Kentucky Equine Research Focus On Equine Nutrition

Delivering Essential Nutrients To Young, Growing Horses

Stephen Duren, Ph.D.
Kentucky Equine Research, Inc.

Introduction
     The goal of raising performance horses is to produce sound athletes. One potential pitfall in raising this type of horse is unsoundness resulting from Developmental Orthopedic Disease. The term Developmental Orthopedic Disease (DOD) includes all general growth disturbances resulting from any alteration in normal bone formation (Lewis, 1995). DOD has many manifestations in growing horses including: 1) physitis, 2) wobblers syndrome, 3) angular limb deformities, 4) flexure leg deformities and 5) osteochondritis dissecans. The causes of DOD have been debated over the past ten years. Currently, the most common causes of DOD are thought to be: trauma to the metaphyseal growth plate or articular cartilage, genetics, rapid growth, nutritional imbalances and environment. With nutrition being one potential factor in DOD, adequately fortifying, and properly balancing the diets for young growing horses is very important.
     To properly fortify and balance a diet for a young growing horse, it is essential to have an understanding of which nutrients are most critical for proper skeletal growth. Further, one must have an understanding of the requirements for these critical nutrients. Understanding the critical nutrients and their requirements is only the first step in properly feeding growing horses. With so many different feedstuffs available, all with very different nutrient profiles, a working knowledge of feeding practices including the amount and type of feed provided and the expected growth rate of the horse is important. The following paper will discuss the essential nutrients for proper skeletal growth along with their requirements. In addition, some of the many different methods available to deliver essential nutrients to growing horses will be explored.
Critical Nutrients For Growth
     The critical nutrients required for growth among different animals are basically similar. All animals require energy, protein (amino acids), minerals and vitamins. However, the actual nutrients which are considered in balancing diets for various farm animals are very different. For example the nutrients which are actively balanced in poultry diets are listed below.

     As one can see, the list of nutrients is considerable. Their are several reasons for this intense accountability of nutrients. First, the digestive system of the chicken does not provide extensive microbial synthesis of nutrients. Second, chickens are typically confined and gather their entire nutrient requirement from a complete diet provided to them.
     In horses, the list of critical nutrients for growth is smaller. Although many nutrients are required, the most critical nutrients for growth of young horses are listed below.

