30 Nov 2015
Andy Durham
Job Title
Figure 2. Proprietary chopped grass made freely available alongside hay to Digby, a horse with few functional teeth left at the age of 32.
Nutritional advice is frequently sought by owners of horses and ponies when discussing various medical complaints and, indeed, proper evidence-based nutritional strategies are important in the management of many medical disorders, such as colic, diarrhoea, respiratory disease, weight loss, liver disease, obesity, laminitis, myopathies and developmental orthopaedic diseases.
Advice obtained from nutritionists employed by feed manufacturers may not always be impartial and it is helpful if the veterinary surgeon is in a position to offer advice and is able to comment knowledgeably on advice offered from other quarters.
Elderly horses may present certain nutritional problems primarily relating to weight loss, malabsorption, poor dentition and insulin resistance (for example, pituitary pars intermedia dysfunction; PPID).
Various theoretical nutritional strategies are proposed to deal with these problems effectively, but they are all underpinned by similar basic principles.
Although it is not unusual for elderly horses to be overweight, poor condition and problems maintaining reasonable condition are probably more common. Thus, perhaps the most important dietary problem to solve in the context of clinical nutrition of the elderly horse is how to achieve greater caloric intake without creating additional medical problems.
Faced with trying to increase the caloric intake of horses, the traditional approach of “reach for the cereals” is likely to create further morbidities, especially in older horses.
Cereal starch may well be calorie dense (for example, the digestible energy [DE] of oats is approximately 13MJ/kg to 15MJ/kg, high-energy mixes and cubes are typically 10MJ/kg to 14MJ/kg), but the high starch content (oats typically 40% to 50%, high-energy mixes/cubes typically 20% to 30%) does create potential problems, with proven links with colic, gastric ulceration, diarrhoea and laminitis, for example.
Starch is simply not a normal dietary constituent for horses. The equine gastrointestinal (GI) tract evolved anatomically and physiologically in the absence of significant starch intake and it is clear that, as a species, horses are very poor at digesting starch compared with most other monogastric species.
Thus, ideally we are trying to provide an increased caloric intake in the form of low sugar and starch feeds, which are well tolerated by the GI tract and do not simulate excessive hyperinsulinaemia, thereby placing them at risk of laminitis.
In the UK we are spoiled for choice when it comes to equine feeds (Figure 1). This is both a blessing and a curse. On the one hand it creates a confusing market for owners and plenty of scope for inappropriate choices based on poor knowledge and advice. On the other hand it creates the opportunity for well-informed owners to source great feeds that can be beautifully customised to their horses’ particular needs.
This article outlines some of the basic nutritional targets we might try to achieve, with examples of feed types appropriate to achieve these targets. The discussion will concentrate on nutritional and dietary principles to enable advice to be offered in such cases.
A simplified approach to dietary formulation requires consideration of the points in Table 1 and is discussed here.
As a general rule, voluntary feed intake of horses will approximate 2% to 3% of their bodyweight (BWT) as dry matter (DM) each day. If highly palatable feeds are offered (for example, grazing) and/or if the individual is a greedy eater then this may be in excess of 5%.
If the diet is 100% preserved forage then it is unlikely to be much greater than 2% to 3%.
Dietary energy requirements in horses are generally expressed as apparent DE (DE; equals ration energy minus faecal energy).
This should be expressed in kJ or MJ, although US publications will use kcal and Mcal (conversion of Jcal × 0.239; calJ × 4.186).
Daily maintenance requirements of non-pregnant adult horses living in a field without additional exercise are considered to typically lie between 127kJ/kg to 152kJ/kg BWT. Clearly, if weight loss is a concern then the upper end of this range is likely to be the target.
The average daily maintenance DE requirements for horses (in MJ/day) can be roughly estimated by dividing BWT (kg) by 7 (for example, 50kg BWT /7 = 71 MJ/day), although an additional 10% might be added to this approximation if weight gain is needed (for example, 78MJ/day for 500kg BWT). Light exercise should be encouraged where possible to maintain musculoskeletal strength and general well-being. However, even light exercise may increase DE requirements by at least 20% (a further 15MJ/day for 500kg BWT).
Daily crude protein (CP) requirements for field maintenance typically lie between 1g/kg to 1.5g/kg BWT. This typically corresponds to a CP concentration of around 6% to 8% in the overall ration. There appears little harm in increasing protein concentration modestly and overall CP of 8% to 10% seems appropriate in geriatric horses, especially where forage is the primary protein source and might lack certain essential amino acids.
High-protein feed balancers are a very useful component of geriatric diets as a means of ensuring good protein intake by adding a small amount of concentrated protein (and minerals/vitamins) to the ration. Remember to examine the nutritional declaration as some so-called balancers appear to have quite low CP content (for example, less than 10%) and are probably inappropriate for this purpose.
