Number 30 April 25, 2006

Transcription

1 Number 30 April 25, What to do with drought-stressed wheat 1 2. Postemerge herbicides for sorghum: Update on Lumax emergency exemption 2 3. Alfalfa management in drought-stressed areas 4 4. Plant analysis for wheat 6 5. Scattered freeze injury reported on wheat 8 1. What to do with drought-stressed wheat In many areas all across the southern half of Kansas, dryland wheat has recently been showing drought stress symptoms. A combination of record heat, dry weather, and high winds in April has sapped the wheat in those areas. Although there was some moisture in late March and early April, soils are extremely dry again. The result is short wheat that has headed out early. Heads are small. And plants are turning blue where they are severely drought-stressed, or even brown where plants have died. In some cases, stress has not been this severe this early since There is still time for wheat to recover if it is just blue and drought-stressed and rains come before it dies. Rains on April 23 and 24 will help out in some cases. Maximum head size is already determined, but kernel weight can still respond to good weather and can make up for some of the loss in yield potential. Obviously, if the wheat is brown, it has died and no amount of rain can revive it. Where the wheat looks bad, producers have a dilemma. What s the yield potential of that wheat? Is the damage irreversible, or will improved conditions help? It is very difficult to estimate yield potential of wheat at this time. There are some formulas used by Kansas Agricultural Statistics, private forecasters, and others, but those formulas are based on tiller counts, spikelet size, and mesh counts. For the most part, those formulas do not take into account the condition of the wheat, and will probably overestimate yields on stressed wheat. 1

2 Producers who have their wheat crop insured cannot take any action until the insurance company loss adjuster releases it, according to Art Barnaby, K-State Extension agricultural economist. So that s the first step a producer would have to take. In southwest and the western part of southcentral Kansas, producers with dryland wheat that is dying should consider just letting it go. There is probably not enough forage present on short, drought-stressed wheat to be worth cutting. Wheat that is only inches tall will probably only yield about ton of forage pre acre. There is considerably more value in letting the crop die and leaving it in place over the summer to prevent blowing. If the field has been released by the insurance adjuster, and some rain comes later, producers could no-till a summer annual forage or grain sorghum into the wheat residue, assuming there are no herbicide carryover problems that would prevent it. Where producers did apply a sulfonylurea herbicide to their wheat, they should consult the label (and all appropriate supplemental labels) to see what cropping restrictions may apply. In southeast and the eastern part of southcentral Kansas, the wheat is a little taller and could be harvested for hay or greenchop if it hasn t yet turned brown and died (and assuming the insurance adjuster has released it). Much of the wheat in those areas had either headed, or the awns have emerged from the boot, however. Wheat awns can reduce the palatability of hay. Ensiling the wheat helps minimize this problem. Producers could also let the wheat stand and hope it recovers with improved conditions. If the entire field dies or is terminated, producers may be able to plant soybeans, grain sorghum, or summer annual forages into the residue, depending on any cropping restrictions from herbicides applied to the wheat. Foxtail millet is a short-season, summer annual forage that could work well in this situation. If the stand of wheat is spotty, with part of the field alive and part dead, then it s probably best to spray the field with glyphosate before planting the summer crop. -- Curtis Thompson, Southwest Area Crops and Soils Specialist cthompso@ksu.edu -- Gary Kilgore, Southeast Area Crops and Soils Specialist gkilgore@ksu.edu -- Jim Shroyer, Extension Agronomy State Leader jshroyer@ksu.edu 2. Postemerge herbicides for sorghum: Update on Lumax emergency exemption A major effort has been underway this winter and spring to get a section 18 emergency exemption from the Environmental Protection Agency for the use of Lumax herbicide for control of triazine- and ALS-resistant pigweed in grain sorghum. To date, the EPA has not granted approval for this exemption, and producers cannot use Lumax as a postemerge treatment on grain sorghum. 2

