Native Horse Pastures
It is the presence of high levels of non-structural carbohydrates (NSC) in the introduced grasses that leads to equine laminitis. These NSC compounds are simple sugars such as fructose and glucose, which are readily absorbed by the gut microflora in ruminants such as cattle and sheep and are rapidly converted into extra body weight, or milk, or fleece. However, their effect on horses is not as beneficial. An elevated content of NSC in horse pastures, and in particular fructose, has been found to be one of the main causes of laminitis / founder in horses and ponies. Anyone viewing the pages of rural magazines aimed at the grazing industries of cattle and sheep will see advertisements featuring the high sugar content of the newest (latest and greatest) varieties of ryegrass, cocksfoot and so forth. These sugars are very readily digestible by ruminants and promote more rapid growth in cattle and sheep than the older, lower sugar varieties. The irony here of course is that the newer (latest and greatest) varieties are more likely to cause issues for horses. Because the horse has a different digestive system to ruminants it does not digest the NSC’s in the same manner and so the feed provided to these different classes of animal affects them differently. It is precisely this high sugar content that is causing so many issues for horses that graze on those grasses and which is causing the laminitis / founder. This is causing many horse owners not only a major expense, but a lot of heartache and grief too. To understand more about the role that pasture grasses play we need to cover some basic biochemistry. Firstly, we need to discuss the basic division of grasses into warm and cool season types, and then secondly we need to discuss how native grasses differ from the introduced grasses. As the main product of photosynthesis, all of the grasses, C3 and C4, introduced and native, produce cellulose. This is the structural compound that makes up the hard bits of leaves and stems. The differences occur when we discuss what happens when the grasses are thriving and producing in excess of the cellulose requirement. In these cases excess carbohydrate is produced and takes one of two alternative paths. In most C4 grasses excess carbohydrate is stored as starch. BUT in all the introduced C3 (cool season) grasses this excess carbohydrate is stored in the cell vacuoles as simple sugars such as fructose and glucose (the NSC’s), while in the native C3 (cool season) types it is stored as starch which has a different digestive pathway and is not harmful to horses. It is the high fructose and its impact on the horses’ digestive process and products that causes equine laminitis/founder.
In our native Australian grasses, both C3 and C4, the excess carbohydrates are stored as starch. So the native grasses cannot cause equine laminitis even when they are growing in favoured conditions.