Matching Diet to Activity Level

When it comes to extracting the maximum effort out of your performance horse, there is no doubting the importance of a sound feeding program. He needs a balanced diet to replenish fuel reserves, repair tissue, and provide a foundation for training and competing successfully. All too often there is a tendency for us to focus on the latest and greatest supplement that has been touted to work wonders in terms of athletic performance, while ignoring the most important dietary consideration for athletic horses -- energy intake.

A couple of things come to mind when we talk about energy. One is a horse’s "energy level." A common complaint is: "My horse has too much energy," meaning he is full of life and perhaps difficult to control when asked to perform various tasks. In this article, though, we are discussing energy in the nutritional sense -- the number of calories in a feed.

The amount of energy (or calories) in the diet should match the horse’s needs. If it doesn’t, the horse will either gain or lose weight -- both can adversely affect exercise performance. The overweight horse is rarely fit enough for the task at hand and can have overheating problems because of the insulating effects of fat, while the underweight horse might be lacking in fuel reserves. To avoid these problems, you need a working knowledge of your horse’s energy requirements and the energy content of the various feeds in his diet.

Calories to Burn

A horse’s dietary energy needs usually are expressed in megacalories (Mcal) of digestible energy (DE), where 1 Mcal equals 1,000 kilocalories (kcal). The goal of your feeding program is to balance "calories in" (energy intake) with "calories out" (energy expenditure) so that body weight and condition are maintained. This is the energy balance equation.

A horse’s daily DE needs equal the sum of his maintenance requirement and the energy he expends during exercise. The maintenance requirement refers to the energy needed for all basic body functions -- for a 500-kg (1,100-pound) horse, this equates to approximately 16.4 Mcal of DE per day (16,400 kcal, or about 16 calories per pound of body weight). For horses weighing between 200 and 600 kg (440 to 1,320 pounds), the following formula can be used to calculate maintenance DE needs (Mcal/day):

DE = 1.4 + (0.03 x body weight in kg)

This equation has only been validated for horses within this range of body weights. Hence, this equation should not be used for very small or very large horses.

When it comes to accounting for the additional energy needed for physical activity, two of the main considerations are running speed and the duration of exercise. The faster the horse runs, the higher his energy usage. Similarly, the longer the horse runs, the more energy used. However, this issue gets complicated because other factors such as ground surface, terrain, and the weight of rider and tack can markedly influence the actual intensity of exercise or work effort.

For example, deep footing will increase energy expenditure because extra effort is required to lift the hooves out of soft or sticky ground, while uneven ground can decrease the efficiency of movement. Endurance riders can attest to the fatiguing effects of soft or uneven footing.

When considering energy use per minute of exercise, an all-out sprint, such as that undertaken by a racehorse during a six- to seven-furlong race, is obviously much more "expensive" than lower intensity exercise such as trotting (see "How Much Energy Does Your Horse Use?" at right). Of course, horses can sustain high-intensity exercise for only a few minutes, while lower intensity work can be endured for several hours. Therefore, the total "calorie burn" will be much greater for endurance activities compared to short bursts.

You can use a heart rate monitor to evaluate energy use by your horse during exercise. There is a linear relationship between heart rate and work intensity as a percentage of maximum aerobic (oxygen use) capacity (VO2max). Because heart rate is affected by factors such as terrain, ground surface, and weight carriage, heart rate measurements are more useful than just exercise level for estimating "calorie burn" (see "Check Your Horse’s Heart Rate" on page 61).

For example, a 500-kg endurance horse working at a heart rate of 150 beats/minute is burning approximately 150 calories per minute (about 9,000 kcal or 9 Mcal) per hour of exercise at this intensity.

Nutritionists assume that about 57% of the digestible energy (DE) in a horse’s diet can actually be used as energy for activity. The other 43% of DE is lost or expended during digestion, absorption, and storage of the nutrients contained in the diet.

Returning to our example, the actual DE intake required for that one hour of exercise is 15.8 Mcal (9 Mcal divided by 0.57=15.8 Mcal)  to get the total amount of energy required for exercise and maintenance). This means a total DE requirement (the maintenance plus exercise components) of approximately 32 Mcal/day -- 16.4 Mcal for maintenance (see "How Much to Feed?" on page 66) and 15.8 Mcal for exercise -- i.e., nearly double his maintenance needs.

