Selecting the right postemergence herbicide for soybeans

Producers who used a preemergence herbicide this year may have more flexibility in their postemergence weed management strategy. With the preemergence herbicide down, producers will generally be able to wait longer before applying postemergence herbicides or cultivating. Several other strategies include the use of an early postemergence herbicide with residual control such as a Roundup + Pursuit (or Extreme) tank mix, a timely postemergence application followed by cultivation or two separate postemergence applications timed 20-30 days apart, letting the weed growth stage dictate application timing.

The critical period of weed control (CPWC) is a period in the crop growth cycle when weeds must be controlled to prevent yield losses. Weeds that emerge before or after this period may not present a threat to crop yields. This information is essential in making decisions on the need for and timing of weed control and in achieving an efficient use of herbicides. Research at the University of Nebraska has shown that each crop has a critical period of weed control; the length of which is influenced by cropping practices. In soybeans, for example, row spacing would influence the length of the period.

Effect of row spacing on CPWC

Studies were conducted in 1999 at Mead and in 2000 at Mead and Concord. Predominant weed species at both locations/years were velvetleaf, common waterhemp and green foxtail, with the densities ranging from 70-100 plants per square yard.

Table 1: The beginning of CPWC in soybean based on 5% yield loss expressed as crop leaf stage (eg. V1) and days after crop emergence (DAE) as affected by the row spacing, at two locations in 1999 and 2000. Row spacing Time to control weeds Time to control weeds inches Soybean leaf stage Days after crop emergence 7.5 V3 19 15 V2 15 30 V1 9

The critical time of weed removal was significantly influenced by row spacing. Generally, an increase in row spacing resulted in a need for earlier weed removal, thus a less competitive crop. For example, beginning of the CPWC in the wide-row soybean (30-inch) was approximately at the 1rd trifoliate stage, based on a 5% acceptable yield loss level (Table 1). This suggests that in the wide-row soybeans the control measure should start early in the season (at the 1st trifoliate stage). In 15-inch rows, the critical period of weed control is delayed and egins with the second trifoliate stage. Not surprisingly, with 7.5-inch rows, the beginning of the CPWC is delayed even further to the 3rd trifoliate stage (Table 1).

This data implies that reducing row spacing delayed the timing of weed control and increased the tolerance of soybean to weed presence. The mechanism of soybean tolerance needs to be determined yet, although we believe it is related to the crop shading effects. The speculation is that even though the weeds are present in the narrow row soybeans they are not growing as vigorously and they are not as competitive against the crop, due to crop shading effects. From a practical standpoint, these results indicate that reducing soybean row spacing increases soybean tolerance to weeds and may require less intensive weed management programs (eg. weed control measure applied perhaps once or twice).

Cost of delaying weed control

The commonly asked question among producers is "How much is it going to cost me if I delay weed control?" Possible reasons for delayed weed control may include weather constrains (rainfall, wind), and time constrains due to large acreage to spray. In order to answer the above question the yield loss data from the above studies were pooled among years-locations and graphed against the crop growth stage at the time of weed removal in corn and soybean (Figure 1).

Figure 1. Soybean yield loss and beginning of CPWC as influenced by the timing of weed removal and row spacing.

The 2% yield loss per every leaf stage of delay passed the critical stage of weed control, was determined as the cost of delaying weed control in soybean. For example, the time to control weeds in 7.5 inch rows soybean is the V3 stage (third trifoliate-Table 1), if weed control is delayed to the V4 (fourth trifoliate), it will cost a producer about 2% in yield losses due to prolonged competition from weeds. The same is true if weed control is delayed past the recommended critical time in other soybean row spacings (Figure 1). This recommendation is applicable up to the R3 stage in soybean (beginning pod). If the weed control is delayed further than these indicated stages the yield losses will be much higher than suggested. In terms of actual economic losses in soybean, it will be about $5 per acre for every soybean leaf stage of delay, assuming a price of $5 bushel and a yield goal of 40 bushels.

Weed size

Weed size at the time of control is another issue. If the weeds emerge four to five days before the crop or if they are taller than the crop, they will shade the crop. In these cases control should begin four to five days (one to two leaves) prior to the beginning of CPWC. If the weeds emerge 5-10 days after the crop, they will not shade the crop that early in the season so the control can be initiated 5-10 days (two to three leaves) after the beginning of critical period.

