* The water content of the soil when rainfall starts Soils have a lower infiltration rate when they are wet than they are dry.

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1 Number 317 September 16, Factors that determine soil infiltration rate 1 2. Control strategy for marestail in soybeans 2 3. Will soybeans setting pods in late August or early September make a harvestable crop? 4 4. Comparative Vegetation Condition Report: August 30 September Factors that determine soil infiltration rate Infiltration is the name of the game when talking about capturing moisture for crops or preventing runoff-induced erosion. There are several factors that determine how fast a soil can absorb rainfall. * The length of time from the start of the rain event Infiltration is usually high at first, decreasing gradually, and eventually reaching a steady-state of slow infiltration as the soil profile fills with moisture. No-till soils usually have a higher infiltration rate at the start of the rain event. But at steady-state, their infiltration rate is often the same as tilled soils. Runoff begins when the precipitation rate exceeds the infiltration rate. * The water content of the soil when rainfall starts Soils have a lower infiltration rate when they are wet than they are dry. * The hydraulic conductivity of the soil Soil texture (the percent sand, silt, and clay) will affect infiltration, as will soil structure. Soils with well-defined structure, stable aggregates, more pores, and higher organic matter content are better able to conduct water through the soil. The structural characteristics are somewhat dependent upon tillage. Tillage breaks down the soil structure and decreases initial infiltration rates throughout the soil profile. Raindrop impacts also break down aggregates during a rain event. Soils that are not tilled gain some benefit from slightly higher levels of organic matter, but their real benefit is the much greater stability of aggregates. * The condition of the soil surface Large soil pores (macropores) such as old root channels or other cracks that extend from the surface well into the soil profile allow for moisture infiltration. Residue lying on the surface slows running water giving it more time to infiltrate, and protects the soil surface from the impact of falling raindrops. Partly buried residue that creates new flow paths into the soil can also aid infiltration. Each of these features are characteristic of no-till soils. 1

2 * The depth and layering of the soil profile When there are different types of soil structure, texture, and original parent material within the soil profile, that can affect the rate of infiltration. Tillage and heavy loads can also change the profile by creating either a subsurface plowpan or a surface crust that will inhibit water movement. Surface crusts can be broken up with freezing and thawing, but plowpans are not. Plowpans may persist in soils for many years and are very difficult to address once present. -- DeAnn Presley, Extension Specialist, Soil Management deann@ksu.edu 2. Control strategy for marestail in soybeans Marestail continues to be 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 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 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 2

3 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. Clarity herbicide has proven to be more effective than 2,4-D for control of marestail, but has more restrictive preplant limitations. However, it may be a good alternative as a fall or very early spring treatment in some areas. Clarity use as a preplant herbicide treatment ahead of soybeans is prohibited in areas that average less than 25 inches of rain per year. In areas with greater than 25 inches of rain, a waiting interval of at least 14 days is required following accumulation of at least 1 inch of rain or irrigation after application of Clarity at rates up to 8 oz per acre. Sharpen is a new herbicide that has provided very good control of maresetail. It can be applied anytime prior to soybean emergence and provides 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 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 or with Outlook in the product Verdict. 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. However, Ignite efficacy is often reduced under lower humidity. 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 3

4 3. Will soybeans setting pods in late August or early September make a harvestable crop? High temperatures and drought conditions have delayed pod set in many soybean fields this year. Where pod set was delayed until mid-september, yields will probably be severely reduced. If pod set occurred in late August, soybean fill and yield will likely be reduced for most locations. The figure below shows data from Kansas Agricultural Statistics indicating that pod set lagged the 5-year average by several days, starting in late July and extending through early September. Temperatures were extremely high during much of the time that beans were trying to set pods, perhaps explaining why pod set was delayed. Some beans have been delayed so much that they are just beginning to set pods in mid- September. This is particularly true for double-crop beans planted after wheat harvest. Will beans that set pods on or about September 15 still be able to produce a harvestable crop? It typically takes about 60 days from flowering to maturity to fill soybean seed of a typical size. Knowing that, we can then calculate how many days remain from September 15 until the average freeze date for a location. We can also use a typical seed filling rate to estimate how large the seeds might get given the time left before freeze. The table below summarizes some of those calculations for select Kansas locations based on information for soybean seed filling rate derived from a 1973 article in Agronomy Journal by J.S. Burris, and assuming a fully developed soybean seed size of roughly 2,500 seeds/pound when it fills in favorable conditions. It also assumes a constant seed filling rate throughout the seed filling period, which may not be entirely accurate but should be close enough for estimation purposes. 4