3 On 17 April, the Biological and Economic Analysis Division (BEAD) at the EPA released a review of the Kansas Department of Agriculture s application for the section 18 emergency exemption for Lumax. It concluded that the application does not meet the criteria for an emergency situation due to the availability of efficacious registered alternatives. However, the door was left open for rebuttals, and this is still being pursued. We will keep you updated on the progress of this request for a section 18 emergency exemption as new developments occur. The main alternatives that BEAD considers adequate are Valor applied days ahead of planting, early postemerge herbicides such as Aim and Buctril, and mid-post herbicides such as 2,4-D and dicamba. Valor can be used to suppress triazine-resistant and ALS-resistant pigweeds. The problem is that it has a 30- to 45-day preplant requirement, and must be used with a split application of Bicep II Magnum. This will reduce weed control effectiveness later in the season, and makes it an expensive program. If Lumax does not get approved quickly, producers will have to look to postemerge herbicides to control resistant pigweed. With Aim-plus-atrazine or Buctril/Atrazine, the key to good performance is very early application. Chances of satisfactory pigweed control go down rapidly as weeds exceed 2-inch height at application. To use these herbicides successfully, it is absolutely essential that sorghum fields be scouted early and diligently. This cannot be done by merely looking at the fields from the cab of a pickup. Waterhemp seedlings are especially hard to spot in no-till fields. If the tallest pigweeds are over 2 inches tall, producers should consider adding about 5 fl oz/acre of 2,4-D amine to Aim, or Aim-plus-atrazine, applications. The growth regulator activity of 2,4-D moves to the growing points, which may not have gotten enough burn from Aim and atrazine to make a kill. If the window for early postemerge control with Aim or Buctril is missed, then herbicides like 2,4-D and dicamba can provide fair to good mid-postemerge control. Crop tolerance of 2,4-D is best when sorghum is 5-8 inches tall, although application of 2,4-D will still cause some degree of crop injury even at that stage. Dicamba is better tolerated by sorghum, but dicamba is not as good at killing pigweed. If pigweed is not ALS-resistant, then Ally-plus-2,4-D is safer and more effective than using a higher rate of 2,4-D by itself. If pigweed is not triazine-resistant, then 2,4-D-plusatrazine or 2,4-D-plus-dicamba are also safer and more effective than a high rate of 2,4-D alone. For pigweed not triazine- or ALS-resistant, a timely application of Peak-plusatrazine-plus-crop oil concentrate can give good control. That combination takes out velvetleaf and other broadleaf weeds as well. 3

4 In a 2005 field experiment with phenoxy herbicides, the best control of 3- to 8-inch Palmer amaranth was with Brash at 1 pt/acre (Brash contains 3/8 lb ae 2,4-D amine plus 1/8 lb dicamba). All pigweeds were stunted and curled over, with lots of callus tissue, but few were actually killed. Control was rated about 80%. This shows how tough pigweed can be once it gets going. If the window for mid-post control is missed, then the highest permitted rates of 2,4-D and dicamba will be required, and should be applied between sorghum rows with drop nozzles. Very few commercial applicators are equipped to do this. -- Dave Regehr, Weed Management Specialist dregehr@ksu.edu 3. Alfalfa management in drought-stressed areas Alfalfa has been under stress in many parts of Kansas during much of April. Drought, alfalfa weevil, and various aphids are causing stunted growth and damaged leaves. This will affect management plans. Normally, the first cutting of alfalfa should be made when regrowth at the crown is apparent. In the spring, this occurs prior to bloom. But this year, the combination of drought and heavy insect pressure has been stressing the dryland alfalfa. Producers may have to consider taking their first cutting earlier than they d like -- even if regrowth at the crown has not yet begun. Leaves contain more nutrients than stems, and it s important to retain as many of the leaves as possible to produce high-quality forage. If cutting is required before the optimum time, root reserves on newly-established stands or even older stands may not be satisfactory to permit rapid regrowth. But left uncut, the hay crop may be lost and damage to the stand may occur. If producers are forced by drought stress to make the first cutting earlier than the ideal time, it s important to delay the second cutting enough to allow nutrient reserves in the roots to replenish. Where weevils are present, producers have to decide whether to spray with a pesticide first and then cut, or to forgo spraying and make the first cutting. If growers decide to cut instead of spray, they need to watch the fields right after making the cutting to make sure adult weevils don t attack the stems. If there s little or no growth a few days after cutting, that s an indication that the stems are under attack. The adult weevils chew around the bark a condition called barking and that restricts growth. If producers determine the weevils are still a problem, growers should go ahead and spray. Producers should make sure alfalfa weevils and aphids are controlled, since this will have long-lasting impacts on productivity. This may require two spray applications this year, since infestations are unusually heavy in many fields. 4