The size of weed species also will affect the herbicide use rates, especially the rates of Roundup in Roundup-Ready soybeans. It is generally known that Roundup has better activity on grassy rather than broadleaf species. The rates of 16-24 ounces should provide control of most common annual grassy species (foxtails, barnyardgrass, field sandbur, woolly cupgrass, panicums) that are 3-8 inches tall. The same rates should control annual broadleaves (velvetleaf, lambs-quarters, pigweeds, mustards) less than 6 inches tall. For taller grasses and broadleaf species, a full rate (32 oz) will be required. Higher rates of Roundup (40-60 ounces) will be needed to control species such as ivy-leaf morning-glory, sweet clover, field bindweed, Venice mellow and various smartweed species (lady's thumb, Pennsylvania smartweed, wild buckwheat, etc).

Practical use of the CPWC in herbicide tolerant crops

The concept of critical period of weed control is an important part of integrated weed management of herbicide-tolerant crops, such as the popular Roundup Ready soybeans and can help answer the fundamental questions of "if" and "when to apply post-emergence herbicide. A generally sound strategy in Roundup-Ready soybeans would be to apply Roundup tank-mixed with a residual herbicide at the beginning of the critical period. This would provide adequate weed throughout the period when weed control is critical to maintaining yields. To select appropriate herbicide mixtures for the weed spectrum at your farm, consult the herbicide efficacy tables from the NU Cooperative Extension publication, Guide for Weed Management in Nebraska (EC-130)

Figure 1. Emergence of morningglory over time in relation to glyphosate application timing. (Click on image to see full-size graph.)

Morningglory is an example of a weed that may use these mechanisms and go on to survive and flourish under the glyphosate system. Morningglory is more tolerant to glyphosate than many weeds with less than 70% control when it is treated at a height of 6 inches. Morningglory's extended emergence period also is a factor in its survival under glyphosate (Figure 1).

To learn more about the factors affecting this the University of Nebraska conducted a study to evaluate the survival mechanisms and success of weeds in the Roundup Ready soybean and glyphosate system The three application timings tested consisted of a single treatment 20 to 25 days after planting (DAP), a single late treatment at 30 to 35 DAP, and sequential glyphosate applications at 20 to 25 DAP and 45 to 50 DAP. Increases in morningglory seedbank size were observed under all treatments, with the greatest increase observed under the single late application. The single early treatment provided good control of the weeds present and prevented early season competition with the crop; however, morningglory continued to emerge after the treatment (Figure 1), resulting in seed production. The single late treatment stunted the large morningglory plants, but many of them recovered and produced seed. The sequential glyphosate treatments resulted in the best morningglory management. The early application of the sequential treatment reduced early season competition and controlled the emerged morningglory. The second application was made after most of the morningglory had emerged and controlled most of the plants that emerged after the first application (Figure 1).

Recommendations

In order to maintain the long-term value of glyphosate as a weed control tool and to reduce weed species shifts, it is important to use integrated weed management (IWM) practices. It is essential to be aware of the species infesting your fields and to match the treatment timing to those species.

• Make applications at the recommended rate and weed size, choosing a key weed species that is tough to control as your guide for treatments.
• Use sequential applications of glyphosate to reduce the probability of increases in species that germinate after early treatments.
• Incorporate other chemistries and weed management practices in your operation.

Leaf rust could continue to develop in the next three to four weeks if we get frequent thunderstorms and southerly winds. It probably won't significantly reduce yields, since most of the wheat in southeast and south central Nebraska is flowering. Leaf rust would need to develop very rapidly these next few weeks to reach high enough severities to damage the crop.

Viruses continue to be the most evident diseases. Symptoms of soil-borne wheat mosaic are still present. The hot weather slows soil-borne mosaic but accentuates symptom development of wheat streak mosaic. Recently, the plant disease clinic received three wheat samples with wheat streak mosaic from southeast and south central Nebraska. One sample from southeast Nebraska tested positive for both soil-borne mosaic and wheat streak mosaic. Symptoms were severe enough that yield will be significantly reduced. I have also observed symptoms of barley yellow dwarf in the field.

