23 Ration evaluation and formulation
Feeding horses is often described as being a combination of art and science and the area of ration evaluation is probably the best illustration of this. Ration evaluation can provide a useful indicator of the appropriateness of a horse or ponies’ diet, in terms of energy, macro- and micronutrient intake, as well as energy source. However, there are limitations to the accuracy of the information obtained from a ration evaluation and interpretation of the data should reflect this.
Practical ration evaluation invariably involves assessment of the suitability of a horse’s current diet relative to its individual circumstances, with suggested changes being made where warranted. In most cases, a nutritionist will be presented with a dietary history of some kind, although there are notable exceptions to this where no dietary history is available, for example in acute welfare cases. Dietary history may also be scant where a radical change in circumstances, such as removal of a horse from pasture, or a recent purchase has occurred. Where available, a horse’s current ration, as well as previous dietary history, is of immense value to a presiding nutritionist and will usually be the starting point for most evaluations.
There is a range of information that must be gathered in order to practically evaluate a current ration or indeed formulate a new ration, all of which will be discussed in this chapter including the factors mentioned in Box 23.1.
Information Required for Ration Formulation Includes the Following
|Young growing, mature, or aged
|Pregnant, lactating, growing, body condition (overweight, ideal or underweight), presence of nutritionally related disease, behavior-related issues.
|Maintenance, level of exercise, type of exercise
|Horses may have a preference for a particular feed type
|Nature, amount and analysis of current ration
|Sensory evaluation (e.g., indication of type of hay)
|e.g., protein, oil and fiber content and calculation of energy content
|Sensory and/or laboratory analysis
|There will be regional variation in the availability of different feedstuffs
The integrity of the information regarding the current diet may be greater when it is gathered by the nutritionist themselves, or by another appropriate person. Being physically present during at least one feeding period is desirable and is likely to increase the accuracy of the dietary information obtained. The client or feeding manager can be questioned regarding the feeds used and their feeding habits and management. A nutritionist should also weigh all of the allocated quantities of feeds, supplements and forages. This presents a good opportunity to sample the components of the diet for any required laboratory analysis.
Remote ration evaluation through telephone, written or web-based questionnaires has become very common in recent years with many feed and supplement companies offering a free advice service. This type of evaluation allows clients to receive useful guidance on the appropriate use of particular products within a product range. However, remote ration evaluation also has the potential to be less robust, as there is reliance on the owner, trainer or other persons to provide accurate information regarding the individual horse, current ration and dietary history. However, the integrity of this information can be improved through skilled interview of the client by telephone and by ensuring that all feeds have been weighed.
The reliance on predicted or published nutrient content of feeds and forages as opposed to actual laboratory analysis may also affect the accuracy of the ration evaluation. With respect to end users, the use of predicted analysis is usually driven by economical constraints. The number of establishments that are prepared to undertake full nutritional laboratory analysis of feedstuffs, forages and grazing is relatively low.
Nutrient analysis of forage, for example alfalfa, can vary considerably depending on factors such as stage of growth at harvest, type of grass, fertility of the soil as well as the soil type and underlying geology of the land (Van Soest et al 1978). All of these factors will also affect the nutrient content of pasture. Whilst historical pasture analysis is sometimes available during ration evaluation, it is usually restricted to large professional establishments. It is therefore more common for a nutritionist to draw on published nutrient values for pasture and forage (NRC 2007), which introduces inherent inaccuracies. Localized regions of pasture inadequacy in trace minerals such as copper, for example in parts of Ireland, or extreme reductions in pasture quality (e.g., in UK moorland) may not be captured.
Proprietary feed and forage companies can usually provide analytical information regarding their feed or forage products, which is likely to be based at least in part on actual laboratory analysis, especially where the legally required declared analysis (e.g., protein, oil, ash and crude fiber) is concerned. Feed companies are also able to provide an accurate prediction of micronutrient content of proprietary feeds, where a vitamin and mineral premix has been used. It is, however, still relatively common for feed manufacturers to use published values for the prediction of other nutrients of importance such as starch, sugars, amino acids, etc.
Laboratory analysis of concentrate feed or forage can be carried out simply to confirm adherence to the declared analysis, or can offer more comprehensive information beyond this. Samples must be as representative of the feed or forage as possible, in order to avoid large discrepancies occurring between predicted and actual analysis due to sampling error.
• Maintaining the integrity or accuracy of gathered information regarding the horse/horses, where ration is being evaluated, as well as the nature of the current ration will improve the evaluation process
The energy content of feeds and forages must be obtained for accurate ration analysis, either through the use of predicted values or calculation from proximate analysis using published equations. The energy value of feeds can also be derived in vivo from feeding trials, although this would usually be beyond the scope of practical ration evaluation. Energy, which is usually described in terms of either joules (e.g. in Europe) or calories (e.g. in USA [1 kcal = 4.184 kJ], is most often expressed as digestible energy (DE), although in Europe net energy (NE) or derivatives thereof as discussed in Chapter 4 are routinely used).
