Ok so this is the million dollar question that needs to be answered with a million questions.
Which herbicide has been used?
How old is your horse, is it a pregnant mare?
How much grass did your horse eat?
When was the exposure?
There are many herbicides on the market, but each herbicide will have instructions for grazing withholding.
My first point is, that these studies are mostly done on farm animals, and we group horses into this catergory. There are not many studies specifically performed for horses. This potentially means it affects them differently.
In addition, the active drug can be researched but once the drug is compounded into a product, it can have different effects on the horses system.
The next important point to make, is that most acute toxicities are found from incorrect use of the product or expose directly to the product either from spillage or access to a storage shed. Less commonly from grazing the pasture during the withholding time.
Acute signs of toxicity commonly involve gastrointestinal disturbance such as colic or diarrhoea, however the clinical signs are very similar to other diseases. This means the history of exposure is very important.
Chronic exposure can also cause disease. This means, over a period of time the horse is exposed to consistently low levels. There is research currently being performed into some of the potential long term effects. One of the diseases highlighted is equine metabolic syndrome. I fine this particularly interesting as many of our patients with this disease commonly live on permanently sprayed paddocks. I have no answer here, other than to say, currently we don’t have a lot of information. Important point: Access to grass for an EMS patient is much more life threatening due to laminitis risk than the concern of chronic exposure.
I would also like to make the point that chronic exposure for horses is still being investigated, but a whole list of diseases related to poor performance have been hypothesised.
Conclusion;
I would recommend looking after paddocks to ensure weeds don’t take over. This would include avoiding drought and flooding where possible, avoid over-grazing the grass, and fertilise where possible which natural products.
If you do need to graze paddocks that have been sprayed, then stick to the manufactures recommended withholding, and provide suitable drinking water rather than run-off water from heavily farmed blocks.
If your horse has had access to a recently sprayed paddock, then remove them straight away, provide them with a product to promote good gastrointestinal microbial balance, and monitor for any signs that are concerning.
I hope one day to be able to provide you with more scientific research. For now please continue to ponder my thoughts