The GI tract of horses is designed to ferment forage and grass and this is generally the best diet for maintenance of GI health. However, forage nutritional quality is highly variable and frequently unknown to the owner, which is a fundamental concern when this is generally the major nutrient source provided.
Forage analysis is readily available and inexpensive via several feed suppliers and is a useful adjunct to dietary planning. Hays might typically contain between 6MJ/kg and 1 MJ/kg DE and 4% to 12% CP as DM. Furthermore, it is not unusual to find water soluble carbohydrate (WSC) content (WSC = sugars and fructans) can be remarkably high (for example, more than 20%) and can be a significant insulin stimulus.
For maintenance requirements most forages will almost invariably represent adequate, and frequently excessive, DE intake in healthy individuals, but older horses in poor condition, and perhaps with dental and absorptive problems, may struggle to keep weight on with even free access to good-quality forage, especially if protein is low.
Clearly, masticatory ability is crucial to forage consumption and digestibility. Where necessary, consideration should be given to using commercially available chopped and processed forage, such as alfalfa, grass cubes or chaffs (Figure 2), which may be generally above average nutritional quality, and also the chopped nature suits horses better with dental problems. These diets can become a partial or total hay substitute where appropriate.
If access to good quality hay or haylage is available then purchase of a garden shredder can enable manufacture of home-made chaff that can be fed to horses with masticatory problems.
When forage alone does not supply adequate calorific intake then further high-fibre feeds should be offered in preference to cereal-based mixes.
Non-molassed sugar beet pulp is one of the highest energy fibre feeds and provides an excellent source of low starch and sugar (less than 5%) calories in horses that have problems maintaining weight or those that cannot chew well. Digestible energy content similar to competition cubes is typical of beet pulp (10MJ/kg to 13MJ/kg) and there is reasonable protein content also (around 8% to 10% CP).
Further high-energy, low sugar/starch, high-fibre feeds include CoolStance, a coconut-based feed with less than 11% sugar and starch, but very high energy (15MJ/kg DE) and protein (20% CP), and also chopped alfalfa products (often around 10MJ/kg to 12 MJ/kg DE and 12% to 16% CP).
Some “complete” pelleted feeds are also available for horses, but it is important to establish sugar/starch content is low (ideally less than 10%) and fibre content is high (preferably more than 25% crude fibre).
Where possible, at least some chopped hay or chaff should be fed alongside these “complete” diets.
Will additional dietary oil meet energy requirements?
Vegetable oils typically provide approximately 40kJ/ml (4MJ per 100ml). They can be fed relatively safely to horses, although they may decrease fibre digestibility in the colon at high levels.
Typically, a gradual increase from 0.1ml/kg BWT up to 1ml/kg BWT over about two weeks will be acceptable to most horses should they require the extra energy.
Palatability can be an issue, although for increased calories any edible oil is fine – such as maize, sunflower, granola, vegetable or coconut. Linseed and soya oil tend to be expensive. Margarine can be given if preferred by the horse.
When fed at 1ml/kg BWT daily, vegetable oil will provide around a quarter of normal dietary energy requirements and tends to produce a very attractive shine on the horse’s coat.
A small amount of cereal-based mix might be useful as an energy and protein supplement and is easily consumed when dentition is poor and requirements are not being met by fibre plus oil diets.
Such mixes and cubes are typically between 10% and 30% starch and 9MJ to 13MJ DE per kg, but should never be offered at a rate of more than 1g/kg starch per meal (about 1kg to 2kg feed per 500kg).
In insulin-resistant subjects (such as PPID cases) these cereal-based starchy feeds should generally be avoided as they are particularly hyperinsulinaemic and may increase the risk of laminitis.
Commercially available “ration balancers” are widely available for provision under such circumstances. Typically, a small quantity of pellets is given that contains a concentrated mix of proteins (for example, 15% to 25%) and minerals and vitamins. Starch may appear high in these balancers (for example, 20% to 25%), but it is generally not much of an issue given the small amount fed. Particular consideration might be given to antioxidant vitamins C and E, especially where high oil levels are being fed. Addition of 1IU to 2IU vitamin E per kilogram BWT is reasonable.
With the nutritional resources widely available to horse owners in the UK, much can be done to maintain condition in elderly horses without creating additional diet-related health risks. The main combined aims of offering high-energy, adequate protein, high-fibre, low-sugar and starch feeds are readily achievable given a multitude of possible feed constituents. Clearly, close monitoring of key age and nutrition-related health concerns, including dentition and endocrinopathies, should also parallel involvement in dietary planning.
