry lagoons, or of fertilizers applied to fields. Steady prevailing winds may concentrate the algae at one end of the pond or lake, increasing the risk of poisoning. The algae may be visible on the water surface or mixed with the water.

Blue-green algae poisoning in domestic livestock may cause sudden death or else photosensitization, tremors, weakness, bloody diarrhea, and convulsions. Clumps of algae may be found in the gastrointestinal contents of animals that die suddenly. Copper sulfate added to pond water up to a concentration of 1 ppm (1 mg/L) has been used successfully to kill algae blooms, but will probably be harmful to other types of aquatic life.

Water high in bacteria is usually also high in nitrates as a result of surface contamination from manure and barnyard runoff. However, high nitrate water levels may come from other nitrate sources, such as crop fertilizers, and not be high in bacteria. Nitrates may build up in well water by leaching down through the soil. Water nitrate levels may fluctuate widely; they are generally highest following wet periods, and lowest during dry periods of the year. Since nitrates dissolve in water, they cannot be filtered out; however, commercially available anion exchange units remove both nitrates and sulfates. Nitrate toxicosis, however, as described in Chapter 19, is rare in horses, if it occurs at all, and in livestock is most often associated with high nitrate levels in forage, not water. Water sulfate concentrations exceeding 1000 ppm may cause diarrhea, although animals develop a tolerance to a constantly high level of sulfates and can tolerate two to three times this concentration after a period of time. Water with low levels of sulfates, however, may have an odor and reduced palatability.

In most areas and situations, bacteria in water pose a greater threat than the contaminants previously discussed and listed in Table 1–3. Most infectious diseases can be transmitted from contaminated water to animals. If water nitrate or phosphate concentrations are low, the water probably does not contain excessive bacteria. However, if either is high, bacterial levels may be elevated and should be checked. The accepted criterium for the sanitary quality of water is the absence of coliform bacteria. Although all coliform bacteria are not disease producing, many are, and their presence indicates that other infectious bacteria and viruses may be in the water. The U.S. Public Health Service considers water containing coliform bacteria (M.P.N.) of 9 or more coliforms per 100 ml unsafe for human consumption. In some countries, levels of 50 coliforms/100 ml are acceptable. What amount is safe for horses isn’t known but, of course, also depends on which organisms are present.

TABLE 1–3 Recommended Upper Safe Level (USL) of Water Contaminants

^aAll values given are in parts of contaminate per million parts of water (ppm or mg/L). For conversion to other units, see Appendix Table 9.

^b These contaminates are not toxic but at concentrations above the amount given may decrease water palatability. In contrast, many of the other contaminates listed may be toxic if water containing concentrations above those given here is the only water consumed.

^c A higher concentration may be safe for horses, as 2.5 ppm results in mottled enamel during teeth development in calves but no observable effects occur in mature cattle at concentrations of less than 8 ppm, and horses are reported to tolerate fluoride intakes two to three times greater than cattle. A concentration of 4 ppm is probably marginally safe for horses, but water with more than 8 ppm should be avoided.

^d High nitrate concentrations in water occur most commonly as a result of fecal contamination.

^e Although chronic selenium toxicosis has been reported as a result of consumption of water containing 0.0005 to 0.002 ppm selenium, concentrations below 0.01 ppm are not generally considered harmful.

^f Or 833 ppm sulfur. Although sulfate concentrations above 300 to 400 ppm can be tasted, and above 750 ppm can have a laxative effect in people, a concentration below 2500 ppm has no effect on growing or reproducing cattle or swine.⁵⁵ The highest no-effect concentration in horses isn’t known but is probably similar to that for cattle and swine.

^g High zinc concentrations may occur where galvanized pipes are connected to copper. This results in electrolysis, releasing zinc from the galvanized pipes into the water.