A complete discussion of energy and protein requirements of young, growing horses is provided elsewhere in these proceedings. Therefore, this paper will focus on the critical minerals necessary for proper skeletal growth.
Calcium
     Calcium is the first mineral often considered in the diets of young horses. Calcium makes up about 35% of bone structure (El Shorafa et al., 1979), with approximately 99% of the calcium in the body found in the bones and teeth (Lewis, 1995). Calcium is also involved in other body functions including muscle contraction and blood clotting mechanisms. Inadequate calcium intake by the developing foal can lead to rickets, which is characterized by poor mineralization of the osteoid tissue and the probability of enlarged joints and crooked long bones (NRC, 1989). This scientific data pointing to the ill-effects of a calcium-deficient diet have also been reported in the field. Knight and co-workers (1985) reported a negative linear relationship between dietary calcium intake and perceived severity of DOD in young horses. Excess calcium has also been fed to young horses. In a study reported by Jordan and co-workers (1975) feeding five times the calcium requirement was not detrimental provided the level of phosphorus in the diet was adequate.
Phosphorus
     Phosphorus is also a critical mineral for proper skeletal development. Phosphorus is often considered with calcium since it also is a major component of bone, making up 14 - 17% of the skeleton (El Shorafa et al., 1979). Two major problems can exist with growing horses relative to phosphorus supplementation. The first is inadequate phosphorus in the diet. A simple phosphorus deficiency can result in DOD and bone demineralization. The second condition is excessive phosphorus content in the diet, a situation in which phosphorus concentration is actually greater than calcium concentration, and phosphorus interferes with calcium absorption.
Copper and Zinc
     Copper is required by growing horses as a component of several copper-dependent enzymes involved with elastin and collagen formation (NRC, 1980). Young growing horses with inadequate copper intake do not suffer from slow growth rate, instead, normal or rapid growth continues but without adequate copper for normal bone and cartilage development. The end result is foals with decreased bone density and DOD (Lewis, 1995). Cymbaluk and co-workers (1981) reported that copper absorption by the horse decreases with increasing copper intake giving horses a high tolerance to excess copper ingestion. The NRC (1989) reported the maximum tolerance level of copper to be 800 mg/kg diet.
     Zinc is required as a component of many metalloenzymes involved in protein and carbohydrate metabolism. Low zinc concentration has been correlated with an increased incidence of DOD in growing horses (Knight et al., 1985). The NRC (1989) reported the maximum tolerance level of zinc to be 500 mg/kg diet.
Nutrient Requirements
     The National Research Council has published a booklet listing the Nutrient Requirements of Horses. The most recent edition of this booklet is the fifth revised edition published in 1989 (NRC, 1989). In this publication, a subcommittee of six equine nutrition research scientists reviewed the equine nutrition literature and updated the nutrient requirements of horses. The requirements stated in the booklet represent the minimum amounts needed to sustain normal health, production and performance of horses. In the introduction of this publication, the subcommittee suggested that consideration be given when applying these recommendations to, among other things, the individual variation in horses, expected performance and different environmental conditions. Therefore, the NRC should be viewed as a good starting place for the formulation of horse rations, and not as the only or best source of information.
     Due to the frequency of publication of updated NRC guidelines, the last revision already seven years old, the NRC cannot contain the most up-to-date information. In an effort to remain current with advances in equine nutrition, Kentucky Equine Research continuously reviews new research, and also conducts and publishes research done in our laboratory. As a result of these efforts, Kentucky Equine Research has modified certain NRC requirements to be more practical in the production of sound, athletic horses. Modifications of NRC requirements which appear in this text along with modifications appearing in the MicroSteed Computer Program are safe additions to horse diets and are currently being used by the staff of Kentucky Equine Research.
     The requirements for those nutrients critical to growth of young horses appear in the following tables. In each table, the minimum requirement established by the NRC, 1989 appears followed by the requirements established by Kentucky Equine Research.
     Now that we have established the critical nutrients for growth and their requirements, it is time to apply this information to feeding growing horses.

