Equine nutrition and metabolic diseases

Chapter 3


Equine nutrition and metabolic diseases





Chapter contents



INTRODUCTION


GUIDE TO TYPICAL FEEDSTUFFS



THE DIGESTIVE TRACT



NUTRIENT REQUIREMENTS



PRACTICAL NUTRITION AND PRACTICAL RATION FORMULATION



FEEDING THE PERFORMANCE HORSE



FEEDING THE PREGNANT/LACTATING MARE



FEEDING THE FOAL AND WEANLING



FEEDING THE AGED HORSE



NUTRITION AND MANAGEMENT IN THE FIELD


NUTRITIONAL ASPECTS OF METABOLIC DISEASES




INTRODUCTION


Nutrition can be defined as the process of feeding and the subsequent digestion and assimilation of food. It is, therefore, of fundamental importance to the well-being of any animal. However, good nutrition will only help a horse to be able to perform optimally; it cannot improve the intrinsic ability of the animal (nor the horse and rider combination). Poor nutrition, on the other hand, may impose limits on an animal’s ability to perform. In most instances it is only when an adult horse is asked to exert itself that the result of imbalanced or inadequate nutrition becomes apparent.


Many of the nutrients we now take for granted were only recognized a relatively short time ago. Although in the early nineteenth century it was appreciated that calcium and phosphorus were needed for “hard bone”, vitamin B12, for example, was not seen as an essential nutrient until the 1940s and selenium not until the 1950s. The original research into the feeding of horses came mainly from army veterinarians and others involved with cavalry, working and pack horses. Mechanization and the replacement of the horse as a means of transport and a power source resulted in a decrease in equine research. The first National Research Council (NRC) report on equine nutrition was published in 1949. The second, in 1961, stated that little new information had been obtained since 1949 and contained many estimates based on extrapolations from ruminants. The next in 1966 was similar. The last one, carried out in 1989, is well overdue for revision. Since then increasing numbers of equine nutrition studies have been carried out, as the popularity of the horse for recreational purposes has increased.


However, many of the equine nutritional practices currently employed have not changed significantly from those followed hundreds of years ago, although the nature and composition of the basic feedstuffs has changed in some instances. The most significant change in the twentieth century was probably the introduction of pelleted feeds around 1920. These became popular in the 1960s when competition, increased knowledge, more ethical companies and government regulations resulted in the evolution of good quality, commercial, compound feeds.


Unfortunately, in many areas of equine nutrition good scientific information is still not available. Confusion and controversy often exist, especially where one research paper’s findings directly contradict those of another. There are many reasons for this confusion. Much of the research has been carried out in ponies and then applied to the horse but such extrapolations may not always be applicable. In other instances, the data available have been extrapolated from another species such as a ruminant. Largely for financial reasons, most of the nutritional research in horses has been carried out on relatively few animals and has concentrated on the effects of short-term alterations in intake.


The adaptive changes that occur over longer periods are therefore not well understood. Differing experimental protocols, basal diets, and exercise regimens may all contribute to the conflicting results. In addition, the normal daily nutrient requirements vary according to several factors, depending on the nutrient involved, and include age, body weight, exercise and environmental conditions. The availability of a particular nutrient may vary not only with the nutrient, its nature in that feedstuff (e.g. of organic or inorganic source) and the presence of other nutrients, but also with the individual animal’s absorptive ability. Nutrition cannot be considered to be an exact science. Considerably more information on the dietary requirements and digestive physiology of domestic ruminants is available than for the horse, however, and more basic and applied scientific research is needed into most aspects of equine nutrition.


Optimal feeding of horses uses both art and science. The science provides the information about the digestive and metabolic processes, the nutrient requirements and the principles behind feeding practices. The art is the ability to convert this theory into practice for the individual horse, its needs, likes and dislikes. In this chapter, a general and, it is hoped, practical guide to equine nutrition is given. Wherever possible, recommendations are based on sound research but interpreted practically. It aims to provide a guideline to follow and to highlight some of the reasons behind the basic rules of feeding. Although, wherever possible, discrepancies between various authors’ recommendations have been removed, some remain because in a number of instances “correct” values are simply not known and the best that exists at the present time is a range of acceptable levels.



GUIDE TO TYPICAL FEEDSTUFFS


Domestication and our increasing demand for horses to perform repeatedly have resulted in energy requirements that, for some horses, are above those able to be provided by their more “natural” diet of fresh forage. Cereals provide more net energy than hay, which in turn provides more than twice the net or usable energy of straw. However, the upper part of the gastrointestinal tract (GIT) has a relatively small capacity and the horse has digestive and metabolic limitations to high grain, starch and sugar based diets. Large grain meals may overwhelm the digestive capacity of the stomach and small intestine leading to the rapid fermentation of the grain carbohydrate in the hindgut. This potentially can result in one of a number of disorders including colic, diarrhea and laminitis.


Therefore, there has been increasing interest in the use of alternative energy sources for horses, especially alternative fiber sources, which do not cause such marked disturbances in the hindgut and yet provide more energy than typical forages. In addition, because vegetable oils provide proportionally more net energy than the cereals, yet contain no starch or sugar and may provide other advantages, there is an increasing use of supplementary vegetable oils.


