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Spider mites are more troublesome during dry weather and we've observed infestations on soybeans. Dr. Ames Herbert will address this issue and practices to manage the pest.
Due to the spotty double-crop stands being experienced; I'll also review some guidelines to help determining whether replanting is a profitable option.
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Lack of soil moisture is the single most important factor affecting soybean yields in Virginia. Soil moisture reserves are usually low following small grain and this year, we may be completely dependent on in-season rainfall for good double-crop yields due to lack of rainfall. Below is a review of the stages in which soil moisture is most critical for yield and some management strategies to help with a drought-stressed crop.
The two periods that soil moisture is most critical for soybean are 1) germination and emergence, and 2) bloom through seed fill. The time from stand establishment through bloom is less important. Although I have stressed the need for the soybean crop to develop adequate leaf area index (LAI), I have seen 40+ bu/A double-crop soybean yields when LAI was low (<3.0 sq. feet of leaves per sq. foot of ground area) if rains came at the right time. On the other hand, I believe these 40 bu/A yields could have been 60 bu/a with adequate canopy development (LAI > 4.0). Unfortunately, much of our full-season soybean crop has not developed adequate LAI and the crop is entering the critical reproductive stages. Unless substantial rain is received within the next 2 weeks, I would expect low yields from full-season soybeans.
The biggest problem with our double-crop planting seems to be stand. Many of us took a chance and planted the crop immediately following small grain harvest under conditions of low soil moisture. In some cases, the crop successfully emerged; in others, stand was very poor and erratic. My main concern with planting into dry soils is the areas of the field where there is just enough moisture to swell and germinate the seed, but not enough to sustain a plant. I generally recommend waiting for a rainfall before planting unless one has very large acreage to plant and its getting late in the season. But, Iım not totally against planting into completely dry soils and waiting on a rain. Note that I said completely dry soils. Soybean seed will wait on a rain for a couple of weeks without losing a lot of viability, but only is the seed is dry.
Obtaining adequate vegetative growth is more important for double-crop plantings; therefore, irrigation (if available) to double-crop soybean during the vegetative stages during drought conditions can be justified. On the following page is the estimated crop water use for soybean with respect to development stage. As one can make out from this chart, the soybean crop will use between 1 to 2 inches of water per week during the reproductive stages. I have heard many times that the crop needs 1 inch of rainfall per week to make maximum potential yields. This can be true in an "average" year since we generally have to soil moisture reserves to make up any deficit when the crop is using 1.5 to 2.0 inches per week. However, 2002 is starting out as a below-average year.
Management Strategies: Many of the management strategies listed below are not necessarily applicable at this time of the year but can serve as a reference for future use. Others can be implemented after stand establishment. Drought will enhance known and unknown problems; therefore a benefit of dry conditions may be discovery of yield-reducing factors in the field.
Soil Fertility. Drought stress will show up first in fields in which nutrients are low and pH is not optimal. A pH of less than 5.5 will show drought stress early due to the high concentration of aluminum, which is toxic to the plant. Low pH results in poor root growth that further restricts exploration to stored soil moisture. On the other hand, high pH (> 6.5) will also show stress early largely due to micronutrient deficiencies, mainly manganese (Mn). The micronutrient deficiency basically is an additive effect to the drought stress. Low levels of other nutrients will also restrict growth.
Tillage/Planting. Soil surface residues will help reduce evaporation, so no-tillage should be the rule on droughty soils or during drought periods in order to conserve soil moisture. No-till land can be driven on before conventional tillage land after rain, but that doesnıt mean that the soil can adequately support equipment without compacting the soil. A general rule of thumb is to wait 1/2 to 1 day after the time you believe you can drive the field. Compacted soils and those with tillage pans will be the first to show drought symptoms. Use the dry weather to identify areas in the field with compaction. To be sure that compaction is the culprit, dig down to a dept of 12-18 inches with a spade immediately adjacent to the soybean row. Observe the root growth. If roots are growing sideways, take a knife and push it into the soil immediately below the horizontal roots, comparing this resistance with the rest of the profile. By doing this, you can observe where in the field compaction is prevalent, and also the depth and thickness of the hardpan. Then map out the areas of the field (an ATV and GPS unit is very valuable for this use) where the compaction is occurring using visual, aboveground crop symptoms (short plants, wilting, etc.). Deep tillage with a minimum-tillage subsoiler (Paratill, DMI Ecolotill, Worksaver Terra-Max, others) may be in order next fall on soils with tillage or naturally forming hardpan. By knowing where in the field that the hardpan is occurring will prevent un-needed and costly deep tillage.
