By Darren Hefty
Roundup weed resistance is on many farms across the U.S. If you remember back from years ago, WHY did you switch to Roundup Ready Soybeans in the first place? Probably because it was a lot easier to spray one product and get 100 percent control vs. spraying four products and getting 80 percent control, right? Well, if Roundup isn’t going to work on some of the weeds on your farm anymore, what can you do?
Post-emerge in soybeans, it is very difficult to kill broadleaves if they are bigger than 2 to 4 inches tall. In crops like corn and wheat, we have awesome post-emerge broadleaf herbicides, but unfortunately we don’t in soybeans, at least until dicamba and 2,4-D-tolerant soybeans come out in the next few years.
Due to the lack of good post alternatives, we strongly encourage you to use a good combination of pre-emerge herbicides to stop your problem broadleaves (and grasses) in soybeans. Here are the 3 families you will likely pick from:
PPOs
The PPO inhibitors are quite effective on most broadleaf weeds in soybeans, especially if you pick the longer residual products like Authority or Valor. Sharpen is also an option here, but you can’t safely use enough Sharpen in soybeans to leave sufficient residual. Authority and Valor each have good activity on waterhemp, lambsquarters, ragweed, and a host of other broadleaves. If you use name brand Roundup, you’ll get a rebate on either product, so that puts the Valor cost down in the $6 to $9 range, depending on if you use the 2 oz. or 3 oz. rate. You can’t buy straight Authority, but Authority MTZ (Authority plus metribuzin) after the rebate will run somewhere around 10 to 14 bucks an acre, depending on your rate.
Triazines
You remember the old Sencor and Lexone, right? Generic metribuzin is the same thing, only a lot less expensive. The hang up a lot of people used to have with metribuzin was in high pH (over 7.4) or sandy soils. However, you can just cut the rate back to one-sixth or one-quarter pound to reduce any crop injury potential, yet you’ll still get good weed activity in those situations. In normal soils, use one-third of a pound for around $3 an acre. The full two-thirds of a pound rate can be used in heavy, low pH soils.
Yellows (or DNAs)
Sonalan is the best, but only Prowl can be used in no-till/strip-till. Treflan is the cheapest of the yellows at about $4 an acre. You may have known these products over the years for grass control, and for the most part they are still great at that. However, they also have good activity on small-seeded broadleaves including waterhemp, kochia, and Russian thistle.
When you look at the worst broadleaf weeds to control in soybeans today, pigweed (including waterhemp and Palmer), ragweed (common and giant), lambsquarters, wild buckwheat, morningglory, and kochia top the list. The Three-Pre approach listed above is your best option for all. You probably won’t get 100% control on any of those weeds, but if you use the right rates for your soil and rotation, you should get most of them. That makes your post-emerge job a lot easier and your odds for great yield dramatically higher.
Now, I do get some questions as to why we don’t commonly recommend Pursuit, Reflex, FirstRate, and Classic pre-emerge. These products would be found in OpTill (Pursuit), Prefix (Reflex), Authority First & Sonic (FirstRate), and Enlite, Envive, Valor XLT, and Authority XL (Classic). Pursuit, Flexstar (which contains Reflex), FirstRate, and Classic can all be used post-emerge, and I prefer to save them for post if at all possible. If I use them pre, I probably don’t want to use them post for fear of carryover. On the other hand, the pre’s I listed above can’t be used post-emerge, so why not use them pre? That way we save ALL our post-emerge options for later, just in case.
I know that using three pre’s may seem like a lot, but it’s a strategy more and more farmers are employing, and depending on the rates and products you pick you may not invest any more than the price of 1 or 1.5 bushels of soybeans. Isn’t that worth it for great weed control?