Feeds can also be described using the horse feed unit (HFU) or in French, l’unite fouragire cheval (UFC). The UFC corresponds to the NE value of 1 kg of standard barley (87% DM) in a horse at maintenance (2250 kcal). The UFC value of other feeds can be calculated by dividing their relative NE content in kcal by that of barley or from prediction from chemical analysis.
There are a number of published equations that can be used to calculate the DE or UFC content of feedstuffs from their chemical composition which have been previously reviewed (Harris 1999) and are summarized below. The energy value assigned to a feed, whether estimated or calculated, must always be in the same format as the target energy requirements, for example DE for NRC requirements (NRC 2007), UFC for INRA requirements. See Box 23.2.
Example of Equations Used to Determine DE and UFC from Feed Chemical Composition (see also Chapter 5)
CC = cytoplasmic carbohydrate; CP = crude protein; DCP = digestible crude protein; ADF = acid detergent fiber; NDF = neutral detergent fiber; CF = crude fiber; DCF = digestible crude fiber; NSc = 100 − CP − EE − NDF − ash; DEE = digestible ether extract; DNFE = digestible nitrogen free extract.
Forage samples should ideally be taken using a hay bore, which consists of a long metal cylinder that can be inserted to a given depth into a bale of hay, haylage or straw usually driven by an electric drill (Fig. 23.1). In this manner, several samples can be taken from many bales to build up a representative mixed sample of a particular batch of forage. Where possible the sub-samples of hay should be mixed but there is also a reliance on the laboratory to resample for analysis in an appropriate way. Bales of haylage should be carefully re-sealed following sampling to avoid aerobic spoilage. Where a hay bore is not available, simple grab samples can be used to give an estimate of forage analysis, but again multiple samples should be taken from multiple bales from varying regions and depths within the bale where possible as this will require bales to be opened. In instances where forage is suspected of contributing to health issues such as colic or sporadic respiratory disease it may be necessary to establish the variability in nutritional value or quality of the forage. In this instance many single bales may need to be analyzed, with composite samples being taken from each individual bale.
Photo courtesy Andrew Theodorou.
For haylage, samples should be placed into sealed bags, expelling the air as far as possible prior to sealing. This is particularly important where fermentation characteristics or microbiological analysis of haylage is required. Due to the variable instability of haylage, once removed from their anaerobic environment (Cecilia 2009), samples of haylage should always be shipped to the laboratory immediately. Ideally, these should be sent on a next day delivery service, avoiding the possibility of haylage samples remaining in the post or being unprocessed over a weekend.
Complementary feed refers to any feed that is fed alongside forage or grazing to provide a balanced diet for the horse and in this context is synonymous with the term concentrate feed. Whilst legally within Europe the term complementary feed also incorporates feed supplements, these have not been included in this part of the discussion and will be addressed separately.
Proprietary complementary feed offers a certain amount of nutritional information as part of the legally required statutory statement. This usually consists of a guaranteed or typical nutritional analysis, plus a list of ingredients which may be expressed in percentage terms or simply in descending order of inclusion. The nutritional analysis may include reference to the content of moisture, crude protein, oil, crude fiber and ash. Additional information pertaining to the level of added fat soluble vitamins A, D, and E, as well as trace minerals such as copper may be provided. Whilst there may be some uniformity between countries (for example in Europe) we can expect a degree of variation in the information legally required across the world.
For complementary feeds, a similar approach to forage is required and multiple grab samples should be taken from several bags of feed. Ideally feed bags should be opened out on the floor, well mixed and then sampled accordingly. A simple protocol for concentrate feed sampling is described below.
In instances where nutritional analysis of feed or forage forms part of a legal dispute which may require evidence being presented before a court or panel of arbitration, it is important to use established protocols for sampling and also to ensure a good chain of custody for the samples. This would ensure that sample integrity is maintained from the point of sampling to the time of analysis. It can therefore be established that the analysis results presented within a legal setting refer to the actual samples taken from a particular feed on a particular day and that these have not been contaminated or adulterated in any way. This process can be simplified by the use of public service individuals with expertise in sample collection who may be called upon by a farmer or owner in situations of a suspected nutritional problem. This may be required during the investigation of potential feed contamination (e.g. following a positive post race or competition urine sample for prohibited substances), as defined by a sport’s governing body. Equally this type of sampling protocol may also be required by trading standards in order to establish conformity to statutory declaration.