Salmonella species is generally the bacterial contaminant in water most likely to cause disease in farm animals. Giardia is the most common cause of water-related illness in people, partly because it survives chlorination. Although giardiasis is rare in farm animals, it can cause diarrhea in young animals. The most common method of destroying bacteria in a water supply is chlorination, although iodine, ozone, exposure to ultraviolet rays or ultrasonics, or filters may be used. Objectionable chlorine taste and odor can be removed from water by an activated carbon filter.

In summary, flowing surface water is most likely to have bacterial contamination, pond or lake water is most likely to contain blue-green algae, and well water, particularly in arid areas, is most likely to have high mineral concentrations. Coliform counts and measures of total dissolved solids are the main indications of water quality.

DIETARY ENERGY

Energy Sources and Use

As described by the first law of thermodynamics, energy can be changed from one form to another but can be neither created nor destroyed. The source of all energy for all living things is sunlight. This energy is captured by plants, which use it via photosynthesis to change carbon dioxide, present in the air, and water to oxygen and the carbon compounds that make up the plant. These carbon, or organic, compounds in plants are carbohydrates, fats, and, along with nitrogen, protein. Nitrogen, like the small amount of minerals needed by plants, is taken up from the soil but is originally from the air. Carbohydrates, fats, and proteins are stored sources of energy.

Animals eat plants, or tissues of others that have eaten plants. The stored sources of energy, carbohydrates, fats, and proteins in the plants are digested, absorbed, and transported to the animal’s body cells. Some are used to make up the structural components of the cell and thus the animal; but, if needed in the future, along with oxygen from the air, they can be converted by chemical reaction to carbon dioxide and water, in the process producing energy. Animals use the energy to produce heat and adenosine triphosphate, or ATP, which cells then use to function. Thus, plants and animals have a mutually sustaining relationship in which plants produce carbon compounds (C-cpds)—organic matter and oxygen (O₂)—that support animals, and animals produce the carbon dioxide (CO₂) and water (H₂O) that support plants Thus, the horse, like us, plants, and all living things on earth, are products of the air and the soil, and function solely as the result of solar energy.

Energy has no measurable dimension or mass, but it can be converted to heat, which can be measured. Oxidation. or burning, converts stored energy (carbohydrates, fats, and proteins) to heat, carbon dioxide, water, and a nitrogen compound. These are returned to the air and soil, where they originated, available to begin the cycle again. When a substance is completely oxidized, the heat produced, called the heat of combustion, is the total or gross amount of energy stored in, and thus available from, that substance. However, as shown in Figure 1–3, the animal cannot use all of the gross energy present in a feed. Some of the stored energy in a feed is not digested and is lost in the feces. Of the remainder, called digestible energy, some is lost in the urine as urea and in the gastrointestinal tract as gases (primarily methane), leaving metabolizable energy, of which some is used in metabolizing feed. What remains is the net energy available for maintenance, growth or fattening, milk production, and physical activity. Most of the total dietary energy needs are for maintenance. Even during heavy lactation or physical activity over 50% of dietary energy is needed for maintenance, and for young horses, 60 to 95% of dietary energy needs are for maintenance, leaving 5 to 40% for their growth.

Fig. 1–3. Dietary Energy Partition.

The amount of heat produced by oxidation (burning) that raises the temperature of one gram of water 1°C is defined as one calorie. This is also known as the small, gram, or standard calorie. However, it is not used in nutrition for animals or people. The calorie used in nutrition is the amount of heat required to raise one kilogram of water 1°C. It is called the large calorie, Calorie, or kilocalorie (kcal) since it is equal to 1000 small calories. In nutrition, the word calorie always refers to kilocalorie, even if it is not capitalized and neither the kilo- or k-prefix is used. For large animals, such as horses, megacalorie (Mcal), therm, or total digestible nutrients (TDN) are usually used. One megacalorie equals one therm, and both equal 1000 kilocalories. Occasionally, primarily in England, instead of calorie, the joule, or, in the physical sciences, the British Thermal Unit (BTU) is used. One megacalorie equals 4.1855 megajoules and 3968 BTU. TDN is a measure of digestible energy expressed in units of weight or percent, with 1 lb TDN equal to about 2.0 Mcal DE (1 kg TDN = 4.4 Mcal). TDN is the sum of digestible carbohydrates, plus digestible protein, plus digestible fats times 2.25, because fats provide about 2.25 times more energy than an equal weight of carbohydrates or proteins. Starch equivalent, or SE, is occasionally used as an energy term by comparing the energy provided by a feed to that provided by starch, which is assigned a value of 100%. Although occasionally used for ruminants, SE is unsuitable for horses because of their different digestive process.

