I have wrestled with Stringhalt over the years and there is no a simple answer as to whether the condition is caused by a plant or some sort of deficiency. I tend to believe that it is actually due to a fungal infection that is found on a variety of plants and may in turn be due to something we put on our pastures.
While Magnesium is important to healthy nerves and many of the herbs I use to treat the condition nowadays are naturally high in Magnesium, I am not at all sure that an inorganic magnesium supplement helps at all.
There are also some suggestions that the condition can be a result of super-phosphate poisoning in susceptible horses.
The stringhalt program consists of the Herbs: Comfrey, Dandelion, Mugwort, Pine Bark, Sage, Saw Palmetto, Valerian (all 1:1 extracts except for Dandelion which is in 1:10 Tincture form.)
Homeopathic: Magnesium Phosphate tissue salt and from the Bach Flower Range; Rescue Remedy, Heather, Vine, Elm and Vervain.
This course is given over a 12 week period and a single dose each day of yoghurt (acidophilus), about a couple of dessertspoons, is given as well.
This whole program is a little complex and crazy sounding but over about 8 years of development I have now got a pretty reliable treatment working. I have probably struggled over stringhalt more than any other condition in either man or beast.