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
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.