When calculating energy intake, or the amount of feed needed to provide a certain amount of energy, such as that necessary to meet the animal’s energy requirements, any of the various energy terms may be used. Of course, the same units must be used for both the energy content of the feed and the animal’s energy needs. Net energy is the most accurate, followed by metabolizable energy (Fig. 1–3). However, they are the most difficult to determine and, as a result, are not routinely available for most horse feeds. Digestible energy values (or TDN) are generally available for most horse feeds and therefore are the most commonly used energy terms. Energy available from forages is usually 5 to 15% higher for cattle than for horses because of ruminants’ more efficient utilization of fiber. Therefore, if the energy content of forages for cattle, or other ruminants, is used to determine the amount of these feeds needed by horses, the amount determined generally will be erroneously low.

Energy Needs

Numerous factors can influence the energy requirements of the horse. These include environmental conditions, the horse’s functions and activity (including intensity and duration of work, weight and ability of the rider, and conditions of the traveling surface), and its physical condition and degree of fatigue. Even when all of these factors are identical, individual horses vary in their energy needs. The average amount of energy needed, as given in Appendix Tables 1, 4, and 5, should therefore be considered only a general guideline—an amount that will be relatively close for a group of horses but either inadequate or excessive for some individuals.

The amount of digestible energy (DE) needed for maintenance (i.e., for no weight change by the mature, idle nonreproducing horse at moderate environmental conditions) by the average horse weighing 1320 lbs (600 kg) or less can be calculated from the following equation.

However, energy needs are lower per unit of body size in horses weighing over 1320 lbs (600 kg) and can be calculated from the following equation.

Estimates of energy requirements for physical activity or work depend primarily on the total weight carried (horse, intestinal fill, rider, and tack) times the distance moved, but increase with decreasing ability of the rider and physical condition of the horse, difficulty of the terrain and surface covered, and other factors. For ponies and light horses, the Mcal DE/day for light, medium, and intense work has been estimated to be respectively 1.25, 1.50, and 2.0 times that needed for maintenance (Appendix Table 4); with light work being activities such as Western and English pleasure, bridle path, hack, and equitation; medium work being ranch work, roping, cutting, barrel racing, and jumping; and intense work being race training, endurance racing, and polo. Digestible energy needs greater than those required at rest have also been estimated in Mcal/hr/100 kg (220 lbs) total weight carried to be 0.17 for a slow walk; 0.25 for a fast walk; 0.6 for a slow trot; 1.0 for a medium trot or slow lope; 1.3 for a fast trot; 2.0 for cantering, galloping or jumping; and 3.9 for a fast gallop (Appendix Table 5).

For draft horses, energy needs depend on factors such as the size of the load pulled and the type of work. Increasing maintenance energy needs by 10%/hr of field work is a reasonable estimate.

For pregnant mares, energy needs do not increase greatly until the last 3 months of gestation, which is when the greatest development of the fetus occurs. Energy requirements for the ninth, tenth, and eleventh months of pregnancy average, respectively, 1.11, 1.13, and 1.20 times that needed for maintenance that needed for. During the first 3 months of lactation, energy needs average 1.8 times, and from 3 months until weaning, averages 1.5 times that needed for maintenance (Appendix Table 4). For young horses, energy needs increase with increasing growth rate and size, but per unit of body weight decreases as the horse gets older and bigger, and growth rate slows (Appendix Table 4).