Table 3.1 provides figures for the typical composition of some common feedstuffs. Individual samples may vary within a range and, for complete accuracy when evaluating a dietary regimen, individual analyses must be undertaken. Pasture and hay analyses vary according to season, soil type, geographic location and other variables such as harvest date. For accuracy, a number of actual representative forage (fresh or preserved) samples must be analyzed when reliable results are required.



The importance of calculating the actual elemental levels of the required mineral from the selected source cannot be overemphasized. The bioavailability of certain minerals will be affected by many factors such as the levels of any antagonistic minerals present. In addition, there will also be variation in utilization between individual horses.


When assessing the nutrient value of particular feedstuffs, it must be remembered that modern methods of feed production have had two major effects on the composition and feed value of equine rations. First, they have allowed a wider range of ingredients to be used as modern production methods destroy many of the harmful substances that might prevent the feedstuff being used in its raw state. Second, they have affected the nutrient availability and digestibility of certain traditional and recently introduced feedstuffs. Moreover, there are several methods of preparation or treatment of feedstuffs that have differing effects on palatability, digestibility and stability in storage.



CEREALS, SUGAR BEET AND OIL SEEDS AS FEEDSTUFFS



Major cereals fed to horses


Oats are the traditional cereal fed to horses in work. They contain significantly higher fiber and lower starch levels than most other cereals and the nature of their starch particles helps to promote a high pre-cecal starch digestibility in contrast, for example, to corn and barley. As with all cereals and cereal by-products, oats provide a low level of calcium and a moderate level of phosphorus (which may be bound in phytate compounds, reducing its availability), giving a reversed calcium to phosphorus ratio (q.v.). Also, in common with most cereals, the level of the essential amino acid lysine is relatively low. There are now varieties of oat produced without husks, colloquially often called naked oats. They have a better amino acid profile, higher oil levels, and thus considerably higher energy levels than the husked varieties.


Barley has a higher energy level than oats and is generally fed rolled or cooked (cooking increases pre-cecal starch digestibility; q.v.). However, the amino acid profile, low calcium level, and phytate-bound phosphorus need correcting when barley is the major ingredient in horse rations.


Maize tends to have the lowest crude protein (CP) of the common cereals fed to horses and the highest energy value. In the UK it is generally micronized or steam flaked before feeding but in the USA and mainland Europe it is often simply cracked to open the outer husk.


Wheat has traditionally not been fed to horses. However, with the introduction of efficient cooking methods, the resultant alteration in the structure of the starch has made it a useful high energy feed for inclusion in commercial coarse mixes and home-mixed cereal rations. Large amounts per meal should be avoided, however, as they may lead to a sticky consistency in the stomach which may favor dysfermentation (q.v.).


Triticale is a hybrid resulting from crossing wheat and rye. In a ground form it should have a feeding value slightly greater than that of barley and evidence indicates that triticale may be better digested by the horse than is wheat. Both rye and triticale are subject to ergot infestation (q.v.), so clean samples should be sought.


Regardless of how triticale, wheat, barley or maize are mechanically processed, the starch will be less well digested in the small intestine than when cooked grains are fed.



Oil seeds


The soya bean is an excellent source of protein in an equine diet, providing good levels of essential amino acids. It must be properly cooked before feeding to help destroy the protease inhibitors as well as potential allergenic, goitrogenic and anticoagulant factors. However, in practice, most soya available in retail outlets has been suitably treated.


Soya is available as an expelled meal, with the oil removed; as the full fat meal, which has added advantages of providing good levels of essential fatty acids; and as the full fat flake, which has usually been micronized. This latter form is the most common to add as a top dressing, as the physical consistency of the meals can be unpalatable to some horses.


Linseed must also be cooked before feeding to destroy the glycoside, which, after soaking, could potentially release hydrocyanic acid. Linseed also has a good amino acid profile and in some methods of processing, where the oil is not extracted, also provides good levels of certain key fatty acids.



Vegetable oils


Horses have been shown to be able to digest and utilize high amounts of oil, but for practical purposes approximately 0.75–1 g of oil/kg BW/day may be considered to be the upper limit. Levels of 5–8% in the total diet are more common in some high performing horses and many performance horses can be fed up to 100 mL/100 kg BW daily in divided doses without any problems provided that it has been introduced gradually, is not rancid, the horse requires such an energy intake, additional vitamin E is provided and the overall diet is re-balanced.


Adding oil to existing feed has the potential to create multiple imbalances (including an inadequate vitamin E intake and a calcium imbalance) and therefore could be considered less safe than feeding a diet where the oil has been balanced in relation to all of the essential nutrients in the feed. Any supplemental oil or oil-supplemented feed should be introduced slowly.


Supplemental fat or oil diets can be supplied in four main ways:



1. As an oil supplemented, manufactured diet. The advantage here is that such diets should be balanced with respect to the protein, vitamin and mineral intake that they provide when fed with forage (and salt as required). This can be a simple, practical and convenient way to feed high oil diets.


