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| Photosynthesis |
| CO² + Water » Light energy » O² + Sugar |
| Sugar = sucrose, glucose, fructose, xylose and sugar alcohols. |
| This sugar is used by the plant for respiration, growth and seed production. |
| The rate of photosynthesis is dependant on levels of CO², water, light and to some extent temperature. The rate of use of the sugar is far more dependant on temperature. |
| If the sugar production is greater than its use, the plant binds the simple sugars together and can store them as starch or fructans. |
These sugars are the non-structural carbohydrates (NSC) as opposed to cellulose and hemicellulose which are structural carbohydrates (fibre).
In the plants that are fed to horses, starch is the main storage sugar in grain and "warm season" grasses (Bermuda grass and tropical grasses) and is also stored in alfalfa and clover. Fructans are the main storage NSC in "cool season" grasses, the type of grasses that make up our pastures in the UK.
Different cool season grasses have different abilities to store fructans, the most effective at storing fructans is perennial ryegrass. Ryegrass is an excellent grass for milk production and weight gain in cattle and sheep and it is one of the reasons so many of our pastures contain it.
| The Connection with Laminitis. |
| Unfortunately high levels of fructans in grass can produce laminitis in horses. |
| Simple sugars and some starch is digested in the small intestines of horses but fructan cannot be digested. Excess sugars and starch and all fructans pass into the caecum and large intestine where they require different gut organisms than the normal flora that deal with fibre. |
| The bacteria that multiply rapidly to deal with the "sugar overload" produce acid and cause a drop in pH. If the population explosion causes the pH to drop below 6, then the fibre fermenting bacteria start to be killed, producing endotoxins and, perhaps more significantly, the lining of the gut is damaged allowing passage of toxins and other factors into the blood stream. |
| Strep. Bovis is one of the bacteria that proliferates and they can produce a toxin that can activate enzymes (MMPs, See Pathogenesis) in the laminae of horses and trigger laminitis. |
It is possible that the Strep Bovis toxin is not the trigger factor that sets off laminitis and that it may be the result of the changes that it produces in the gut wall, and substances released from there, that results in the laminitis developing.
The resulting laminitis, and the severity of it, does seem to correlate to the size of the sugar overdose i.e. is dose dependant. Knowing the levels of the NSCs in the complete diet of a horse should be helpful in the control of laminitis, and particularly in the obese animals with Metabolic Syndrome.
| The factors affecting fructan content of grass: |
| The type of grass – Ryegrass > Fescue > Cocksfoot > Timothy |
| The temperature – Higher NSC when sunny and cold, particularly if for a number of consecutive days. |
| Water availability – respiration is limited before photosynthesis. |
| Light intensity – Cloudy weather & lower NSC. |
| The time of day – sugars are used up overnight if conditions favourable for respiration and growth. (NSC lowest from 3am to 10am) |
| The day length. |
| The stage of growth – Whole plant NSC highest at flower stage. |
| The plant part – flower parts > lower part of stem > upper stem > leaves. |
Optimum growing conditions will give lower NSC. "Stressed" grass will give high NSC.
| The factors affecting fructan content in hay: |
| The type of grass |
| Low nutrient status – stressed grass |
| Water shortage – stressed grass |
| Intense sunlight during growth hay curing – photosynthesis can still occur |
| Frost |
| Flower stage |
| Cut in the afternoon – When fructan level at highest |
| Fast drying – stops respiratory loss so retains more fructan |
Fructans are water soluble so can be "washed" out of dead grass and hay.
NSC content of grass, hay and even weeds, under certain conditions, can be too high for laminitic horses.
| Suggestions: |
| Graze early in the morning |
| Manage pasture and hay crops for optimum but not excessive growth |
| Manage grazing to prevent heading (production of flowers) |
| Have an area of restricted grazing for times when the fructan levels are likely to be very high. |
| The NSC content of hay is dependant on many factors – have it tested. |
| Soaking of hay should reduce the NSC content of grass. |
These notes are a very brief resumé of the presentations given by Kathryn A. Watts of Rocky Mountain Research & Consulting Inc "Forage and Pasture Management" and "Dietary Strategies to Prevent Laminitis" at The Third International Equine Conference on Laminitis and Diseases of the Foot in Florida Nov 2005.
Her work can also be found in Journal of Equine Science Vol. 25 No. 8 August 2005 pp338-344 A Review of Unlikely Sources of Excess Carbohydrate in Equine Diets. Or at www.safergrass.org
Some other useful sources of information on the subject.
| I recommend obtaining a copy of "Equitalk Edition 1" from Dodson & Horrell Dodson & Horrell Ltd, Ringstead, Kettering, Northants NN14 4BX 01832 737300 Helpline 0870 4423322 (in UK) |
Equitalk 1 gives easy to follow details particularly on fructans and their link to laminitis and also advice on grazing.
Fructans induced laminitis is in my practice the most common trigger of the condition.
(Dodson & Horrell can test hay samples for NSC)
Dodson & Horrell, since 1998, have organised a number of conferences - "International Conference on Feeding Horses". There have been a number of presentations increasing our knowledge on the role of fructans in laminitis.
Sugars in grass – an overview of sucrose and fructans accumulation in temperate grasses. A.Longland & A.Cairns (1998) Proceedings of the Dodson & Horrell International Research Conference on Laminitis 1998 pp1-3.
Fructans and their implications in the development of laminitis in horses at grass. A.Longland (2000) Proceedings of the Dodson & Horrell 3rd International Conference on Feeding Horses. March 2000 pp3-5.
Aetiology of fructans-induced laminitis; mechanism of fructans involvement, alteration of hindgut microflora and quantities required. Dr. Chris Pollitt (2002) Proceedings of the Dodson & Horrell 4th International Conference on Feeding Horses. March 2002 pp3-6
Comments from Kathryn Watts in response to this piece. www.safergrass.org
Under – [The factors affecting fructan content of grass] it appears that you are suggesting that timothy will have less fructan than cocksfoot and this is less than fescue. These may all be very high under certain conditions and those conditions may vary with species.
I have found timothy with 31% NSC so I don’t want people to think that timothy is safe. I’ve seen some high levels in orchard grass after drought stress. Orchard grass got its name because it grows well in the shade under trees. Put it in full sun and it does even better. They all have high potential to accumulate sugar and fructan although ryegrass is definitely the worst.
When you say, "Water availability – respiration is limited before photosynthesis" I would rather say "Drought stress causes increase of sugar and fructan".