Jeff Gunsolus advised crop producers to consider using soil-applied herbicides even during times of cheap glyphosate.
While there can be risks, such as no timely rainfall, a pre-emergence herbicide can get control of early-season weeds so glyphosate can work on pesky plants at more equal stages of growth, says the University of minnesota weed specialist. Beginning weed control early can also reduce the potential for yield loss and provide chemical diversification to avoid glyphosate resistance.
Gunsolus says it was more difficult last year to control common waterhemp, giant ragweed, common ragweed and common lambsquarters. Some growers are using an increasing amount of glyphosate due to less effective weed control.
"Don't follow glyphosate failure with more glyphosate," Gunsolus says.
Biotypes of common waterhemp, common ragweed and giant ragweed are confirmed to be resistant to glyphosate at four to eight times the labeled use rate. Common lambsquarters isn't confirmed as resistant, but it's hard to control with many post-emergence glyphosate applications, Gunsolus says.
Trials at the university's research and outreach centers found economic returns on pre-emergence herbicides are showing up in some corn systems. A pre plus glyphosate system can be just as good for the checkbook as a two-pass glyphosate system. Glufosinate-based systems showed good returns, but some were more costly than glyphosate, Gunsolus says.
He says weeds compete significantly with corn for nitrogen. A pre-emergence or early post-emergence herbicide that removes weeds by the time they're 4 inches tall will be more profitable than using post-emergence herbicide alone. If a producer is worried about being timely with a post-only system, a pre-emergence herbicide is good risk management tool to protect inputs and yields, Gunsolus says.
In one study, weeds were removed off a corn field at 3 to 4 inches in height, while another field kept its weeds for an additional 13 days. The first field produced 204 bushels per acre. The field with taller weeds only made 170 bushels per acre and produced stunted corn.
Other research compared how much nitrogen was used by 4- or 12-inch tall weeds. The growth period between them was 8 days, which means the weeds grew an inch each day. In the extra days, nitrogen uptake by weeds at least doubled. During 2 years, corn yields with 12-inch weeds dropped an average of 12 to 18 bushels per acre compared to fields with weeds removed at 4 inches.
Producers can also fight early competitive weeds with a healthy crop and good row spacing.
Soybeans can make their own nitrogen and handle weeds longer than corn if they have good moisture, Gunsolus says. Pre-emergence herbicides with soybeans can give better weed control coverage than post-emergence/glyphosate tankmixes. Similar to corn, a pre-emergence system can get control of early season weeds.
With Roundup Ready soybeans, the pre-emergence plus glyphosate treatment showed the highest return on investment.
"The pre-plus-glyphosate treatment is looking good," Gunsolus says.