Variety Selection. Planting soybean varieties of two or three different maturity groups (MG) can spread the risk of drought. The most critical stages of a MG V variety will be approximately 5 to 10 days later than a MG IV variety. By planting both, one may spread the critical period of development over a longer period.
Row Spacing / Plant Population. Narrow row spacing will tolerate drought better due to more rapid canopy formation, preventing evaporation losses, and a more uniform removal of soil moisture. Our research also indicates that higher plant populations are more appropriate on droughty soils (see Soybean Update Vol. 5, No.2). It is true that higher populations will deplete soil moisture faster, but the soybean crop will produce more leaf area and compensate by adjusting its pod load per plant to an optimal level. Drought conditions are excellent for mapping soil types that differ in plant-available water-holding capacity via aboveground stress symptoms. Such maps could be used for future variable-rate seeding.
Pest Management. Weed competition can be most severe in times of drought. Do not let weeds utilize limited soil moisture. Apply herbicides before weeds reach the maximum-labeled stage for control. Use adequate rates to get complete control. Be aware that postemergence herbicides will not work as well during times of drought; therefore, do not cut rates. Preemergence herbicides need rain for activation. In no-till fields, apply a non-selective herbicide immediately after planting to kill existing weeds. If drought conditions make stand establishment uncertain, a postemergence program may be the most risk-free.
Insect and nematode problems will enhance drought effects. Drought stress will likely show up early in nematode infested fields. In some cases, light to moderate nematode infestations may only be noticed in a drought year. Once again, use this opportunity to map out the infestations in order to take appropriate future action. If the crop is small (LAI < 4.0), defoliating insects will be more likely to cause yield declines. Therefore, do not risk additional lost to these plants. Spider mites, thrips, and leafhoppers may also be more problematic in drought-stressed soybean.
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So, what do you do? First, realize that the plants growing in the field are going to be higher yielding that any that emerges after replanting. If you destroy those plants, you've just writing off some profit. So, do you leave those in the field, and plant through them? In many cases, this is the best choice. If you're using wider rows, then split the rows with the new plants. But, remember, a drill will cut up many of these healthy plants and make them less productive than the ones that you just replanted. I would suggest planting through the old stand with a drill only if you've suffered well over 50% loss. Change to an earlier maturing variety so that harvest maturity will match. A general rule of thumb is that a 5-day delay in planting (after mid-June) will result in a 1-day delay in maturity. Therefore, if planting 1 month late, chose variety with a relative maturity of about 1/2 maturity group earlier. If you originally planted a variety with a relative maturity of 5.0, then replant with a 4.5 if replanting 1 month later. This will bring the harvest maturity of the old and new plants into sync. Also realize that you'll need a higher plant population when planting late. Your final stand with a July planting should be 200,000 to 220,000 plants/acre.
But, how do you decide whether or not to replant? Below are some guidelines that incorporate plant population with gaps in the field. We've found that large gaps (3 feet or greater) causes as much or more yield loss that from low plant populations.
Table 1. Plant populations of different row spacing with different plant counts per foot.
| Row Spacing | |||||
|---|---|---|---|---|---|
| Plants/foot | 30 | 24 | 20 | 15 | 7.5 |
| -------------------- Plant Population (1,000's/acre) --------------- | |||||
| 1 | 17 | 22 | 26 | 35 | 70 |
| 2 | 35 | 44 | 52 | 70 | 140 |
| 3 | 52 | 65 | 78 | 105 | 210 |
| 4 | 70 | 87 | 105 | 139 | 278 |
| 5 | 87 | 109 | 131 | 174 | --- |
| 6 | 105 | 131 | 157 | 209 | --- |
| 7 | 122 | 152 | 183 | 244 | --- |
Table 2. Hula-hoop method for determining drilled soybean populations.