2. High oil supplemental feedstuffs, such as rice bran, which are also high in fiber and usually low in starch. However, many of the rice brans available have the same disadvantages as wheat bran in that they have a very imbalanced calcium-phosphorus content.


3. Supplemental animal fat. Many horses find most animal fats to be unpalatable and these fats seem often to be more likely to cause digestive upsets. Their use is not to be recommended.


4. Supplemental vegetable oils. The exact type of oil that may be preferred will depend on the individual horse and the nature of the processing to which the oil has been subjected. Corn oil and soy oil are probably the most commonly used vegetable oils in Europe.


It has been suggested that feeding oil supplemented diets, with appropriate training, can result in a range of effects on a variety of physiologic, metabolic parameters as well as on performance. These include:



In order to have the potential to obtain metabolic benefits from the feeding of oil or oil supplemented diets, in addition to those associated with its high energy density and lack of starch content, the oil needs to be fed for several months.


Linoleic acid and alpha-linolenic acid are considered dietary essential fatty acids. However, no evidence of deficiency has been described for the horse and it must be assumed that a normal diet of cereal grains and natural forage, or of pasture herbage, will provide the dietary requirements.



FORAGES AND OTHER FIBER SOURCES


The choice of fiber source is now much wider with the advent of new methods of grass preservation, improved grass and other forage plant species as well as other technological advances. There are two equally important aspects when selecting the most appropriate fiber source:




Hay


Hay is the most commonly used long fiber source and may be divided into several types.


Meadow hay is generally made from permanent pasture and has a great diversity of species, including several different grasses, herbs and other plants. This hay is often termed “soft”, as the diversity of plants results in different rates of growth and stages of maturity at the time of cutting.


Meadow hay will, on average, also provide higher protein (typically 8–12% dry matter [DM]) and digestible energy levels (typically 9–11 MJ/kg DM) than most seed hays. Meadow hays also often have higher mineral levels than seed hay, as they contain a greater proportion of deeper rooting herbaceous species.


Seed hay is produced from specially seeded leys, usually 1–3 yr old, and contains predominantly one or two grass species (often rye grass or timothy). This hay is more uniform and, due to the growth rate and cutting time, is usually fairly mature, with a lower proportion of leafy material and higher proportion of stalk than meadow hay. Typically, seed hay has lower energy (for example approximately 8–10 MJ/kg DM) and protein levels (approximately 4–8% DM) and a lower digestibility than meadow hay. The mineral levels in seed hay also tend to be lower than for meadow hay, although this will depend on the soil upon which the hay was grown.


Legume hay (usually alfalfa or lucerne, clover or sainfoin) has, due to nitrogen-fixing properties, higher protein levels than seed or meadow hay. Legume hay may be grown as a pure stand, but can be difficult to dry in cool wet climates as the stalks are very moist and thick so that cutting is often left until later in the season, when temperatures increase. However, by then the hay may have become very mature and stalky, with low energy and digestibility values. An exception to this is alfalfa, which can be cut earlier, barn dried and packaged in a short chopped form in plastic-covered bales. Clover and sainfoin are commonly mixed with grass species such as timothy to make conventional hay, but care must be taken with the leys, as competition between the species will alter the relative proportions over a number of years.


Barn-dried hay is wilted in the field for 2–3 days when weather conditions are good and loose packed into special buildings; air of a particular temperature and humidity is then blown through the stored grass for 7–10 days before baling. When correctly practiced, this results in hay with a high DM, thus limiting the development of fungal spores.


Sometimes hay is baled with too high a moisture content due to unsuitable weather conditions at cutting time, insufficient turning in the rows, or other factors. A high moisture content will allow the development of large quantities of fungal spores (q.v.), which can be inhaled by the horse. Hay with a DM ≤87% should always be assessed for spore level.




Semi-wilted forages


In an attempt to reduce the level of fungal spores in forage for horses, methods of preserving grass have diversified. “Dust-free” grass and other forage plants such as alfalfa are wilted to approximately 50–60% DM and packaged in semi-permeable plastic, where a mild lactic fermentation occurs owing to the limited amounts of oxygen present. This lactic fermentation stabilizes at approximately pH 4.5–5.5, depending upon the amount of oxygen and substrate available for the microbial fermentation and helps to inhibit the proliferation of fungal spores. (Water activities <0.985 combined with a low pH may help to control in particular the sporulation of Clostridium botulinum, q.v.).


These forages exert a lower challenge to the horse’s respiratory system and may be a valuable way to provide long fiber to a horse suffering from allergic respiratory disease (q.v.).


Damage to packaging allows the influx of oxygen, permitting further microbial activity to occur. This can be identified as a patch of mold, which is limited by the extent of the oxygen diffusion through the hole in the packaging. There may also be an increase in temperature as a result of the microbial activity. Bags in which this has occurred should be discarded.



Silage


Some larger horse keepers now find it economical to use silage. The rapid pH drop, the greater water activity, the decrease in available oxygen and soluble carbohydrates as fermentation progresses all can help inhibit the development of fungal spores. There are, however, several important factors to be considered before using silage.


