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I believe that this is an indication of the valuable work that your soybean associations and checkoff boards are doing to increase the use of soybeans. Please note the request on page 6 from the American Soybean Association concerning biodiesel. There's one way to keep our prices at reasonable levels. That is to continue to actively en-courage the use of soybean products.
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"How late can I plant before yields are reduced?" This is a very common question this time of year. Unfortunately, the answer is not as clear cut as one may think. If we look at average yield reductions due to late planting, we can summarize the following:
The problem with the above statements is that the actual amount of yield decrease will vary with the growing season, weather patterns, maturity group, and location. Occasionally, a late-planted crop will yield just as well as a May-planted crop, especially in a productive growing environment (high water-holding capacity soil, high fertility, high rainfall, etc.). As you may recall from previous discussions, the key to maximizing yield potential is to produce 4 complete layers of leaves before flowering. If grown on a productive environment, the crop can produce enough leaf area for maximum potential yield, even if planted late.
The below graph compiled from research in North Carolina illustrates this phenomenon. Note that the X-axis is not an actual date of planting, but the weeks before or after the last planting date that a crop under a specific environment is able to meet minimum leaf area requirements. Before this date, little yield loss occurs; but afterwards, significant losses occur. Unfortunately, the graph only explains why late planting dates reduce yield and not the actual planting date at which significant yield losses will occur. So, the graph is not predictive. Week "0" will move around, varying from early June to mid-July, but usually takes place in mid-June.
While the graph on the previous page can be a little confusing, it illustrates the point that there is no definitive answer to the question, "How late can I plant before yields are reduced?" If you are planting soybeans on a very productive soil, chances are that the latest planting date to avoid significant yield losses will be later than if planting on a droughty soil. More details of the effects of late planting dates can be found in Soybean Update Vol. 4, No. 2 (June, 2001) located on the web at http://www.vaes.vt.edu/tidewater/soybean/soybeanup/0106/0106.html .
Past field records can help decide whether or not a given field will not meet canopy requirements. If, in the past, soybeans have not completely covered the ground area by flowering (looking down on the canopy, you can see no or very little soil), then do not delay planting in this field. On the other hand, if in the past, soybeans have always produced adequate canopy by flowering, then delayed planting in field will not likely result in as large of yield losses.
Higher seeding rates and narrow row spacing will alleviate much of the effect of delayed planting. I'll reiterate some double-crop guidelines for seeding rate stated in last month's issue. Use the table in Vol. 6, No. 3 (http://www.vaes.vt.edu/tidewater/soybean/soybeanup/0305/0305.html) for more detailed seeding rate information.
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May weather was a bit unusual to say the least. We were wet all month, but soil temperatures were fairly warm at the beginning of the month. Recent cold fronts then lowered soil temperatures. Oddly enough, the seed were in a better environment for rapid emergence in early May than later in the month.
This cool, wet May has led to delayed planting and caused slow emergence of what has been planted. In some cases, the stands are very spotty in nature. More emergence took place and filled in some of the gaps as temperatures warmed, but some of the problem was likely disease. I won't go into detail of the poor seed quality issue again, but I'll refer you to previous issues of the Soybean Update that can be found at http://www.vaes.vt.edu/tidewater/soybean/soybeanup/soybeanup.html.
Instead, I'll try to provide you with some idea of how to diagnose whether seedling disease is or is not the culprit. Dig up the seedling with a trowel or small spade; don't pull up by hand. Examine the roots, the cotyledons or seed leaves, and the area between the roots and cotyledons called the hypocotyl. The roots of a healthy seedling should appear white. The cotyledons should be green in color without blemishes. Don't confuse a purple hypocotyl with seedling disease; this is normal. Some have asked is this is a symptom of purple seed stain. It is not. Actually, soybean varieties with purple flowers will have a purple cotyledon; varieties with white flowers will have a green hypocotyl.
With all the rain that we've had, flooding injury is possible. You'll be able to tell this by weak seedlings with a gray cast to the roots. On the other hand, light tan to brown, sunken, and discolored roots are signs of disease.
