Even if seeding is two or three weeks away, a pre-seed burnoff now could keep these weeds from getting too big to control. We have reports of gigantic winter annuals in fields where they haven’t been sprayed.
Weeds established ahead of the crop also take up nutrients the crop could use. While this may not be much more than 10 lb./ac. of nitrogen (which isn’t lost, but just tied up until weed biomass breaks down again), it does represent some nitrogen that is no longer available to this year’s crop.
Weed seedlings can also provide a green bridge for disease. Pathogens on weeds and saprophytes on decaying sprayed weeds can move over to crop plants and infect them. High levels of seedling diseases have been observed in canola crops where volunteer canola (untreated obviously) were sprayed out shortly before crop establishment and in fields where a related weed — stinkweed in this case — was sprayed out and was decaying. In these cases, the canola crop could have been diseased due to the green bridge for disease that weeds and volunteer canola provided. Another factor could be something called allelopathy. Chemicals from decomposing plants can inhibit germination of other plant seedlings. When this occurs within the same species, the term is “autoallelopathy”. Read more.
Are these factors enough to change weed management practices? No. Spray weeds early. Seed as soon as you can. And take your chances with these other factors. However, when scouting fields with unexpected delayed in emergence, these factors could be considered.
On the other hand if soils are saturated, growing weeds can act as pumps to dry the soil quicker than if the burnoff is force earlier than the field is traversable. A good rule of thumb is hold off on the burnoff until you can access the field with equipment. Aerial application is a last resort.