First, the DM content is very important to help inhibit clostridial activity. Horses also find very low DM material unpalatable, therefore it is recommended that silage for horses should have a DM >35% and preferably >40%.


Second, pH should ideally be <6.0 in order to inhibit undesirable microbial activity. At pH values between 5.0 and 6.0, the DM should exceed 40% as an added inhibitory factor to clostridial activity. Horses may find very low pH material unpalatable, and silage with a pH <4.5 may be rejected by some horses. There are also anecdotal reports that low pH silage may be extremely unsuitable for donkeys.


Third, it should be appreciated that if the silage was made to meet the nutrient requirements of dairy cattle the nutrient levels may be unsuitable for horses: for example, lactating dairy cattle require a higher daily protein intake than most horses. Finally, once a “big bale” of silage has been opened it should be used within 2–3 days in order to prevent secondary microbial activity from occurring.


Where large amounts of silage or haylage are used, supplementary vitamins D and E are necessary as ensiling destroys vitamin E and vitamin D2 is not synthesized during the ensiling process. It is always advisable to get professional help when considering producing silage for horse feeding.



Straw


Straw provides a low nutrient level forage, which may be used to provide a portion of the daily forage intake for some horses, although problems with spores and dust must be taken into consideration. In addition, due to the high silica and indigestible fiber content there is a risk of impactions (q.v.) especially in Thoroughbreds and Thoroughbred crosses.


Chemical treatment of straw with sodium hydroxide and ammonia may increase nutrient value but this requires specialized equipment and expertise and the practice has not gained widespread popularity. Straw may be sprayed with molasses to increase its palatability but the nutrient values are still very low. There have been some concerns expressed regarding the various chemicals that may be used in grain production (e.g. to restrict the growth in height and to prolong the vegetative stage). It has been thought, for example, that they may leave unwanted residues on the straw; however, currently there is no evidence to support this concern.


Significant intakes of straw should also be avoided in young animals, where the hindgut microflora may not be fully established and the highly indigestible straw may lead to impactions.




Soya hulls and sugar beet pulp


It has been suggested that certain fiber sources (sometimes referred to as highly digestible fibers) such as sugar beet pulp practically provide more digestible energy to the horse than their traditional crude fiber, protein, fat, etc., analysis would suggest. This is in part because sugar beet pulp contains major fractions of pectins, arabinans and galactans, etc., which are lost during the crude fiber analysis, yet these carbohydrates can be fermented and thereby utilized by the horse. In addition, the fiber or more specifically the non-starch polysaccharide (NSP) in beet pulp is highly digestible over the total tract with a significant proportion being degraded in the small intestine during transit to the hindgut.


Various digestibility studies suggest that not only is sugar beet pulp well fermented in the horse (>60% digestibility of organic matter) but that this degradation occurs to a large extent within the time period that such a feedstuff would remain within the gut. This explains why sugar beet pulp and a similar feedstuff, soybean hulls, are increasingly being used as fiber-based energy sources in modern horse feeds.


In the UK, sugar beet is usually molassed and presented as dehydrated shreds of compressed pellets. When rehydrated, there is a considerable increase in volume, and thus the practice of soaking sugar beet shreds or pellets in at least twice their dehydrated volume of water is essential before feeding. Small quantities of extruded unmolassed sugar beet in compound mixes need not be soaked before feeding, as the extrusion process has already expanded the material.



METHODS OF FEED PROCESSING


Mechanical rolling, bruising or grinding of cereal grains aims to break open the outer husk, thereby releasing the floury kernel for enzymic digestion. The disadvantages include a greater predisposition for oxidation and an increase in dust. In addition, mold growth occurs more readily in grains where the kernel has been broken.


Micronizing cereal grains and vegetable protein seeds is a rapid method of cooking and rolling the grains to gelatinize the starch and improve enzymic digestibility within the small intestine.


Steam flaking of cereal grains is a method of mechanically and biochemically altering the structure of the starch molecules in order to improve digestibility.


Extrusion of cereal grains and vegetable protein seeds involves grinding and increasing the moisture of the cereal grains and “cooking” the resultant slurry at very high temperatures and pressures before forcing the cooked material through a die, where the resultant drop in pressure forces a rapid expansion of the material as air enters the mixture. Typically this process leads to a moisture content of 8–10% (or DM of 90–92%).


Oil extraction and “roasting” of high oil seeds is an industrial process. The resultant oils extracted are then available for use in human or animal feeds, depending upon the degree of refinement.


Grinding and steam pelleting: individual ingredients are ground, mixed, steamed and forced under mechanical pressure.


Coarse mixing combines feedstuffs, often processed by one of the above methods, usually mixing them with a sugary syrup such as molasses or glucose.



FEED STORAGE


Correct storage of feedstuffs is essential to preserve their nutrient value, ensure palatability is retained, and help prevent fungal or bacterial contamination, which may affect the horse’s health. In order to help prevent fungal growth, feedstuffs should be of the correct moisture level; cereals with a moisture content >16% must be considered suspect. The treatment of high moisture cereal grains with propionic acid to act as a mold inhibitor and preservative has been practiced, although the effects of this compound on horses have not been widely studied.


