Intense use of Bt hybrids is also anticipated for the 2012 growing season. "I have questioned the wisdom of applying such intense selection pressure on insect populations when many of the pest species are well below economic levels in most producers' fields," said Gray. Nonetheless, this pattern is not expected to change.
When Bt hybrids entered the market place in 1996 and for many years thereafter, the use of a 20 percent refuge was the standard protocol for the Corn Belt, based upon the use of Bt hybrids aimed primarily at the European corn borer, which express a high dosage level of Cry proteins. In 2003, Bt hybrids were commercialized for corn rootworms, and similar refuge requirements were implemented across the Midwest, even though the Bt hybrids targeted at corn rootworms were not high dose and the mating characteristics, along with dispersal patterns of adult corn rootworms, are different than those of corn borers.
Why were the refuge requirements similar for such distinctly different insects?
"Because of familiarity, convenience, and thus, the greater likelihood of implementation of the 20 percent structured refuge by producers rather than tailoring refuge requirements to the unique biological characteristics of corn rootworms," said Gray.
Today producers have more flexibility with respect to the type of refuge they implement. Although more than half of the producers at the 2012 Classics indicate they intend to use the 20 percent structured refuge this growing season, the seed blend (refuge-in-a-bag) strategy is gaining popularity as more pyramided Bt hybrids enter the marketplace.
At the 2012 Classics, producers were asked if they planted a refuge in 2011. On average, 83 percent of producers said they had established a refuge. The proper establishment of refuges will become increasingly important as more acres are planted to Bt hybrids, selection pressure increases, and the threat of the development of western corn rootworm resistance looms.
Approximately 37 percent of the producers who took part in the 2012 Classics will use a seed blend as their refuge and hedge against insect resistance development.
"From a convenience angle, it's easy to see why this approach will increase in popularity," said Gray. Of concern is the anticipated reduction in the volume of non-Bt seed produced by the seed industry as refuge requirements drop from 20 percent levels, which could make it more difficult for producers to purchase elite germplasm from non-Bt product lines. Access to non-Bt hybrids by producers is important if the industry wants to maintain an integrated approach to pest management across the Corn Belt.
Black cutworm moths abundant and on the move
Editor's note: High-resolution digital files are available to use with this story at http://www.aces.uiuc.edu/news/News_Photos/weeds
Professor of entomology and crop sciences Extension coordinator Mike Gray urges producers to look for early signs of leaf-feeding injury from black cutworm larvae. The migratory moths, which lay eggs on winter annual weeds in producers' fields, have been captured in pheromone traps throughout much of Illinois. Following hatch from the eggs, the larvae begin to feed on weeds, but they eventually have the potential to cut seedling corn plants.
Record-breaking warm temperatures in March persisted into early April, and as a consequence many fields across southern and central Illinois have been planted and are beginning to emerge. Corn in the one- to four-leaf stage of development is most susceptible to cutting by black cutworm larvae.
"Even if you planted a Bt hybrid, don't be lulled into complacency," said Gray. Under heavy infestations, control afforded by some Bt hybrids may be inadequate. University of Illinois Extension personnel Dale Baird (Lee County) and John Fulton (Logan, Menard, and Sangamon) reported captures of nine or more moths over a one- to two-day period on March 24 and March 30. Retired crop systems Extension educator Jim Morrison reported that 16 moths were caught on April 2, the earliest and most significant capture in many years.
Fields most at risk from black cutworm injury include those heavily infested with winter annual weeds. Favorite targets for egg-laying black cutworm moths include mouse-eared chickweed, bitter cress, shepherd's purse, yellow rocket, and pepper grass. More information about the biology, life history, and scouting procedures for the black cutworm is available at: http://extension.cropsci.illinois.edu/content/black-cutworm.
Gray encourages readers to visit the following North Central IPM PIPE website (http://apps.csi.iastate.edu/pipe/?c=entry) to view captures of black cutworm moths. According to Kelly Estes, State Survey Coordinator, Illinois Natural History Survey, trapping for other insect pests -- the European corn borer, corn earworm, western bean cutworm, and fall armyworm -- will also be reported on this site throughout the summer.
NOW is the time to plant?
According to the Illinois Weather & Crops report issued by the Illinois office of the National Agricultural Statistics Service, an astounding 5 percent of the Illinois corn crop was planted by April 1, with 1 percent planted by March 25. Over the past 20 years, there were only two in which there were reports of corn being planted by the first Sunday in April. In a few other years with dry weather in early April, farmers were described as "anxious to start planting" but it is clear that almost no one thought that planting that early was a good idea.
This year, most of Illinois remains dry in the middle of the first week of April. "While planting is getting a serious start in some areas, others are still waiting until the crop insurance date at the end of this week, or until after Easter," said Nafziger.
One reason for waiting is that the weather forecasts, particularly models of temperatures over the next seven to ten days, will be soon be updated to extend past the middle of the month. According to Nafziger, "While we all know that weather forecasts can change suddenly, as we approach the middle of April, we should get a clearer idea of whether or not the emerged crop could be in trouble."
