Horseweed is a noxious weed that proliferates and can use up natural resources in the soil that your lawn needs. Glyphosate-resistant horseweed is especially bad because it resists the most common and typically effective chemical weed control. Individual flower heads can produce horseweed seed in quantities of 200,000 that are easily dispersed, making this problematic weed challenging to eradicate, especially where it thrives in certain western states.