Be careful how long you will let weeds grow with your soybeans, even if you put down a residual herbicide and they are weed escapes. There is accepted, quantified evidence which may surprise you that indicates how much soybean yield can be hurt in the early stages from competition from weeds.
Try out these scenarios. Which would be worse? Would you rather put up with one pigweed per 10 feet of row in soybeans, or five giant foxtail plants? If you said the foxtail, you're right, and it's not even close. Weed scientists estimate that five giant foxtail plants per 10 feet of row will cost about 3% in yield potential. However, one pigweed plant can cost up to 30% of yield potential.
If there's a catch, it's that you will seldom find just five foxtail plants per foot of row if weeds have escaped or not yet been sprayed. Often, they're almost too numerous to count. Likewise, you may have more than one pigweed plant per foot of row.
Try this one. Would you rather have 2 pigweeds or 3 velvetleaf plants per 10 feet of row? Pigweeds are a bushier broadleaf, while velvetleaf usually put a couple of large leaves on a tall stem. According to weed data, published in the Purdue University Corn & Soybean Field Guide, you're going to be better off with the 3 velvetleaf plants, although both scenarios could cause significant yield loss.
Two pigweeds per 10 feet of row can drop soybean yield potential by 50%. This assumes the pigweeds are there early from the start, and allowed to grow. Meanwhile, 3 velvetleaf plants could clip yield potential to about 60% of normal.
You would think jimsonweeds, with their big, bushy leaves, would be more competitive than pigweeds. But in studies, jimsonweeds were tolerated better than the other broadleaves in the study, primarily pigweed and velvetleaf. Two jimsonweed plants per 10 feet of row could knock off about one-fifth of the soybean yield potential.
The bottom line is that while you may still have soybeans to plant and maybe even corn to plant, or anhydrous ammonia to apply, be sure you scout early-planted soybean fields for weeds. Based on these numbers, time spent scouting and then treating the fields with herbicides if necessary can be profitable time.