Feed supplements can also be analyzed for adherence to their specification, in order to ensure that they contain the declared level of active ingredients. This is particularly important where a product is reputed to help maintain an aspect of health e.g. the reputed ability of a product to support joint health may depend on the level of particular active ingredients being present. Previously glucosamine-containing oral joint supplements have been shown to have poor adherence to their declared specification in terms of the amount of this active ingredient they contained (Oke et al 2006). In this study, of the products analyzed, 39% were shown to contain less glucosamine than claimed by the manufacturer and 17% had less than 30% of the amount claimed. It is also not uncommon for specialized supplement ingredients to exhibit a reduced or even absent level of the bioactive substance, (e.g., harpagosides, which are the active ingredient in Devils claw; Dunnett 2010).
Pasture sampling should be carried out using an established format, which may include walking a large “W” or “M” within each paddock and cutting representative samples of grass at numerous points. Areas of heavy contamination with urine or dung should be avoided and in order to minimize soil contamination samples should not be cut too close to the ground or pulled rather than cut (Fig. 23.2). Additionally samples should ideally be taken on dry days to avoid soil contamination from splash back. Further details on pasture sampling procedures and precautions have been previously provided (MacNaeidhe 1995).
Appropriate sample preparation, extraction and analysis of the analytes requested is then required, ideally using an established, experienced laboratory with appropriate external accreditation. Where laboratory results show any unexpected deviation from declared analysis or established levels, repeat analysis should always be requested.
The analysis requested from the laboratory will depend on what information is required and the specific ration evaluation questions being asked. For example, analysis may be undertaken in order to assess the suitability and/or quality of a particular batch of forage, or a feed mix may be analyzed to enable calculation of a DE value. Alternatively laboratory analysis of a proprietary feed may be required to check for adherence to the declared analysis or typical analysis provided by the manufacturer. Individual raw materials may be analyzed so that they can be added to a database enabling the formulation of proprietary feeds or a ration of straights. “Straights” are a colloquial term that refers to a mixture of straight feeding stuffs such as oats, barley, alfalfa, sugar beet, etc. that can be mixed to provide a complementary or concentrate feed for horses. Table 23-1 gives examples of some of the analytical profiles that could be requested for different situations.
Assessment of forage intake requires a record of both the weight of forage offered, which can be easily achieved when horses are fed individually, but is more problematic in group fed animals. Wastage of forage should also be recorded in order to increase accuracy. Wastage of forage can be particularly high in performance horses (Geor 2005), that may self-limit forage intake when a high level of energy rich concentrate feed is fed. Where hay is soaked or steamed, the weight offered should be recorded prior to such treatment.
An indication of the dry matter of forage is advantageous and is a necessity where haylage is considered. The dry matter content of haylage can vary considerably both between suppliers and to a lesser extent from batch to batch from the same supplier. A moisture probe can be used to provide an estimate of the dry matter content of haylage by sampling at multiple sites. This enables increased accuracy of a ration assessment that is carried out on a yard. In addition it will provide important information for the horse owner or trainer to help ensure a more consistent intake of forage dry matter. This is essential information during ration evaluation as large differences in the dry matter content of haylage between batches will affect the delivery of energy and nutrients to the horse. Dry matter analysis is also relevant when soaked feeds such as sugar beet are used.
For horses or ponies that spend time at pasture, an assessment of grass intake must be made. This is often problematic and represents a significant area for inaccuracy within the overall ration evaluation, both in terms of energy and macro- and micronutrient intake. However, if the animal is maintaining bodyweight when out at pasture (or being fed conserved forages) this reflects adequate energy intake. In contrast, animals increasing or losing bodyweight in such circumstances may have access to too much or too little pasture or forage, respectively. This may be due to factors such as herbage yield, stocking density, etc. or may reflect a clinical issue. This is of course more significant in animals where grass represents a significant portion of their diet including brood mares, leisure horses or ponies and endurance horses. Voluntary dry matter intake (VDMI) will be influenced by grazing behavior, which can be affected by a number of factors such as quality and quantity of grazing, gender, age, breed, breeding status and also whether the animals are at grass in isolation or with a group. This has been reviewed extensively in Chapters 3 and 18, and also previously (NRC 2007). The effect of forage quality on voluntary intake is not simple in horses and appears to vary between individuals with some increasing and others decreasing their voluntary intake in response to a reduction in forage quality and dry matter digestibility (Edouard et al 2008). The effect of sward height was more straightforward, with horses choosing grass with a higher sward when patches are of an equal quality (Edouard et al 2009), presumably to maximize intake.