Areas of brown discoloration and a soft, watery rot is a symptom of pythium seed decay and damping off. Seedlings may rot before it emerges or emerge, wilt, turn brown and die. Damping off can also be caused by Fusarium. Cotyledons will usually be yellow. Light to dark brown or reddish brown to black sunken lesions can also be found on the cotyledons, hypocotyls, and roots. If plants survive, the discolored and infected roots will continue to rot and lead to poor growth. This fungus is one of the culprits involved in "Essex Syndrome", common in northeast Virginia coastal plain soils.
Plants infected with rhizoctonia root rot can also lead to damping off and poor stands. Visual symptoms of preemergence damping off will not usually be recognized because these plants do not emerge. However, this disease can be recognized after emergence by the reddish brown lesions on the hypocotyls at the soil line. This is most evident after the plants have been removed from the soil. The plants may or may not die, but infected plants will be weak because of decaying lateral roots.
With purple seed stain, cotyledons may shrivel, turn dark purple and fall prematurely. Infection can spread from the cotyledon to the stem, producing necrotic areas that girdle the stem and kill the seedling. Infected plants that survive will be weak and stunted.
The photos to the left were obtained from the U.S. Soybean Diagnostic Guide developed by the soybean checkoff program. Copies of this publication can be obtained from the United Soybean Board. A request form can be found at http://www.unitedsoybean.org/ .
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One risk is allowing weeds to become too large for adequate control. Certain weeds, such as morninglory may not be adequately controlled once they become too large, even if higher herbicide rates are used. Therefore, don't delay application past size restrictions on the label. You'll be dealing with a second application, just to salvage the crop. Plus, uncontrolled weeds just produce more seed and add to next year's problems.
Another risk, just as important, is the risk of yield loss due to delayed applications. What is the cost of delayed herbicide application? Research has determined that the critical period of weed control, or the period in the crop growth cycle during which weeds must be controlled to prevent yield losses, can vary from 3 to 6 weeks after planting. Of course the exact time depends on weed numbers and species present.
Another factor related to the critical period of weed control is row spacing. The ability of the crop to rapidly canopy and shade out weeds in a Roundup-Ready weed control program is critical for success. Narrow rows will close the canopy sooner, therefore should more effectively compete with weeds. Intuitively, we should assume that this would also affect the critical period of weed control. But, until recently, hard data has not been available to confirm this.
Recent research by Dr. Steve Knezevic's program at the University of Nebraska addressed this question. In this study, researchers planted soybean at 175,000 seed/acre in 7.5-, 15-, and 30-inch row spacing. Naturally occurring weed populations were comprised mainly of velvetleaf, pigweed, lambsquarters, and foxtails in a fairly high density (85-93 shoots per yard2). Weeds were removed with glyphosate herbicide according to soybean development stage beginning at V1 (one true soybean leaf) through R3 (beginning pod formation). The results are shown below.
The first thing that you'll notice is the rapid increase in yield loss as glyphosate application is delayed. The second thing is the effect that row spacing had on yield loss. Narrow row spacing resulted in less yield loss as applications were delayed. Weeds are better competitors in wide rows.
With data presented above, the researchers determined that the critical soybean stage that weeds needed to be removed to prevent greater than 5% less yield loss were the V1, V2, and V3 stage for 30-, 15-, and 7.5-inch row spacing. The table that follows shows this critical time of removal expressed in crop development stage and days after emergence (DAE). Note that this is days after emergence, not planting.
| Row Spacing (inches) | Weed Freed Period (crop growth stage) | DAE (days) |
|---|---|---|
| 7.5 | V3 | 19 |
| 15 | V2 | 15 |
| 30 | V1 | 9 |
I know most growers would rather use the calendar to implement weed control, but this is not the best approach. You just can't say that the best time to control weeds is x weeks after planting. Soybeans that I planted on the first of May are now only barely in the V2 stage (34 days after planting), but soybeans that I planted only 10 days ago (mid- to late-May) are almost in the V1 stage. Double-crop plantings can reach the V2 stage in 10-14 days after planting. As the graph shows, yield loss depends on the stage of the crop (and weeds), not on the number of days after planting.
