What weed is this and what controls it?
From the photo I can tell it is a low growing perennial forb with odd pinnate leaves and reproductive stems that bend outward and up. Also, given that it is green and growing now I know it is a cool season plant that will be flowering early this spring. The most likely plant: Lambert's Locoweed.
This plant can be toxic to cattle if eaten in moderate or greater than moderate amounts. If there are only a few plants here and there across a pasture, then it won't be a major concern, especially if there are a lot of other green grasses to choose from. However, if this is one of the few green plants in the pasture, they will seek this plant out and it will become a higher in proportion within their daily diet and could lead to possible toxicity issues. If the flowers of this plant are purple, the toxicity is fairly low, but if the flowers arrive and are white, it is significantly more toxic and be more important to avoid or control.
Here is a great link for this species for description and control - http://www.ars.usda.gov/is/np/PoisonousPlants/locoweedtoxicity.pdf
I hope this helps. If you have any other questions, feel free to email me directly.