| No. of Plants | Inside Diameter of Hula Hoop | ||||
|---|---|---|---|---|---|
| 30" | 32" | 34" | 36" | 38" | |
| (Plants in 1,000's per acre)* | |||||
| 6 | 53 | 47 | 41 | 37 | 33 |
| 10 | 89 | 78 | 69 | 62 | 55 |
| 14 | 124 | 109 | 97 | 86 | 77 |
| 18 | 160 | 140 | 124 | 111 | 100 |
| 22 | 196 | 172 | 152 | 136 | 122 |
| 26 | 231 | 203 | 179 | 160 | 144 |
| * Plants/acre = no. plants ÷ (3.14 * r2 ÷ 43,560 ft2) where r = radius of hula hoop in feet. | |||||
Table 3. Yield response (% of maximum) of double-crop soybeans to deficit stands.
| % Stand lost to gaps | Remaining Plant Pop (1,000's/A) | |||
|---|---|---|---|---|
| 220 | 180 | 140 | 100 | |
| 0 | 100 | 93 | 88 | 74 |
| 10 | 96 | 89 | 84 | 70 |
| 20 | 91 | 85 | 80 | 67 |
| 30 | 85 | 79 | 75 | 62 |
| 40 | 78 | 73 | 69 | 58 |
| 50 | 75 | 70 | 65 | 55 |
| 60 | 71 | 66 | 62 | 53 |
| *Data from 15-inch row spacing. | ||||
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Twospotted spider mite has become a chronic pest of soybean in Virginia. This pest is associated with hot, dry weather, conditions that have certainly prevailed during the past several growing seasons.
Why do mites do best under these conditions? They simply lay more eggs, survive better, develop at a faster rate, and live longer all of which results in faster growing and larger populations. Drought stressed plants are preferred by mites because the water content is lower, and the nutrient levels higher, in the plant juices they feed on. Soybean is also more attractive to mites when in the flowering and pod development stages rather than the vegetative stages. However, we are seeing vegetative stage beans under attack. Dry weather also favors mite populations because these conditions tend to suppress the development of a particular mite fungal disease that, under moister conditions, can devastate populations. It is also common to see more mite activity in fields where soils are sandier, because those soils lose moisture more quickly creating droughty conditions for plants in those areas.
Mite populations may also be high this year because of a series of relatively mild winters. Mites overwinter on non-crop and weedy hosts such as grassy waterways, roadsides, set-aside acres, and pastures. As temperatures warm in the spring, they feed and begin migrating to corn and other hosts, and then to soybean. Warmer winters mean larger winter weed host populations and better mite survival.
Mite infestations often occur first on field edges, as they move away from field-edge weeds that have either dried down, been mowed, or burned down with herbicides. In these cases, edge-treating fields with acaricides (mite killing pesticides) may prevent further movement into the field. This is a good approach in many cases and can save a lot of money and product. When treating, treatment should overlap considerably (2 spray boom widths) into apparently healthy beans adjacent to the damaged areas. This will remove those mites that you may not have seen, and lay down a protective barrier between the infested and clean areas, thus slowing spread.
However, when mite-favoring conditions continue (hot, dry, windy), infestations may show up both on field edges and in spots throughout the field. Mites that have 'ballooned' into the field create these'hot' spots. When mites 'want' to leave an old food source that is no longer palatable, or when populations get high, they will climb to the highest point and send out a tiny silk strand that acts as a balloon. Wind will carry the mites via these balloons to a place in a field where they 'seed' out onto new host material. If fields have several of these hot spots, treatment of the entire field is warranted.
Mites injure soybean by feeding on the green leaves. They have needle-like mouthparts that are used to puncture individual leaf tissue cells and consume the entire content. The presence of numerous damaged cells results in the yellow/white stippled appearance. These mite feeding symptoms are usually first noticed at the base of the leaf. If populations get larger on that leaf and feeding continues, leaves will turn yellow, brown on the leaf margins, eventually die and drop off the plant. Extensive damage can kill plants.
Treatment thresholds are somewhat vague but generally, if 50% of the plants in a field are showing mite-feeding symptoms, as described above, on 1/3 to 2/3 of the leaf surface, a field treatment is warranted. Of course the onset of timely rainfall or cooler conditions can help plants recover.
