How Sweet is Your horse?

By Dr Simone Hoskin,  PhD (Animal Nutrition)

In equine diets, ‘sugar’ is found in the contents of plant cells.  Technical terms for sugars include water soluble carbohydrate (WSC), non-structural carbohydrate (NSC), fructans in pasture and starch, primarily found in cereal grains.  Most sugars and starches are rapidly digested in and absorbed from the small intestine, increasing blood glucose and triggering increased insulin secretion.  However, if the sugar or starch content in the diet exceeds digestive capacity in the small intestine, overflow into the hindgut upsets the microflora balance, which then increases the acidity of the hindgut. Hindgut acidosis is a trigger for development of disorders like laminitis, colic and tying up.

Structural carbohydrates or fibre comprise the walls of plant cells and can only be slowly digested and utilised through fermentation action of the microbial population in the equine hindgut.  This produces volatile fatty acids, which are absorbed and used as slow-release energy by the horse.  Fructans are an exception, because although classified as NSC, they can only be digested by hindgut fermentation.  But because fermentation of fructans is relatively rapid, the acidifying effect can be similar to excess undigested starch reaching the hindgut.  Fibre is critical in the diet of the horse, primarily to maintain healthy gut and metabolic function, with the recommended daily minimum fibre consumption recently increased from 1 to 1.5% of body weight(1).

Evolution of neural mechanisms for seeking sugar and other foods high in calories in the wild is a survival strategy.  But in recent horse management, the preference of horses for high-sugar ‘modern’ horse foods including improved pastures and cereal-based concentrates, usually combined with inadequate fibre intake, has contributed to or caused a range of digestive, metabolic and endocrine disorders largely associated with high blood insulin and/or gut acidity including laminitis, tying up, obesity, gastric ulcers, PSSM and Cushings disease.  Many horse owners are becoming increasingly paranoid about sugar in their horse’s diet, avoiding all feeds containing molasses, for example.  But an important principle of nutrition is not to consider dietary constituents in isolation, and many problems in equine nutrition can be solved by increasing the ratio of structural to non-structural carbohydrates in the diet – more fibre, less sugar.  Horses do require sugar in their diet for normal physiological functions, and it is impossible to formulate a horse’s diet with no sugar.

Table 1:  Sugar (as WSC + starch) content of some common equine feeds.

Equine Feed Sugar content (% dry matter)
Extruded cereal-based compound ration 50
Whole oats 45
Pelleted sport horse compound ration 25
Oat hay/oaten chaff 22 (range 18-28)
Fresh ryegrass-based pasture 18 (range 9-30+)
Meadow hay 13 (range 7-30)
Lucerne hay 11 (range 9-22)
Low starch laminitic horse compound ration 8
FiberEzy®: equine-specific fermented Lucerne and timothy grass 6 (range 5-8)
FiberProtect®: equine-specific fermented Lucerne with Xanotyde® 3 (range 2-4)

High sugar feeds include those based on cereal grains, also called compound or concentrate rations, that have a high starch content and added molasses.

Intermediate are concentrate rations with reduced cereal grain content and increased protein and structural fibre content.  This fibre is usually from inclusion of legume seeds or hulls and lucerne leaf meal, and fresh or dried forages.

Grasses can often be higher in sugar content than legumes, which have more protein and minerals such as calcium, hence the use of lucerne to balance diets based on soaked grass or meadow hay for equines at elevated risk of laminitis.  However, the difference in sugar content of immature leafy versus mature stalky forage can be 2-3-fold, eclipsing the subtler differences that might exist between legumes and grasses, with most pastures and meadow hays comprising a mixture of grasses and legumes anyway.  Improved ryegrass, especially annual or Italian ryegrass and cereal hays e.g. oaten, tend to be highest in sugar.  But because of the wide variation in forage sugar content, herbage testing is justified to accurately determine sugar content.

Horse feeds low in sugar are classified as less than 10% sugar (NSC).  These include specialised low sugar/starch concentrate rations, very low quality (stalky) hay and best of all in terms of consistently low sugar; fermented forages, especially legumes such as lucerne.  This is because any fermentation process, such as ensiling, consumes sugar.

Controlled Fermentation is the stringent manufacturing process that is used to produce all Fiber Fresh products.  This involves fresh forage harvesting, followed by blending, further processing and fermentation of forage crops in a highly controlled factory environment.  Controlled Fermentation consistently ensures that the quality of the forage crop is managed, that the optimised anaerobic fermentation cycle is consistent and is free from contamination or spoilage.

One of the principle processes driving Fiber Fresh Controlled Fermentation is the buffering capacity of the forage used.  This is dependent on the supply of sugar to produce sufficient fermentation acids to overcome the buffering capacity of the forage, which is comprised of the inorganic bases (potassium and calcium), protein and free amino acids.  A vital characteristic of the forage used in each product is the sugar to protein ratio.  High sugar (or starch) content favours acid production, whereas a high protein content can develop a higher pH through ammonia production, if fermentation is not controlled and the moisture content is too high(2).

Lucerne has a high buffering capacity and a reliably low sugar content, so molasses (<1%) is added to optimise fermentation pH.  The microbial population fermenting the lucerne need this added molasses sugar for their own energy demands, so by the time fermentation is complete at 14 days, this sugar has been largely utilised (see Figure 1).

References:

(1)            Harris, P.A. et al.  Review: Feeding conserved forages to horses: recent advances and recommendations.  2016.  Animal, Vol 11 (6), pp. 958-967.

(2)            Jennings, J.  Principles of Silage Making.  University of Arkansas, Division of Agriculture. FSA3052.