Volunteer corn has become a common weed in the Upper Midwest in recent years, and due to last year’s drought, there’s an even greater chance of seeing unwanted corn in fields this year, says Lizabeth Stahl, an Extension crops educator with the University of Minnesota. Not only does volunteer corn cut into yield, but it also impacts long-term management of corn rootworm (CRW).
Some rootworms can survive on Bt-CRW hybrids and resulting volunteers. Exposure of western corn rootworm to sub-lethal doses of Bt toxin in soybean and corn fields can potentially hasten development of this pest’s resistance to Bt-CRW traits.
Thus, volunteer corn in corn reduces the effectiveness of rotating corn genetics. Further, volunteer corn in soybeans minimizes the benefits of crop rotation. Beyond corn CRW, other insect and disease problems might be increased because of volunteer corn, too.
“In areas most affected by the drought of 2012, stalk quality issues may have led to increased stalk breakage and dropped ears. Grain moisture was also very low in areas, reaching 13 percent or less… by the end of the harvest season. Harvesting grain at a low grain moisture content can lead to increased mechanical harvest losses due to increased kernel shattering and ear droppage. These factors can lead to higher-than-normal volunteer corn populations the following year,” she says.
Stahl has teamed up with University of Minnesota Integrated Pest Management (IPM) Specialist Brue Potter and Extension weed scientist Jeff Gunsolus to take a look at the issue of volunteer corn.
They cite Nebraska research revealing that 3,500 to 7,000 volunteer corn plants per acre can cut into soybean yields 10 to 27 percent respectively. Volunteer “clumps” of corn are even more competitive than individual plants. In soybeans, 3,500 clumps of corn per acre reduced yield 40 percent.
In corn, a volunteer population of 7,000 clumps per acre reduced yield 14 percent (while the same population of individual plants trimmed yield 5 percent), according to Nebraska work. Minnesota trials showed that volunteer corn populations had to hit at least 8,000 plants per acre before yield was reduced an average of 8 percent. Although that population didn’t consistently result in yield loss, that level of volunteer corn, notes Stahl, could be used as a guide to help determine at what point control of volunteer corn, in corn, is warranted to protect yield.
Even though volunteer corn in a cornfield could potentially help offset yield losses by contributing to yield, Minnesota researchers found that volunteer corn typically lags 1 to 6 leaf stages behind the planted hybrid, and has small ears (if any) with poor kernel set. (At high populations, volunteer plants are typically barren.)
As noted, volunteer corn can also affect CRW management. Roughly 75 percent of volunteer corn plants from a CRW-resistant hybrid can be expected to carry the Bt-CRW trait, according to this University of Minnesota trio, who say “this poses a challenge in the management of CRW and in the management of resistance to Bt-CRW traits.”
Performance issues in Bt-CRW corn were first reported a couple years ago, but according to these Minnesota crop experts, “the scope and intensity of complaints increased markedly in 2012.” To date, the performance issues in Wisconsin’s neighbor state have been confined to western corn rootworm (WCRW). They haven’t documented significant Bt-CRW control issues with the more common northern corn rootworm (NCRW).
The WCRW behavioral variant (i.e. rotation-resistant WCRW) that lays eggs in soybeans to hatch out in corn the following year has been documented in Wisconsin. (It renders crop rotation ineffective as a control measure.) It’s a concern in southeast Wisconsin.
While small CRW larvae might survive on roots of seedling corn and some grassy weeds, they require nodal corn roots to complete their development, according to Stahl and the others. This is why rotating to a non-host crop like soybeans is the most effective CRW management option in fields with Bt-CRW performance issues, they say.
It’s only successful, however, if volunteer corn isn’t a problem the following year. “In soybean, volunteer corn nullifies the benefit of crop rotation in CRW management,” say these Minnesota researchers. “Larvae are able to feed and mature, and late-pollinating volunteer corn is very attractive as a feeding and egg-laying site for CRW beetles. As a result, the presence of volunteer corn in soybean creates the potential for CRW problems in the field the following year.”
Current Bt-CRW “events” are low-dose; some larvae develop to adults. Consequently, some CRW larvae should be expected to feed and survive on volunteer corn plants the following year. Not all plants will express the Bt toxin, but those that do have also been show to express it at variable and reduced levels.
Purdue University researchers demonstrated that WCRW larvae are able to feed on roots of volunteer corn that tested positive for the Cry 3Bb1-Bt toxin at levels similar to plants testing negative for the Bt toxin. WCRW beetles also emerged from volunteer corn that expressed the Bt toxin.
The concern is that exposure of CRW larvae in soybeans and corn to sub-lethal doses of Bt toxin also has the potential to hasten development of resistance to Bt-CRW traits.
Early control of volunteer corn in soybeans is critical, as the nodal root system – needed for larvae to mature – begins to develop at V6. Control of volunteer corn at V4 to V5 will starve out CRW larvae. Further, early control also prevents volunteer corn from becoming a feeding and egg-laying site for female CRW beetles later in the season. Volunteer corn that survive to silk attracts CRW females to feed and lay their eggs, negating the beneficial effect of rotating out of corn in order to manage this pest. According to this University of Minnesota team, more than one herbicide application may be needed to control later-emerging volunteer corn.
Volunteer corn control is complicated by the fact that most hybrids have at least one herbicide-resistant trait, if not more. For instance, glyphosate won’t control volunteer corn the following year if a Roundup Ready (RR) hybrid was planted the previous year. The same goes for glufosinate and Liberty Link. Many hybrids are now stacked with both traits (RR and LL), so neither chemical can be used to control volunteer corn. Of course, volunteer RR corn could be controlled with glufosinate and vice versa.
There are a number of good post-emerge options to use on both stacked and conventional corn that volunteers in soybeans. According to Purdue University, these include: Select Max, Clethodim (Arrow/Section), Assure II/Targa and Fusilade DX. Activity will be improved with a crop oil concentrate (COC) or methylated seed oil (MSO) plus ammonium sulfate. However, control can be more variable when these herbicides are mixed with a glyphosate product containing surfactants in the absence of COC or MSO. Control can sometimes also be reduced when those herbicides are applied with a post-emerge broadleaf herbicide.
Control options in corn are limited, however, if, as noted, the volunteer corn contains RR, LL or RR/LL traits. Cultivation may be the only option left. Consider cultivating when populations exceed 8,000 plants per acre (to protect yield potential).
Consider planting soybeans in fields where volunteer corn populations are projected to be high. Then control any volunteer corn in those bean fields when the corn is V4 to V5 to both protect yield and better manage for CRW.
Planting continuous corn or rotating soybeans and corn with the same herbicide traits allows volunteer corn plants to persist in fields, unless cultivation is incorporated into management. That’s a practice not many farmers are very keen on anymore.