Of course, our example horse likely does not work this hard every day -- there will be rest/pasture turnout days, and other days with predominantly walking exercise. On the other hand, energy use (as DE) during a 50-mile race can be as much as 25 Mcal. This horse’s actual DE needs should be calculated based on the volume of training and racing over a longer period (e.g., four to six weeks) and will depend on the horse’s body weight and condition. Additional calories will be needed if some weight gain is desired, or calorie intake might be restricted if weight loss is the goal (see below).

A Starting Point

For most of us, these computations are not a practical means for estimating a horse’s DE requirements. The National Research Council’s (NRC) 1989 Nutrient Requirements of Horses recommends a much simpler, more user-friendly approach. It first categorizes horses based on the amount of exercise performed on a daily basis -- light, moderate, or intense. These would be where light work is Western and English pleasure, hack, and equitation; moderate work is ranch work, barrel racing, roping, cutting, and jumping; and intense work is race training, polo, three-day eventing, and endurance training and racing. Then, for light, moderate, and intense work, the DE requirement (Mcal/day) is estimated to be respectively 1.25, 1.5, and 2.0 times that needed for maintenance.

Although these recommendations are a good starting point, energy needs for individual horses within the three categories can vary tremendously, perhaps by as much as plus or minus 20%. Whereas the "average" 500-kg (1,100-pound) Thoroughbred racehorse needs about 34-35 Mcal of DE per day, some horses undertaking the same training and racing program might maintain body condition when consuming only 27-28 Mcal/day, while still others might need 40-42 Mcal/day! This is the classic "easy keeper" vs. "hard keeper" scenario.

The bottom line is that the NRC guidelines are just that -- guidelines. Use the NRC numbers as a starting point, but recognize that you will need to tailor the feeding program to suit each horse’s needs. You should utilize body condition scoring (see "How Does Your Horse Score?" in the November 2001 issue of The Horse, article #2861 at on a regular basis, perhaps as frequently as every week. This is particularly useful when there has been a big increase in the demands placed on the horse, e.g., an increase in training volume or the start of the competition season.

It is not uncommon for owners and riders to overestimate the calorie needs of their horses, particularly those which are used for light riding activities. Although these horses do fit into the NRC light work category (1.25 x maintenance DE requirement), many are ridden only once or twice per week. As well, these horses often have restricted turnout time, so there is limited voluntary activity. So, in reality their actual DE needs are not much different from maintenance. Don’t fall into this trap. On the other hand, for the higher-level performance horse the tendency is to underfeed, eventually leading to weight loss.

A moderate body condition score (5 on a scale of 1 to 9, where 1 is extremely thin and 9 is obese) is a reasonable target for most horses. At this score, the horse has adequate fat reserves to draw on during times of stress, such as the cold winter months, but not so much weight that light or moderate exercise performance is compromised.

For some types of performance, however, trainers and horsemen prefer their horses to be leaner, closer to a body condition score (BCS) of 4. There are concerns that excess weight could impair exercise performance. However, a BCS of less than 4 can contribute to performance problems because of a lack of energy reserves -- the best example is the endurance horse required to go 50 to 100 miles. For the show and hunter circuit, a BCS of around 6 (moderate to fleshy) is often desired.

In the cold northern climes, a change in season can also affect DE needs. In the northern plains of the United States and in Canada, DE requirements might increase by as much as 30-50% in very cold conditions -- although some of this increase can be offset by a decrease in physical activity during the very cold months.

Calories In

Having estimated your horse’s DE needs, the next step is to find the right mix of feeds that will fulfill that requirement. Bear in mind that the amount of feed that a horse can consume in a day is a finite amount, typically 2-3% of his body weight. If you provide low-quality, low-energy feeds, chances are the horse will not be able to meet his energy needs. So, rule number one is to start with a good-quality forage. For horses in the light and moderate work categories, forage (hay) should be the major dietary component, as much as 70% of the ration (see "How Much to Feed?" on page 66). A reasonable goal is for the horse to consume 1.5-2.0% of his body weight in hay each day. For the 500-kg (1,100-pound) horse, that’s 7.5-10 kg or 16.5-22 pounds (1 kg=2.2 pounds). Remember that 1% of body weight consumed as forage is an absolute minimum.