5 Alfalfa weevils have started pupating as of April 21 south of U.S. Highway 36 so very little additional damage should be expected from larval feeding. If producers need to spray and swath early they can refer to the K-State Research and Extension publication, Alfalfa Insect Management 2006 (MF809) for insecticides with a very short pre harvest interval (PHI), some with 0-1 day after treatment. Swathing will help with the aphids also as pea and cowpea aphids are usually earlier season pests. Regarding insect problems, Randy Higgins, Extension entomologist, issued the following advice in an April 12, 2006 release from K-State s Department of Communications ( With the warm weather in April, Growing Degree Days are accumulating fast, so insect development is progressing rapidly. A field that may not have shown much damage three or four days ago can start to take on a silver or frosted appearance from a distance as alfalfa weevil larvae emerge from eggs, grow larger, and inflict more damage. Treatment of alfalfa 3 to 7 inches tall may be justified when feeding is evident on the top inch of growth and one to two alfalfa weevil larvae per stem are present. When alfalfa is 8 to 14 inches tall, growers should consider treatment when larvae are found causing significant feeding damage to the top 1 to 2 inches of growth on 30 to 50 percent of terminals or about 1.5 to 2 (or more) larvae are present per stem. Heavy survival of fall-laid eggs may be an indicator that large numbers of larvae from spring-laid eggs may eventually develop in some areas. If this happens and a grower chose to treat small alfalfa, re-treatment may eventually become necessary in some fields, Higgins said. With luck, one treatment will be all that is required in most locations, but growers are advised to keep watch as their hay crop develops. Compared with northern states, Kansas often experiences an extended larval damage interval following sequential and partly overlapping hatches of fall- and spring-laid eggs, he said. In addition to weevils, producers should pay attention to aphids. Pea aphids have a dark band encircling the base of each antennal segment, but blue alfalfa aphids do not. Cowpea aphids are much darker - sometimes almost black - and they frequently seem to congregate near the tips of infested stems. Evaluate plant vigor and aphid densities to determine if treatment is warranted. Fifty pea aphids per 10-inch-tall alfalfa is thought to justify treatment, whereas 20 blue alfalfa aphids should be considered threatening. Although there is less research data regarding the importance of cowpea aphids, recommendations suggest that treatment thresholds for pea aphids be followed if these dark aphids are present in significant numbers. In most cases, fewer aphids are required to trigger concern about smaller alfalfa. 5

6 In past years, we have sometimes seen aphids develop to significant levels in Kansas when cold weather held back or killed the initial alfalfa growth, Higgins said. At low levels, aphids may not cause the crop significant harm, and they can serve as important food sources for beneficial insects that overwintered in the field before dispersing into other crops. Higgins encourages producers concerned about alfalfa weevils and aphids, to refer to recommendations in the K-State Research and Extension publication, Alfalfa Insect Management 2006 (MF809) for more specific recommendations and treatment options. The publication is available at or in print from most K- State Research and Extension county and district offices. -- Jim Shroyer, Extension Agronomy State Leader -- Jeff Whitworth, Extension Entomology 4. Plant analysis for wheat Plant analysis is an excellent "quality control" tool for wheat growers interested in high yield wheat management. There are two primary ways plant analysis can be used: as a routine monitoring tool to ensure nutrient levels are adequate, and as a diagnostic tool to help explain some of the variability in wheat growth we see in fields this time of year. Keep in mind however that any plant stress (drought, heat, frost, etc.) can have a serious impact on nutrient uptake and plant tissue nutrient concentrations. Sampling under stress conditions for monitoring purposes can give misleading results, and is not advisable. For monitoring purposes, flag leaves should be collected at random from the field at the boot to initial heading stage of growth. The leaves should be allowed to wilt over night to remove excess moisture, placed in a paper bag or mailing envelope, and shipped to a lab for analysis. Do not place the leaves in a plastic bag or other tightly sealed container, as they will begin to rot and decompose during transport, and the sample won't be usable. The data returned from the lab will be reported as the concentration of nutrient elements, or potentially toxic elements, in the plants. Most labs/agronomists compare plant nutrient concentrations to published sufficiency ranges. A sufficiency range is simply the range of concentrations normally found in healthy, productive plants during surveys. It can be thought of as the range of values optimum for plant growth. The medical profession uses a similar range of normal values to evaluate blood work. The sufficiency ranges change with plant age (generally being higher in young plants), vary between plant parts, and can differ between varieties or hybrids. So a value slightly below the sufficiency range does not always mean the plant is deficient in that nutrient, 6

