How long was the weed out of the ground before you snapped the photo? The weed looks dessicated, like it lost a lot of water for being out of the ground too long. When weeds are dessicated, their leaves shrivel up, and the weed or grass becomes very hard to identify. Some weeds I've pulled looked dessicated after just 15 minutes, IIRC, especially if placed in a warm room. Too keep the weed or grass fresher looking, I've wrapped a small, wet paper towel around the roots. Sometimes they can look good for days if wrapped like that, and they might still be identifiable, but it's best to take the photo quickly if pulling the weed out of the ground.

Thanks - your link said Bidens pilosa is also a host and vector to harmful parasites such as root knot nematodes (Meloidogyne species) and tomato spotted wilt virus (GISD 2010) I will definitely discourage it! Thanks

It looks like a Trifolium species, i.e. a clover, but without seeing the whole plant it's difficult to give a definite id. To get rid of it use any method for broad leaved weeds. Personally, I remove weeds by hand - pulling or digging.

Im guessing you are in the British Isles? If so you have Anthriscus sylvestris, aka Cow Parsley. This website http://your-natural-horse.com/2010/05/cow-parsley-a-great-free-herb/ says it's fine for horses. But , since they illustrate their article with a picture of a different plant, I wouldn't rely on the information without getting a second opinion.