A major cause of concern is the possibility of a late frost harming the emerged crop. According to 30-year weather data summarized by the Midwest Regional Climate Center (http://mrcc.isws.illinois.edu), the median dates at which temperatures of 32 degrees last occur in the spring are in late April in northern Illinois and in mid-April farther south in the state, with the earliest last freeze of the spring occurring in late March.
"If we're optimistic and assume that the pattern we have had so far this spring will hold, we might not have to lose too much sleep over concerns that the emerged crop will freeze," Nafziger said.
With temperatures continuing to stay well above normal, the crop continues to emerge rapidly. "The plot we planted at Urbana on March 16 has reached to two-leaf stage, with good stands and in very good condition," said Nafziger. "That crop has received about 275 growing degree days since planting."
There are indications that temperatures may drop over the next week, meaning that growing degree days (GDD) accumulations will return to more normal levels. GDD accumulation for March has averaged only 67 GDD over the past ten years at Urbana; in 2012 it was 233, about 50 percent higher than in any of the previous years. Average GDD accumulations in April and May have been 166 and 374. "That means that corn planted on April 1 in a normal year will not grow as much by May 1 as the corn planted in mid-March grew by April 1 in 2012," explained Nafziger. A return to normal temperatures will slow growth, but the effects of the warm temperatures up to now will continue to have the crop developing ahead of normal.
Soil temperatures remain above average, but soils are dry and daily temperature fluctuations are relatively large. This is because water in soil holds heat better than mineral material or air. If night temperatures drop into the 30s, the warm soil protects the leaves from frost damage by radiating heat to them even as the leaves radiate to the sky. This will not help for long if temperatures drop to freezing or below.
The question is: Now that it's April, is there any reason to wait much longer to start planting?
According to Nafziger's planting date data from the last five years, "Planting on April 20 produced the highest yield of 201 bushels an acre and planting on April 30, May 10, May 20, and May 30 yielded about 2, 7, 15, and 27 bushels less than the highest yield, respectively." If this year follows the same pattern, corn planted on April 1 or April 10 will yield 7 and 2 bushels less, respectively, than corn planted on April 20.
The only agronomic issue at this point besides the ongoing concern about whether the weather pattern will change for the worse is whether there is enough soil moisture to allow seeds to germinate. This is a particular concern in fields where soils were tilled some time ago and where the lack of rain has meant that surface soil has continued to dry.
"Whether to plant into dry soil with the idea that seeds will germinate quickly once it rains is a common issue in late May with soybeans, but not in early April with corn," explained Nafziger. Corn seed needs to take up less water than soybean seed to emerge. If soils stay relatively dry, corn seeds tend to stay viable even if the weather is too cool or too dry for emergence.
"It's probably not a good idea to till again to try to bring up moisture; that could result in uneven distribution of soil moisture down the row, causing unevenness in emergence, which can reduce yields," he said. "Soils in most tilled fields are already fairly fine by now, even after only one tillage pass, and more tillage would lead to more soil moisture loss and would add to the risk of crust formation after heavy rain."
So -- it looks as if, in 2012, the corn crop planting is on course to be the earliest ever, assuming that the current weather pattern continues. Concerns related to the possibility of deteriorating weather conditions diminish with each day. Planting should accelerate over the next week. With luck, including getting some rain in the coming weeks, the crop will be off to a good start.
For now, the focus should be on getting corn planted first. Recent data show good yields from planting soybeans in April, and if they can be planted under good conditions, there is little need to wait until May to start to plant soybeans.
Multiple vs. effective modes of action
However, each component of a herbicide premix or tankmix with multiple modes of action is not necessarily effective for every weed or under all application conditions.
For example, giant ragweed, a large-seeded, summer annual, broadleaf weed species, can be difficult to control with a single herbicide. Because of its extended germination and emergence characteristics, farmers may have to use more than one herbicide or multiple herbicide applications.
While herbicide resistance has been found in the Illinois giant ragweed populations, it is not as common or widespread as herbicide resistance in other weed species. Many soil-residual—herbicide premixes containing two or more active ingredients are available to farmers who want to be proactive and use multiple modes of action to reduce the selection for herbicide resistance in giant ragweed.
However, "To effectively reduce the selection intensity for resistance to a particular active ingredient, each component of the premix or tankmix should have similar efficacy against the target weed species," Hager said. "In addition, each component should demonstrate similar soil persistence." Few commercial soil-applied herbicide premixes satisfy these criteria when giant ragweed is the target species.
The timing of a herbicide premix or tankmix application can also influence the effectiveness of each component. For example, a farmer may choose to use Harness Xtra (or any other premix of a chloroacetamide and atrazine) to control waterhemp, a small-seeded summer annual broadleaf weed species that has developed resistance and is becoming a serious problem in Illinois fields.
If it is applied before waterhemp seed has germinated, each component of this premix (acetochlor and atrazine) provides effective residual control. However, if the Harness Xtra is not applied until after waterhemp emerges, only the atrazine can effectively control the emerged waterhemp plants.
Similarly, said Hager, "A tankmix of a chloroacetamide herbicide and glyphosate applied after waterhemp emergence does not contain more than one effective mode of action to control emerged waterhemp, nor does it contain more than one effective mode of action for soil-residual control of later-germinating waterhemp."
Illinois waterhemp populations have evolved resistance to herbicides from at least five herbicide site-of-action families, including inhibitors of acetolactate synthase (ALS), photosystem II (PSII), protoporphyrinogen oxidase (PPO), enolpyruvyl shikimate-3-phosphate (EPSPS) and hydroxyphenyl pyruvate dioxygenase (HPPD). "While not every individual waterhemp plant is resistant to one or more herbicides, most field-level waterhemp populations contain one or more types of herbicide resistance," said Hager.
Perhaps even more daunting is the occurrence of multiple herbicide resistances within individual plants or fields. Waterhemp plants and populations demonstrating multiple herbicide resistance are becoming increasingly common, can be more difficult to manage, and few herbicide modes of action are effective for their control.
For example, most Illinois waterhemp populations have developed resistance to ALS-inhibiting herbicides, which is often "stacked" with resistance to herbicides from other mode-of-action families. Soil- and foliar-applied herbicide premixes containing various ALS-inhibiting herbicides are numerous. While many of these products do have multiple modes of action, the ALS inhibitor component cannot control ALS-resistant plants.
In a postemergence scenario, applying a premix or tankmix of a diphenylether (PPO inhibitor) and glyphosate (EPSPS inhibitor) provides two effective modes of action against sensitive waterhemp, one effective mode of action against waterhemp resistant to either PPO inhibitors or glyphosate, and no effective modes of action against waterhemp resistant to both PPO inhibitors and glyphosate.
In summary, using herbicides from multiple mode-of-action groups may help reduce the selection intensity for resistance to a particular active ingredient of a herbicide. However, many factors determine the effectiveness of each premix or tankmix ingredient against the weed species of concern.
Growth regulator herbicides for burndown applications
Weed control may be improved when more than one active herbicide ingredient is included in the burndown application. Burndown applications often include growth-regulator herbicides, such as 2,4-D. Both amine and ester formulations of 2,4-D are labeled for preplanting burndown applications, but the ester formulation is usually preferred over the amine formulation.
Hager explains, "The low water solubility of an ester makes it less likely to be moved into the soil by precipitation, where it could injure germinating crop seeds." Also, esters are better able to penetrate the waxy surfaces of weed leaves, so they provide better control of large weeds, especially when air temperatures are cool.
Some 2,4-D ester formulations can be applied without a waiting period before planting corn, while a seven-day wait is recommended for others. "When applied prior to planting soybean, wait at least seven days between application of one pint per acre and planting, or 15 to 30 days between application of up to two pints per acre and planting," said Hager. "Pay careful attention to label statements of any 2,4-D formulation used prior to crop planting."
Some retail applicators might be experiencing difficulty acquiring sufficient quantities of 2,4-D to meet the need for burndown applications. "Substituting an amine formulation of 2,4-D for an ester formulation is possible," Hager said, "but remember, there are significant differences between these formulations." Specifically:
- Higher application rates might be necessary because an amine formulation does not penetrate the weed's leaf cuticle as easily as an ester formulation does,
- Amine formulations are less volatile but more water soluble than ester formulations, meaning that they are more likely to move into the soil after precipitation, increasing the likelihood of crop injury,
- The interval between application of 2,4-D amine and soybean planting is longer (15 days for one pint or less, 30 days for greater than one pint) than the interval with ester formulations.
Dicamba is another growth-regulator herbicide used to control existing vegetation before planting. Several commercially available products contain dicamba, but not all of them are labeled for application prior to crop (especially soybean) planting.
"In comparison with 2,4-D, dicamba provides greater control of chickweed, henbit and prickly lettuce, comparable control of dandelion, and less control of horseweed and mustard species," said Hager.
Application rates used in burndown applications can impact the interval between application and planting. For example, Clarity can be applied at up to 16 fluid ounces per acre before planting corn (up to eight fluid ounces on coarse soils or medium- and fine-textured soils with less than 2.5 percent organic matter) with no interval between application and planting. However, the 16-ounce use rate requires a minimum accumulation of one inch of precipitation and a 28-day waiting interval between application and soybean planting. An eight-ounce application rate still requires a minimum accumulation of one inch of precipitation, but the waiting interval is only 14 days. These intervals must be observed prior to planting soybean or crop injury might occur.
Orr Agricultural Research and Demonstration Center Field Day
Please mark your calendars for the Orr Agricultural Research and Demonstration Center Field Day on Tuesday, July 17th. More information will be available closer to the date.