Another very important point that can be gleaned from this data is the amount of additional yield loss that occurred with each delay in weed control past the critical times stated. An average of 2% yield loss per every development stage of delay past the critical time (5% yield loss) was determined as the cost of delaying weed control. The actual economic loss per every stage of delayed weed control can be calculated using crop price and yield. For example, at a price of $5 per bushel and 30 bushel yield potential, each development-stage delay in weed control could cost you $4 per acre.
Some may argue that weed height or stage may be more important than crop height. But weed height varies significantly with year and location. The researchers correctly stated that weed height does not provide enough information unless it is coupled with crop development stage. Weed height should instead be used to adjust the herbicide rate. "Weeds are controlled because we try to protect the crop," states the researchers, "therefore the crop should be the focus of the program."
One final note is needed. Weeds emerged with the crop in the above research. If weeds emerge before the crop, control the weeds 4 to 5 days (1 to 2 stages) earlier. If weeds emerge after the crop, you can probably wait an equal number of days before control is necessary. I've noticed that it's becoming an increasingly common practice that a burndown herbicide application is being forgone in no-till Roundup Ready soybeans, especially in our double-crop plantings. In this situation, timing of weed removal becomes particularly important. Don't let weeds rob you of yield, especially when this can be prevented.
For more details on this research and further information on how nitrogen rate affects the critical period of weed control in corn, refer to Dr. Knezevic's paper:
Knezevic, S.Z., S.P. Evans, and M. Mainz. 2003. Yield penalty due to delayed weed control in corn and soybean. Crop Management doi:10.1094/CM-2003-0219-01-RS.
This electronic article can be found at the Crop Management web site: www.plantmanagementnetwork.org/cm/.
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First, when will AMS improves glyphosate activity? The simple answer is in situations of "hard" water. Hard water is water that contains significant amounts of calcium (Ca2+) or magnesium (Mg2+) cations. Iron (Fe2+) can also lead to hard water, but its effect on glyphosate has not been confirmed to the extent of Ca2+ and Mg2+. Think about hard water in this way. When washing your hands, is it hard to form lather with soap? If it is, then the water is hard. Some places such as the Midwestern U.S., you cannot get any lather out of soap due to the high levels of Ca2+ in the water. Therefore water softeners in the home are very common. The addition of AMS to glyphosate in areas with this type of water is always beneficial.
Why does AMS improve glyphosate control under hard water situations? Research has shown that increased control is due to increased uptake of the herbicide. This results in an increase in the total amount of glyphosate reaching its site of action. The increased uptake is attributed to preventing the formation of calcium and magnesium salts with glyphosate. Chemically, AMS delays or prevents the crystallization of glyphosate on the leaf surface, allowing more time for the herbicide to penetrate the leaf.
The ability to overcome hard water problems with AMS is not disputed. But what about those who do not have hard water problems? Many claim that AMS helps even when hard water is not a problem. Some recent research addressed this question.
Researchers from Illinois compared the translocation of glyphosate in common lambsquarters and velvetleaf in response to AMS. In these experiments, distilled water was used, therefore eliminating any effect of calcium, magnesium, or other elements. In a whole-plant bioassay experiment, they found little response of lambsquarters from the addition of AMS to Roundup. However, the rate of glyphosate needed to reduce the growth of velvetleaf by 50% was 4 times more than the rate needed when AMS was included. This gave a good indication that something other than formation of calcium and magnesium salts is responsible for the increased control with glyphosate by adding AMS. Laboratory experiments using 14C indicated the improvement in control was due to increased translocation of glyphosate within the velvetleaf plant. Two to threefold increase in total 14C-glyphosate was observed. Therefore, for this species, something other than the hard-water effect is taking place.