Oxidation is a potential problem in rolled, cracked or bruised cereals. This rancidity will clearly affect palatability and may also affect nutrient availability, especially of certain vitamins. There may also be metabolic implications such as an increased level of peroxides. Storing feed at low humidity and low temperatures will reduce the rate at which oxidation occurs. The importance of maintaining feedstuffs at a suitable temperature, humidity and with adequate ventilation must therefore be strongly emphasized.


Infestation with mites is another common cause of feed spoilage. The practice of keeping feeds in galvanized bins can be useful, but care must be taken to ensure the bins are fully emptied and cleaned of all the previous material before a new bag or load is tipped in. In hot weather, high moisture feeds may cause condensation in the galvanized bins, so caution should be exercised.



FEEDSTUFFS ANALYSIS


From time to time it may be necessary to undertake more detailed analyses of specific feedstuffs. Such costly analysis will only be of value if representative samples, especially of pastures or roughage, are taken. Sampling may be required because of a suspected nutritional involvement in a particular disease or disorder where typical, tabulated analyses do not provide the necessary degree of detail or accuracy. Analysis may also be required in order to confirm that dietary ingredients meet the required or designated specification, or to assist with future purchases. If there is any suspicion of a feed related problem the samples should obviously be taken from the feedstuffs that have been fed.


Caution should be exercised as to whether the analyses are given on the fresh as fed material, or on a dry matter basis. “Dry matter” basis refers to the feed or forage after the moisture has been taken out, whereas the term “as fed” refers to a feed as it would be fed to a horse. Most concentrate feeds such as cereals, cubes, pellets, etc., contain approximately 10% moisture with a dry matter content of 88–92% and fresh forage from 20% to 60%. It is important to realize that only the dry matter contains nutrients, so more feed will need to be fed, on an as fed basis, to match requirements if the feed contains more water.


Box 3.1 gives suggestions for routine analyses in order to assess general feed quality and major nutrient levels. In special circumstances, more detailed analyses will be required.



Box 3.1   Suggestions for routine analysis in assessing general feed quality


Cereal grains and by-products, vegetable and animal proteins



1. Dry matter.


2. Crude protein (CP).


3. Digestible crude protein (often based on ruminant data so caution needed with interpretation); or, as a guide,


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4. NDF (and ADF).


5. Ash (as a possible guide to soil contamination).


6. Oil (various methods available, ether extract is usually adequate).


7. Energy (check if given in terms of ruminant ME [metabolizable energy], and conversion coefficients to horse DE should be used—current suggested conversions: divide by 0.9 for high fiber material, divide by 0.85 for low fiber material—very approximate). Many equations are available to estimate DE (i.e. there is no definitive equation) e.g.:


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8. Micronutrient levels for these ingredients may be obtained from published tables.


9. Certain feedstuffs in the above category contain anti-metabolites in their raw state, and where toxic effects are suspected, analyses for the presence of the anti-metabolites should be made.


10. Results should be interpreted in terms of units per day intake (e.g. g/day protein, or MJ/day DE), rather than percentages as, unless intake quantity is known, the figures are meaningless in terms of effect upon the animal.


Forages (including hay, haylage, silage, straw)



Where forages are the main source of nutrients, further mineral analysis may be advisable. In addition to the above, for silage and haylages:



N.B. If abnormally high levels are suspected, nitrate levels should be assessed. Caution should be taken with the interpretation of the nitrate data, as laboratories describe nitrate levels differently.


Pasture and soil


Whilst it may be necessary sometimes to determine the protein and energy levels in pasture, mineral levels are required from both pasture and soil; in the former to ensure any necessary deficiencies are corrected in the concentrate feed, and in the latter to ensure correct fertilizer programs are maintained. Several standard texts on soil analyses and fertilization programs are available.


ADF, acid detergent fiber; NDF, neutral detergent fiber; NfE, nitrogen free extract.



THE DIGESTIVE TRACT



PHYSIOLOGY AND PATHOLOGY OF DIGESTION


The empty alimentary tract accounts for about 5% of the total BW of a horse. The weight of the gut contents varies according to feeding, from 5% (concentrate) to 10% (roughage). In the small intestine, digestion is primarily by the body’s own enzymes, while in the voluminous large intestine the feed components are fermented by microorganisms. It is important to appreciate, however, that some fermentation will occur in the stomach and the small intestine. The approximate sizes of the various sections of the gastrointestinal tract are shown in Table 3.2.




Mastication


The duration of feed intake ( Table 3.3) depends on the type of feed and the size of the animal. By feeding mainly concentrates, the time taken to ingest the feed and the number of chewing movements are greatly reduced. This may lead to a change in behavioral patterns: for example, animals may bite and lick objects within their reach. Horses should be fed sufficient amounts of roughage (preferably long fiber or chop) daily in order to help prevent such abnormal behavior.



