Many nutrition texts and feeding standard publications (e.g., National Research Council: 2007) present RCs or a mixture of both RQs and RCs. The main rationale for RCs is to include safety margins or to compensate for absence of complete data on strict RQs. Although it may seem pedantic, it is very important to differentiate between RQ and RC. Feeding below RC is not in conflict with sound nutritional practice, but feeding below RQ is in conflict with scientific evidence and often problematic from a legal standpoint (not of minor importance).

However, it is still possible to feed below the RQ without clinically apparent adverse effects. Conversely, problems related to nutrition may become apparent when horses are fed at or above the RQ. In part, these divergent scenarios can occur because the system used to define requirement data assumes that all horses are the same and therefore, for example, the needs of a Clydesdale are identical to those of a Thoroughbred. Unless a diet is obviously imbalanced, it can often be difficult to practically determine whether the ration is adequate or not unless a full nutritional evaluation is carried out. Full nutritional evaluations are expensive, can only represent a snapshot in time, and can be difficult to achieve especially if pasture forage is part of the ration. Day-to-day variability in nutrient profile in feedstuffs (pasture, preserved forages, even commercial feeds) coupled with individual differences in intake make it impossible to achieve complete accuracy. That said, as an initial step it is possible to use generic tables of analytical data to get an indication of whether the diet is likely to be limiting in one or more nutrients.

Published nutritional surveys in which in-depth nutritional analyses were used have reported that many categories of horse, even elite performance animals, are often not being fed the RQ intakes of several key nutrients – let alone more “optimal levels” recommended by some. Yet, these animals are still competing and, according to their owners, apparently healthy. There are several possible interpretations: (1) the recommended RQ intakes are not valid in general or for those individuals specifically; (2) the imbalance is being masked by an oversupply of a complementary nutrient; or (3) the imbalance has not been present for long enough to manifest as a clinical problem. Alternatively, the horse may have a problem associated with the diet but this link has not been made by the owner/trainer. As many horses change owners relatively frequently – and young thoroughbred racehorses rarely stay with the same trainer until they are geriatric – it is very difficult to determine the long-term chronic effects on health, etc. of suboptimal nutrient intakes at various times within an animal’s life. There has been little or no work on the concept of epigenetics in the horse but perhaps we should consider this more seriously in the future. Ideally, it would be great to raise, train, compete and retire large number of animals on diets that provide all the nutrients at the level we consider to be suboptimal, at recommended levels and at optimal levels. This, however, remains an unrealistic goal.

Do we know what the true RQ is? There are differences in RQ data published by the various authorities. Although the concept of an RQ is not debatable, in reality there is a range of values that represent RQ for any nutrient and there are differences in what can be considered an RQ because of the following:

1. Dimensions: It is usual to standardize RQ data by referring to BW or metabolic BW (i.e. BW^0.75). As discussed above, the use of actual BW implies a linear change in requirements e.g. from a 100-kg pony to an 800-kg draft horse. Many of the published RQ values were derived from studies that used animals in the middle of this BW range and consequently the RQs may not be applicable to animals at the extremes of BW. This is why some authorities apply allometric scaling, such as BW^0.75 (or another exponent number), in the development of RQ values. Another issue is the use of unit per day or unit per kg DMI for expression of RQs. The latter is traditionally used for trace elements and is very convenient in daily use; however, this dimension fixes the nutrient requirement to another nutritional factor (feed intake) that has its own flexibility – as discussed above.

2. Basic method for defining RQ: The basic principle is to calculate RQ = endogenous losses/utilization + product/utilization. This is commonly referred to as the factorial approach. In many instances, RQ values have been derived from data for other species rather than the equid, and this approach may not be valid. The utilization rate must be evaluated in animals in different physiologic states across a range of intakes. Variance in the applied utilization rate has a significant impact on the calculated RQs.

3. The method for defining RQ described above ignores the interaction between different metabolic systems, as it takes a nutrient in isolation and then balances input with minimal output to reach the point of equilibrium. Individual nutrients, however, have important impacts on other systems e.g. feeding at the RQ for protein may limit the capability of the immune system. Calculating RQs based on purely the factorial approach may have adverse effects on body homeostatic mechanisms. Chloride is a simple example; feeding Cl strictly according to the RQ derived from the factorial approach may result in lowered Cl concentration in blood and an alkalemic shift in acid-base balance.

4. Experimental data: Differences arise in transforming experimental data, especially if results across different studies are inconsistent. A good example is milk yield in mares. The published data show a very wide range even when they are standardized to BW or BW^0.75. As a result, there is no single universal model that can be used for incorporation of milk yield into RQ calculations.

5. How to quantify other biological effects such as the impact of exercise? Exercise effort should be readily quantifiable; a horse of known BW moves from A to B in a certain time. In contrast to daily gain or milk yield, however, exercise is difficult to quantify in terms of additional requirements for energy and nutrients. There are several external factors that contribute to marked variation in the metabolic cost of converting chemical energy into kinetic energy, including fitness, dietary regimen, environmental conditions, terrain etc.

Our opinion

1. A primary goal in the development of requirement data is to define the energy and nutrient provision that drives any metabolic process without activating compensatory strategies in the animal in order to cope with either limited or excess availability of that nutrient. In some situations, health and performance, at least in the short term, will be maintained in the face of limited nutrient supply due to the impact of compensatory mechanisms (e.g., enhanced absorption efficiency and/or reduced excretion). This does not imply that nutrient provision below the guidelines provided in this book and elsewhere is recommended.

2. Clarity is needed in recommendations on the amount of any nutrient that should be provided in the ration. It should be made clear whether the value is a requirement and therefore should be considered (practically) as a minimal level to sustain life (with the provisos above) vs. a recommendation, which implies that a safety margin has been built-in. There also may be justification for developing rations on the basis of optimal intakes, especially when a particular system (such as the immune system) may benefit from increased intakes (e.g. horses in stressful circumstances such as intense physical activity and travel for athletic competition). Nonetheless, at present most optimal recommendations are based on personal experiences or beliefs and have little or no scientific basis.

3. For most end-users (nutritionists, veterinarians, etc.), we recommend the selection and use of one reference publication (NRC, GEH or INRA). Tables 1-5 in Appendix show energy and nutrient recommendations as published by the NRC and GEH and as suggested in this book for a 500-kg horse at various life stages (together with values used by one of the editors (P.H.) that represent a mix of recommended and optimal requirements).

The dilemma of “supplementation”

Even the term supplement is controversial as it can be taken to include products that: (1) restore nutritional balance to a ration, e.g., forage or cereal balancer feeds; or (2) provide a safety margin when the nutrient quality of a ration is not known (see Chapter 19). The utility of such supplements is clear and is not further discussed here. Instead, this discussion focuses on compounds or plant material that are used to provide one or more nutrients in a specific preparation (e.g., amino acids in gelatin) or one or more non-nutritional factors (e.g., flavonoids) in order to support metabolic processes (e.g. antioxidative capacity) or provide some other purported benefit. This type of supplementation is one of the most hotly debated topics in equine nutrition. The challenge for science and practice is to find the balance – on the one hand, not immediately dismissing the possible advantages of a given supplement (even if there is a dearth of efficacy and safety data), while on the other hand, not accepting all “claims” that lack any supporting evidence.