The rate used to cause a 50% rate reduction in this experiment is however much lower than that which we would normally apply in the field. No one would be satisfied with only 50% control. Extrapolating greenhouse results to the field is risky business. But, I'll do it anyway and assume that the greenhouse results are similar to those one may encounter in the field. A closer examination of the rate-response revealed that AMS increased velvetleaf control 20%, 15%, and 6% at 0.6, 0.9 and 1.0 lb ai of glyphosate, respectively. This is roughly equal to 20, 28, and 32 oz. of 4 lb/gal formulations of glyphosate. In other words the benefit of AMS decreased with increasing rates of glyphosate. Another way of looking at this indicated that control with a rate of 1.0 lb ai of glyphosate alone would be equivalent to a 0.892 lb ai rate with AMS. Let's assume that a 4 lb/gal formulation of glyphosate costs $32 per gallon. A quick calculation reveals that the addition of AMS to 28 oz of glyphosate (versus 32 oz alone) would be beneficial only if the cost of the AMS was less that $1 per acre.
I'll remind you that the above calculations only apply to actively growing plants under greenhouse conditions. Less-than-optimal field conditions could show a larger monetary benefit from AMS. A greater benefit would also be obtained if using lower rates of glyphosate. Combining the conclusions of this research with the benefits of increased translocation if using hard water makes a better case for adding AMS. More practically, the addition of AMS may improve control under adverse conditions commonly found in field settings.
I will remind the reader that these results only relate to velvetleaf; control of lambsquarters was not improved. Other research (Hall et al., 2000) concluded that increased glyphosate activity with AMS on velvetleaf was due to higher calcium content of velvetleaf leaves versus other species. Therefore, one should not assume that AMS will always improve overall weed control with glyphosate, unless hard water problems exist.
So, what is my conclusion regarding AMS and glyphosate? If you have hard water, use it. Also use it if velvetleaf is a problem. With other weed species and if not using hard water as a carrier, the only benefit would be insurance. Like most insurance, we hope we don't need it; but it makes us sleep better. We don't have enough information to insure that control would increase. Higher rates of glyphosate seem to give as much benefit as AMS. It then becomes a question of economics.
The journal article that the above information was taken is: Young, B.G., A.W. Knepp, L.M. Wax, and S.E. Hart. 2003. Glyphosate translocation in common lambsquarters and velvetleaf in response to ammonium sulfate. Weed Science 51:151-156.
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Dr. Henry Wilson's research group at the Virginia Tech Eastern Shore AREC has shown that the addition of manganese to glyphosate will result in reduced control of several weed species. Reduced control caused by the addition of Mn could be overcome with higher rates of glyphosate with some species, but not all. Lambsquarters control was one weed in particular that higher glyphosate rates did not help. Furthermore, chelated Mn caused the greatest reduction in control of all most species.
This effect is not unlike that we see when AMS is added to glyphosate in hard water solutions. Dr. Wilson's group suggested that reduced weed control caused by the addition of Mn may be due to the complexing of the herbicide, which could result in the formation of insoluble glyphosate-salt complexes that are not readily absorbed into the plant. Manganese, instead of calcium or magnesium is the problem.
Therefore, make separate applications of glyphosate and Mn. The risk of inadequate weed control which can lead to reduced yields is too great.
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Two Kinds of Spray Drift - Vapor Drift and Particle Drift.
Vapor drift is the volatilization of pesticide molecules and their movement off-target. It occurs independently from the application. The potential for vapor drift of any pesticide can be predicted by its vapor pressure, the air temperature, size of treated area and climatic conditions. Vapor drift can travel much further than particle drift. Avoid vapor drift by not using pesticide formulations whose potential volatility is high, given the air temperature and climatic conditions. These include pesticides which will volatilize rapidly from moist soil or higher temperatures. (See Table 1)
Particle drift is the off-target movement of spray particles formed during application. It is affected by the environment, application equipment, and how the equipment is used. Spray particles less than 200 microns can drift considerable distances.
Agricultural spray nozzles produce a wide range of spray droplet sizes (10 to >1000 microns). As a comparison, a pencil lead is approximately 2000 microns in diameter. A paper clip wire is 850 microns, a staple wire is 420 microns, a toothbrush bristle is 300 microns, a sewing thread is 150 microns, and a human hair is approximately 100 microns in diameter.
