Number 266 October 8, 2010

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1 Number 266 October 8, Control strategy for marestail in soybeans 1 2. Wheat planting decisions under dry conditions 3 3. Evaluating wheat stands 4 4. Comparative Vegetation Condition Report: September 20 October Control strategy for marestail in soybeans Marestail has become one of the biggest problem weeds on many soybean fields in Kansas. It is a little easier to control in corn since it is still susceptible to atrazine, dicamba, and other herbicides that can be used in corn. Marestail has historically been considered a winter annual weed, but can also germinate in the spring or summer and act as a summer annual. In fact, marestail appears to be shifting to more spring and summer germination in Kansas. Individual plants can produce an abundance of tiny seed that can be easily dispersed by wind. Seed can germinate soon after it is produced, but also can remain viable in the soil for several years, making it a hard weed to control with crop rotation. In addition to those problems, many populations of marestail in Kansas now appear to have some level of glyphosate resistance, while some populations may also be resistant to ALSinhibiting herbicides. Herbicide effectiveness on marestail depends largely on the stage of growth and size of plants. Marestail generally is most susceptible to herbicides when it is small and still in the rosette stage of growth. Once marestail starts to bolt and exceed 4 to 6 inches tall, it becomes very difficult to kill with most herbicides. Since marestail can germinate throughout much of the year, a single herbicide application probably will not provide season-long control, particularly in no-till. In soybeans, marestail control should begin with a burndown application of 2,4-D while the marestail is in the rosette stage which may be in the fall or any time in the spring and early summer. A tankmix of 2,4-D and glyphosate can burn down a broad spectrum of annual broadleaf and grass weeds. Fall applications can be effective even into December as long as applications are made to actively growing weeds during a stretch of mild temperatures. In fact, for fall applications, it may be better to wait until November to allow most of the fall-germinating winter annuals to emerge. A residual herbicide such as the Valor or Classic (unless ALS resistant) containing 1

2 products can be added to help control marestail through winter and early spring, but don t expect a residual herbicide applied in the fall to provide residual marestail control through the spring and summer of the next year. If a fall treatment isn t applied, early spring treatments in March to early April should be applied to help control the fall-germinating marestail. For marestail that germinates in the spring or summer, 2,4-D is generally very beneficial for early-season control, but its use is limited as planting time approaches. A waiting period of 7 days is required after application of up to 1 pt/a 2,4-D LV4; 15 days for up to 1 pt/a 2,4-D amine; and 30 days between application and planting of soybeans for rates greater than 1 pt/a for either ester or amine 2,4-D products. Sharpen is a new herbicide that generally has provided very good control of maresetail. It can be applied anytime prior to soybean emergence and will provide a short period of residual control. Sharpen works best if applied with methylated seed oil and in combination with 2,4-D or glyphosate. Because Sharpen is a contact herbicide, using higher spray volumes (15-20 gal/a) will help increase herbicide coverage on the plants. Sharpen works very fast and quickly desiccates marestail foliage, but larger bolted marestail can sometimes regrow from axillary buds one to two weeks after treatment. Sharpen can not be applied postemergence to soybeans. Sharpen is also available as a premix with Pursuit in the product OpTill. In addition to a burndown application made in fall or early spring, most fields will benefit from use of residual herbicides that include a Valor, Classic, or FirstRate component in the spring, along with another dose of a burndown herbicide if needed. The use of a residual preplant or preemergence herbicide at planting time, tankmixed with a burndown herbicide, will help provide additional control of marestail, as well provide early-season weed control and help manage or prevent the development of other glyphosate resistant weeds such as waterhemp, ragweed, Palmer amaranth, or kochia. If marestail are not controlled in fall or early spring and have started to bolt before they are treated, Ignite herbicide has proven to be one of the best treatments for control of larger bolted marestail. Ignite can be used as a burndown treatment prior to emergence of any type of soybeans, or as a postemergence treatment in Liberty Link soybeans. Postemergence control of large marestail in soybeans can be very difficult, especially if the marestail is glyphosate resistant. FirstRate, Classic, and Synchrony herbicides are probably the best postemergence options, unless marestail is also ALS-resistant. The combination of these herbicides with glyphosate on Roundup Ready soybeans seems to work best, even on glyphosate-resistant marestail. Glyphosate-resistant marestail and other glyphosate resistant weeds have developed due to overreliance on glyphosate for weed control. Integration of other herbicides into the weed control program and proper timing of herbicide applications is a key factor to help manage and prevent the development of glyphosate-resistant weeds. -- Dallas Peterson, Weed Management Specialist dpeterso@ksu.edu -- Doug Shoup, Southeast Area Crops and Soils Specialist dshoup@ksu.edu 2

