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.

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.

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.