The key is to set up the sprayer to produce the largest droplets that will provide adequate control of the target pest. This is a balancing act between the particle size that is best for drift control and the one that is best for product efficacy. Large spray particles may reduce drift, but may not provide the coverage needed. In some situations, efficacy may have to be sacrificed to avoid drift. The drift potential depends not only on the volume medium diameter (VMD), but also the total spectrum of droplet size. A VMD of 300 could mean that half the droplets were 250 micron in diameter and half were 350 microns. Or it could mean half the droplets were 50 microns (very susceptible to drift) and half were 650 microns. The VMD plus the droplet spectrum gives a more accurate estimate of the droplet size relative to drift. If the droplet size is doubles, there is 1/8 as many spray droplets. Spray droplets leave the nozzle at a certain speed. Small droplets (<200 microns), lose momentum faster than large particles and slow to the fall of gravity; when slowed by other forces such as air movement.
Engineers analyzed data for more than 100 studies involving drift from ground sprayers. Of the 16 variables he considered, three were the most important: wind speed, boom height, and distance downwind.
Wind speed. When the wind speed was doubled, there was almost a 700% increase in drift when the readings were taken 90 feet downwind from the sprayer. Hence the recommendations of spraying in 10 mph wind or less. Be aware that drift potential also may be high at low wind speeds. This is because light winds (0-3 mph) tend to be unpredictable and variable in direction. Calm or low wind conditions may indicate the presence of a temperature inversion. Drift potential is actually lowest at wind speeds of 3 to 10 mph (gentle but steady breeze) blowing away from sensitive areas.
Boom height. When the boom height is doubled, for example from 24 to 48 inches, the amount of drift increased 350% at 90 feet downwind.
Distance downwind. If the distance downwind is doubled, the amount of drift decreases five-fold. If the distance downwind goes from 100 to 200 feet, there is only 20% as much drift. If the downwind distance goes to 400 feet, only 4% of the drift occurs than at 100 feet. Check wind direction and speed when starting to spray a field. Start spraying one side of the field when the wind is lower or choose to only spray part of a field because of wind speed, wind direction or distance to susceptible vegetation. The rest of the field can be sprayed when conditions change.
Also consider slowing down on the passes closest to susceptible vegetation, etc. By slowing down from 10 mph to 7 mph (70% of the speed), while spraying at 40 psi with a rate controller, will drop the pressure by one-half or to 20 psi. These changes can also be made manually. Do not drop below the minimum recommended pressure for the nozzle tip being used. Other important factors to consider when reducing drift include spray pressure, nozzle size, nozzle orientation, operating speed, air temperature, relative humidity, shields on sprayers and nozzles, application rate and instructions from the manufacturer of the spray product.
Temperature and vapor pressure during herbicide application also influence drift. Atmospheric stability is an important factor that is often ignored. Temperature inversions occur when a layer of cool air near the soil surface is trapped under a layer of warmer air. With temperature inversions the temperature increases as you move upward. This prevents air from mixing with the air above it. This causes small-suspended droplets to form a concentrated cloud that can move long distances. If large numbers of small droplets are captured in this warm or inversion layer, the deposition control is lost. Records indicate that movement of these inversion layers may transport chemicals several miles.
The most common cause of temperature inversions close to ground level is radiant cooling of the ground -- the ground cools off quicker than the air above it. Clear skies favor radiant cooling and therefore favor the formation of surface inversions. Early morning and late afternoon are the times when surface inversions are most likely to occur. Conditions not favoring inversions include low heavy cloud cover, strong to moderate winds (greater than 5-6 mph), a temperature increase of 5 degrees, and bright sunshine.
It's important to recognize when inversions are present. Bodies of water or well-irrigated fields both favor the formation of inversions. Under clear to partly cloudy skies and light winds, a surface inversion can form as the sun sets. Under these conditions a surface inversion will continue into the morning until the sun begins to heat the ground. Wait for a 5-degree increase in temperature after sun up usually reduces the chances for an inversion. Inversions only affect the small pesticide droplets that don't settle quickly. There is a higher potential drift and therefore off-target effects if the application is made during a surface inversion. The small droplets can remain in a concentrated cloud until the inversion dissipates or the cloud of droplets moves out of the area where inversion conditions exist. Minimizing the production of small droplets will minimize the potential or drift under inversion conditions. It may be illegal to start a fire to determine the presence of an inversion or wind direction. Instead smoke bombs or smoke generators are recommended.
