The buttercup species (Ranunculus species) includes several annual and perennial plants. They are common in overgrazed horse pastures. You are right to be concerned as buttercups are poisonous. They contain protoanemonin which is a irritant of the GI tract and also causes oral irritation. It also can cause dermatitis is people so avoid picking without gloves. The toxic component is in the fresh leaves and flowers. If you make hay then the toxicity will not carry over as it is lost when dried.

The general consensus is that horses will avoid buttercups and other weeds. But if the field contains little grass then they will consume the weeds.

OK, so how to control?

The best control for all weeds in forage is to have a thick stand of grass. Buttercups establish in bare patches. They also grow in areas of excessive damage following heavy rain, which has been common this spring. Also, horses will use sections of the field as latrines and this is where buttercups grow in abundance. See page 7 from this great Teagasc report (Irish Ag Board http://www.teagasc.ie/publications/20...)

It is difficult for them to establish in taller vegetation. Buttercups propagate by seeds and this occurs late in summer so ensuring healthy pastures before then will help prevent the weed from spreading. Sowing grass seeds now is good. If you also grow beneficial plants that serve as forage this can help.
Here is a great (GB) resource on planting grass http://www.pavohorses.co.uk/advice/pa...+

Note that you want to reduce seeds entering your pasture and this can happen with animals and vehicles.

Since you dont wish to use chemicals* So, you best solution is manually removing the weeds. This is hard but as you keep at it you will reduce the amount of weeds. Especially if you focus on this now, before the buttercups set seed.
Here is a video from the RHS on weed removal. https://www.youtube.com/watch?v=Fw0Hd...


Another useful approach that can occur alongside planting new grass is mowing. By cutting the weeds with a mower you prevent them flowering and cause them to expend energy growing new vegetative parts (which you mow again). Set the mower just above the height of the grass.
“Mow at a height above the grass seedlings when weeds are 8 to 10 inches in height to reduce shading by weeds.” A single mowing will not satisfactorily control most weeds. However, mowing three or four times per year over several years can greatly reduce and occasionally eliminate certain weeds. Also, mow along fences and borders to help prevent the introduction of new weed seeds. Regular mowing helps prevent weeds from establishing, spreading, and competing with desirable grasses and legumes.”
http://extension.psu.edu/pests/weeds/...

Is your pasture at high productivity? Overgrazed pastures are much more likely to show heavy infestation in the spring. - See more at: http://extension.umd.edu/learn/contro...

Could you give the pasture a break? Or section of parts to give the grass a chance.


Finally, people do use vinegar and/or clove oild as a spray to kill weeds. These work well with some science to back them up http://extension.psu.edu/pests/weeds/...

http://mysare.sare.org/MySare/Project...

With a pump spray it would be an addition to manual control.

Good luck

* I understand why you would not like to use commercial products (I hesitate to say chemicals as that is what vinegar is). Or course stuff I have read in preparing this has stated that the commercial chemicals for control of broadleaves in pastures are fine. But I haven’t been able to find good studies on this. Would be happy to hear from people on this (i.e. for horses)