NRC guidelines show how age, weather affect nutrition needs

Several conditions may promote an unusual nutrient demand in horses, aside from that normally required for maintenance, growth, reproduction and activity.

The National Research Council's recently updated Nutrient Requirements of Horses, 6th Edition, discusses several of those conditions in a chapter on "Unique Aspects of Equine Nutrition."

Two key issues are the special nutritional needs of foals and older horses, and those of all horses during extreme cold and hot weather.

Foals' unique demands

During the first 6 to 8 weeks of life, foals primarily are supported by the dam's milk, though creep feeding may be added within the first 10 weeks. Foals even at a few days of age can consume some solid feed.

"During the first 24 hours of life, foals consume approximately 15 percent of body weight as milk, increasing to 22 percent to 23 percent on Day 2, and approximately 25 percent of body weight (15 liters for a 50-kg foal) by seven days postpartum," NRC states.

Mare's milk is 98 percent digestible (from a study in Thoroughbred foals). Its composition is 10.7 percent solids, 2.7 percent protein, 1.8 percent fat, and 6.2 percent lactose during the first four weeks of lactation (Table 1). However, in Table 1, note the great variability/range in the data, and understand that the composition of mare's milk varies with nutrient intake, metabolism and sampling variability.

Table 1 Mare's milk composition

Not only do foals seek creep feed if available, but, according to a study noted in the NRC, those that become accustomed to eating dry feed prior to weaning have reduced weaning stress. Creep rations usually are 16 percent to 20 percent high-quality crude protein, 0.8 percent to 1 percent calcium and 0.6 percent to 0.8 percent phosphorus, although there is only limited research on what is optimal.

Management of orphaned foals and their nutrient needs depends on when they lose their dams. Younger foals should be maintained on milk or a milk replacement, though older foals (greater than 10 weeks of age) can be maintained on creep feed and high-quality forage.

Fostering of foals less than 6 to 8 weeks of age to a "nurse-mare," or collected mare's milk, is preferred. If cow's milk is substituted, it should be 2 percent fat-skimmed milk, with dextrose added at 20g/L (40 mls of a 50 percent dextrose per liter of milk). Milk replacers should be about 15 percent fat and 22 percent crude protein, with a fiber content of <0.5 percent, fed as a 10 percent to 15 percent solution.

According to the NRC, "it is advisable to gradually increase the volume of milk fed over a seven-to 10-day period." One suggested method is to feed at 5 percent to 10 percent of body weight on Day 1, increasing to 20 percent to 25 percent body weight by Day 10. "Orphan foals can be weaned from milk at 10 to 12 weeks of age," according to one study NRC cited.

Effects of aging

Older horses first must be defined as to chronological, physiological and demographic age to determine their nutritional needs.

According to studies cited by the NRC, 20 years is the threshold of old age, and it is further defined by the physical signs of aging (low BCS, loss of muscle mass over the top line, yielding a swayback appearance, hollowing out of the grooves above the eyes, graying of the coat and dental disease).

Once age is defined, its affect on digestion, absorption and metabolism must be considered to determine an older horse's nutrient needs, as nutrient requirements are a function of those variables.

The NRC notes that "finally, the effect of aging-associated disease on nutrient requirements must be determined and should be viewed separately from the effects of aging itself."

The ability of older horses to consume feed easily is noted in dental changes. Their "ability to prehend and chew feed decreases the digestibility of nutrients, and leads to substantial loss of body weight." Feeding options for such older horses with dental disease include complete processed feeds, ensiled forage (haylage) or chopped hay or forage cubes. Water can be added to feed to make it easier to consume and oil added to increase energy density.

The energy requirements of older horses "are a function of energy expenditure and the efficiency with which gross energy (GE) present in feeds is converted to net energy (NE)." With less physical activity, older horses may exhibit a lower fat-free mass, along with an associated lower maintenance energy requirement, suspected from data from dogs and humans, though unknown from specific studies in horses.

It has been shown, however, that horses' fecal energy output increases from a decreased use of fiber. It has been suggested that aged horses "have a reduced absorptive and/or digestive function in the large intestine." It is "likely that age-related changes in teeth may impair a horse's ability to masticate feed, subsequently decreasing digestibility in the remainder of the digestive tract," NRC states.

"The effect of aging and age-related disease on protein requirements of horses is unknown," NRC adds. One study reported lower crude protein apparent digestibility (67 percent vs. 73 percent, 3 percent) in aged horses (26, 5 years of age), when compared to younger horses (2.3, 0.5 years of age). "Whether this finding reflects the old horse population in general and significantly impacts protein requirements of old horses remains to be determined," NRC states.

Changes in micronutrient requirements of older horses remain "relatively uninvestigated." One study cited shows a decreased apparent phosphorus digestibility in older horses. Another study regarding micronutrients was considered controversial, though it suggests a change in vitamin C status in aged horses.

"The true effect of aging and age-related disease on nutrient requirements remains to be determined in horses," NRC concludes. Yet, "chronological age alone is not sufficient to categorize horses relative to age-related changes in nutrient requirements."

Feeding in cold or hot weather

According to the NRC, "for horses kept in environments outside the thermoneutral zone, adjustments in nutrient requirements will occur and changes in feeding management may be necessary. "Cold weather creates an increased demand, mainly for energy."

It is necessary to ensure that most idle, adult horses in cold weather have increased feed, including additional good-quality hay free-choice, "to supply additional DE (digestible energy)."

Grains also may have to be added to "ensure adequate energy intake, especially for growing, thin, worked or aged horses." NRC says, "Growing horses may require an additional 1.3 percent DE for each degree Celsius below the lower critical temperature, plus the DE required for weight gain. Adult horses should be given an additional 2.5 percent DE for maintenance per degree below LCT."

With persistent cold weather, increased dietary energy is critical. In addition, NRC recommends keeping horses warm with blankets, rugs and/or shelters to reduce heat loss.

Hot weather requires a significant increase in water intake – 30 percent to 75 percent – to compensate for excessive sweat and respiratory losses. "At ambient temperatures above the upper critical temperature, "water should be supplied in a manner that allows voluntary intake by horses."

Horse feed intake should be managed to minimize heat load. Increasing fat intake to decrease the heat load is suggested, but studies are limited. However, increased fat raises concerns about decreasing the digestibility of fiber, dry matter and protein.

Fat supplementation has been shown to increase Vitamin E and beta-carotene absorption in horses.

It also is critical to make free-choice salt available for horses in warmer climates.

Besides changes in feed management, the NRC recommends "shade, preferably that allowing unimpeded air movement."

Ed Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.