Foliar diseases have increased significantly due to earlier rainfall. Tan spot and leaf rust were common. In southeast Kansas disease pressure was significantly higher than in central Kansas. The wheat was more advanced in maturity and all fields were at the soft dough stage and waiting only on some sunshine and wind to mature. Tan spot was perhaps the most common and the most severe, although it varied from field to field. Leaf rust was high on about one of ten fields and had severities ranging from 5% to 25%. Some stripe rust was occasionally noted in fields at about 5% severity.

Outside of Kansas, Karnal bunt appears to be at low levels as compared to last year in Texas and Arizona. Custom cutters were reported to be avoiding fields even near those where samples had tested positive to avoid spreading the disease.

Jon A. Appel
Kansas Department of Agriculture

The governor's declaration provides for:

• state officials to assess the developing situation on an ongoing basis;
• an inventory of public and private resources to address drought issues; and
• creation of a network to allow for timely collection of information from - and distribution of information to - cities and counties facing drought issues.

Federal assistance

Sixteen counties have been named primary disaster areas due to drought, unseasonably cold spring temperatures, high winds and frost. They are Banner, Chase, Cheyenne, Custer, Dawson, Deuel, Dundy, Frontier, Hayes, Hitchcock, Keith, Kimball, Perkins, Red Willow, Scotts Bluff, and Sioux.

In addition, seven counties are eligible for assistance because they are contiguous to the primary disaster areas. They are Arthur, Blaine, Box Butte, Buffalo, Dawes, Furnas, Garden, Garfield, Gosper, Lincoln, Logan, Loup, McPherson, Morrill, Phelps, Sherman, and Valley.

Federal low-interest loans are the primary resources being made available to residents in these counties.

Twelve counties have been released for Conservation Reserve Program (CRP) emergency grazing: Arthur, Banner, Chase, Cheyenne, Deuel, Garden, Hayes, Keith, Kimball, Perkins, Scotts Bluff, and Sioux.

Climate report

Drought conditions are being experienced throughout the Rocky Mountains from Mexico to Canada. The worst hit areas appear to be from Mexico to south central Wyoming where most areas are experiencing severe or extreme drought. North central Montana is the only area of the country experiencing exceptional drought conditions, the highest classification on the U.S. Drought Monitor scale. This area has experienced almost five years of drought.

Precipitation across Nebraska during the 60 days prior to May 19 was less than 20% of normal in the southern Panhandle and southwest Nebraska. A broad area of 20-40% of normal precipitation extends eastward through the Sandhills to Broken Bow and south to Kansas. Much of the rest of the state has had precipitation at 40-80%of normal.

Since September periodic wet periods with heavy rainfall have kept the eastern fourth of the state generally within normal ranges; however, most of central, north central, south central, and north central Nebraska have received less than 80% of normal precipitation for the period, further intensifying drought conditions. Southwestern Nebraska has near normal numbers for the period, however, that may be somewhat misleading since most of the precipitation fell last fall.

Snowpack/streamflow/reservoirs

Seasonal snowfall within areas of the Rockies feeding the Platte River System were abysmal. Snowpack estimates are 20%-50% of the historical normals and less than 80% of last year's snowpack. Snowpack elevations are above 10,000 feet and won't likely provide moisture until mid-June. As a result, inflow rates into Lake McConaughy were running about 300 cubic feet per second (cfs) as of May 21. This compares to 1,000 cfs for the same date last year. Streamflow rates entering Swanson Lake appear to have been at historical lows for May and set historical lows in March and April. Every river gauge on the Republican River in Nebraska is either setting daily historical lows or is within the lowest 10% of flow rates ever measured. This has been consistent through 2002.