During chewing, the feed is thoroughly ground by the molar teeth; at the same time the secretion of saliva is stimulated. The grinding of whole grains is necessary for their optimal digestion in the small intestine. The intensity of the grinding of the roughage may be important for the passage of digesta through the ileocecal orifice and the large intestine. Short chopped straw or hay (≤20 mm) as well as very fine grasses (e.g. wind bent grass [or silky bent-grass], Agrostis spica-venti) may be swallowed without intensive chewing and grinding. This increases the risk of obstructions as well as increasing the risk of dental enamel points and hooks developing due to restricted chewing movements. The ingestion of lawn mower cuttings increases the risk of colic either due to an obstruction (q.v.) or dysfermentation (q.v.).


The ground feed is mixed with varying amounts of saliva depending on the duration of the feed intake ( Table 3.4). This means that the DM of the boluses swallowed is higher after feeding concentrates than roughage. The occurrence of esophageal obstructions depends not only on the swelling capacity of the feedstuff (for example, dried sugar beet pulp), but also the speed of the feed intake and the size and DM content of the boluses swallowed.




Stomach and small intestine


The horse has a small, simple stomach, which is suited to the intake of rather small quantities of feed per meal. The cranial region of the stomach is non-glandular and is lined by stratified squamous epithelium similar to the esophagus. As this region fills, bacterial fermentation of the feed starts. This principally involves lactobacteria, which convert easily soluble carbohydrates to lactic acid.


Microbial activity and degradation is stopped when the gastric contents reach the fundic gland region and mix with the acid stomach juice containing pepsinogen.


Large quantities of digestive fluids are secreted into the small intestine, in particular from the liver (bile) and the pancreas into the duodenum. The main functions of the pancreatic secretion are to neutralize the acid chyme and to provide proteolytic, amylolytic and lipolytic enzymes. Bile also helps to alkalize the digesta, and the bile acids are required for emulsification and digestion of lipids.


Although the pancreatic enzyme amylase hydrolyses starch to disaccharides and trisaccharides, these have to be further digested by the mucosal enzymes before the resultant hexoses can be absorbed. Mucosal enzymes are also important for protein digestion and absorption.


In the small intestine of the adult horse, the digestive processes (i.e. the enzymatic degradation of proteins, fats, starch and sugar) are similar to those of other monogastric animals. However, the activity of most of the enzymes in the chyme, especially amylase, is lower than in other monogastric animals. The type of feedstuff affects the amount of soluble carbohydrate absorbed as glucose: up to 85% of the starch content of whole oats will be digested by the end of the ileum but only 30% of the starch from whole maize (for heat processed maize grains digestibility increases to approximately 90%).


Adult horses (500 kg BW) secrete >100 L fluid/day into the pre-cecal gut at approximately 70–100 mL/min. The DM content of the small intestine is about 5% so that even indigestible fibrous particles can be easily passed to the cecum. At the ileocecal junction, the chyme flow is stopped and the contents are discontinuously pressed into the cecum (5–7 times/h; up to 1 L at a time), which means that obstruction is a potential risk.


For the commonly fed diets consisting of hay and oats, approximately two thirds of the completely digestible parts of the feed will have been broken down and absorbed by the time the ingesta reaches the large intestine.



Large intestine


The large intestine does not possess mucosal enzymes and does not have active transport mechanisms for hexoses and amino acids. Digestion and absorption of residual carbohydrates and proteins relies instead on microbial action and absorption of the end products of microbial fermentation. The intensity of this process depends on the amount and the temporal influx of fermentable material arriving from the small intestine.


This bacterial degradation mainly produces volatile fatty acids (VFA), i.e. acetate, propionate and butyrate, plus amino acids, ammonia, sulfides, etc., and, after a high influx of easily fermentable carbohydrates, lactic acid. The rate of VFA absorption increases with decreasing pH. Disturbances of the digestive processes in the large intestine are marked on the one hand by insufficient microbial activity and on the other hand by accelerated degradation rates, particularly in the cecum.


Excluding damage to the flora (e.g. by antibiotics or mycotoxins) low microbial activity in the large intestine occurs when animals are fed rations consisting mainly of poorly fermentable components such as straw or late harvested hay. If large amounts of these feeds, which are difficult to break down, are ingested, obstruction of the colon (q.v.) may occur due to slow and incomplete microbial activity. This will be aggravated by any factor that decreases the rate of the passage of ingesta, such as lack of water, little exercise, parasites, and intake of soil and toxins which influence the flora of the large intestine, as well as gastrointestinal motility. On the other hand, if large amounts of easily fermentable substances that escaped digestion in the small intestine flow into the cecum, abnormal fermentation in the cecum may result in digestive disturbances and acidosis. This may happen with large amounts of mixed feed per meal or if carbohydrates such as maize starch or lactose are fed. Disturbances are especially likely if the animal has not adapted to a high grain diet.


Undigested proteins and urea that enter the large intestine are broken down by microbial enzymes. The main end product is ammonia, which is absorbed particularly at alkaline pH. Microbial protein, which is synthesized in the large intestine, fundamentally cannot be utilized by the horse. Animals with a high demand for protein (e.g. foals or lactating mares) must therefore be fed high quality protein that can be broken down and absorbed primarily in the pre-cecal section of the gut.


