Are you confused about when the sugar content of grass is highest?   It makes me crazy when people say, within the same article, to avoid fast growing grass, yet the next paragraph explains why stressed, slow growing grass is higher in sugar.   These seem like conflicting statements.   This deserves a deeper dive into the interaction between how you manage your grass and how you manage grazing your sugar sensitive horses. 

Plant scientists know that rapidly growing grass, on a whole plant basis, is lower in sugar concentration, when environmental conditions are most conducive to growth.  For the reasons why, review these articles on basic sugar metabolism in grass: and the factors affecting the NSC content of grass.

So why does it seem like laminitis increases when grass is growing rapidly?

Sugar per bite vs. sugar per acre

A lot of horse pastures I have seen around the world are overgrazed. A lot of people consider paddocks to be more like a playground than a significant contribution to their horse’s nutrition.  If this is the case, you get used to looking across the paddock and seeing grass that is 2 inches tall.  You get complacent because it looks like there is not much to eat.   When grass is growing slowly, the intake rate by the horses is limited by the slow growth rate of the grass.  Even though the sugar content is increased per bite of grass, you might get by because the horses spend a lot of time moving around eating teeny tiny mouthfuls of really short grass.  Generally speaking, when sugar content per mouthful decreases, sugar per acre increases.  Well managed grass is lower in sugar concentration, but there is more of it.   So the way you manage grazing really matters when deciding which is better or worse. 

Managing intake rate

If you manage your horse’s intake rate all year round, by strip grazing, limiting time at pasture, or using a grazing muzzle, you don’t have to worry so much about your horse overeating.   Fast growing grass becomes a good thing.  When cold nights or drought make grass higher in sugar per bite due to slower growth rate, you can just make narrower strips for grazing, lessen or eliminate turnout time, or put an insert into your grazing muzzle to make the hole smaller. 

Don’t depend on grass growth rate to limit intake

  When the limitations to growth are removed by rain, warm temperatures, or fertilizer then grass will grow faster, and horses have more to eat within the same space and time.  When grass growth rates increase, they may finally have an opportunity to overeat.  When you look across the paddock it may still look like there’s not too much to eat, but the horses may be eating twice as much.  You can’t see more grass because it has now become a layer of fat all over your horse.   Horses prone to laminitis often relapse when allowed to eat too much grass.  The paddock that was OK when grass growth rate was low has now become high risk.

Stem elongation phase

Now we need to talk about the concentration of sugar in different parts of a grass plant.  When growing conditions are good for rapid growth (no cold nights, plenty of sunshine, nutrients, and moisture) the first new leaves are lower in sugar concentration.  There is not enough photosynthetic tissue available to keep up with the demand of the new growth.   Once there is enough leaf surface area, around 3 leaves, the overall total plant content of sugar starts to increase.  However, after the 3 leaf stage, the plant starts to concentrate sugar for the eventual production of a seed head.   This embryonic seed head first forms at the bottom of the stem, with leaves forming all around it like a telescope in its short form.   At around 6-7 leaf stage this developing seed head rapidly emerges in what is called ‘stem elongation phase’.   Once this sugar rich embryonic seed head rises to the top of the plant, horses now have the opportunity to selectively graze only the portions of the plant that are highest in sugar.    I’ve always wondered if this rapid stem elongation phase is what nutritionists are cautioning us about when they say ‘avoid rapidly growing, lush grass’.  Pastures can be managed to keep grass in a vegetative stage, which means it is kept in the leafy stage with no heads.  I think this is better way to keep sugar concentrations lower over the whole paddock and within each plant.   When grass heads start to emerge, paddocks should be topped, slashed, or mowed as high as your equipment can accommodate. If you don’t have appropriate equipment, low risk animals should graze that section first.

How would I correct these common contradictions?  Just say ‘avoid too much grass’. (Don’t get me started on the ‘lush’ word.  That’s a subject for another blog post.)  Pay more attention when the amount of grass, or the sugar concentration of grass is higher.  If there is both more grass, and some near freezing nights, get vigilant because this double whammy can really cause problems in horses prone to laminitis.

Generalization and over-simplification creates confusion and misunderstanding.  Hopefully, this more detailed explanation can empower you to make better management decisions for your horses prone to obesity or laminitis.   While you have no control over the environmental conditions that affect the sugar content of your grass, you CAN take control of the amount of grass your horse eats.