Most plants increase their growth in spring — photosynthesis ramps up when sunshine increases. Plants are sugar factories - photosynthesis produces carbohydrates (CHO) which promotes growth. Horses become sleek and the need to feed is reduced – at last feeding costs go down! Great for the budget and great for horses – except those at risk of insulin resistance and prone to laminitis! And although we recognise many, many breeds of pony – there are really only 2 groups: those that have foundered and those that are going to founder.
Because the CHO or ‘carbs’ (starch, sugar and fructans) concentration of forages are highly variable, and because they have a role in laminitis/founder, equine polysaccharide storage myopathy (EPSM), and equine metabolic syndrome (ie insulin resistance or peripheral Cushing’s) they must be considered when deciding which horse should graze, when and for how long.
The amount of CHO in the leaves and stems depends on plant growth rate – fast growth burns a lot of carbs so levels are lower during rapid growth. When growth is slowed by water and nutrient deficiencies, salty soil and cool temperature (especially <5°C) but sunny days accelerate photosynthesis, carbs skyrocket by as much as 5 times. Whenever photosynthesis exceeds growth, carbs accumulate. A frost or a hot, dry wind may cause rapid changes in carb levels within a very short time. Climate and plant species are so important in determining carb levels that it is difficult to generalize. So while the general rule-of-thumb for at risk horses is to ‘..avoid grazing lush grass…’ it does not guarantee low carb levels.
At night, photosynthesis stops (due to lack of sunlight) and the plants burn stored sugar to stay alive. What this means to horses and us as owners, is that carb levels are lowest from 3am to 10am, peak in the afternoon (doubling every 3 hours on warm, sunny days) and decline as darkness descends – making early morning the safest time for at-risk horses to graze. Grasses in shady, low-sunlight areas have less carbs (ryegrass in shady areas has less than half the carbs of ryegrass in sunny places), stress such as cool temperature or frosts (not uncommon in spring) increase carb levels, as does overgrazing (the bottom few centimetres are a reservoir for carbs), and don’t allow at-risk horses access to seed heads – which are very high in carbs. Weeds like chicory, dandelion and thistles can also be dangerously high in carbs.
Other strategies include keeping grass short (6-8cm) and leafy; adequate moisture and fertiliser to encourage growth and utilisation of carb. There is a belief that pasture grown without fertiliser is more “natural” and healthier. In fact, grass grown under nitrogen or phosphorus deficiency are higher in NSC than that grown with optimum amounts of fertiliser. Preventing horses from grazing on sunny, cool spring days (when growth slows and carbs accumulate), and avoiding stubble (high in fructans), and grazing muzzles also have a role. Muzzles reduce bite mass and only allow access to the tops of leaves where carb concentrations are lowest.
If grazing is restricted, hay must be supplied. But like the plants from which it is made, hay can have wildly varying carb levels, so ask for NSC (the sum of starch, sugar and fructan) levels when you buy hay – and only get hay that is <12%, preferably 10% NSC. Carb levels vary between batches and published values for oat hay range over a 2-fold difference in carbs (up to 29%) and so some hay and even straw will be too high in carbs for susceptible horses. Oat hay is often thought to be ‘less rich’ than lucerne hay, but although protein levels are higher, the carb levels are usually ½ that of oat hay – the best Lucerne hay for horses is stemmy with lots of purple flowers. Bran can be up to 31% carb – nearly 3x the amount in oat hay – and not advisable for at risk, carb-intolerant equines.
Horses whether on and off pasture likely require mineral and vitamin supplementation – but grain-based (and grain by-product mill-run or hominy meal) feeds are not recommended due to their carb content – stick with fibre-oil rich feeds, or pasture lick blocks that have been balanced for the vitamins and minerals deficient in all pasture and hay. Magnesium deficiency often occurs on spring pasture, which is low in magnesium – so consider supplementing magnesium. Colic also increases in horses on spring pasture due to gas production and low fibre intake – feeding some beet-pulp or hay will increase fibre and the weight of the gut – helping prevent sections from floating off in to the wrong location – predisposing to displacements, torsions and twists.
Weed growth increases along with pasture growth – check for toxic weeds. There is lots of useful information on weeds on departments of agriculture/primary industry websites and agronomists – so you can easily check for any risky plants in your area, so you’ll have the spring pasture risks licked.
EQUINE CLINICAL NUTRITION
Dr Jennifer Stewart