I did find this information for these two plants.
White clover comes with some special problems, especially if it has black mould in it (whether the clover is fresh or fed as hay). This mould contains a toxin that causes a condition known as ‘slobbers’, where the horse exudes liquid from its eyes and mouth, as excess urine and as diarrhoea. They also typically suffer from bloating colic
Buttercups may seem an odd plant to be considered toxic, and most paddocks carry a few. However, many horse paddocks become infested with them if not managed properly, leaving horses surrounded by a sea of yellow – with little choice but to eat them. Buttercups contain a chemical called ranunculin, which burns the inside of the mouth and the digestive tract as it passed through the gut. This can lead to colic as well as gastric and mouth inflammation. Interestingly, dried buttercup does not appear to have this toxicity – so can safely be consumed in hay or silage.
How to get rid of Buttercups
Fortunately, Ally/Cimarron is a very safe and effective herbicide for controlling buttercups. Ally is a liquid, soil-applied herbicide that moves into the plant through the root system. Ally can be applied in spring or early summer at the rate of .1 to .3 oz per acre. There is no grazing restriction for Ally and horses can be returned to the pastures immediately. It is also possible to tank-mix Ally with liquid fertilizers in spring. Ally should only be applied to grasses that have been established for at least 6 months. For timothy, at least 12 months is desirable and tall fescue should be established for at least 24 months. Ally is very persistent in the soil, therefore crop rotation guidelines must be adhered to. Ally should not be used if you plan to overseed the pastures, since the herbicide will remove new grass seedlings as well as the weeds.
Remember that Ally and 2,4-D/Banvel are broadleaf herbicides that will also eliminate existing clover plants in pastures.