It's been 18 years since the National Research Council (NRC) revised its Nutrient Requirements for Horses – a manual widely
used by equine practitioners. . Now the Sixth Revised Edition (2007) is out, published by the NRC's Committee on Nutrient
Requirements for Horses, chaired by Laurie Lawrence, PhD, professor at the University of Kentucky's Department of Animal Sciences.
The volume, which this time comes with a Web-based computer program, updates what was published in 1989 on the various nutrient
It offers expanded chapters on energy, carbohydrates, fats and fatty acids, protein and amino acids, minerals and vitamins.
Besides covering basic nutrients, there is a chapter on water and water-quality issues; feeding and feed processing, including
pasture and forage; feed additives; feed analysis; and on formulating and evaluating rations.
The most detailed new information is in the chapters on "Feeding Behavior and General Considerations for Feeding Management,"
"Unique Aspects of Equine Nutrition" and one on nutrient needs of donkeys and other equids.
The innovative computer program helps users determine nutrient requirements and formulate rations for horses and ponies of
various physiological classes (maintenance, exercise, reproduction, lactation and growth).
Most information is aimed at light horse breeds, so the authors suggest that "the recommendations for ponies and draft horses
be applied with discretion."
As in past editions, most information is said to reflect the latest scientific knowledge with a proviso that it is subject
to further review. Here are some highlights:
Energy is discussed mainly as apparent digestible energy (DE) – the amount of energy in a food as determined by comparing
gross energy or energy intake minus that contained in the feces, which includes not only that not absorbed, but also sloughed-off
intestinal material and digestive secretions.
The result is therefore apparent DE, not truly non-absorbed energy, since it includes endogenous losses.
Although for some animal species energy is discussed as metabolizable energy (ME) or net or recovered energy (NE = GE-[DE+
UE+HE]), the revised volume shows DE is sufficient and a good basis of the energy needs of horses.
The committee decided that metabolizable energy or net energy was difficult to discern, given the variability and complication
of calculating and evaluating energy lost via urine and gaseous losses, and that HE (heat energy or heat production) was even
more convoluted for the horse.
Heat energy – energy lost to the environment – includes that for basal metabolism, digestion and absorption, the heat of fermentation,
product formation (anabolism), voluntary activity (chewing, movement to retrieve nutrients, i.e., grazing), thermal regulation
and waste formation and excretion.
The net energy, by definition, is a recovered entity or product, as in the energy of work, or in milk, or in fetal tissues.
With this unit as the basis of energy requirements for horses, the apparent DE for maintenance, growth, reproduction and exercise
Maintenance energy is that exclusive of any of the other physiological states. There is an extensive section on the energy
needs for exercise, including that for light, moderate, heavy and very heavy work. Heavy work is defined as four to five hours
per week – 20 percent walk, 50 percent trot, 15 percent canter and 15 percent gallop – in ranch work, polo, show horses (frequent,
strenuous), low to medium eventing and race training (middle stages).
Very heavy work is defined as that ranging from one hour per week of speed work to six to 12 hours per week of slow work,
including racing (Thoroughbred, Standardbred, Quarter Horse, endurance) and elite three-day event work.
The main energy providers in horse diets are discussed in chapters on carbohydrate and fat.
Carbohydrates, from pasture, stored forage and grain, are the prime suppliers of energy for the horse, absorbed in the small
intestine, and those converted to volatile fatty acids (VFAs) (proprionic, butyric, acetic) and absorbed in the hindgut (cecum
and colon). The microbial population in the cecum can meet up to 30 percent of the horse's maintenance energy needs with additional
VFAs produced in the colon.