What’s the link between frosted grass and laminitis?

Horses in winter

Winter is well and truly here and scraping the frost off my windscreen the other morning reminded me of a common question from my clients at this time of the year; that is whether or not frosty grass is more likely to cause laminitis. There are many misconceptions about laminitis, the commonest being that lush pasture is the primary cause. The reality is that this is incorrect and I have seen many cases occur in horses and ponies grazing barer paddocks, both in summer and winter. In fact, studies have shown there’s an approximately four times higher risk of laminitis in both the summer (June–August) and winter (December–February) months compared to the lusher growth months of spring (March–May).

During warm and sunny weather grass makes and stores sugars by the process of photosynthesis and at night, during another physiological process called respiration, the grass is able to convert these sugars into stem growth. But by looking at summer and winter from the grass’ point of view, we can appreciate that being over grazed and frozen are both stressful situations and the grass plants’ response is to store its sugar rather than converting it into new growth.

 

Whilst the exact underlying cause remains unclear, previous explanations for a seasonal variation in laminitis risk have focused on changes in grass growth, grass starch and sugar levels, and fructan accumulation corresponding to certain climatic conditions such as morning sunlight following overnight frost with air temperatures below 5 degrees Celsius. These growth conditions can significantly increase the nonstructural carbohydrates (NSC) (simple sugars, starch, and fructan) production in grasses. A broad generalisation is that sugar levels are at their lowest between midnight and dawn. It’s the increased level of fructans that are responsible for triggering a laminitic episode in a susceptible equine.

 

Once owners of laminitis prone animals are aware of this knowledge they can use it to their advantage by either preventing altogether or allowing limited grazing only during and after sunny, frosty weather. Turnout should only be allowed in the early morning and prohibited at times of high light intensity and low temperatures. It is imperative that if the ambient temperature is below 5 degrees Celsius then avoid turnout altogether even if it is not actually freezing. Once milder nights and overcast days return, the grass will be able to respire and use up its stored sugar, thus lowering the grass’ overall fructan content.

 

Below are my other recommendations to owners to prevent or alleviate laminitis when the weather is frosty:

  • Careful timing of grazing can also be combined with physical prevention of excessive intakes through the use of grazing muzzles. The use of such devices not only reduces the overall grass intake but also restricts the intake to the tops of leaves where the concentrations of NSC tends to be lowest.
  • Ensure hooves are well trimmed and balanced and protected with laminitic shoes or pads or alternatively use boots, deep bedding or turnout in a sand pen. During frosty weather horses who have previously suffered a pedal bone rotation may show increased signs of foot soreness because their solar surface is more sensitive to hard, frozen ground. This will only be exacerbated by overgrown heels and long toes because even the slightest forward tipping of the pedal bone against the solar surface or strain on the laminae can worsen the pain and discomfort of walking on frozen and often uneven ground.
  • For overweight horses and certain types like Native Breeds and cobs, leaving them unclipped and not rugging them up in the cold weather can be used to owners’ advantage by encouraging weight loss as they use calories to keep warm.
  • The commonly used preventative measure of hay soaking to remove water soluble sugars is not as effective at colder temperatures, therefore hay should be soaked in warm water if possible.
  • Since horses will tend to get less exercise or turnout and therefore utilise less calories the amount of concentrate feed given per day should be reduced or even omitted altogether, depending on the nutrient content of the forage being fed. Ideally, the major source of fibre in the diet should have sugar and starch levels below 10%. For this reason, it is highly recommended to have a complete nutritional value analysis undertaken by a specialist company before the hay or haylage is fed.

 

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