Has the behaviour of your horse changed this spring? If we consider how grass grows, then a greater understanding can be gained of our horses’ behaviour.

Many customers from around the country (more than 20 now this year) have contacted me asking for advice as their horses are displaying odd contrary behaviour, which is significantly different to their norm. This has meant that they are just not fun to be with anymore, and in many cases I encounter expensive veterinary intervention has taken place, but without any improvement.

horseThe lad on the right is a case in point. The face says it all! But actually, it is not his fault. It is because the grass he was eating was full of the wrong (for a horse, not the grass) nutrients which have an adverse effect on his whole physiology. This is in turn having a significant detrimental effect on his behaviour and is creating a horse that is incapable of being ridden safely, and certainly no fun to be with.

Horses as sentient beings have evolved to access nutrients from locations within their environment irrespective of wherever the plant based nutrients may be located. Many plants have evolved to interact with their environment to specifically access a nutrient, which is then available for horses to use when they eat it.

Of necessity we have to keep horses in a restricted environment, and hence they often need a mineral supplement. This requirement is exacerbated by a dry spring and the corresponding restricted grass growth which changes the plant nutrient content.

If we consider how grass grows, then a greater understanding can be gained of our horses’ behaviour. It is especially important to gain understanding of grass growth and development in adverse weather conditions, as that will have a deleterious effect on nutrient availability for the grass that the horses eat.

If first we consider how a plant interacts with its environment to access the nutrients it needs from the soil. Essentially the root produces a slime layer (exudate) that contains essential nutrition for the Arbuscular Mycorrhizal Fungi (AMF) in the soil to feed on.

In return the AMF are able to generate a network in the soil that enables  the nutrition that is held in the root zone (rhizosphere) to be liberated and made accessible for the plant roots to absorb by osmosis.


In preparation for the forthcoming growth, plants absorb extra potassium in the sporing so that nutrition can be utilised throughout the plant as they grow larger.

In a dry spring, (as witnessed in the extremely dry April in 2020 & 2021) the plant has absorbed the extra potassium, but now has nothing to do with it as growth has been stunted by the drought. Therefore, the grass contains excessive levels of potassium that then induces Grass Tetany symptoms in horses.

Grass Tetany occurs when the ungulates diet is too high in potassium, which essentially means that the extra cellular fluid is permanently high in potassium. This upsets the delicate sodium / potassium ratio and as a consequence the nerves and muscles cannot relax, creating restless hyperactive animals that lack body condition, display a lack of ability to increase weight, and are often impossible to interact with as they are so distressed.

Imagine stinging nettle rash happening in all your nervous system!

Horses are particularly prone to Grass Tetany due to their unusual digestive system (see diagram below) which relies on the autonomous  peristaltic action of the hind gut to move fodder through and then allow the animal to gradually access the nutrition. In contrast the digestive system of ruminants, that have efficient stomachs, enable them to break down cellulose more efficiently and rapidly.

Grass Tetany rapidly disrupts this crucial peristaltic action, adding to the animal’s distress.

This interruption to the peristaltic action is often accompanied by poorly formed ‘chestnuts’ as the fodder isn’t broken down properly and the horse is consequently extremely short of manganese.

Inside the horse

The most effective way to reduce the extremely negative effect of induced Grass Tetany is to balance the horse’s nutritional intake with magnesium.

Magnesium in the plant regulates the uptake of phosphorous; structural component of ribosomes; major constituent of chlorophyll production; and plays a crucial part in transforming sugar and starch to utilisable energy.

The over-winter hydrated magnesium ions are larger than their natural ionic size as they are hygroscopic, and that they become relatively easy to leach.

But in a dry spring plant availability of magnesium will be severely restricted or even reduced completely, as the magnesium returns to its normal size, but now there is no available water to transport it to, or up, the plant,

Hence the excessive levels of potassium, in relation to the negligible levels of magnesium in tissue tests. My conjecture is always confirmed by taking a tissue test.

tissue test soil

The other EXTREMLEY important point to note is the high molybdenum level in the grass sample above.

Molybdenum is required by grass to naturally access and process nitrogen from the soil. In the spring the demand will obviously be high in preparation for growth, but the drought meant that the plant had absorbed it in the wet weather but now that it was ‘drought stressed’ there was nowhere to utilise the molybdenum.

What does that mean for the horse (or any stock grazed on the grass)?

The excessive levels prevent animals, and especially horses with their badly designed digestive system, from absorbing copper and manganese.

Copper is essential for efficient renal function, and manganese is required for extracting energy from forage.

Andrea Thomason called me a month ago because she suspected her horses had COPD (breathing disorder) and so did the Vet, and they were “restless, listless, and all constantly flicking their heads. Standing weaving at the field gate, and off their heads” And she was afraid that they would all have to be put down.

During a long explanatory conversation, I suggested that we organise soil and tissue tests (see above chart), but meanwhile get some Epsom Salts (magnesium sulphate in a rapidly available form) in a bucket of water and see if the horses drink it. Straight away they all drank a bucket each – “faster than I have ever seen them drink before!”. They innately knew what they needed.

Andrea said: “Last night I was extremely distressed and hadn’t slept for days. But after talking to you, and thanks for answering at the weekend, I have a better understanding of what is happening, and the support to put the problems right.

Extra manganese was also added to their ration as none of the horses were forming ‘chestnuts’ I suspected that the grass would have a high molybdenum level, which was proved to be correct.

Within a few days the horses were calmer and more assured. The soil tests confirmed the extremely low magnesium levels, and those low levels were exacerbated in the leaf due to the drought. Once all the information from the soil and tissue analysis were collated, I suggested that Andrea spread Kieserite (magnesium sulphate) fertiliser on the paddocks. Within two days “the horses were full of the joys of spring. And thank you so much Jonathan for the support and advice

How can you help your horses to be happy?

  1. Be aware of your horses change in demeanour, other than just having an ‘off day’.
  2. Understand the impact that weather can have on grass and forage nutrition.
  4. If you don’t know what is happening in the soil and grass you are unlikely to have Happy Horses.
  5. Speak to Lordington Park Agronomy to find out how we can help.