There are many reasons for the success of the weedy foxtails. These include biodiversity, competitive ability, tolerance to stress, and herbicide resistance.
A broad biodiversity within and among foxtail species give it the advantage of having phenotypes and genotypes ready to exploit any opportunity provided by any weed management system. To see more of the global biodiversity of this exciting species-group:
Foxtails are excellent competitors with annual crops. Below foxtails are interfering with corn. Left is some green foxtail below a mature corn crop, possibly late-emerging plants capable of restocking the seed bank at the end of the season. Notice the corn in the picture on the right. Its leaves have curled up indicating moisture stress from the foxtails (and the common lambsquarters) choking it.
The foxtails are resistant to many kinds of stress. Below, on the left are some green foxtail plants growing in a compost pile, not the most favorable soil environment. Below, in the picture to the right, notice the foxtail plant (as well as the sunflower plant, left side) growing out of the solid concrete (in its bag); not a favorable soil either.
Many herbicides effectively control most foxtail infestations. Few herbicides control
all the foxtails in a field or on a farm. Biodiversity among variants allow some the
escape these herbicides, in addition to other causes like application errrors. Below are
some examples of foxtails that escaped, and also the sad story of some that didn't:
These small foxtail seedlings are twisted and curved due to treatment by 2,4-D. This herbicide rarely kills foxtails, even these little ones.
This foxtail plant's leaves are stuck together, a good symptom of alachlor and other acetanilide herbicides.
This foxtail plant was treated with diclofop, an ACCase inhibiting herbicide. Notice the light chlorosis at the base of the most recently emerged leaf, new growth.
These foxtail plants didn't survive treatment by another ACCase inhibiting herbicide.
This green foxtail plant shows injury from still another ACCase inhibiting herbicide, sethoxydim. Notice the necrotic upper leaves, the panicle of the main axis as well as a small secondary panicle. Green foxtail has the ability to tiller profusely, allowing it to produce even more seed. Possibly the sethoxydim inhibited nodal meristems, stimulating other branching nodes to tiller and flower.
These little foxtail seedling got hit with acifluorfen. They probably will die, but notice the green tissue on the lower part of the plant on the left, it may yet live on to haunt this grower later with its seed.