26 Controversial areas in equine nutrition and feeding management
The Editors’ views
Practical feeding of horses is both an art and a science – the science provides foundation knowledge for the formulation of a ration and the art is adjusting this ration to best fit the circumstances of an individual horse, taking into account the needs and beliefs of the owner as well as the availability of feedstuffs, environmental influences etc. There is not one correct way to feed any horse, providing some flexibility to individualize rations but also enabling the development of myths and feeding traditions that become established practice and yet are based on little or no objective evidence.
This chapter looks at some of the arguments commonly put forward for and against a range of nutritional and feeding management practices/beliefs. Wherever possible, our comments are based on scientific evidence and refer to the state-of-the-art as we perceive it; however, where scientific evidence is not available or is inconclusive our comments will of necessity be derived from individual personal experiences and even beliefs. Even when scientific evidence is available, conclusions often vary according to the circumstances and the individual. In this chapter, therefore, we have tried to highlight some of these controversial areas and provide our current opinion. For this reason, reference citations have been kept to a minimum.
Questions concerning feed intake
Can a healthy horse lose condition due to its biological limitation in feed intake?
The answer to this important question is simply NO, even for horses with high energy requirements (e.g. lactation or heavy athletic training and competition). In healthy horses, weight loss will reflect problems with feed quality and/or adequacy of supply.
Should dry matter intake be expressed as a percentage of bodyweight (BW) rather than metabolic BW (kg0.75 BW)?
Here, the answer is less straightforward – some of the key arguments for and against are given below:
The determination of metabolic body weight is an allometric scaling procedure that provides an across species term for energy requirements at maintenance level. The scaling factor commonly used (0.75) is not necessarily optimal for the horse (and certainly is not always valid for other biological aspects, e.g. heart size). Perhaps more importantly from a practical viewpoint, the expression of dry matter (DM) intake as a % of actual BW is much easier to apply in the field, e.g. DM intake = 2% of BW is much more applicable than the artificial term of 120 g DM/kg0.75 BW.
Over a wide range of BW (e.g., 100 to 1000 kg) DM intake cannot be a simple linear function of BW. Ponies show different chewing and swallowing characteristics than horses; the ratio of feed mass to dental surface area differs between small and large equids and the ingesta transit time will also vary.
Several biological phenomena related to intake, such as oropharyngeal feed processing, are not linearly related to BW i.e. they are scaled by a factor <1. Although 0.75 may not be optimal, given the biological relationship between DM intake and energy requirement there is rationale for use of metabolic BW (i.e., DM kg0.75 BW). Even so, further adjustments may be needed for certain breeds/individuals and circumstances. For example, ponies maintained on pasture may have proportionally a higher DM intake than a horse even when expressed on a metabolic BW basis; thin ponies in the summer with ad libitum access to feed may have much higher intake than an obese animal of the same breed and size offered the same feed (see Dugdale et al 2011). Expressing the requirements for other nutrients as a percentage of BW or DM intake is similarly problematical and currently not recommended (see also Chapter 9).
Has the limit of dry matter intake in horses been determined?
Much of the relevant literature is discussed in Chapter 3.
We have sufficient information available for most practical purposes but recent work has suggested that average intakes are likely to be higher than previously suggested (e.g., up to 12 kg DM for a 250 kg nonobese pony or 4.8% of BW – Dugdale et al 2011). It seems unlikely that there is much more flexibility for higher intakes considering the time required for ingestion of 1 kg DM.
Not in all circumstances and for all types of feed. Complex interactions between type of feed, available time and physiological status are likely to impact maximal DM intake.
We have sufficient information to define limits of DM intake by a healthy horse/pony with free access to typical feedstuffs and with no restriction in time budget. It is important to understand that, from evolutionary, physiological and behavioral viewpoints; the horse is adapted to spend long periods of time engaged in foraging/feed ingestion activity. Management practices that encourage prolonged feeding activity are beneficial to health and welfare as long as energy overconsumption is avoided.
Should we be using minimal requirements (RQ) or basic recommendations (RC) or “optimal” recommendations (OR)?
The adequacy of energy intake can be readily assessed by repeated evaluation of body condition; however, it is not possible to visually determine whether or not the horse is being fed a balanced diet providing recommended levels of amino acids, macro- or micronutrients – unless the intakes are sufficiently low or high to cause overt signs of deficiency/toxicity. Overt deficiency or toxicity is rare in the developed world although conditions such as big head disease (nutritional secondary hyperparathyroidism due to a marked imbalance in the Ca:P ratio of a diet through excessive phosphorus intake) are still reported in some countries. Animals may appear to be fit and healthy, yet their diet at the time of examination may not meet one or more nutritional requirements.
In discussions on how to feed horses, it is frequently stated that “they have had horses for years, never ensured that they were receiving adequate micronutrients and never had any problems”; and therefore: “the recommendations must be wrong”. This reflects a common misunderstanding of RQ and RCs which is discussed below. For the purpose of the following discussions, we have used the following definitions:
• Minimum requirement (RQ) is taken to be representative of the minimal intakes to sustain life in an average healthy horse within the population;
• Basic recommendations (RC) include a “safety margin” so that they should cover the minimal needs for the majority of healthy animals in a population; and
• Optimal recommendations (OR) enable the needs of particular individuals or groups of individuals with specific needs to be met (e.g. those animals with certain clinical conditions or experiencing increased “stress”). By necessity, ORs tend to be more opinion based.
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. BW0.75). 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 BW0.75 (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 BW0.75. 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.
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.
Do we need supplements?
Many arguments for and against the use of supplements have been given over the years – examples are given below:
One of the tenets of good feeding is to feed the horse as an individual, and supplements are a means by which the diet can be tailored. A supplement enables one to consider specific individual conditions (e.g. a horse with a nervous temperament or poor hooves) or environmental risks (e.g. the effect of housing on lung health; the impact of travel on the immune system). Supplementation in such circumstances may work to counteract the imbalance between nutrient supply and demand and help to restore “normality”. In other situations supplementation may help to provide additional boost or support to enable enhanced performance or reduce the risk of certain clinical conditions developing.
Do they work?
While there has been a marked increase in the number of specialized dietary supplements on the market, this has not been matched by a concomitant increase in research on the safety, efficacy etc. of these products. Many substances are marketed without adequate understanding of their function in the horse. Too often there is little or no evidence regarding the effect of a supplement on a particular metabolic or physiological mechanism, nor data to suggest that this process is of any relevance to the health, behavior or athletic performance of a horse. For most supplements, there is little scientific evidence derived from the horse to support marketing claims. Reported positive effects can reflect owner perceptions (placebo effect).
There is increasingly evidence that nutrition can play a very important role in the management of certain disease conditions in man and other animals – nutritional requirements may change or the affected individual may respond to the inclusion of particular nutrients or feedstuffs in the diet. In the equine field too, there is prior and emerging scientific evidence for a few products/supplements (e.g. high doses of biotin and certain hoof problems; antioxidant supplement and lung health; Kirschvink et al 2002). Even if scientific support is currently lacking, this should not be taken as indication that a given supplement is not of benefit to the horse.
• There can be considerable misinformation and hype associated with some supplements, and this makes it very difficult for owners/feeders to make informed decisions regarding the “value” of a particular supplement.