Most water-soluble vitamins as well as the fat-soluble vitamin K are synthesized in the large intestine. The horse appears to be able to utilize these so that oral supply is only necessary under certain circumstances (q.v.).



Water and electrolytes


There is a large water and electrolyte turnover in the gastrointestinal tract. While considerable amounts of water, sodium (Na) and chloride (Cl) enter the small intestine via the saliva, stomach juices, pancreatic juice and bile, only about 50% of water, 35% Na and 80% Cl will be absorbed by the end of the ileum. Therefore a large ileocecal flow of water and Na (and to a lesser extent chloride) takes place ( Table 3.5), but most of the water and electrolytes that enter the large intestine will be absorbed.



Absorption of calcium (Ca) and magnesium (Mg) mainly takes place in the small intestine, while phosphorus (P) absorption occurs predominantly in the large intestine. Therefore a high P intake disturbs Ca absorption to a greater extent than a high Ca intake affects P absorption.


The large water and electrolyte turnover in the intestine has two consequences:





Normal fermentation in meal-fed horses


Ingesta is propelled quite rapidly through the small intestine in a fluid form in the adult horse, some appearing in the cecum within 45 min and much of it reaching that point within 3 h of eating. Protein that escapes digestion in the small intestine is degraded to ammonia by bacteria in the ileum and to a much larger extent in the large intestine. The carbon skeletons are utilized as energy sources by the bacteria yielding acetic, propionic and butyric VFA as by-products. Starch that escapes digestion in the small intestine and most structural carbohydrates are subjected to fermentation by the large gut bacteria, again yielding VFA.


Bacterial fermentation also produces some longer chain fatty acids, as well as lactic acid. These acids and some of the ammonium ions are absorbed from the large intestine and enter the systemic circulation. Much of the ammonia, however, is reutilized by bacteria in the synthesis of bacterial protein, stimulating rapid bacterial growth. The bacterial population therefore ebbs and flows between the surges of ingesta reaching the large intestine.


It is important to note that there is a fluctuation in the hindgut bacterial populations in any meal-fed horse. The microflora population within the hindgut does adapt to a certain extent to the type of feed being fed. However, if the dietary fluctuations are too marked, or excessive starch or rapidly fermentable carbohydrates reach the hindgut even in the concentrate adapted horse, this will result in a significant change in the microbial population, which may have clinical consequences.



Dysfermentation


As well as obstructions, conditions associated with dysfermentation are the main cause of digestive upsets. Mistakes in feeding technique, selecting incompatible feeds or insufficient preparation of some feeds may induce inappropriate microbial growth and/or dysfermentation (q.v.), potentially with severe consequences for the health of the horse.


After ingestion of large meals of starch or sugar, fermentation is more extensive in the first part of the stomach, because the stomach contents have a higher DM content and the mixing of feed and gastric juice is therefore slower. This means that either the normal reduction in pH is delayed or the pH remains >4.5. Large amounts of lactic acid will be produced. The same process can occur with feeds that have a sticky consistency or potentially whenever gastric acid secretion is reduced. A further risk comes from feeds that are contaminated, especially with yeasts, when gas production may be so extensive that there is a risk that a stomach rupture (q.v.) may occur.


Mistakes in feeding technique and the selection of incompatible feeds may also have consequences in the small intestine. Again, inappropriate microbial fermentation (e.g. of easily digestible starch provided in large amounts) may produce high amounts of organic acids, resulting in a reduction in the pH and disruption of normal digestion. Spasmodic colic (q.v.) may be the final clinical result. All factors that reduce passage in the small intestine (excitement, stress, parasites, etc.) favor microbial activity and potentially increase the risk of colic.


As described above, large amounts of starch and, to a lesser extent, protein, ingested in a single meal may have an effect on digestion both in the stomach and the small intestine. In addition, significant amounts reach the large intestine, stimulating an almost explosive growth of microorganisms. Gas production can exceed the rate at which the methane, hydrogen and carbon dioxide are normally absorbed into the blood and expelled through the lungs so that the lumen of the large intestine becomes distended. Moreover, the rapid production of VFA and lactic acid in particular causes a rapid decrease in pH of the fluid, increases the permeability of the mucosa and upsets the microbial balance. This favors the growth of organisms that can withstand a lower pH, stimulating more lactic acid production and causing the death of certain bacteria that cannot survive under such conditions, thereby releasing endotoxins (non-protein lipopolysaccharide fragments of the cell wall of Gram-negative bacteria) and other compounds. These endotoxins, together with the other unwanted compounds produced as a consequence of this change in the conditions of the hindgut, may be absorbed into the blood and have further adverse effects. The blood flow to the feet, for example, may be particularly sensitive to some of these factors that may in turn trigger the development of laminitis (q.v.).



