By Matt Morris, Agriculture Extension Educator
University of Maryland Extension, Frederick County
mjmorris@umd.edu
While I am usually very cautious to complain about the rain, enough is enough! There isn’t one farm in this State that isn’t weeks behind because of our recent weather. Most all the hay is nearing straw quality and there’s still a good amount of corn that is yet to be planted. Not to mention the sheer number of soybean acres that haven’t gone in the ground. One thing this weather has done a good job of is ruining the best laid herbicide plans for corn and soybeans. I’ll outline some scenarios and what post-emergence options you might have.
Let’s say you planted corn May 15th and were planning to spray after you planted, but before the plants emerged. Then it started raining. Now your corn is up and at the V3 or V4 stage (3 or 4 visible leaf collars). Weeds are emerged and racing the corn. The obvious go-to would traditionally have been glyphosate (RoundUp) for tolerant corn varieties. However, in the day and age of herbicide resistance this probably won’t be enough. For tough to control broadleaf weeds I’d suggest throwing a product like Status or Diflexx Duo in with your glyphosate to pick up weeds like marestail, lambsquarter, or pigweeds. These are “safened” dicamba products. The caution is that these products cannot be applied to corn before the V2 stage. Also remember, most herbicides have a maximum height for application ranging anywhere from V2-V8.
You’ll also want to think about throwing a residual product in the tank since we missed that before planting. This is where atrazine or S-metolachlor (Dual) can work well. Atrazine can help control a lot of our broadleaf weeds and works really well on morning glory if you have pressure in your fields. S-metolachlor won’t kill emerged weeds, but will keep any small seeded weeds from germinating for several weeks. A word of caution: atrazine cannot be applied to corn greater than 12” in height. S-metolachlor has a maximum application height of 40”.
Soybeans present a much more challenging problem due to herbicide resistant broadleaf weeds and their widespread prevalence in the area. Here’s our first scenario: You sprayed a combination of burndown and residual herbicides on May 15th with hopes to plant that weekend. Then it started raining. You still can’t get into the field to plant. Whatever you do, resist the temptation to plant without another residual application. If you can’t, the weeds will come up long before your crop can canopy. You may need to go back in with a product like S-metolachlor, metribuzin, or Valor to hold weeds back until the crop canopies. If you need another burndown because weeds are emerged, adding in a product like paraquat with your residual can help. The key is starting clean. A word of caution: most herbicides have a maximum application rate per season so read the label to make sure you don’t exceed that amount.
Here’s the second scenario: you planted soybeans on May15th and planned to come in and spray the next day. Then it started raining. Now your soybeans are up and no burndown or residual herbicides were applied. The first thing to do is remember that most all of our marestail are glyphosate and ALS (Firstrate, Synchrony, Raptor, etc.) resistant so I will not consider them here. That being said there are areas they still may work, but I caution against them being used alone. Post-emergence options are limited and work best on 4” or smaller weeds. The PPO herbicides like Reflex or Blazer can be applied post-emergence for small, annual broadleaf weeds, but can cause minor crop injury. Since there was no opportunity to apply a residual herbicide it may be a good idea to include S-metolachlor or Zidua to try and hold back any weeds yet to germinate. Zidua must be applied by the 3rd trifoliate growth stage.
If you planted dicamba tolerant soybeans or Liberty-Link soybeans they can provide excellent post-emergent weed control. Again, I caution using dicamba or Liberty on weeds greater than 4” or in the tank alone as that is the path to herbicide resistance.
There are a million different scenarios you can be faced with in the field and we’re dealing with Mother Nature. That means we can still get rains delaying us even further. If you have questions about your options don’t hesitate to give your local Extension office a call and one of us can work up something specific for your scenario.
Matt Morris
301-600-3578
mjmorris@umd.edu