Drift reduction nozzles
Many new spray nozzles are designed to reduce drift. Many of these use a pre-orifice which controls the flow rate. The exit orifice controls the pattern formation. The result is larger spray droplets which are less susceptible to drift. Also, some of these nozzles can be used over a wider pressure range, which produces large droplets at low pressure and small droplets at high pressures. The ability of these nozzles to produce good spray patterns over a wide pressure range makes them an excellent choice to use with rate controllers which control the application rate by pressure changes.
While helpful these drift reduction nozzles can still create drift under some conditions, such as when the sprayer speed is increased and the resulting pressure is increased, resulting in smaller spray droplets. At slow speeds the spray droplets may be too large for good coverage. Air induction or venturi style nozzle (See Figure 1) produces droplets with a VMD of 400 to 600 microns, practically eliminating the more driftable droplets less than 200 micron particles. However, particles larger than 400 microns are problematic because they have a tendency to bounce off vegetation and may provide minimal coverage of weed foliage. The spray pattern produced by venturi style nozzles generally was more variable than the pattern produced the XR flat fan nozzles.
Drift retardants reduce drift by increasing the viscosity or surface tension of a spray solution. The viscosity of spray mixtures can greatly influence the size of spray droplets produced by atomizers. It has been demonstrated many times that drift retardants significantly increase the VMD of a sprayed solution when added to the water in a spray tank. After several trips through the sprayer pump some of the drift retardants lose their efficiency and the size of the spray particles is the same as before. Research shows that while some drift retardants may help under some conditions, most drift management should include nozzle selection, boom height, pressure, etc.
The environment and human safety are the top priority of any activity. Handle pesticide safety. With the larger number of people coming into contact with agriculture, we need to be sensitive to their lack of knowledge of agricultural issues. Understanding drift and knowing how to manage it will help most producers avoid problems.
Summary
| Table 1. Relative damage to tomatoes by vapors from 2,4-D. (Baskin and Walker) | ||||||
|---|---|---|---|---|---|---|
| Temperature and hours of exposure | ||||||
| 70-75 F | 90 F | 120 F | ||||
| 2,4-D Formulation | 2 hr | 16 hr | 2 hr | 16 hr | 2 hr | 16 hr |
| Butyl ester (High volatile) | 3.5 | 6.0 | 5.8 | 6.0 | 6.0 | 6.0 |
| Butoxyethanol ester (Low volatile) | 1.0 | 1.0 | 2.3 | 5.7 | 6.0 | 6.0 |
| Dimethylamine (Non-volatile) | 1.0 | 1.0 | 1.0 | 1.1 | 1.2 | 1.2 |
Upcoming Field Days & Conferences
Virginia Ag Expo: Corbin Hall Farm, Urbana, August 13.
Soybean Field Day: Eastern Virginia AREC, Warsaw, Aug. 19
Tidewater AREC Field Day: Hare Road Research Farm, Suffolk, Aug. 21.
Virginia Soybean Association Winter Educational Meeting: Williamsburg, Jan. 30, 2004.
American Soybean Association News
The American Soybean Association (ASA) needs your help! ASA is urging all soybean producers and those interested in biodiesel to e-mail their U.S. Senators and ask them to contact Senate Leadership and encourage them to move forward with a Comprehensive Energy Bill that contains strong biodiesel incentives.
As you may know, our goal is to include the biodiesel tax incentive and other biodiesel-friendly measures in the Comprehensive Energy Bill pending consideration in the U.S. Senate. We have heard reports, however, that Senate consideration of the Energy Bill may be postponed. Given our country's need for a comprehensive energy policy, the ASA finds any decision to delay consideration of this issue simply unacceptable.
You can send a prepared e-mail to your Senators by logging onto ASA's Legislative Action Center at http://capwiz.com/soy/home/. Senate leadership is weighing the decision to postpone debate on the Energy as you read this message, so e-mail your concerns today!
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Sincerely,
David L. Holshouser
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