It is commonly thought that the big problem with spring grass is magnesium deficiency. Magnesium is the central molecule of chlorophyll, which makes grass green, so you would think that the greener the grass, the calmer the horse. We have all found out that isn’t true!
So what is happening with Spring grass?
Ground temperatures have warmed up to the point where as soon as moisture is added (it rains), the grass can uptake nutrients needed for growth, from the soil.
Spring grass becomes ‘lush’ and is comparatively high in water. This exacerbates the lack of salt because it means the horse is taking in more water but no more salt unless you supply it. This can lead to metabolic issues in itself.
There is then a lot of leaf area for making sugars via the process of photosynthesis so the grass becomes high in NSC’s.
Two of the major nutrients needed by plants are potassium and nitrogen. These two link together to form potassium nitrate and any livestock consuming such grass has to be able to either utilize or excrete these nutrients.
The nitrogen content is measured as the crude protein (CP). High levels in the grass does not mean it is good protein for your horse. Cattle can metabolize this excess nitrogen because their rumen contains the enzymes needed to incorporate the nitrogen into proper protein. (A protein molecule is a string of amino acids with a nitrogen molecule at either end).
Horses, having neither the rumen nor the enzyme, cannot perform this function. The excess nitrogen is converted to ammonia which is detoxified in the liver to urea (urea is less toxic than ammonia) and excreted in the urine. This is why your horse’s urine can become ‘strong’ in spring, even killing the grass as evidenced by brown patches.