Whilst it is recognized that the rate of intake of pasture is commonly affected by body size and mouth morphology in other mammals, the effect in horses and ponies has only recently been reported to conform to this model. Ponies appear to be able to increase their rate of consumption by increasing bite size; however, their overall intake seems to be limited by the maximum achievable processing speed (the time taken to bite and masticate), which is greater in larger horses as it increases with body size (Fleurance et al 2009).
Ultimately, dry matter intake of horses and ponies at grass, which is discussed in greater depth in Chapter 18, can vary widely between individuals and most estimates suggest a range of between 1.6–3.6% of bodyweight (NRC 2007) with lactating mares typically at the higher end of this range for VDMI (2.6–3.1%). Ponies are estimated to have a high VDMI in comparison to horses, which may approach 5% of body weight when stabled and provided with ad-libitum fiber based complete feed. Ponies also appear to have the capability to consume a high proportion of their predicted VDMI in a relatively short period of time when grazing time is restricted (Dugdale et al 2011). It has been reported that ponies are capable of ingesting nearly 50% of their total daily dry matter intake during only 3 hours of grazing (Ince et al 2011).
In young horses (yearlings and 2-year-old Salle Francais breed), intake relative to metabolic bodyweight was 82 g OM/kg LW0.75/day and did not vary with age. However, the 2-year-old horses exhibited a reduced foraging time and a higher rate of intake (Pascal et al 2000).
When using ration evaluation software programs, the contribution of grass to total daily dry matter intake can either be indicated manually by the user, or in some instances will be predicted by the software. This is often achieved simply by subtraction of the dry matter contribution of other feeds from a predicted total dry matter intake. Time spent at grass is also sometimes used to predict grass intake by ration evaluation programs, although the derivation of the calculations used for this practice is questionable given the paucity of data in this area.
An estimate or assumption of VDMI of grass must be made and used to assess the contribution of grass to energy, protein and macro- and micronutrient intake. Information regarding the nutrient analysis of grazing is also therefore required. For horses or ponies at grass that exhibit optimal body condition, maintenance requirements for energy and protein can be assumed to be satisfied. However, comparable assumptions cannot be made for broodmares or youngstock.
Grass intake will obviously be affected by factors such as availability, palatability and digestibility of the grazing. This in turn will be affected by the stocking density and also the pasture maintenance program including use of fertilizer, overseeding and the removal of droppings. A common sense approach needs to be taken to adjust the likely VDMI upwards or downwards based on at least the availability of grass and stocking density. For example, overgrazed or drought stricken pasture and/or a high stocking density will severely reduce the VDMI compared to well managed grazing with a low stocking density.
Once all of the information regarding feed intake and analysis of feeds and forage has been gathered, the task of assessing the ability of the ration to satisfy daily requirements for a particular horse or group of horses begins. Other information such as age, body condition and level of exercise undertaken as well as breeding or growth status must of course also be established. Whilst difficult to quantify, environmental influences on energy requirement should not be ignored (Cymbaluk & Christison 1990).
In harsh weather conditions a regular assessment of bodyweight and/or condition is required, in order to react to any early indication of change in bodyweight through small adjustments to the ration. The assessment of body condition and determination of the level of exercise per day represents a further area for potential inaccuracy. Preferentially such information should be obtained directly by the nutritionist.
There is a lack of awareness of body condition and also some resistance amongst horse owners to accept that horses or ponies are overweight, leading to inaccuracy of owner assessment (Wyse et al 2008). Body weight can be assessed easily where there is access to a weighbridge, or alternatively an estimation of bodyweight can be made using measurements of heart girth, length, and wither height (Carroll & Huntington 1988). A number of commercially available weigh tapes are also available, which simplifies this process. A good correlation exists between bodyweight calculated using a weight tape in comparison to that measured on a weighbridge (Ellis & and Hollands 1998). However, weigh tapes may be more useful for determining differences in bodyweigh over time for individual animals as their accuracy can be variable. In addition, this method is less useful in pregnant mares, growing horses, very fit as well as extremely thin or fat animals, as discussed in Chapter 22.
Body condition score should also be assessed and there are a number of scoring systems available. A condition score system can involve an overall assessment of the whole body according to an arbitrary scale of, for example, 1–5. Alternatively, condition score of different regions of the body, such as neck, withers, loin, tail head and ribs can be assessed in isolation and a mean condition score calculated accordingly. The latter method may be more robust where horses are not of a standard shape such as broodmares and horses in race training. However, in obese animals that require caloric restriction, changes in body condition may not always be apparent with initial weight loss as shown previously in ponies (Dugdale et al 2010, Henneke et al 1983). Body condition score should not be relied upon in isolation to trigger further caloric restriction and is therefore ideally used in conjunction with actual bodyweight. (Further recommendations can be found in Chapter 28.)