The digestible energy content of hay can vary considerably -- the more mature the hay, the lower the energy value (as well as other nutrients such as crude protein, minerals, and vitamins). Ideally you should submit a hay sample to a laboratory for analysis of its DE and protein content. In the real world, however, it is not realistic to analyze hay samples because a different batch is being fed every few weeks.

Still, it is reasonable to use published values (see "Energy Content of Popular Horse Feeds" at left) in your calculations. Timothy hay has a published DE content of 0.8 Mcal/lb, while an average alfalfa hay has about 1.0 Mcal/lb.

Getting back to our example 500-kg (1,100-pound) horse, let’s say we are feeding a 50:50 timothy:alfalfa mix at 2.0% of his body weight. This gives us a DE value of 19.8 Mcal (8.8 Mcal from the timothy, 11 Mcal from the alfalfa). We will also assume that this horse needs about 25 Mcal of DE to account for exercise, so a further 5.2 Mcal are required.

Commercial grain concentrates, such as a sweet feed with added fat, provide energy in the range of 1.4 to 1.6 Mcal/lb, whereas straight oats provide 1.45 Mcal/lb. So, 3-3.5 pounds of a typical commercial sweet feed or 3.4 pounds of oats will meet the horse’s additional energy needs of 5 Mcal. The advantage of the commercial feed over oats is that it is fortified with high-quality protein, minerals, and vitamins, thus ensuring that the requirements for other essential nutrients are met.

An alternative approach is to feed a so-called complete feed -- i.e., one that contains both fortified grain concentrate components and fiber/forage (e.g., alfalfa or beet pulp), and can thus be fed without other fiber sources such as hay. These feeds are very useful when the goal is to reduce the amount of dust in the horse’s environment, such as for horses with chronic respiratory problems, including heaves. It is important to adhere to the feeding instructions when using complete feeds -- they are less calorie-dense than grain-concentrates. A common pitfall is to feed these products at a level equivalent to oats or a grain-concentrate, with the end result being weight loss and poor performance.

Which brings us to rule number two -- it is imperative that you weigh the hay and concentrates you feed your horse, and feed by weight, not volume (the National Research Council’s DE values are based on weight). If you consistently use one type of grain or concentrate, it is easy to determine the weight of a can or scoop of this feed. Hay is a bit trickier, but for each new batch, try to measure the weight of a few flakes or biscuits and use those values to determine how much hay to feed.

What about calorie intake from pasture? This will depend on the quality of the pasture and the amount of time available for grazing. When a horse has only 1-2 hours per day available for grazing, you can realistically ignore this intake when calculating DE needs. On the other hand, a horse in the light work category that has 24-hour access to quality pasture (in spring/early summer when pasture quality is highest) might only need a vitamin-mineral supplement to meet all his nutritional requirements.

Slim Jim or Tubby Tim

If you alter your horse’s activity pattern, you should anticipate and implement changes in the feeding level. This approach, together with regular use of body condition scoring, should help keep your horse near his ideal or target body condition. The easy or hard keeper horses can, however, present more of a challenge.

For the easy keeper horse, it might be necessary to reduce or eliminate grain concentrate feeding while maintaining or even increasing his exercise routine. A vitamin and mineral product should be fed along with his hay to ensure that his dietary needs are met. Grazing time might have to be restricted.

The underweight, hard keeper will need more groceries. For example, let’s assume a 450-kg (990-pound) roping horse needs to gain about 20 kg (44 pounds) and he is currently consuming 26 Mcal of DE per day -- 16 Mcal from hay and 10 Mcal from six pounds of concentrate. For this amount of weight gain over a six- to eight-week period, he will require an additional two to three pounds of grain/concentrate per day (an extra 3.0-4.5 Mcal). As always, this change in feeding level should be made slowly (over 10 days to two weeks) to avoid digestive upsets.

Find out your horse’s realistic energy needs, know the energy provided by his feeds, and plan his diet accordingly. Account for any change in exercise level or body condition as it occurs. If you do all of this, you should never have to look at your horse and suddenly think, "How’d he get so fat/skinny?"

About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University

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