26 Controversial areas in equine nutrition and feeding management
The Editors’ views
Questions concerning feed intake
Should dry matter intake be expressed as a percentage of bodyweight (BW) rather than metabolic BW (kg0.75 BW)?
Our opinion
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.
Should we be using minimal requirements (RQ) or basic recommendations (RC) or “optimal” recommendations (OR)?
• 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.
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.
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”
Do they work?
Our opinion
• 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.