3 2. Wheat planting decisions under dry conditions Soils are generally very dry in much of the western half of Kansas. Producers who have not yet planted their wheat basically have three main options: * Plant into dry soil ( dust it in ) now at the normal seeding depth and hope for rain. This is probably the best option. The seed will remain viable in the soil until it gets enough moisture. Before planting, producers should look at the long-term forecast and try to estimate how long the dry conditions will persist. If it looks like there s a good chance the dry weather will continue until at least the back end of the optimum range of planting dates, producers should treat the fields as if they were planting later than the optimum time. Rather than cutting back on seeding rates and fertilizer to save money on a lost cause, producers should increase seeding rates, consider using a fungicide seed treatment, and consider using a starter fertilizer. The idea is to make sure the wheat gets off to a good start and will have enough heads to have good yield potential, assuming it will eventually rain and the crop will emerge late. Wheat that emerges in November almost always has fewer fall tillers than wheat that emerges in September or October. There are some risks to this option. For one thing, a hard rain could crust over the soil or wash soil off planting ridges and into the seed furrows, potentially causing emergence problems. Another factor is the potential for wind erosion is the field lies unprotected with no ridges. Also, the wheat may not come up until spring, in which case it may have been better not to plant the wheat at all and plant a spring crop instead. Probably the worst-case scenario for this option would be if a light rain occurs and the seed gets just enough moisture to germinate the seed but not enough for the seedlings to emerge through the soil or to survive very long if dry conditions return. This could result in a loss of the stand. * Plant deeper-than-normal into moisture now, if possible. This option can work if the variety to be planted has a long coleoptile (it s crucial to know the coleoptile length of the variety you re using when planting deep to moisture), the producer is using a hoe drill, and there is good moisture within reach. The advantage of this option is that the crop should come up and make a stand during the optimum time in the fall. This would keep the soil from blowing. The main risk of this option is poor emergence. Deep-planted wheat normally has below-normal emergence, so a higher seeding rate should be used. Any rain that occurs before the seedlings have emerged could add additional soil into the seed furrow, making it even harder for the coleoptile to reach the soil surface. Any time you increase the seeding depth, the seedling will have to stay within the soil just that much longer before emerging through the soil surface. Delayed emergence leads to more potential for disease and pest problems, and reduced tillering potential late in the season. It s even possible that the wheat would get planted so deep that it would germinate but never emerge at all, especially if the coleoptile length is too short for the depth of planting. * Wait for a rain, and then plant. To overcome the risk of crusting or stand failure, producers may decide to wait until it has rained and soil moisture conditions are adequate before planting. Under the right conditions, this would result in good stands, assuming the producer uses a high seeding rate and a starter fertilizer, if appropriate. If it remains dry well past the optimum range of planting dates, the producer would then have the option of just keeping the wheat seed in the shed until next fall and planting spring crop next year instead. 3

4 The risk of this option is that the weather may turn rainy and stay wet later this fall, preventing the producer from planting the wheat at all while those who dusted their wheat in have a good stand. There is also the risk of leaving the soil unprotected from the wind through the winter until the spring crop is planted. Crop insurance considerations and deadlines will play a role in these decisions. -- Jim Shroyer, Extension Agronomy State Leader jshroyer@ksu.edu 3. Evaluating wheat stands Where wheat emergence is uneven or poor, producers will have to decide whether to replant. This decision is not always clear-cut, but some guidelines can help with the decision. Is the stand count below normal, and if so, by how much? The first step is to determine what is normal. If a producer uses a drill with 12-inch row spacings, plants at a 60-pounds-per-acre seeding rate with a variety that has 15,000 seeds per pound, and expects a germination and emergence rate of 75 to 80 percent, there should be 675,000 to 720,000 plants per acre. This amounts to about 15.5 to 16.5 plants per foot of row. If a grower planted 60 pounds of seed per acre using 7.5-inch rows, and a germination rate of 75 to 80 percent, that would be about 9 plants per foot or row. The next step is to determine the average number of plants per foot of row that is present by taking numerous plant counts across the field. This assumes the stand is more or less uniform throughout the field, with no large gaps. Generally, if the average number of plants is about 50 percent or more of normal, the recommendation is to keep the stand. With less than 40 percent of normal, the recommendation is to replant the field. With a stand that is between 40 and 50 percent of normal, the decision is more difficult. There are two major concerns to consider other than yield potential in deciding whether to replant: the susceptibility of the ground to wind erosion and the potential for weed and grass infestations. Where stands are less than 40 percent of normal, these become major concerns, even if yield potential is not a concern. In fact, research in western Kansas indicates that 260,000 to 320,000 plants per acre (or about six to seven plants per foot of row) can produce within 90 percent of expected yields especially if the plants are able to tiller well and the stand is uniform. But if the soil is blowing or weeds and grass infestations become severe, the stand should probably have been replanted and thickened. Where the stand is 30 to 40 percent of normal, the yield potential will have been reduced enough that replanting will usually pay off. If possible, replanting should be done at a 45 degree angle to the original stand to minimize damage to the existing stand. 4