Understanding The Variables
     To begin the process of providing essential nutrients to growing horses, one must realize that methods of feeding vary greatly throughout the United States and the world. Many feeding variables exist, each providing a series of challenges for delivering the proper amount of diet fortification. The following are several examples of common feeding variables.
Availability and quality of natural and/or stored forage
     The types of forages (pasture and/or hay) which are available to young growing horses have a significant impact on diet fortification. Young, growing horses are capable of eating 1.5 - 2% of body weight in high quality pasture or hay per horse per day. Depending on the nutritive value of the hay or pasture, this can have a profound influence on the nutrient intake of the growing horses, and thus the remaining nutrients which need to be supplied by grain supplementation. For example, the difference in nutritive value between alfalfa and timothy hay is immense, with alfalfa typically having more energy, protein and calcium than timothy hay. Further, the difference between hay or pasture utilized in a young, vegetative state vs. a mature state is important, since the nutritive value and the intake of forage decreases with increased maturity.
Amount of supplemental feed (grain) typically fed
     The amount of grain fed to young, growing horses varies widely throughout the world. For example, the normal amount of grain fed to a yearling Thoroughbred in Central Kentucky is much greater than typically fed to a yearling Thoroughbred in Washington. The amount of grain excepted as a "normal" intake for a Quarter Horse weanling halter prospect is much greater than fed to a Quarter Horse weanling not intended for show. These basic differences in the amount of grain considered to be "acceptable" will have large implications on the amount of fortification which should be contained in these grain mixtures. Unfortunately, many manufacturers pay little attention to the amount of grain which is actually being fed by the horse-owner.
Desired growth rate
     The rate at which growing horses gain weight is a function of the amount of feed provided and their genetic capacity for growth. The body weight of a growing horse can be controlled by adjusting the intake of calories. Horse-owners who desire rapid weight gain in young horses will typically provide a larger proportion of calories from grain concentrates. Since these horses are eating more pounds of grain per day, the concentration of nutrients in that grain can be less. On the other hand, horse breeders who do not stress rapid weight gain in young horses typically feed fewer pounds of grain. Grain concentrates for these horse breeders must be more concentrated since fewer pounds are provided to the horse. Both the fast-growing and slower-growing horses need proper dietary fortification; however, the amount of energy (calories) provided with this fortification must be different.
     Ability to individually feed horses
     In many areas of the world growing horses are fed individually a measured amount of feed on a daily basis. This is the best case scenario for feeding young horses. Unfortunately, many breeders of horses are unable to individually feed their young stock. In these situations young horses are fed in groups where one horse potentially can monopolize the feed. A feed product destined for use in this type of situation would need to have a low energy content, or a low intake, to prevent excessive growth, but still have a safe level of fortification to provide each horse with critical nutrients for growth.
     Each of these variables provide a series of challenges for delivering the proper amount of diet fortification. The following are actual diets which can be formulated to address these common feeding variables.
The Diets
     In the following examples, several feeding programs will be developed for a 12 month-old yearling weighing 715 lbs, gaining 1.1 lbs per day with an expected mature weight of 1100 lbs. The nutrient requirements for this horse are listed in Table 4.
Example 1.
     The first feeding situation is an example for supplying critical nutrients using three different levels of grain intake (Moderate, Low, and Minimal). In this example, the yearling diet consists of free-choice access to good quality pasture, with supplemental grain feeding twice daily. In Figure 1, the yearling is on a moderate grain intake (8 lbs/horse/day) with an estimated intake of pasture dry matter of 12 lbs/horse/day. The level of fortification found in the pasture (DM) and the level of fortification necessary in the grain concentrate (As-Fed) to balance the remainder of critical nutrients is shown below.

     The same yearling on a low grain intake is shown in Figure 2. In this example, the yearling is receiving 4.5 lbs of grain/horse/day with pasture dry matter intake estimated at 15.5 lbs/horse/day. Since the yearling is eating fewer pounds of grain/day, the concentration of nutrients in that grain must be higher to satisfy the nutrient requirements. The level of fortification necessary in the low intake grain concentrate is shown below, compared with the nutrient profile of the grain used in the previous example.

     Finally, their are horse feed clients who want to feed the absolute minimum amount of grain necessary to their horses. On good quality pastures, it is possible for the yearling in this example to consume enough pasture to satisfy requirements for digestible energy and protein to maintain a moderate level of growth (1.1 lbs/horse/day). However, the concentration of critical nutrients (calcium, phosphorus, copper and zinc) will not be adequate in a pasture only diet. To proper balance a diet in this situation, it is estimated the yearling will consume nearly 16 lbs of pasture dry matter/day along with 1.25 lbs of supplement/horse/day. The nutrient profile of this diet is depicted in Figure 3. The nutrient profile of the supplement pellet is shown below.

Example 2.
     In this scenario, the horse owner is free-choice feeding, good quality alfalfa hay to our example yearling. The alfalfa diet is supplying adequate energy, protein and calcium to support the desired moderate growth rate, but is marginal in phosphorus, copper and zinc. This is a situation very common to young growing horses in the west, where high quality alfalfa hay is common. To properly balance this diet, one would want to feed a low intake supplement pellet which provided essential phosphorus, copper and zinc, but did not add a significant amount of energy, protein or calcium since these nutrients are already in excess. In Figure 4, a final diet consisting of alfalfa (15.5 lbs/horse/day) and 1.5 lbs of a specially designed mixing pellet to be fed with alfalfa hay is shown. The nutrient profile of this mixing pellet is unique since it contains low protein (9%), an inverted ratio of calcium to phosphorus and high trace mineral concentrations. In formulating such a product, it is essential this supplement pellet be used only in diets for horses eating predominately alfalfa hay (greater than 50% of the forage consisting of alfalfa).