7 but it is just an indication that the nutrient is relatively low. However, if that nutrient is significantly below the sufficiency range, then one should ask some serious questions about the availability and supply of that nutrient. Levels above sufficiency can also indicate problems. High values might indicate over fertilization and luxury consumption. Plants will also sometimes try to compensate for a shortage of one nutrient by loading up on another. This occurs at times with nutrients such as iron, zinc, and manganese. In some situations very high levels of a required nutrient can lead to toxicity. Manganese is an example of an essential nutrient which can be toxic when present in excess. Plant analysis is also an excellent diagnostic tool to help understand some of the variation seen in the field. When using plant analysis to diagnose field problems, producers should take comparison samples from both good/normal areas of the field, and problem spots. Collect soil samples from the same good and bad areas. Don't wait for boot to take diagnostic samples. Early in the season (prior to stem elongation) collect whole plants from different places in your sampling area. Later in the season take the uppermost, fully developed leaves (those with leaf collars visible). Handle the samples the same as those for monitoring. The following table gives broad sufficiency ranges for wheat early in the season, prior to jointing (Feekes 4-6), and later in the season at boot to early heading (Feekes 9-10). Keep in mind that these are the ranges normally found in healthy, productive wheat. Growth stage Nutrient Unit Whole plant at tillering-jointing Flag leaf at boot to heading Nitrogen % Phosphorus % Potassium % Calcium % Magnesium % Sulfur % Growth stage Nutrient Unit Tillering-jointing Boot Iron ppm Manganese ppm Zinc ppm Copper ppm Boron ppm Aluminum ppm <200 <200 7

8 Plant analysis is an excellent tool to monitor the effectiveness of your fertilizer and lime program, and a very effective diagnostic tool. Producers should consider adding this to their toolbox. -- Dave Mengel, Extension Soil Fertility Specialist dmengel@ksu.edu 5. Scattered freeze injury reported on wheat Late last week, April 19-20, there was some scattered freeze injury reported on wheat in some areas of northcentral Kansas as nighttime temperatures got down to as low as 28 degrees F. The wheat was in the boot stage, and symptoms so far consist of leaves that were burned brown. It is unknown yet if the heads sustained any damage. In general, wheat was hurt most severely where it had been no-tilled after soybeans. This wheat was late planted, drought-stressed, and stands were thinner than other wheat. Notill soils also radiate less heat at ground level, so temperatures in the plant canopy are often lower in early spring in no-till fields. We will monitor this situation, as well as any damage that may occur from freezes in the coming days, to see if there has been any head damage and loss of yield potential. Give me a call or send an if you suspect freeze injury to the wheat in your area. -- Jim Shroyer, Extension Agronomy State Leader jshroyer@ksu.edu These e-updates are a regular weekly item from K-State Extension Agronomy. All of the Research and Extension faculty in Agronomy will be involved as sources from time to time. If you have any questions or suggestions for topics you'd like to have us address in this weekly update, contact Jim Shroyer, Research and Extension Crop Production Specialist and State Extension Agronomy Leader jshroyer@ksu.edu 8