Hay Hotline links buyers and sellers The Nebraska Hay Hotline was activated in May and now has almost 90 sellers registered. Producers who have hay, alfalfa, or other types of forage to sell or donate may call the hotline to be included in a database of available suppliers. Pastures also will be included in the database, which is updated daily. The list is available by calling the hotline at 800-422-6692 or accessing the web site. "I have received some reports that producers are selling down cattle herds because of increasing forage concerns," said Meryln Carlson, director of the Nebraska Department of Agriculture, in a news release announcing the hotline. "Hopefully farmers and ranchers will be able to find available forage close to their homes by placing a call to this hotline." According to a report by the National Agricultural Statistics Service, Nebraska hay stocks, as of May 1, 2002, were 1,280,000 tons, more than double the stocks available on May 1, 2001, following the drought of 2000. "If the situation continues to decline like it did in 2000, the available stocks may be used quickly," Carlson said.

In late May Lake McConaughy near Ogallala was at 65% of capacity. That's 92% of last year's level on the same date and 78% of the 2000 level. Projections for this summer indicate inflows into the lake will be at 30% of the historical normal. Only three years have seen lower inflow rates at this point in time: 1956, 1963, and 1992.

Lake levels for Hugh Butler, Swanson, and Enders continue to be disappointing. As of May 21, Swanson Reservoir contained only 57% of its normal pool, Swanson stood at 29%, and Enders came in at 35%. Harlan County Reservoir has had record low inflows since March.

Irrigation restrictions

As of May 20, users in the Central Nebraska Public Power District or Nebraska Public Power District are not expected to be restricted. State Line Canals expects to deliver less than 15 inches, compared to a normal supply of 18 inches. No restrictions are forecasted for delivery systems originating from the Loup River. The Ainsworth Irrigation District anticipates full allocations this summer.

Restrictions are projected for several districts. The affected districts, the normal water supply and the expected supply (in inches) are: Nebraska Bostwick (12, 11); Cambridge Canal (12, 9); Bartley Canal (12, 6); Red Willow Canal (12, 6); Meeker-Driftwood Canal (12, 6). The H&RW Irrigation district does not now expect to be able to deliver water this season, and the Frenchman Valley Irrigation district expects to deliver just 4 inches. No normal delivery quantities were given for these two systems.

How much to apply depends on your situation. Will you actually use the extra grass to graze more cattle or increase hay yields? It's surprising how often fertilizer is applied and the extra growth is wasted by poor grazing management. Unless you economically harvest extra growth as hay or with livestock, do not fertilize.

Also consider the ability of your warm-season grass to yield more. Taller warm-season grasses like big bluestem, switchgrass, and indiangrass will respond to nitrogen better than shorter grasses like little bluestem, sideoats grama, or blue grama.

In considering how much to apply, remember to adjust nitrogen for moisture conditions. In eastern Nebraska, 50 to 60 pounds of nitrogen per acre works well when rain and soil moisture is high, but during drought it may be advisable to skip fertilizing altogether. In western Nebraska only subirrigated meadows have enough growth potential to respond to added nitrogen; there about 40 pounds per acre will do.

Soluble salts in manure consist of positively (ammonium, calcium, magnesium, potassium and sodium) and negatively (bicarbonate, chloride, sulfate and nitrate) charged ions. Soluble salt concentration can be estimated using electrical conductivity (EC) in millimho per cm (mmho/cm) or decisiemens per meter (dS/m); mmho/cm = (S/m). The soluble salt concentration (in parts per million) can be estimated as 640 * mmho/cm.

Electrical conductivity as a measure of ammonium Electrical conductivity is used to estimate the salt content of manure. As ammonium is generally the main ion in liquid manure, EC can be used to estimate ammonium-N concentration. If manure is less than 1% dry matter content, or if EC of manure is between 0 and 6 mmho/cm, ammonium-N concentration can be estimated as: NH4-N (ppm) = -9 + 77 * EC.

Liquid manure is considered safe for application to a growing crop when the EC is less than 3 mmho/cm, although manure with an EC greater than 5 has been applied without leaf burn. The thresholds for injury are not well established and vary with crop and growth stage. Corn and wheat are less susceptible to leaf burn than legumes and corn taller than 18 inches is less susceptible than younger corn.

When the electrical conductivity of manure effluent is greater than 3, effluent can be diluted with fresh water to reduce the salt effect if appropriate check valves (required by law) are used to prevent contamination of the fresh water source.

When diluting manure through the irrigation system, begin pumping fresh water first and then add the effluent to avoid applying effluent at full concentration.