We recommend only two product options for controlling mites in soybean: Dimethoate (either 4EC, or 267EC) or Lorsban 4E. Although Karate Z is listed as an option, it is listed as providing suppression only, which in most cases is not sufficient to control active mite infestations. Dimethoate 4E can be applied at 1 pt/acre, Dimethoate 267 at 1.5 pt/acre, and Lorsban 4E at 0.5 - 1 pt/acre.
It is critical when making mite treatments to apply so that the materials wet the soybean leaves and stems. We call it the 'Three C's' of mite control - 'Coverage, Coverage, and Coverage'. High spray volume, around 20 gallon/acre, and low pressure are desired.
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The theme for this year is "Systems for Success." This year attendees have a special educational opportunity because we will be featuring the Regional Cropping Systems project that has been coordinated by Dr. Mark Alley of Virginia Tech the past five years at Camden. The cropping systems project is supported by United Soybean Board, the Foundation for Agronomic Research and USDA.
The Regional Cropping Systems project is a multi-state project with the main research site at Camden. Cropping systems evaluated include: (3 crops/2 years) no-till corn, conventional till wheat - no-till double crop soybeans; (4 crops/3 years) no-till corn, no-till full season soybeans, no-till wheat - no-till double crop soybeans; and (4 crops/2 years), no-till barley - no-till double crop corn, no-till wheat - no-till double crop soybeans. The above cropping systems have been conducted on the same 3-acre strips with three replications of each treatment for five years. Each cropping strip includes both highly productive and extremely sandy soils. Modern crop production technologies including variable rate seeding of corn and soybeans, variable rate nitrogen on corn and yield mapping at harvest were incorporated into these studies.
Dr. Mark Alley, W. G. Wysor Professor of Crop and Soil Management at Virginia Tech, will discuss yield results over the past five years for wheat, barley, corn, and soybeans for each cropping system, soil differences, cropping systems differences and how to apply what has been learned to other farms in the region. Dr. Alley will focus his comments on corn and wheat. Dr. David Holshouser, leader of the soybean aspects of the cropping systems project, will discuss the influence of the above cropping systems on soybean yields. Conclusions of five years work are quite different on the highly productive compared to the sandy droughty soil. Mr. Ron Mulford, Crop Management Specialist from the University of Maryland and Regional Project Cooperator from the University of Maryland, will discuss results from tillage research on Marylandıs Eastern Shore.
Dr. Bobby Grisso, Extension Agricultural Engineer at Virginia Tech, will discuss the equipment technologies used in the cropping systems study including variable rate seeding, variable rate fertilizer application, and light bar navigation systems. The light bar navigation system will include "hands on" experience.
Interest in corn seed treatments as an addition/substitution for at-planting insecticides is increasing as new seed treatment products become available. Dr. Rod Youngman, Extension Entomologist, will discuss plots at Camden evaluating corn seed treatments for control of soil and above ground insect pests. Corn hybrids from 13 seed companies will be available for individual tours, as will soybean varieties from 11 companies. Replicated large plots of corn were seeded with a Great Plains precision drill comparing 30 inch rows with twin rows planted on 30 inch centers. A large block of 15" corn was also planted with the Great Plains drill for general observation. Ms. Liz Rucker, Extension Crop Testing Research Associate and Mr. Harry Behl, Research Technician both of whom work with Dr. Dan Brann, Extension Grains Specialist, will be on hand to answer questions about the corn plots.
Additional attractions in the morning include over 70 exhibitors including many with large equipment. In the afternoon there will be an informal tour of the new irrigation systems installed at Camden covering about 400 acres with four pivots.
General field tours will begin at 8 a.m. with cropping systems research tours beginning at 9 a.m. The last tour will be leaving at 11:30 a.m. Field tours include three 30 minute presentations so come early if you want to experience this excellent learning experience regarding cropping systems as well as visit with exhibitors. Dr. Dan Brann, Extension Grains Specialist, will be on hand to explain the field tour detail plans to attendees. Lunch will be available to the first 1,000 people who pay the $6 registration fee. Camden Farm is located 2.8 miles south of Port Royal on Route 17. Signs will be posted the day of the event. Pre-registration is not necessary. Anyone desiring to be an exhibitor or needing additional information regarding the field day should contact Mr. Bill Snyder at 800/511-0671.
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Sincerely,
David L. Holshouser
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