Feeding Young Horses: Graduating to a Grown-Up Diet

Consider growth rate and nutrient balance when deciding what to feed young horses.
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Graduating to a Grown-Up Diet
Forage alone can't fully meet the nutritional requirements for growth; therefore, youngsters have higher nutrient needs than do mature horses. | Photo: iStock

Consider growth rate and nutrient balance when deciding what to feed young horses

While perusing shelves of dog food at your local pet store, you’ll likely see designations on bags and cans denoting specific formulations: adult, senior, small breed, large breed. This degree of nutritional precision also applies to horses. Foals, weanlings, adults, and seniors need different amounts of protein, energy, and minerals.  

Mare’s milk and solid food provide excellent nutrition and make feeding fairly simple before a youngster reaches weaning age. Questions arise when he’s on the cusp of the next age bracket. How do you select the right diet for a young horse that is maturing into an adult?

Growth Rates

Karen Davison, PhD, an equine nutritionist and director of equine technical solutions for Purina Animal Nutrition, in Gray Summit, Missouri, is well-versed in the complexities of feeding horses of all ages. She shares considerations for transitioning young horses to adult feed:

“The level of nutrition, protein, vitamins, and minerals relative to calorie requirements is much higher for a growing horse than for a mature horse,” she says. “As the horse ages, there is a shift from nutrition needed to develop tissue and grow, to more nutrition devoted to maintaining the body.”

Based on National Research Council estimates, an average 1,000-pound horse is 64% of his mature height at 12 months, 77% at 18 months, and 86% at 24 months.

“Skeletal growth occurs only as long as the physes (growth plates) remain open; once they mature and close, long bones cannot increase in length,” says Davison.

Growth plate closure occurs from the ground up—those in the lower limbs close earliest, around nine to 11 months of age; the knees and hocks around 24 months; and the shoulders and stifles usually in the third year. Variations occur due to genetics and breed, nutrition, and management.

“Breed type modifies specifics on how to feed, mostly due to differences in rate of growth and age at maturity,” says Davison. “All horses continue to mature and get heavier for a couple of years following the end of growth in height.”

Kathleen Crandell, PhD, is an equine nutritionist at Kentucky Equine Research, in Versailles, with a special interest in feeding growing horses for athletic development. “Research demonstrates that we can influence growth rate with the amount of energy supplied in the diet, as long as all other nutrients are supplied in adequate amounts,” she says. “What we cannot change is the final mature size of an individual beyond its genetic potential.”

When Is it Time for a Diet Change?

Once a young horse reaches 65-70% of its mature weight—usually around a year of age—growth slows and your nutritional strategies need to change. Yearlings should generally consume 50% forage (hay and pasture) and 50% concentrate or a “junior” supplement by weight. (For the purposes of this article, “concentrate” or “supplement” refer to a manufactured, balanced feed combining forage and grain, often called a complete feed. “Grain” refers to corn, oats, and/or barley.)

Horses younger than 2 might develop a hay belly when ingesting more than 50% forage, says Davison. “This isn’t necessarily body fat but indicates a youngster’s less-efficient forage digestion,” she says. “Support lean tissue development in the youngster while not overfeeding. Body condition scoring is a great management tool to monitor growth and fat deposition.” Ideally, keep your growing horse’s body condition score around 5 or 6 on the 1-9 Henneke scale.

The proper forage-to-concentrate ratio depends on your forage quality. This is where you might want to have your hay analyzed to determine its nutrient content. As growth rate slows, a horse voluntarily consumes more forage. Because a young horse typically won’t eat enough forage to meet his protein, vitamin, and mineral requirements without getting too fat, Davison recommends feeding a ration balancer, which provides concentrated levels of protein, vitamins, and minerals without too many additional calories.

By about age 2, a horse has reached nearly 90% of his mature weight and can transition from the 50% hay and 50% supplement diet to free-choice quality hay and however much supplement or ration balancer he needs to maintain an appropriate body condition score.

How a young horse develops depends to an extent on his genetics and how the owner feeds him. “A tendency toward early development needs to be supported with good nutrition,” says Davison. “A horse that is genetically programmed to slower maturity still needs good nutrition but fewer calories. An oft-made mistake is the attempt to slow growth rate below a horse’s ‘preferred’ genetic programming, with a misguided idea that the slower the growth rate, the better. However, slow growth achieved at the expense of balanced nutrition won’t prevent developmental disorders; it simply delays when musculoskeletal abnormalities appear.”

Must-Haves: Minerals and Vitamins

Horses need a balance of specific ­minerals—particularly calcium and phosphorus—for bone and cartilage development. Our sources suggest ensuring horses get as much calcium as phosphorus, ideally with a calcium-to-phosphorus ratio of 1.1-1.25. In areas where horses subsist primarily on calcium-rich alfalfa-based diets, Crandell suggests supplementing at least 0.6% dietary ­phosphorus.

Researchers have found that ­calcium-to-phosphorus ratios as high as 6:1 don’t cause developmental orthopedic disease (DOD), provided the horse receives adequate amounts of both ­minerals.

Trying to compensate for imbalances by adding minerals has its limitations. “Adding calcium to a calcium-deficient diet likely reduces the incidence of DOD, but adding more calcium to a diet that already contains adequate calcium is not likely to prevent DOD,” Davison says.

“A growing horse that receives the minimum recommended feeding rate of commercial concentrate growth feed or amounts above recommended feeding rates of mature horse formulations shouldn’t need additional mineral supplementation—minerals are already included in those formulations,” Crandell adds. Otherwise, you can feed 1-2 pounds of a ration balancer daily to mitigate nutrient deficiencies.

Feed Fat Over Carbs

Nutritionists consider fat to be “safer” than carbohydrates to feed young horses. This is because blood glucose levels don’t tend to rise following the ingestion of fat calories as much as they do after carbohydrate (grain) calorie consumption.

“Added dietary fat, such as vegetable oil, is a concentrated source of energy, providing 2.5-3 times the calories as similar weight of grain,” says Crandell. “However, there is a limit to how much fat can be fed—an excess of 12% of the total diet risks disruption to the intestinal microbial ecosystem. Most total diets—forage and concentrates/supplements—rarely exceed 6% fat.”

Rice bran is a popular fat source. However, Davison notes its potential issues: “While the fat and nutrients in rice bran keep a horse shiny, its high phosphorus content is problematic and could contribute to poor skeletal development due to an inverted calcium-to-phosphorus ratio.”

With that in mind, most manufacturers of commercial stabilized rice bran products add calcium to balance the calcium-to-phosphorus ratio.

If you offer dietary fat using oil rather than a concentrate feed source that contains vitamins and minerals, supplement with 100 IU of natural vitamin E per 100 mL (~3 ounces) of vegetable oil, says Crandell. “There is an upper limit to the amount of fat a horse will tolerate,” she adds. “Horses are highly sensitive to rancidity in fat, which will turn them off their feed. Possible disruptions in the digestive tract caused by excess dietary fat also can contribute to poor performance.”

Crandell says fat should not replace all calories from starch; otherwise, growth and maturation could slow. On the other hand, if oversupplied, especially when necessary nutrients are missing, fat calories accelerate fat deposition.

“A young horse that reaches a body condition score of 7 or greater is at a much greater risk for developing DOD and insulin insensitivity, regardless of the source of dietary calories,” says Davison. 

Preventing DOD

Dietary imbalances, management, and genetics make growing horses prone to DODs, which include physitis, angular limb and flexural deformities, osteochondrosis, and vertebral ­malformations.

Davison says that when a population of young horses has been fed a poorly balanced diet or excessive calories, or if they’ve been overfed after a period of improper feeding, subsequent accelerated growth rates can lead to a higher-than-normal incidence of DOD. “Significant time in confinement is also deleterious to the growing skeleton,” she says. “Correcting these issues likely reduces the incidence of DOD in young horses.” That said, she points out that DOD can still show up in youngsters despite excellent management and diet.

“Cutting back on protein, vitamins, and minerals slows growth rate without interrupting growth,” she continues. “Deficiencies in important nutrients potentially lead to delayed onset of DOD. Steady and proper growth can be optimized by controlling calories, providing properly balanced nutrition and adequate free-choice ­exercise.”

“Excess weight on bones and joints of a growing horse is more detrimental than being underweight,” says Crandell. “Use of a ration balancer provides a low-­calorie option to balance out the forage for the easy keeper.”