Guidelines on the amount of feed needed to meet the horse’s energy requirements are given in the feeding programs described throughout this book. How to calculate the amount of energy and feed needed by the horse is described in Chapter 6.

Energy Deficiency

After an animal’s initial adjustment to the palatability of the feeds in its diet, the average amount consumed by the healthy animal will be an amount adequate to meet its energy needs if the feed is available and its gastrointestinal tract will hold that amount. The maximum daily amount of air dry feed that a horse can consume is equal to 3 to 3.5% of its body weight. If this amount of feed does not meet the horse’s energy needs, the energy concentration of the diet must be increased. For the horse, this is accomplished by feeding more grain, adding fat to the diet or feeding a more digestible, better-quality forage.

There are four reasons the horse may not consume enough dietary energy to meet its needs: (1) a sufficient amount of feed is not available, (2) its gastrointestinal tract won’t hold enough of the available feed because the digestible energy density of the feed is too low, (3) it can’t consume enough because of a physical problem (e.g., injury or bad teeth), or (4) it doesn’t want to consume enough because of illness, stress, inadequate water intake, or poorly palatable feed. Regardless of the reason for inadequate feed intake, the first and most noticeable effect is lassitude. This is because horses need 80 to 90% of all the feed ingested for energy, 8 to 14.5% for protein, 2 to 3% for minerals, and less than 1% for vitamins. With inadequate feed intake, the greatest deficit will be dietary energy, followed by protein. Unless there is a disease-related increase in the loss of minerals or vitamins, signs and effects of deficiencies of these nutrients, during periods of inadequate feed intake, occur much later, to a lesser degree, and are masked by signs of energy and protein deficiency.

Inadequate feed and, therefore energy, intake causes hormonal changes that decrease the body’s energy utilization by reducing physical activity, milk production, and growth rate. The hormonal changes increase utilization of the body’s stored and structural sources of energy (carbohydrates, fats, and proteins) resulting in weight loss. The utilization, or deposition, of excess body fat and protein alter the horse’s appearance, as described in Table 1–4

The horse’s small stores of carbohydrates are depleted within the first few days of total food deprivation. Within 1 week, the body adapts by increasing body fat utilization, thus conserving body protein. Because the principal function of body fat is as a storage source of energy, its loss during starvation does not impair critical body functions. For this reason, loss of body fat does not seriously threaten survival, unless peripheral body fat stores are mobilized very rapidly, resulting in hyperlipidemia, i.e., excessive lipids in the blood. This generally occurs in ponies, particularly if they are obese, and in ponies and horses when there is both a decrease in feed intake and an increase in stress, such as transit, systemic illness, pregnancy, or lactation. Depression, weakness, decreased food intake, incoordination, recumbency, and death may occur. In most cases of inadequate feed intake, however, hyperlipemia doesn’t occur sufficiently to cause these effects.

Once body fat stores near depletion, utilization of the body’s only remaining source of energy, protein, accelerates. Body protein use is not random. Proteins providing structural support in the form of bones, ligaments, tendons, and cartilage are used after those in the blood, intestines, and muscle. During feed deprivation, a loss of function occurs earlier in the tissues and organs whose protein is used first. The order of occurrence of decreased organ function either as a result of feed and, therefore, energy deprivation or as a result of protein deprivation is as follows.

TABLE 1–4 Horse’s Appearance Associated with Dietary Energy Intake^a

^a A body condition score of 5 indicates the proper amount of dietary energy intake, 3 or less inadequate energy intake, and 7 or greater indicates excess energy intake. (From Ott EA, Chairman, Subcommittee on Horse Nutrition: Nutrient Requirements of Horses. 5th ed. National Academy Press, Washington, DC (1989).)

Liver and plasma proteins decrease. If sufficiently severe, the decrease allows fluid to leave the plasma, resulting in edema and stocking.

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