Example 3.
     Despite the best efforts of the owners, our example yearling has developed DOD. The veterinary surgeon involved has suggested an energy restricted diet to avoid any further rapid weight gain. It is important to realize that an energy restricted diet will decrease the rate of gain; however, the skeleton of the yearling will continue to grow. The end result is a yearling which has grown taller, but has become progressively thinner. Since the skeleton of the yearling continues to grow even on an energy restricted diet, it is important the horse receive adequate levels of essential nutrients required for growth. In figure 5, the yearling can be fed at approximately 70% of energy requirements with adequate nutrients to support continued skeletal growth. The diet consists of 11 lbs of mixed hay (alfalfa/grass) plus 2.5 lbs of a protein, vitamin and mineral supplement pellet (All-Phase).

Example 4.
     The directions on the feed bag suggest that our example yearling receive the grain concentrate at a minimum rate of 8 lbs/horse/day. These directions were placed on the bag in order for the horse to get adequate diet fortification. Unfortunately, the owners of the horse do not want to feed anymore than 5 lbs of grain/horse/day. If they only feed 5 lbs of this grain/horse/day along with a mixed hay (14 lbs/horse/day) the yearling will be marginal in phosphorus, copper and zinc intake (Figure 6). A method which can be used to provide the requirement of essential nutrients while still adhering to the owners maximum of 5 lbs of grain/horse/day rule is shown in Figure 7. In this situation, the intake of mixed hay remains constant while the level of grain concentrate is dropped from 5 lbs/day to 3.5 lbs/day. The remaining 1.5 lbs, which has been set aside for grain intake, is provided as a supplement pellet rather than the normal grain. The finished diet will then consist of 14 lbs of mixed hay/horse/day, 3.5 lbs of grain concentrate and 1.5 lbs of supplement pellet (All-Phase).
Example 5.
     Finally, their are horse owners who raise young horses in situations which do not allow them to feed grain and/or supplement pellet on a daily basis. Hay and/or pasture are available free-choice, but supplemental grain feeding is not possible. The young horses still require diet fortification, but this fortification must be provided with a free-choice supplement. This free-choice supplement must be palatable and self limit intake. Fortified molasses blocks can be used to accomplish this goal. Figure 8 depicts a diet for our example yearling. In this diet, the yearling is eating approximately 17 lbs of mixed hay/day along with 3 lbs/day of a well-fortified molasses block. It is important the concentration of nutrients which are contained in the block are appropriate for the actual intake. In other words, the intake of these blocks by young horses needs to be monitored to insure proper fortification. If intake of these blocks is grossly over or under the recommended intake, the blocks will do a poor job of balancing the diet. The appropriate level of nutrient fortification in a free-choice molasses block with a targeted intake of 3 lbs/horse/day is shown below.