5 Until the end of October, producers could cross-drill at the rate of pounds per acre in western Kansas and pounds per acre in central and eastern Kansas, using a double-disc opener drill if at all possible to minimize damage to the existing stand. If the replanting is done in November or later, increase the seeding rates to pounds per acre in western Kansas and pounds per acre in central Kansas. If stands are less than 30 percent of normal, increase the seeding rates just mentioned by pounds per acre. If a hoe drill is used, the seeding rate should be higher than if a disc drill is used because the hoe drill will destroy much of the original stand. With a hoe drill, add about pounds per acre to the seeding rates mentioned above. Where there was no emergence in all or parts of the field, producers would have to use a slightly higher seeding rate than used initially to 90 pounds per acre in western Kansas and 100 to 120 pounds in eastern and central Kansas, using the higher end of those ranges when planting in November or later. Seeding rates on non-irrigated fields should not be higher than 90 lbs/acre in western Kansas or 120 lbs/acre in central and eastern Kansas. Under irrigation, seeding rates should never be higher than 150 lbs/acre. Before replanting, producers should first dig through the soil crust to determine why the seed did not emerge. The most common emergence problems are dry soils, crusting, poor quality seed, seedling rot diseases, and wireworms. If dry soils are the cause of the problem, which is the most likely situation this year, replanting will do no good unless the seed has partially germinated and stalled out before emerging. If the seed are still hard and viable, or have a very short coleoptile emerging from the seed, the best advice is to leave the field alone and wait for rain. Where crusting has occurred, producers should determine whether the seeds or seedlings are still viable or the coleoptiles have become bent or crinkled due to the crusting. Sometimes a light rain on crusted soil will soften the crust so seedlings can emerge. Otherwise, a rotary hoe will break up the crust, allowing them to emerge. If there has been adequate moisture and no crusting, but little or no emergence, poor quality seed, seedling rot diseases, or soil insects are possible causes of the problem. In this case, the field will need to be replanted with good quality, treated seed. -- Jim Shroyer, Extension Agronomy State Leader jshroyer@ksu.edu 4. Comparative Vegetation Condition Report: September 20 October 5 K-State s Ecology and Agriculture Spatial Analysis Laboratory (EASAL) produces weekly Vegetation Condition Report maps. These maps can be a valuable tool for making crop selection and marketing decisions. The most recent VCR maps from EASAL are below: 5

6 Map 1. The Vegetation Condition Report for September 20 October 5, from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that while the last two weeks have been drier than normal, the vegetative condition continues to be greener than normal in the eastern half of the state. Areas in western Kansas show less greenness than the long-term average, as winter wheat is slower to emerge. The latest crop condition report from Kansas Agricultural Statistics shows only 12 percent of the wheat emerged compared to a fiveyear average of 23 percent. 6

7 Map 2. The U.S. Corn Belt comparison to the 21-year average shows the drier-than-normal conditions continuing in much of the center of the Corn Belt. The area from southern Michigan through Illinois, Indiana, Ohio, and Kentucky saw less than 75 percent of their normal precipitation in September. 7

8 Map 3. During this period compared to the 21-year average, the Vegetative Condition Comparison shows the lower-than-normal vegetative production extending from the central Corn Belt down the Ohio River and Upper Mississippi Valley region. In contrast, the upper Great Plains and parts of Texas are much greener than usual. We thought it might be interesting to add a map this week that compares this year s vegetation condition to last year s (in addition to comparing this year to the 21-year average). You may recall that the fall weather last year was so wet that much of the corn and soybeans were still in the field at this point in the season. This year, the weather has been so dry in most areas that row crop harvest has been unusually early. As a result, there is less green area showing up on the map this year than last year for this time period. 8

9 Map 4. During this period compared to last year at this time, the Vegetative Condition Comparison shows the Eastern half of the country is a lot less green this year than last. The hot, dry summer took a particular toll in the New England region. Exceptions can be seen along the New York/Pennsylvania border, and the western portion of Upper Michigan. -- Mary Knapp, State Climatologist mknapp@ksu.edu -- Kevin Price, Agronomy and Geography, Remote Sensing, Natural Resources, GIS kpprice@ksu.edu -- Nan An, Graduate Research Assistant, Ecology & Agriculture Spatial Analysis Laboratory (EASAL) nanan@ksu.edu These e-updates are a regular weekly item from K-State Extension Agronomy and Steve Watson, Agronomy e- Update Editor. 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 Steve Watson, swatson@ksu.edu, or Jim Shroyer, Research and Extension Crop Production Specialist and State Extension Agronomy Leader jshroyer@ksu.edu 9