5 Estimates of the effect of average fall freeze date on seed filling period and predicted soybean seed size for selected Kansas locations. Days Available for Pod Fill if Pod Set Begins on Sept. 15 (days from Average Date of Fall Freeze Date City Fall Freeze 1-60 Days 2 Sept. 15 to Average Fall Freeze % of Full Seed Size 4 Date) 3 Tribune 7-Oct 8-Aug 22 6 Hays 10-Oct 11-Aug Topeka 11-Oct 12-Aug Manhattan 15-Oct 16-Aug Ottawa 17-Oct 18-Aug McPherson 19-Oct 20-Aug Salina 20-Oct 21-Aug Winfield 20-Oct 21-Aug Great Bend 21-Oct 22-Aug Parsons 23-Oct 24-Aug Chanute 24-Oct 25-Aug Wichita 25-Oct 26-Aug Yates Center 28-Oct 29-Aug No restriction to grain filling period: From Weather Data Library: 2 Date that would allow full seed fill before average freeze date. 3 Days available for seed fill with pod set on September Percent of full seed size for each seed filling period, based on daily seed fill rate from Burris (1973). Assuming a constant number of seeds per acre for a given field, the reductions in seed size presented in this table are likely to have a similar impact on soybean yield. Once pods have been set and seeds begin to fill, the number of seeds can only be reduced, so this is not a risky assumption. These estimates clearly indicate that soybean yield will be severely reduced with pod set as late as September 15. Even if pod set occurred in late August, bean fill and yield will likely be reduced for most locations. The estimates in this table assume favorable weather conditions and soil moisture. Many locations have been drought stressed since mid-july. Also, as we move seed fill later into the fall, temperatures will fall below the 75 to 85 degree F optimum range. Both of these factors will likely slow the seed fill rate, further reducing seed size for a given seed fill period. Ironically, the seed fill rate may have been slowed early in the seed fill period by excessively high temperatures. -- Kraig Roozeboom, Crop Production and Cropping Systems Specialist kraig@ksu.edu 5

6 4. Comparative Vegetation Condition Report: August 30 September 12 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. Two short videos of Dr. Kevin Price explaining the development of these maps can be viewed on YouTube at: The objective of these reports is to provide users with a means of assessing the relative condition of crops and grassland. The maps can be used to assess current plant growth rates, as well as comparisons to the previous year and relative to the 21-year average. The report is used by individual farmers and ranchers, the commodities market, and political leaders for assessing factors such as production potential and drought impact across their state. The maps below show the current vegetation conditions in Kansas, the Corn Belt, and the continental U.S, with comments from Mary Knapp, state climatologist: 6

7 Map 1. The Vegetation Condition Report for Kansas for August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that photosynthetic activity has begun to decline as vegetation reaches maturity. The northeastern area of the state continues to have the highest level of photosynthetic activity, as adequate soil moisture has favored plant development in this region. Decreased photosynthetic activity is beginning to spread northward into northwestern Kansas. 7

8 Map 2. Compared to the previous year at this time for Kansas, the current Vegetation Condition Report for August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that the northern counties are experiencing a greater level of photosynthetic activity this year. In particular, Norton and Graham counties are experiencing a greater level of photosynthetic activity than last year. Southeast Kansas biomass condition is much below that of last year, particularly in the Flint Hills region. 8

9 Map 3. Compared to the 22-year average at this time for Kansas, this year s Vegetation Condition Report for August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that the North Central and the Northeastern divisions continue to experience above-average photosynthetic activity. The reduced photosynthetic activity in southwest Kansas is also being experienced now in the southeast division. In the southwest and southeast divisions, soil moisture is reported 99 percent short to very short, while in the southwest subsurface soil moisture is reported at 100 percent short to very short. 9

10 Map 4. The Vegetation Condition Report for the Corn Belt for August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that the highest photosynthetic activity is occurring from northeastern Nebraska through the Upper Peninsula of Michigan. Another area of high photosynthetic activity can be seen in south central Missouri. In contrast, photosynthetic activity has begun to decline in Illinois, Indiana, and Ohio, as well as the western areas of the High Plains. In the Ohio River Valley, early high temperatures hastened crop maturity. The cooler weather of the last week was too late to provide much relief. 10

11 Map 5. The comparison to last year in the Corn Belt for the period August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that biomass production is similar. A higher level of photosynthetic activity continues to be experienced in the Northern Plains through northeastern Iowa and northern Illinois. 11

12 Map 6. Compared to the 22-year average at this time for the Corn Belt, this year s Vegetation Condition Report for August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that greater-than-average photosynthetic activity continues to dominate the Northern High Plains. Also of note is higher photosynthetic activity across Central Missouri. While southwestern Missouri continues to experience moderate drought conditions, the central portion of the state has experienced more favorable conditions. Cooler weather has slowed pasture growth. 12

13 Map 7. The Vegetation Condition Report for the U.S. for August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that there is a general east/west divide in photosynthetic activity. Despite warmer-than-normal conditions in the Pacific Northwest, the area continues to see a high level of photosynthetic activity. The extremely low photosynthetic activity that has plagued the Southwest has moved northward along the eastern edge of the Rockies. Low photosynthetic activity can be seen in the Nebraska Panhandle and northward into western North and South Dakota. 13

14 Map 8. The U.S. comparison to last year at this time for the period August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that even though photosynthetic activity is low in the western High Plains, it is above last year s level. Photosynthetic activity in the Ohio River Valley, on the other hand, is much below last year. As flood waters retreat along the Mississippi River Valley, photosynthetic activity shows greater activity than last year. 14

15 Map 9. The U.S. comparison to the 22-year average for the period August 30 September 12 from K-State s Ecology and Agriculture Spatial Analysis Laboratory shows that areas of Arkansas, Louisiana, and Mississippi have benefitted from the rains of the various tropical systems and have a higher level photosynthetic activity than average. The Southern Plains continues the pattern of below-average productivity. As we move into fall, this lack of productivity will begin to be masked by the normal dormancy of Fall and Winter. Note to readers: The maps above represent a subset of the maps available from the EASAL group. If you d like digital copies of the entire map series please contact us at kpprice@ksu.edu and we can place you on our list to receive the entire dataset each week as they are produced. The maps are normally first available on Wednesday of each week, unless there is a delay in the posting of the data by EROS Data Center where we obtain the raw data used to make the maps. These maps are provided for free as a service of the Department of Agronomy and K-State Research and Extension. -- 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 15