A low-starch, high-fat concentrate with a ration balancer might benefit horses with specific growth or metabolic issues, says Crandell. Always offer free-choice salt, as well.

Once a horse has matured, he is not likely to incur a new “developmental” orthopedic disease. “However,” says Davison, “unsoundness caused by DOD may not become apparent until skeletal structures are stressed by concussion or repetitive work, as when a young horse enters into training or competition.”

Preventing Gastric Ulcer Syndrome

The equine gastrointestinal tract evolved to handle small frequent meals throughout the day. Equine gastric ulcer syndrome (EGUS) can result when any age horse consumes abundant carbohydrates (grain products) and/or is subject to long periods of fasting between meals.  Providing steady access to forage is an important strategy for lowering a horse’s risk of developing EGUS.

Free Download: At a Glance: Equine Gastric Ulcers
Free Download: At a Glance: Equine Gastric Ulcers

“Feeding free-choice hay to any age horse is appropriate when they’re working hard enough to burn calories and don’t exceed body condition scores of 5-6,” says Davison.

Crandell refers to a 2011 study of young Standardbreds to illustrate the importance of forage type in the growing horse’s diet: “Youngsters that had been on a pasture-only diet at the start of the study were tested on two diets high in concentrate—the ulcer scores worsened. The first diet was 50% hay cubes and 50% commercial grain concentrate. The second diet used the same ingredients but ground into a complete pelleted feed. Ulcer scores were highest with the complete pelleted feed, even though it had the exact same ingredients as the cube and concentrate diet.”

She says this is probably due to horses’ decreased chewing and saliva production when consuming a complete pelleted diet, as well as the interval without feed because horses consume pellets quickly.

Feeding recommendations to prevent EGUS in adult horses are relevant to the growing horse, says Crandell:

  • Provide smaller, more frequent meals when feeding large amounts of concentrate or grain;
  • Limit starch content to less than 1 gram of starch per kg body weight per feeding. For example, a 450-kg (1,000-pound) horse should receive no more than 5 pounds of grain or concentrate per feeding;
  • Provide free-choice access to forage if possible, or offer at least 1.5% of the horse’s body weight in daily forage;
  • Maximize chewing time using slow feeder hay bags or bale boxes that allow for periodic eating without ­overconsumption;
  • Add alfalfa to the diet to help reduce ulcer scores.

Tackling the Transition

Understanding gastrointestinal function and offering the proper balance of nutrients are key to transitioning a young horse to an adult diet. Davison says the transition might not come through feeding a different ration but, rather, offering less total feed than you would to an adult horse. Or, the shift might be to a higher proportion of forage and less supplement. 

“Protein provided in the transition period is a little higher than for the mature horse because of ongoing development and building of body tissues, albeit at a slower rate,” says Crandell.

Davison says there’s no hard rule about what ­percentage-protein feeds horses need. “This is often affected by the calorie content or recommended feeding rates of supplements,” she says, to meet his total protein requirements.

In general, growing horses 1 to 2 years old need about 10-15% more protein than do mature horses. “Usually protein requirements for the transitioning period are addressed by good-quality hay with a 12% protein concentrate given at the recommended feeding rate,” says Crandell.

“If a horse matures early in size, weight, and substance, then transitioning to an adult feed may be done with little to no transition time, but if the horse still seems immature with growing yet to do, then staying a little longer with growth feed is advantageous,” she adds.

Also consider nutrient quality when modifying diet: An “adult” diet of a supplement of 10% protein with 8-10% protein in grass hay is not sufficient for a growing youngster. Davison doesn’t recommend transitioning youngsters to adult diets until all growth and development is complete.

Take-Home Message

Dietary decisions aren’t necessarily about good or bad feeds, calorie sources, or ingredients, says Davison. Rather, they’re based on the total diet’s balance. Proper feeding management and nutrition should support growth without overfeeding and fattening the young horse.

“Successful transitioning depends on the individual horse and its rate of maturation,” says Crandell. “Transitioning a horse is more an art than a science. As the old saying goes, ‘It is the eye of the master that fattens the calf.’ ”

Practice good husbandry and feeding to maintain a steady growth rate. Provide appropriate amounts of correctly balanced, lower-starch rations intended for growing horses, and weigh and measure horses regularly during their growth period.

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Written by:

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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