The importance of fiber


In addition to functioning as a source of energy, the fibrous components of feed have other values. In the long form (pieces in excess of 2–3 cm in length), fiber occupies the stabled horse’s time in chewing, so that it is less inclined to what are often referred to as “boredom-related” stable vices. The gastric contents have a higher moisture content and are more friable, allowing more immediate penetration of gastric juices, including HCl, and promoting better digestion further down the digestive tract. Also the microbial fermentation of fiber (fed as short or long material) proceeds at a slower pace than does the fermentation of starch or protein. This in turn has two advantages:




Gastric ulceration


Modern management practices that include meal feeding, low fiber/high concentrate diets, early weaning and intensive training programs help to produce a poorly buffered, acidic environment in the stomach. This has been linked to the high prevalence of gastrointestinal ulcers (q.v.) particularly in intensively managed horses such as performance horses.


Foals are highly susceptible to ulceration because they start secreting gastrin just after birth before the gastric mucosa has fully developed. In addition, stressful weaning programs may act as an exacerbating factor in the development of gastrointestinal tract ulcers. There is a strong correlation between the diet fed and the pH of the stomach. Concentrate diets have always been implicated but ensuring that these are fed in small amounts, possibly in combination with forages such as alfalfa hay, may help. A high forage intake, which encourages chewing and stimulates salivation, may also be advantageous. Turning out to pasture can be very beneficial for those that are affected. Medical treatment is often required.



NUTRIENT REQUIREMENTS



INTRODUCTION


The principal function of feed is to provide the nutrient requirements of the horse and its symbiotic gastrointestinal microorganisms. Maintenance requirements can be defined as the daily intake that maintains constant body weight (BW) and body composition as well as the health of a healthy adult horse with zero energy retention at a defined level of low activity in comfortable surroundings.


Nutrient requirements are typically stated as an amount per kg of feed or amount per kg BW daily. These amounts are the minimum needed to sustain normal health, production and performance of an average healthy animal. The amounts vary widely amongst horse groups with differing physiologic demands, i.e. growth, age, lactation, physical activity and workload (rider weight and ability, terrain and intensity of activity). They will also be affected by other factors including the environmental conditions. For example, energy requirements increase in animals exposed to very low temperatures, particularly where there is considerable air movement, and rain decreases the insulation properties of the coat.


As well as varying according to breed, body composition, stage of training, etc., it is also very important to remember that horses are individuals and differ in their metabolic efficiency (e.g. some horses are “good doers”), temperament, health status (including level of parasitic burden), appetite, likes and dislikes and other variables. It should be noted that there can be a difference between what a horse can eat and what it might need for maintenance, which in practice means that many mature horses will gain weight if fed free choice hay and not exercised.


Guidelines to requirements only can be provided; these then need to be tailored to the individual circumstances. It is important to note that one of the main reference materials used and referred to are the National Research Council (NRC) requirements, which recommend minimal rather than optimal requirements.


Imprecision in the assessment of nutrient requirements is compounded by uncertainty in feed analysis. The bioavailability of nutrients also varies between feed sources. The values given in Table 3.6 assume a high bioavailability from the ration.



Requirements can be given in a variety of ways. Two of the most common are per kg DM feed intake, and amounts per day on a dry matter or as fed basis. It is important to check what units are being used. Either can be suitable but obviously the DM intake guidelines rely on horses being fed appropriate amounts of feed for their workload, age, reproductive status, etc.



BODY WEIGHT ESTIMATIONS


Many nutrient requirements are proportional to body weight. Unfortunately, judgment by eye can be inaccurate; calibrated weighbridges are the most accurate but are not commonly available. Weigh tapes and estimations based on linear measurements provide an approximation but these can only be taken as a guide or used for monitoring purposes where standardized procedures are followed.


There are a number of equations available such as that to estimate body weight in kg (w) from heart girth in cm (hg) and length of the body in cm (l) from the point of the shoulder to the point of buttock for the adult horse only:


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This will tend to overestimate the weight, for example, of those horses with reduced gut fill in hard work such as the fit racehorse and is not reliable for the pregnant mare in late gestation or for young, growing animals, etc.



WATER


The requirements for water are given in Table 3.7. The water content of the body should remain within the approximate limits of 68–72% of fat-free mass. Values below this represent a dehydrated state. Water is lost by excretion in urine, feces and sweat, as well as by evaporation from the lungs and in milk. Lactation can increase needs by 50–70% above maintenance.



Table 3.7


Guide to the minimum 1 water needs (kg) of horses per kg of feed DM consumed and the water (kg) provided by pasture herbage per kg herbage DM (1 kg water is equivalent to 1 L)


image


1Quantities of water given assume ideal environmental conditions.


The requirement for supplementary water is influenced both by the amount of DM consumed and by the moisture content of feeds available. Cereals and hay contain approximately 10% moisture whereas pasture herbage contains 40–80% moisture depending on season and rainfall. These sources affect the supplementary amounts of water required ( Tables 3.7 and 3.8). Dry mares (or other non-lactating horses) on lush pastures with shade, undertaking no work, can thrive without additional water, although it is always advisable that a clean supply be made available. Pregnant and lactating mares should be provided with supplementary water at all times (see Tables 3.7 and 3.8). Foals should have adequate access from around 2 weeks of age. The requirements of the breeding stallion are similar to those of the pregnant mare.


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Related posts:

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  2. Fundamentals of the equine pre-purchase examination
  3. Anesthesiology
  4. Perinatology

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Equine nutrition and metabolic diseases

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