Wind, high air temperature and/or low humidity may increase potential leaf burn. Leaf burn is less likely if liquid manure is sprinkler applied in the evening or at night, however night application may contribute to increased odors since odors do not disperse as quickly as during the morning and early afternoon hours.

Soluble salt content can be determined by having liquid manure samples analyzed by a laboratory. Producers also can take their own electrical conductivity measurements with pocket meters that are available for about $75.

In summary:

1. Test the electrical conductivity of liquid manure or effluent before applying through a sprinkler system to a growing crop.
2. If the electrical conductivity is greater than 3 mmho/cm, take one or more of these precautions.
   • Dilute with fresh water.
   • Apply at night.
   • Apply when corn is over 18 inches tall.
   • Apply to cereal crops.
   • Apply to a small area and wait a few days to see if leaf burn occurs.

Guests on the Extension broadcast include Paul Jasa, NU extension engineer, and Mark Schroeder, director of farm operations at NU's Agricultural Research and Development Center at Ithaca. The program is hosted by Doug Jose, UNL ag economist and farm management specialist. Managing machinery costs is a challenge for almost every farmer, Jasa said, but every farmer looks at the challenge from a slightly different perspective.

"Some like nuts and bolts and like making repairs. They'll run a piece of equipment a lot longer than others," Jasa said. "Other producers buy a new piece of equipment any time their neighbor does."

?Knowing when to buy and what to buy is also important. NU's Mark Schroeder runs his full-size equipment until it reaches a point of maximum efficiency based on cost per acre - usually 500 to 800 hours for a combine and 1000 hours or less for a tractor. Schroeder then looks for a dealer program and sells.

“I try to wait until demand is high for good, late-model equipment,” said Schroeder. “You’ll get a higher price for something that can be turned around and sold real quickly.”

Jasa and Schroeder will talk about the logic of buying versus leasing, equipment integration, maintenance strategies, the importance of anticipating costs and the changes brought by new technologies.

The clinics offer individual and confidential information and education on farm finances, the laws, regulations and policies governing Farm Services Agency (FSA); debt restructuring and other legal options; and how the mediation program can help work with lenders to find an agreeable and workable solution. To make an appointment and learn specific clinic locations contact the Farm Hotline at 800-464-0258.

Producers were able to get into their fields this week in eastern Nebraska to take their first cutting of alfalfa. (Photo by Brett Hampton)

• Bob Wright, Extension Entomologist at the South Central Research and Extension Center: Corn rootworm larvae were found in UNL research fields near Ithaca and Clay Center on June 3. Egg hatch began between May 31 and June 3. (No larvae were found when fields were sampled May 30, but several large first instars and some smaller first instars were found June 3.)

• Ron Seymour, Extension Educator in Adams County: The crops continue to improve with moderate rainfall and a significant increase in temperature. The soil moisture conditions are adequate in the top foot but very dry in the subsurface. Numerous grasshopper nymphs were noticed in the border of several fields. Much of the corn crop is in the 3- to 4-leaf stage. Soybeans have emerged in many fields and some plants have had minor feeding damage due to bean leaf beetles and grasshoppers. Wheat plants have reached their full height and are blooming. The crop looks good with few pest problems. The first cutting of alfalfa has begun. Alfalfa weevils are present but the infestation is minimal.

• Jim Schild, Extension Educator in Scotts Bluff County: The dry conditions continue and wheat ground is being converted to pasture. Native grass is starting to turn brown and has had little growth. Corn stands are uneven and ground being worked for dry beans is pretty much dry beans. The current talk is how long the irrigation water will last this summer. The current prediction is 50 days, although it may drop to 40 days if we don't receive rainfall. In a normal year, the water runs from mid to late June to mid September. There also may be an "alfalfa run" for two weeks in mid April.

• In the news: This week Successful Farming’s Ag Online and Reuters reported that of the 38 million acres in Brazil planted to soybeans, 2.6% have tested positive for the Asian rust fungus. The fungus, which thrives in humid areas, can cause yield losses of up to 70% in legumes; however, losses in Brazil are estimated at about 10%. The fungus traveled on air currents from Africa and Asia to South and Central America.

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Copyright 2002 by the University of Nebraska

Published by University of Nebraska Cooperative Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture

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