Conclusion
     Nutrition imbalances have been recognized as one potential cause of DOD in young, growing horses. Therefore, it is important the diets of young horses be properly balanced with nutrients known to be critical to proper development. In young, growing horses the major nutrients of concern are protein, energy, calcium, phosphorus, copper and zinc. Each of these nutrients have minimum requirements put forth by the National Research Council (NRC, 1989). Kentucky Equine Research has elevated the requirements for these essential nutrients to allow an increased margin of safety, and to make them more practical in the production of sound, athletic horses. Understanding the essential nutrients and their requirements is the first step in properly feeding young horses. Next, one must understand the many variables associated with feeding including: 1) the availability and quality of natural and/or stored forage, 2) the amount of supplemental feed (gain) typically fed, 3) the desired growth rate and 4) the ability to individually feed horses. Once this information is put together, a proper balanced grain or supplement can be designed to balance the diet.
References
Cymbaluk, N.F., H.F. Schryver, and H.F. Hintz. 1981. Copper metabolism and requirements in mature ponies. J. Nutr. 111:87.
El Shorafa, W.M., J.P. Feaster, and E.A. Ott. 1979. Horse metacarpal bone: Age, ash content, cortical area, and failure-stress interrelationships. J. Anim. Sci. 49:979.
Jordan, R.M., V.S. Meyers, B. Yoho, and F.A. Spurrell. 1975. Effect of calcium and phosphorus levels on growth, reproduction and bone development of ponies. J. Anim. Sci. 40:78.
Knight, D.A., A.A. Gabel, S.M. Reed, L.R. Bramlage, W.J. Tyznik, and R.M. Embertson. 1985. Correlation of dietary mineral to incidence and severity of metabolic bone disease in Ohio and Kentucky. P. 445 in Proc. 31st Am. Assoc. Eq. Pract., F.J. Milne ed. Lexington, KY.
Lewis, L.D. 1995. Equine Clinical Nutrition: Feeding and Care. Williams and Wilkins, Baltimore, MD, USA.
N.R.C., 1980. Mineral Tolerance of Domestic Animals. National Academy Press. Washington, DC, USA.
N.R.C., 1989. Nutrient Requirements of Horses. 5th revised edition. National Academy Press. Washington, DC, USA.

PROTEIN AND ENERGY REQUIREMENTS OF GROWING HORSES
Les Breuer, PhD
L.H. Breuer & Assoc.
East Alton, IL
Introduction
There are several good sources of information on feeding young horses including the NRC Nutrient Requirement of Horses (14), textbooks (7,8,13) and various university extension publications. Since most feeding recommendations are based directly or indirectly on NRC nutrient recommendations, the NRC recommendations will be evaluated for accuracy and adequacy for feeding horses in the real world of the commercial horse industry. This paper will attempt to approach the subject with practical concerns of the horse producer in mind.
S ome questions of practical concern are:

1. How important is mare feeding to milk production and foal growth?
2. Is it necessary to supplemental feed (creep feed) suckling foals?
3. How much protein should creep feeds and weanling feeds contain?
4. How much hay and how much concentrate should be fed to weanling and yearlings?
5. Do colts and fillies need to be fed differently?
6. Can young horses be harmed by over-feeding? By under- feeding?

