Twentyfive years ago, wild proso millet was not a
major grassy weed problem in most of midwestern North American corn and row-crop fields.
It is now. Why?
Population shifts in weed species in an area are caused by powerful selective forces in our production system, especially in our weed management systems. As new herbicides were introduced in the 1960's and 1970's many weed species that dominated before that time were now tamed, like the foxtails for an example. This highly effective herbicide weed control left bare spots in our fields: good for the grower, but unfortunately an empty apartment house to wild proso millet.
The seed, dormant in the soil seed bank, grabbed the opportunity offered and spread like wildfire in places like southern Wisconsin and Southern Ontario. Below is that nasty seed. The range of seed color is a reflection of biodiversity with the species. There is both a crop proso millet (primarily birdseed in North America), and the weed. This biodiversity could be either due to genetic or somatic polymorphism. In North America the black and darker seed usually is associated with the weedy types, while the lighter brown and cream color seed is from the crop. A caution though, I noticed very black proso millet crop seed being grown for food in northern China (Manchuria), so seed color might be deceiving:
If left to reproduce, this species is a prolific seed producer, as seen in the carpet of seed on the ground under a corn crop in the late part of the growing season:
Escaping many common herbicides, the little seedlings get a start in corn:
Below can be seen the root system of a young plant, if you look carefully you can still see the seed attached to the root sytem below. Because the seed will often stay with the roots, this can be used in field ID, especially when the you are unsure if it is wild proso millet or woolly cupgrass. The seedlings of these two can be similar: broad, almost corn-like, leaves.
The hairy ligule and collar region:
The plant can look like a wimpy corn plant (left).
Stems (hairy) and leaf arrangement. Notice the flowering plant on the right; even though it is riddled with smut disease it still managed to set seed. Nothing can stop a determined wild proso millet plant.
Besides prolific and dormant seed waiting to take over a corn field, wild proso millet dominates because it has an unusually well developed herbicide resistance system. Once I applied 99 lbs. atrazine (10-100 times recommended rates) to wild proso millet in the greenhouse. It grew bigger than the untreated plants, possibly because it used the rapidly degraded herbicide as a nitrogen source. The weeds always win. Below are two plants that seem to be surviving a dose of an ACCase inhibiting herbicide:
Below can be seen some wild proso millet plants injured by alachlor. Notice the rippled leaves, the leaves stuck together in the whorl (not unfolded and spread properly), and the interesting deformities. The picture on the left also shows a whitish soybean cotyledon injured by frost early in the season. This picture reveals an important fact about alachlor, it can cause more injury to plants when growing conditions are unfavorable (cool):
Wild proso millet causes very large crop yield losses if allowed to:
Another (of very many) reason wild proso millet is such a successful weed is its ability to change and adapt to what resources it can find each season: plasticity. If the resources are small and it is late in the season it can set a couple of seeds from a very tiny plant (left). If it has lots of goodies to grow and it starts early it can grow very large, tillering to form more seedheads and seed (right).
Seedheads, and more seedheads: