Field activity across the state ranges from replanting to weed control and spraying. Photo by Brett Hampton

May 27, 2005

Rootless corn reported in central Nebraska

In addition to drought stress, several diseases appear to be widespread. Tan spot and stripe rust (see below) can be found in fields throughout the Panhandle. In some cases, spraying may be called for to prevent serious injury. Recent warm weather also brought out the symptoms of wheat streak mosaic virus -- yellow streaking and mottling of leaves. This disease is widespread in the Panhandle and may pose a serious threat to grain yield. Growers are encouraged to scout their wheat fields now to determine if they have disease issues that need to be addressed.

For scouting and treatment recommendations for wheat diseases, see previous issues of CropWatch or the UNL Wheat Production Handbook, available on-line at http://www.panhandle.unl.edu/personnel/lyon/

Drew Lyon Extension Dryland Crops Specialist

Delaying weed control decreases soybean yield

Currently, we don’t know of any glyphosate-resistant weeds being found in Nebraska; however, it appears that our fields are experiencing a slow shift in weed species. In the last three years, university weed extension specialists have been receiving phone calls and complaints on glyphosate failing to control certain weed species, including some “new weeds.”

Table 1. Weed species and their heights at the time of herbicide application, levels of weed control with 22 oz rate of Roundup WeatherMax (at 21 days after application), and the rate of Roundup WeatherMax rate needed to provide 90% control of respective species at Concord in 2004 (preliminary data). Note that while rates higher than the labeled rates were used in this research, always read and follow the label. Weed species Weed height Weed control with 22 oz of WeatherMax Rate of WeatherMax needed to achieve 90% control (inches) (%) (in fl. oz) Field bindweed 3 95% 22 6 70% 35 9 60% 35 Ivyleaf morningglory 4 50% 37 8 30% 50 12 21% >60 Kochia 4 100% 15 12 95% 20 20 90% 22 Russian thistle 5 100% 12 10 90% 22 15 90% 22 Yellow sweetclover 4 50% 35 8 35% 48 14 10% >60 Velvetleaf 5 85% 28 10 65% 34 21 60% 40 Venice mallow 6 70% 32 10 50% 40 18 45% 58 Common waterhemp 4 98% 16 9 90% 28 14 85% 30 Wild buckwheat 3 70% 28 6" tall/ 12" runners 60% 33 6" tall/ 24" runners 45% 40

Weed species shift is not a new thing -- it has happened since man started cultivating crops. Weedy and invasive species easily adapt to changes in production practices to take advantage of the change. Species that do not adapt become “less frequent” compared to those that do adapt. Despite the fact that glyphosate controls many weed species, especially grasses, many broadleaf species are naturally tolerant to label rates of glyphosate. It appears that as a result of repeated use of glyphosate in Nebraska, there is a slow shift in weed species from those easily controlled by glyphosate to those more tolerant of this herbicide.

Based on phone calls and questions from producers, crop consultants and agronomists, we compiled the following list of species which appear to be taking advantage of this shift and becoming more numerous: marestail (horseweed), morningglory (common and ivyleaf), wild buckwheat, Pennsylvania smartweed, lady’s thumb, venice mallow, yellow sweetclover, field bindweed, waterhemp, kochia, Russian thistle, primrose and volunteer Roundup Ready^® corn.

If these weeds are not controlled, their seeds will be a major problem in the future, especially in no-till systems, due to lack of tillage as a tool for weed control. Such shifts in weed populations to more tolerant weeds is already resulting in increased weed control costs due to additional herbicide applications or increased glyphosate rates.

Research on glyphosate rates

This article summarizes preliminary data from studies conducted at Concord and North Platte in 2004 to determine the appropriate dose of glyphosate to control these weed species.

We tested seven rates of Roundup WeatherMax® ranging from 4.6-60.0 oz with 2% v/v AMS. Each glyphosate rate was applied at three growth stages of the weed, targeting 1) 2- to 5-inch tall weeds (early POST), 2) 6- to 12-inch weeds (mid POST) and 3) 12- to 20-inch weeds (late POST). Visual ratings of percent weed control were conducted about 21 days after glyphosate treatment, based on a scale from 0 to 100 (where 0 = no injury and 100 = plant death).

Most of these weeds survived the label rate of Roundup WeatherMax (22 oz/acre). Weed size was the most important factor that determined the level of control for each species (Table 1). Ivyleaf morning-glory and sweet clover were the hardest species to control. For example, the 22-ounch rate provided only 50% control of ivyleaf morningglory that was 4 inches tall. The control level was further reduced with taller morningglory, resulting in 30% and 21% control for 8- and 12- inch tall plants, respectively (Table 1). A similar trend was observed for other weed species (Table 1). The label rate of Roundup WeatherMax provided good control (more than 85%) of kochia and Russian thistle, regardless of plant height (Table 1).

Since the label rate of WeatherMax did not provide adequate control of most species over 3 inches tall, we developed dose response curves (not shown) to determine how much glyphosate is needed to achieve at least 90% control of taller plants (6 to 20 inches tall). This information also will help determine rates for late glyphosate applications in Roundup Ready soybeans (not an uncommon practice in Nebraska). Based on our data from the dose response curves, to achieve at least 90% control of taller weeds, much higher rates of Roundup WeatherMax ranging from 1.5- 4 times the label rate are required. About 1.5-2 times the rate was needed to control 3- to 6-inch tall wild buckwheat, Venice mallow, velvetleaf, waterhemp, sweet clover, ivyleaf mornigglory and field bindweed. About 3-4 times the rate was needed to control 12- to 15-inch tall ivyleaf morningglory and yellow sweetclover. For weed sizes and respective rates, see Table 1.

This data reaffirms what many practitioners have observed, that glyphosate used alone does not work as well today as it did five to six years ago. The label rate of glyphosate did not provide adequate control of most problem weed species tested. If the trends in weed shifts continue, glyphosate used alone will no longer be a viable tool for weed control in Roundup Ready systems. Mixing glyphosate with other post-emergence broadleaf herbicides, or using soil-applied herbicides after soybean planting, indicates a potential to effectively control most of these species.

Using various weed control tools is not a new thing, we only “forgot” about it since the introduction of Roundup-Ready crops. Changing modes of actions in your herbicide program is also one of the basic ideas in an Integrated Weed Management (IWM) program, especially to combat weed resistance/tolerance issues. Integrated Weed Management provides a system for integrating several tools for weed control.

I believe that Roundup-Ready technology only fits well when used with other weed control methods under the umbrella of an IWM system. The value of this technology can be preserved only by proper management and reduced overuse. It is easy to fall into a trap of overusing glyphosate when one glyphosate-tolerant crop is grown after another. Proper use of this technology, as a component of an IWM program, is the key to preserving the long-term benefits of this technology while avoiding many of the concerns about its use or overuse.

Stevan Knezevic
Extension Integrated Weed Management Specialist

European corn borer moths are flying

Female European corn borer moth European corn borer egg hatch

European corn borer moths have been active since the second week in May in south central Nebraska, based on catches in black light traps at Aurora and Clay Center, and activity likely has extended northward.

As non-Bt corn grows it will become susceptible to the first generation of corn borers. While we haven’t seen a bumper crop of moths so far in our black light traps, we should not be complacent about the European corn borer in non-Bt cornfields.

Timely and accurate scouting is the key to managing European corn borer in standard (non-Bt) corn hybrids. Remember that conditions are localized and each field should be scouted to make accurate decisions.

Corn borer larval survival depends on several factors. High humidity and warm temperatures are ideal for establishment of larvae in the whorl. Egg masses are white, with 5 to 40 eggs in each mass, and laid on the underside of leaves near the midrib. The masses look like fish scales flattened against the leaf. In four to seven days the heads of the developing larvae will be visible, and the eggs will appear spotted. This is the “blackhead” stage, and these eggs normally hatch within 24 hours. As the larvae enter the whorl to feed on the developing tissue, the feeding scars (shot-holes) appear as the leaves emerge from the whorl. Larvae will remain within the whorl for 7 to 14 days before boring into the stalk.

Corn that is less than 16 inches with extended leaf height (distance from the ground to the tip of the leaf pulled up vertically, about six-leaf stage) is unlikely to support young larvae because of the presence of a substance known as DIMBOA, a natural resistance factor. As the plants grow, the level of DIMBOA decreases, so plants above the 16-inch extended leaf height will generally support corn borers. First generation corn borers prefer taller plants for egg laying, therefore, the earliest planted fields are more likely to have higher populations. Scout these fields first, but do not neglect other fields because any cornfield is a potential target and should be scouted.

Now that Bt corn is being planted widely, be sure you know whether the field you are scouting was planted to Bt corn. In Bt corn, corn borer injury to whorl stage plants should be limited to a few tiny pin-holes where larvae initially fed before ingesting a lethal dose of Bt toxin. However, seed lots may contain a small percentage of off-type seed (typically less than 4%) which does not produce sufficient toxin levels to kill corn borer larvae. If greater than 4% of plants show significant leaf feeding damage in a Bt cornfield, check to confirm it is corn borer causing the injury. (Other caterpillars such as corn earworms or common stalk borer are not controlled by Bt corns currently available.)

If you believe that corn borer is causing the injury, contact a representative of the company that sold the seed to investigate the situation more completely.

Light trap reports Black light trap data can help monitor timing of insect activity and suggest when to more closely monitor for pest insects. Results from black light traps operated by UNL Extension are available at http://entomology.unl.edu/fldcrops/index.htm Additional sites will be added later. Bob Wright Extension Entomologist

To determine the need for treatment, scout at least 20-25 consecutive plants in at least 4-5 places in the field (100 plants minimum per field). The scouting locations should be randomly selected and representative of the field as a whole. At each scouting location, randomly select the first plant that will be sampled. If you do not and always start sampling at an infested plant, the counts may be inflated by as much as 5%. Count the number of plants showing shot-hole feeding and determine the percent of infested plants. Next, pull the whorls from at least two randomly selected infested plants in each set of 20-25 plants. Unroll the leaves and count the number of larvae in the whorl and determine the number of larvae per infested plant. Young corn borers usually suffer from 60-85% or higher mortality due to natural enemies, weather and disease, so try to wait to make treatment decisions until most of the borers are second instar. This will allow you to take advantage of natural larval mortality.

If you find new shoots, how tall are they? Are many of them a couple inches taller than your usual cutting height? If you cut these new shoots off along with the first growth, your alfalfa plants will have to start a whole new set of shoots. This could delay the second cutting by as much as a week. Fortunately, you can avoid this delay. All you need to do is raise your cutting height a couple inches so that you avoid clipping off most of these new shoots. Your regrowth then will have a head start toward the next cutting. And since the stubble you leave behind has low feed value, the yield you temporarily sacrifice is mostly just filler.

Normally I suggest leaving as short a stubble as possible when cutting alfalfa because that maximizes yield and doesn’t affect regrowth, but the situation this year is different. Don’t blindly start cutting alfalfa when harvest is delayed. First look for new shoots, then raise cutting height if needed.

Bruce Anderson
Extension Forage Specialist

Spraying for musk thistle needs to occur before Memorial Day. Plants that have started to grow tall usually are not completely killed by spraying. Digging is your best option now. Fertilizing warm-season grasses also should have been completed by Memorial Day or soon afterward.

Your alfalfa should have been cut already if you are south of the Platte River and as soon as possible elsewhere for good quality hay. Late cutting might give good hay for many livestock, but there is little chance of getting dairy quality hay any more this cutting. And start planning now for any possible shortages.

Memorial Day also marks the start of the planting season for summer annual grasses for many folks. Sudans and forage sorghums can be planted now. Millets should be planted in a couple of weeks.

This week also is a good time to estimate if your pastures will have enough moisture to produce the growth needed by your livestock this year. If drought has caused reduced growth, adjust animal numbers now before it's too late. Summer rains will not catch up completely. If growth is abundant, you may be able to cut some for hay.

Taking advantage of this time to evaluate and address forage status can help you avoid complications later in the season.

Bruce Anderson
Extension Forage Specialist

Blends present several beneficial effects. They can combine several varieties that complement each other in various attributes, making them more stable from one year to the next while presenting a more diverse target for invading diseases and insects. One of the real keys to the success of blends will be how well they perform relative to pure lines. This year 10 blends were planted.

These blends are in addition to the 55 varieties in each plot. Each meeting will feature sessions on how to select winter wheat varieties, wheat fertility, management of winter wheat residues, winter wheat diseases, and a tour of the plots. For more information on tours, contact the extension educator in the host county.

Bob Klein
Extension Cropping Systems Specialist

Table 1. Blends used in the 2005 UNL winter wheat variety trials. Blend Number Type Varieties involved SmDf Blend #1 3 semi dwarf varieties Millennium, Alliance, Wahoo SmDf Blend #2 3 semi dwarf varieties Millennium, Wesley, Wahoo SmDf Blend #3 3 semi dwarf varieties Millennium, Halt, Wahoo SmDf Blend #4 3 semi dwarf varieties Millennium, Harry, Wahoo TallBlend #5 3 tall varieties Pronghorn, Goodstreak, Buckskin SmDf Blend #6 2 semi dwarf varieties Millennium, Wahoo SmDf Blend #7 2 semi dwarf varieties Infinity CL, 2137 WhtBlend #8 2 white varieties Nuplains, Antelope WhtBlend #9 2 white varieties Trego, NW99L7068 IrrBlend #10 3 irrigated varieties Jagalene, Wesley, 2137

Kevin will talk about their various grass-based enterprises, including stocker cattle, sheep, chickens and horse hay. Tall fescue — both endophyte free and novel endophyte varieties — have been planted for grazing and hay.

Other featured speakers will include Steve Wallace, Barenbrug USA sales manager; Bob Scriven, Kearney, irrigated pasture grazing consultant; Bruce Anderson, Extension Forage Specialist; Terry Gompert, Knox County extension educator; and Doug Anderson, Nuckolls and Thayer counties extension educator.

Wallace has traveled across the United States, New Zealand and Europe observing and learning about grass based agriculture systems. He will discuss forage species selection and management. Scriven has worked with many irrigated pasture producers in Nebraska and surrounding states and will talk about the economics of establishing and managing irrigated pasture.

Directions to the tour site are from Nebraska Hwy 74 at Ohiowa: travel 4 miles south on County Road 20 and 1/4 mile west on Road X.

For more information contact one of the following tour sponsors: Brad Young at Prairie States Seed, 866-373-2514; Doug Anderson, 402-768-7212; Duane Lienemann, 402-746-3417; or Kevin Kimbrough, 402-295-2530.

L. Mark Lagrimini

A biotechnology researcher has been hired to head the University of Nebraska-Lincoln's Department of Agronomy and Horticulture. L. Mark Lagrimini will assume the job Aug. 1.

Lagrimini is a former project leader at Syngenta Biotechnology Inc. at Research Triangle Park in North Carolina and spent 12 years (1987-1999) at Ohio State as a faculty member in the Department of Horticulture and Crop Science. At Ohio State Lagrimini led a research program on plant enzymes.

At Syngenta, from 1999-2004, he was principal scientist and group leader, working on western corn rootworm resistance and drought tolerance and nitrogen efficiency in corn. Most recently, he's been a project leader at Athenix Corp., an agricultural biotechnology firm working to develop novel traits for herbicide tolerance and insect resistance.

Lagrimini received a bachelor of science degree in biochemistry at the University of Illinois and his doctorate in biochemistry from the University of Iowa.

Lagrimini takes over for Lowell Moser, who has served as interim head since Kenneth Cassman returned to the faculty last year. Lagrimini will be paid $150,000 a year.

The Department of Agronomy and Horticulture is part of the university's Institute of Agriculture and Natural Resources.

Randy Pryor, Extension Educator in Saline County: Growers here and in Gage County are beginning to see widespread, longer term damage to winter wheat from the early May freeze. Leaves are burning from the bottom of the plant up and while green heads pushed through, many were incomplete or smaller than usual. We're seeing a lot of stem damage.

Gary Zoubeck, Extension Educator in York County: We've found "rootless corn" here in several fields that were pounded by earlier rains and then faced hot, dry conditions.

Dewey Lienemann, Extension Educator reporting for Webster and southern Adams counties: Most of the corn has been planted and has emerged and the last fields of soybeans were being planted Monday. Some corn and soybean were replanted due to the storm that brought too much rain, hail and flooding to parts of southern Adams County. Most of Webster County got 1.5-2.5 inches of rain without the heavy hail and flood. We were starting to show effects of dry con-ditions until then. A lot of pivots are running as the ground is crusted and beans and some corn are having trouble breaking through the soil. Some wheat across the county is still having trouble and is yellow, stunted and slow to develop; however, a lot of wheat has headed.

Unfortunately we have found several fields, especially in lowland areas, exhibiting severe frost damage. We had five straight nights of below freezing temperatures just before boot stage and then a reprieve for a week with another frost coming a couple of weeks ago. We have had reports of split stems, barren heads and even no heads. I anticipate we will have a lot of white caps in this year’s wheat, diminished yields in spotted fields, and an early harvest.

Alfalfa was hit with both freeze and more alfalfa weevil than I can remember. Some producers sprayed early, but most cut their alfalfa when they found damage. Some grain sorghum has been planted but many producers are waiting for warmer soils. Pastures are starting to really come on as are mullein and musk thistle in areas that haven’t been sprayed.

Ralph Kulm, Extension Educator in Holt and Boyd counties: Moisture and cold temperatures have been the big stories for this area. Precipitation is more than 3 inches above normal for the year.

Freezing temperatures (11 out of 12 nights) from April 23 to May 4 damaged alfalfa and cool season grasses and lowered soil temperatures to the point where nearly all of the corn emerged about May 14, regardless of when it was planted. About 90% of the alfalfa has recovered and will produce a fair first cutting. The other 10% is trying to make regrowth from the crowns. Alfalfa weevil populations are building, but it is too late to control them. Producers will need to harvest and then scout the regrowth. Soybean planting is nearly completed. Pastures are in better condition than at any time in the past five years.

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