Renewable Farming

Soybeans: Is season-long weed control possible without glyphosate, glufosinate, dicamba?

That’s a logical question because season-long weed control is getting more difficult even with GMO traits and their linked herbicides. Already this season we’re seeing resistant weed “escapes” popping through soybean canopies in soybean fields which were always 100% weed-free a decade ago, before the phrase “resistant weeds” emerged.

July 24, 2017 — Weed scientists are among some of agriculture’s most sought-after advisors — and we make no pretense of knowing any significant slice of today’s complex cocktails of chemical weedkillers.  Yet, July and August hit us with the most urgent calls from farmers: “What can I do to cope with late weeds in my soybeans?” 

Interestingly, we’re not hearing any more weed escape cases from non-GMO soybean growers than we are the growers who’ve relied totally on GMO traits and related chemistries for 20 years.

But now, with the dicamba damage news rippling up from the Southern states, there’s an increased sense of urgency. Will every grower be forced into buying dicamba-resistant beans just for insurance against dicamba drift? 

Meanwhile, we can report on some ways our non-GMO soybean growers are managing weeds quite acceptably. We’ve never sprayed glyphosate on our place, and have had little difficulty with serious weed invasions in beans. Our non-GMO farmers who don’t grow cover crops typically use a premerge on beans, with the best timing they can arrange so rain showers enhance its effectiveness. Then, weed escapes are hit early with long-proven contact herbicides — Flexstar, Cobra. Even Poast and Treflan; remember those? 

The only postmerge herbicide we have detailed experience with is Cobra, so we never feel competent to offer a “prescription” on the appropriate weedkiller chemistry. Most non-GMO growers are far more seasoned with the various modes of herbicide action than we’ve ever attempted to be. 

Our main experience with our own product, WakeUP, is that if a foliar-applied herbicide needs to be systemic, WakeUP Summer in the spray tank will help coat the leaf more uniformly, lift the cuticle barrier temporarily and carry the herbicide deep into circulation more thoroughly. By this means, as an adjuvant, it can make an effective contact weedkiller more effective. It can’t fix a herbicide blend that’s ineffective.  We usually recommend to our first-time users that they make their usual spray mix without WakeUP Summer, then in the next identical spray tankful, induct enough WakeUP Summer to deliver 5 ounces per acre.

We like to see 15 to 20 gallons of spray solution applied per acre on later beans, which will make weed leaves almost glossy. No dewdrops. Just sheets of spray mix spreading across even the most fuzzy weeds, like velvetleaf.

Unless the herbicide label requires a crop oil, we encourage growers to leave out the MSO when they use WakeUP Summer. It’s designed to form colloidal micelles in water — tiny water “ball bearings” that repel each other but attach to hydrocarbons and the positive ions in herbicides. These colloids do the heavy lifting on the waxy cuticles of weed leaves, and the presence of MSO ties up some of the colloidal micelles that WakeUP builds.

In our own experience dealing with weed escapes in non-GMO soybeans, we’ve noted that using WakeUP without MSO appears to result in less burn on soybeans, but improved weed kill of larger weeds than when MSO is used as an adjuvant. The “sticker” type adjuvants may hold herbicides on the leaf longer, but it makes more sense to carry the chemical into weed circulation. Gumming herbicides onto the leaf surface may be one of the reasons that Cobra exhibits leaf burn on beans. A cell membrane disruptor (PPO inhibitor) will show phytoxicity on the bean leaf surface. We’ve always heard that “beans grow out of it” but still, there must be a bit of a pause in photosynthesis if parts of the chlorophyll are scorched in some leaves.

We’re often asked if AMS is necessary. It’s a pH adjuster, and if the herbicide label calls for it, the label should be followed.

When farmers ask us for the “best herbicide,” we try to refer them to someone who does know. For your reference, you can download a detailed herbicide mode-of-action chart assembled by the University of Wisconsin’s Nutrient and Pest Management program. It’s a three-page PDF at this link.

Weed-free soybeans growing out of
“terminated” cereal rye. Photo in Iowa, mid-July.

The exciting prospect for non-GMO weed control is to build soil biological activity and nutrient content so effectively that weed pressure gradually eases away over the seasons. Also, rapidly rising management knowhow with cover crops can be a big help in suppressing early weeds. We’ve seen that work among organic soybean growers. Crimping cereal rye flat, then planting beans into it, appears promising. This substitutes more management care in place of more chemicals. We hear objections that more field operations like crimping and cover crops are obstacles when farming thousands of acres with a plant-spray-harvest formula. Right. Corn-soybean growers of about 1,500 acres and less appear to be moving most aggressively into the realm of biological farming and cover crops.

We’re actively pursuing and testing that elusive cover-crop burndown formula that’s ecologically friendly. We can flatten young cereal rye and it stays down. Older, taller cereal rye hit with our experimental juices tends to grow back from the roots. Hang in there, we’re not giving up.