The values used in the discussion to follow are intended to apply to typical Thoroughbred, Standardbred, Quarter Horse and other light breeds with mature body weights of 1150 to 1200 lbs. The values can be lowered or raised for smaller and larger horses.
Rates of gain are used which have been observed in horse operations which are competitive in their segment of the horse industry. Discussions about whether moderate growth would be better than rapid growth are useful and this area deserves much more study. However, horse operations which are producing animals for weanling and yearling sales, halter shows, 2 and 3 year-old racing, or just want horses which "look" competitive really have little choice. Their goal has to be rapid growth, if not maximum growth, with as few developmental and health problems as possible. Rapid growth rates obviously require liberal amounts of energy and protein. The questions are: When? How much? In what form? To answer these questions, it is helpful to break the entire growth period into themanagement phases of suckling, weanling, yearling and 2 year old.
Suckling Foals
The most growth in the shortest period of time occurs in the foal from birth to weaning. It is normal on successful commercial horse farms for foals to grow from about 10% of mature body weight at birth to nearly 50% of mature body weight at weaning (12,18). Daily gains are 2.3 to 2.4 lb. per day during the suckling phase for foals weaned at 6 months of age and even greater for earlier weaned foals. The weights shown in the NRC tables for weanlings appear to be 10% to 15% below the industry norm.
Mare milk is the only significant food the suckling foal receives during the first few weeks of its life and normally continues to be its major source of nutrients until the foal is 4 to 5 months of age. NRC concluded from their data review that mares produce an amount of milk equivalent to about 3% of their body weight for the first 12 weeks of lactation and an average of about 2% of their body weight during the next 12 weeks. Lactation studies (19) in which mare caloric intakes required to maintain body weight are determined while producing milk to support rapid growth rates in their foals indicate that NRC DE recommendations for lactating mares are 10% to 12% low. Calculations based on weight gains of foals receiving mare milk and known amounts of supplemental feed indicate that mares are producing milk in the 4%-3% of body weight range rather than the 3%-2% range assummed by NRC. The additional milk would account for the higher DE requirements observed in the lactating mare studies.
Rapid growth rates in foals result in rapid increases in body weight and consequently, rapid increases in DE requirements for maintenance while at the same time milk production by the mare begins to decline. Clearly, the foal must have other sources of nutrients if it is to continue to grow at rapid rates. Pasture studies with mares and foals (5) indicate that older foals get significant amounts of nutrients from high quality pasture. The same would be expected when high quality hay is available to the foal. Under such conditions, foals consume supplemental concentrate at a rate of about 1% of body weight. Calculations indicate that under pasture conditions, older foals probably consume about 3% of body weight as feed dry matter, approximately equally divided between mare milk, pasture or hay, and concentrate. When pasture or hay is limited or poor quality, or mare milk production is low, voluntary concentrate intake will increase (19).
Weanlings
To properly feed weanlings requires good management and the application of more nutritional knowledge than that required for any other class of horse. It is not unusual to see weanlings which are thin and pot-bellied with rough hair. Foals are seen which have been weaned 2 or 3 months and have gained little or no weight since weaning. Some owners accept such poor performance inweanlings as normal and unavoidable. It may be normal for some and it is arguable as to whether this is harmful to the horse but it can be avoided by paying attention to the following:

1. Psychology of weaning.
2. Health care.
3. Rations with proper nutrient density and balance.

The first two factors are outside the scope of this paper but are important in order to reduce psychological and physiological stressors to a minimum during the weaning period. Conversely, a balanced diet and a full belly will probably increase immunity to and/or help with recovery from some of the disease problems and possibly help with the psychological problems as well.
Daily weight gains for weanlings of 1.4 to 1.9 lbs. per day suggested by NRC appear to be reasonable based on experimental (3,5) and farm observations(12,18). However, NRC suggested dry matter intakes for weanlings of up to 3.5% of body weight appear to be too high. Practical and experimental observations indicate that 450 lb. to 500 lb. weanlings will consume a maximum of 2.8% to 2.9% of body weight of air dry matter. Thus, the total air dry matter intake for a 500 lb. foal can be expected to be about 14.5 lb. per day after a 2 to 3 week adjustment period. NRC estimates that the DE requirements of a 500 lb. weanling are 15.2 Mcal and 17.7 Mcal DE per day for weight gains of 1.4 and 1.9 lbs. per day, respectively. Therefore, the caloric densities required in 14.5 lbs. of feed theoretically should be about 1.05 and 1.22 Mcal DE per lb. for weight gains of 1.4 and 1.9 pounds per day. Using good hay and grain, a caloric density of 1.05 Mcal per lb. can be achieved with a ration composed of about 70% hay and 30% grain. Any one with experience feeding weanlings knows they won't gain 1.4 lbs. per day on this ration. It will definitely take a lower hay:grain ratio to realize this moderate rate of gain. So where does theory go wrong? A couple of possibilities are: (a) hay DE is used less efficiently for gain than grain DE, i.e., net energy for gain in young horses is a lower percentage of digestible energy in hay than in grain, as is well recognized in ruminants and/or, (b) weanlings need more DE for gain than estimated by NRC.
Experimental data indicate that weanlings require about 5% more DE for gain than NRC recommends and that the percentage hay in the ration can be no more than 20% to 25% for rapid gains of 1.8 lbs. to 1.9 lbs. per day and 35% to 40% hay for more moderate gains of 1.3 lbs. to 1.4 lbs. per day. NRC recommends a constant 30% hay in the ration regardless of whether moderate or rapid gains are expected.
Yearlings/2 Year Olds
Normal weight gains in yearlings range from 0.75 to 1.25 lbs. per day depending on prior feeding and development history. Yearlings which have been fed for rapid gains during the suckling and weanling phases and fed for rapid gains as yearlings will become obese, especially in the case of fillies. Yearlings which have been underfed previously may need to be fed for rapid growth similar to weanlings according to body weight rather than age. This should be done carefully to avoid DOD problems.
Experiments with yearlings which have been fed for rapid gains as suckling foals and for moderate or rapid gains as weanlings similar to common industry practice indicate that NRC recommendations for DE for gain in yearlings are as much as 15% below requirements. It may be that the NRC equation for DE requirements for growth which has a correction factor based on age doesn't adequately account for differences in previous feeding and growth histories. Possibly the equation could be improved by using body weight as a correction factor. Higher maintenance requirements due to the high level of activity in yearlings may also contribute to the apparent higher DE requirement for growth in yearlings.
Data from breeding farms (12,18) and experimental results show that colts and fillies grow at similar rates up to about a year of age but colts appear to continue to grow into their yearling year at slightly higher rates than fillies. Colts need to be fed enough additional feed to account for higher gains of 0.2 lbs. to 0.3 lbs. per day as well as any higher maintenance requirement due to the large amount of voluntary activity in yearling colts which are kept in groups or are allowed to exercise in groups.
Similar to yearlings, amounts to feed 2 year olds depend on how close they are to reaching mature weights which in turn depends on previous feeding and development history. A gain of 0.25 lbs. per day will add 100 lbs. of weight a year which should be adequate under most practical conditions. This will need to be modified if horses are put in training.
Protein Requirements of Growing Horses
Protein requirements for growth in horses are primarily determined by requirements for the amino acids contained in the protein. This author and coworkers (1,2,3,4) demonstrated a lysine requirement in young horses thirty years ago which has been confirmed by numerous researchers (6,10,11,15,16,17). The NRC lysine recommendations are based on a ratio of lysine to digestible energy which is decreased slightly as horses progress from weanlings to yearlings and 2 year olds. Actually these ratios only apply to a rather narrow range of weight gains and will underestimate lysine requirements in young, rapidly growing horses and overestimate requirements in older, slow growing horses. An analysis of studies on lysine requirements in horses shows that the lysine to digestible energy ratio varies widely with differences in growth rates as shown in the following table:
Daily Gain Lys:DE Ratio Ration Protein
(lb./day) (gm/Mcal) (%)
1.0 1.27 11.0
1.25 1.59 12.5
1.5 1.91 14.0
1.75 2.22 15.5
2.0 2.54 17.0
2.25 2.86 18.5
2.5 3.18 20.0
The NRC values for lysine to DE ratios of around 2 gm lysine per Mcal DE would be expected to support weight gains of 1.5 lbs. to 1.75 lbs. per day which are usually satisfactory gains for 6 month old foals. However, higher lysine levels are needed in rations for orphan foals or early weaned foals and in creep feeds, especially if mare milk is severely restricted.
The ration protein levels required to meet the amino acid requirements are determined by the amino acid content and availability in the ingredients used in the ration. The values shown in the table are based on a corn-oats-soybean meal feed. Values can be lowered if higher quality protein sources or synthetic amino acid supplements are used. It should be noted that the protein levels apply to the complete ration. If a significant amount of hay is fed, the protein level in the grain feed may need to be adjusted depending on the protein content of the hay.
Studies of other amino acids in horses are limited. The other amino acids do not appear to be of much practical significance in feeding young horses. Lysine is the first-limiting amino acid in typical horse ration ingredients and if the lysine requirement is met using these ingredients, the requirements for the other amino acids will likely be met. If large amounts of unusual ingredients or synthetic amino acid supplements are used, then levels of other essential amino acids may be a concern. A recent study (9) in yearling horses showed that threonine at 80% of the lysine level was adequate. Adequacy of other essential amino acids levels have to be evaluated in animal studies and/or by comparing to known requirements in simple stomached species such as swine, rats, and humans.
Lowering protein levels to reduce growth rates to help with DOD problems in young horses is a common recommendation in the horse industry. This practice will reduce weight gains as well as bone growth as measured by wither height. There is no question that gross over-feeding or over-consumption of high protein grain feeds will help precipitate DOD problems in susceptible horses. However, there are no definitive data showing that the feeding levels and growth rates recommended by NRC or in this paper are likely to raise the incidence of DOD problems. Practical experience with numerous feeding trials, controlled and uncontrolled, indicate that selection of the proper ration for the class of animal with respect to its amino acid and mineral content,and feeding the ration at the recommended level will result in minimal DOD problems. There should be a concern of how nutrient restrictions will affect the expression of the genetic potential of the horse for size and structure. Reducing protein intake without a concurrent reduction in caloric intake should change the composition of gain to more fat and less muscle. If possible, it would be preferable to reduce both caloric and protein intake and try to maintain a normal muscle and fat content at the lower rate of gain.
Summary
Protein and energy nutrition can be used in horse farm management to regulate the growth and development of young horses. It does not have to be a largely uncontrolled process as is often the case. At a time when horse farm economics are a great concern, the proper application of nutritional knowledge in developing horses can result in the production of the desired animal with the highest possible feed efficiency.
Literature Cited

