Two years ago, a pretty yellow plant infested one of our alfalfa fields. We called it mustard, but it wasn't mustard. The sheep refused to eat any part of it. We dug up that field last fall and planted a cover crop of turnips and radishes, and then oats this spring.
Now the pretty yellow plant is blooming in the alfalfa field behind the barn. It's also in the north end of our pasture. I walked through the pasture yesterday with Caroline and her boyfriend, John. His dad farms in North Dakota. John immediately identified the invasive species as leafy spurge, a much-hated plant in the Plains.
This morning I researched leafy spurge and I don't like anything I see. Here are some links with good info:
This noxious weed from Europe and Asia is destroying pastures and it very hard to control. Guess we better start attacking.
UPDATE: This is not leafy spurge, but wild mustard (Bob was right). See more here: http://www.livingthecountrylife.com/blogs/betsys-backyard/2014/05/14/its-wild-mustard/
Leafy spurge (yellow plants) in our pasture