How to Properly Conduct an Equine Nutrition Evaluation: Figuring Requirements

Ration evaluations are intended to compare a horse’s daily nutrient intake to a set of requirements to see how well the feeding program meets the horse’s nutrient needs. This seems a pretty straightforward accounting exercise, but what nutrient requirements should be used.

The National Research Council (NRC) publishes a set of requirements for horses as it does for other species of livestock. For the most part, NRC values represent minimum requirements for most nutrients. These are levels of intake that are required to prevent frank deficiency symptoms. There is no allowance included as a safety margin to take into account factors that may increase the requirement for a nutrient. Different sources of nutrients may have different bioavailabilities, and there may be other substances within a ration that interfere with the digestibility or utilization of the nutrient.

While digestible energy and protein are two NRC requirements that fairly accurately describe the needs of horses maintained under practical management conditions, for most of the vitamins and minerals, many nutritionists will recommend values 1.25 to3 times higher than NRC. Kentucky Equine Research (KER) developed its own set of recommended daily nutrient intakes for different classes of horses. These more liberal recommendations are based on KER’s own research and experience in the field. All of these recommended nutrient intakes are far from absolute, and they will continue to evolve as more data become available. For now, though, these recommendations adequately reflect what is needed by the horse under a wide range of conditions.

Energy Balance

Of primary importance in developing or evaluating a ration is ensuring that the appropriate amount of digestible energy is supplied. While energy is not a nutrient, it is a need that the ration must meet and is derived from nutrients. Nutrition evaluations depend on the concept of energy balance to match feed intake to requirements. Calculating energy balance allows a primary overall evaluation of how well a ration meets the horse’s caloric needs. Only after we are satisfied that the horse is in a reasonable state of energy balance should we proceed with the rest of the evaluation.

If evaluating an existing ration for nutrient adequacy, the most expedient method to determine energy balance is to assign a body condition score to the horse as well as question the caretaker. A horse that is in zero energy balance maintains its weight. A horse in negative energy balance uses more calories than it consumes and is likely underweight or losing weight. A horse in positive energy balance gains weight and may be overweight.

Once you’ve determined the horse’s current energy balance, you now need to determine if it is desirable to maintain the status quo. For instance, though a horse may be in a positive energy balance and be gaining weight, if it is already overconditioned, it may be more desirable to develop a ration to put the horse in a negative energy balance so that it will lose weight. The opposite may hold for a horse that has been losing weight or is carrying less condition than desired.

Energy Is the Key

The first step for every evaluation is to calculate the energy requirement of the horse. We currently use units of digestible energy (DE) to describe dietary energy requirements. DE requirements can be expressed as kilocalories (kcal) or megacalories (Mcal), where 1 Mcal equals 1000 kcal. Joules are another unit of measure used to describe DE. To convert calories to Joules, simply multiply by 4.18. Therefore, 1 kcal = 4.18 kJ, and 1 Mcal = 4.18 MJ.

Digestible energy is actually a fairly crude method of describing the energy content of feed, because it is calculated by subtracting the energy lost in the manure from the gross energy (GE) content of the feed. There are large differences in how efficiently the digested energy is utilized from different feedstuffs, so DE requirements can only be viewed as estimates that must be refined depending on the type of diet being fed. For example, a horse will require more DE to meet its energy requirement if it is eating an all-forage diet than if it is eating a ration high in grain or added fat due to the differences in efficiency of utilization. More accurate systems of energy evaluation utilizing net energy (NE) have been developed, but have not been placed into widespread use because of a lack of information about NE requirements for various classes of horses and NE contents of different feedstuffs.

The following table lists the DE requirements for various classes of horses. The maintenance DE requirements for horses is DE (Mcal/day) = 1.4 + 0.03 x body weight (kg). Requirements for other classes of horses will depend on age, growth rate, reproductive status, or work intensity.

Energy requirements for different classes of horses.

¹ADG = average daily gain (kg/day); ²BW = body weight (kg) ; ³Pregnant T1 = first four months of pregnancy; ⁴Pregnant T2 = second four months of pregnancy; ⁵Late pregnant = last 3 months of pregnancy; ⁶Early lactation = first three months of lactation; ⁷Late lactation = second three months of lactation

Read the first part of this series: Conducting an Equine Nutrition Evaluation: Getting the Information.

Adapted from: Pagan, J.D. 1998. Computing horse nutrition: How to properly conduct an equine nutrition evaluation. Advances in Equine Nutrition, Volume I. Nottingham University Press, Nottingham, U.K., pp. 111-123.