1. Breuer, L.H., and J.D. Word. 1967. Studies on complete pelleted rations for horses. Proc. 22nd Texas Nutr. Conf., p.188.Breuer, L.H., and J.D.Word. 1968. Studies on the dietary amino acid requirement of the equine. Fed. Proc. 27:730.
2. Breuer, L.H., L.H. Kasten and J.D. Word. 1970. Protein and amino acid utilization in the young horse. Proc. 2nd Equine Nutr. Physiol. Symp., p.16.
3. Breuer, L.H., and D.L. Golden. 1971. Lysine requirements of the immature equine. J. Anim. Sci.33:227.
4. Breuer, L.H. 1974. Feeding the breeding and the growing- developing horse. Proc. Maryland Nutr. Conf., p.102.
5. Borton, A., D.L. Anderson and S. Lyford. 1973. Studies of protein quality and quantity in the early weaned foal. Proc. 3rd Equine Nutr. Physiol. Symp., p.19.
6. Cunha, T.J. 1991. Horse Feeding and Nutrition, 2nd ed., Academic Press, NY.
7. Evans, J.W., A. Borton, H.F. Hintz and L.D. Van Vleck. 1977. The Horse. W.H. Freeman, San Francisco.
8. Graham, P.M., E.A. Ott, J.H. Brendemuhl and S.H. TenBroeck. 1994. The effect of supplemental lysine and threonine on growth and development of yearling horses. J. Anim. Sci. 72:380
9. Hintz, H.F., J.E. Lowe and H.F. Schryver. 1969. Protein sources for horses. Proc. Cornell Nutr. Conf., p.65.
10. Hintz, H.F., H.F. Schryver and J.E. Lowe. 1971. Comparison of a blend of milk products and linseed meal as protein supplements for young growing horses. J. Anim. Sci.33:1274.
11. Hintz, H.F., R.L Hintz, L.D. Van Vleck. 1979. Growth rate of Thoroughbreds:Effect of age of dam, year and month of birth. J. Anim. Sci. 48:480.
12. Lewis, L.D. 1995. Horse Feeding and Nutrition, 2nd ed., Academic Press, NY.
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