VIRGINIA SOYBEAN UPDATE

Index

• Virginia Soybean Crop Condition
• Production Outlook
• Corn Earworm/Spider Mite Update
• 1998 CEW Blacklight Trap Data
• Corn Earworm Thresholds in Soybeans
• Estimating Soybean Yields Prior to Harvest

Virginia Soybean Crop Condition

As I write this article, most of Virginia received much needed rain over the past couple of days ranging from 0.25 to 2.0 inches. This rainfall may have salvaged what was shaping up to be a rather disastrous full-season soybean crop. In many areas, the crop is generally shorter than normal, but that is not my main concern. There is still enough growth to maximize our yield potential (full-season soybeans usually produce more foliage than is needed in an average year). In many areas, growth is very good, the crop is producing an abundance of pods, and it looked as if yield potential is high. But, the recent drought could not have come at a worse time. Our earlier maturity groups had begun setting seed, only to not have sufficient soil moisture to do so. Therefore, pods on the plant are "flat" or only contained one seed. Later maturity groups are setting pods, but aborting many of them a few days later. However, the recent rains should help minimize the damage. But, more rain is needed. During pod and seed fill, the soybean crop can use up to 0.3 inches of water daily (seed figure below). Therefore, an inch of rain may only last 3 to 4 days.

Our double-crop plantings look worse than the full-season crop, but I must remind you that the most critical stages of development for this crop will not arrive for a another couple of weeks; therefore, rainfall in late August and in September can result in a good crop. Hopefully, Mother Nature will be kinder to us in the second half of the growing season than the first.

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Production Outlook

Virginia farmers not only have had the recent drought to contend with, but also low soybean prices (less than $5.75 for new crop soybeans). The low prices are generally due to good growing conditions in parts of the Midwest and due to the Asian financial crisis that is reducing export potential. Any potential for price improvements? This will probably solely depend on the probability of early frost or freeze damage to the Midwestern crop. Due to wet fields at planting, much of the Midwest crop was planted late which will increase the likelihood of frost or freeze damage, especially in the northern growing areas like South Dakota, Minnesota, Wisconsin, and Michigan. Cool weather conditions (especially nighttime temperatures) over the next few weeks could slow the crop. Upward price movement could result. Other than this, the chance of better soybean prices are slim.

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Corn Earworm/Spider Mite Update

Corn earworm is generally the most destructive insect pest in Virginia soybeans. A corn earworm advisory is issued on a weekly basis to Virginia Cooperative Extension County Agents and to some local newspapers during August and September. The advisory summarizes emergence of the insect from corn in July and current moth activity as monitored by a system of blacklight and pheromone traps. Earworm infestations, if they occur will most likely follow peak moth activity periods by 8-10 days. By staying informed of the moth activity, one can effectively begin to intensify their scouting activity during critical periods.

Percent corn earworm infestation in corn was generally high this year, especially compared with last year. The average for the 10 southeastern counties was over 54.8% (P. George, Surry, Sussex, Dinwiddie, Greensville, Southampton, Isle of Wight, Suffolk, Chesapeake, and VA Beach). Based on these averages, high to moderate levels of earworm pressure can be expected in soybeans in the different regions. High levels can be expected in the southeast and moderate pressure should occur in the remainder of the survey area. Although larvae ranged from small to large, many were either large or had already pupated and dropped out of the ears at the time of the survey. This pointed to an earlier moth flight than usual. Corn was also drying down quickly this year which may have drove moths out to search for more susceptible host food sources such as cotton or soybeans. As of August 7, moth activity generally remained low or decreased somewhat over the previous week. Cooler night temperatures experienced during early August might have inhibited activity, but as daytime temperatures increase, moths will become more active. The amount of moth activity still does not account for the large number of worms found in field corn. Therefore, expect to see moths flying until first frost. Of course, those late flights will not pose any threat to crops. As of this writing (Aug 12) moth catch has started to go up again. We have begun scouting soybean fields and have discovered a lot of moths, some eggs and a few worms. We recommend that folks begin spot checking fields, especially those that are beginning to set pods. Moths will be attracted to fields that are flowering and setting young pods. No worm thresholds have been seen or reported yet, but I think it would be wise to begin looking. We have seen some fields with spider mite infestations. If fields develop worms and mites, we will have to consider insecticides that will control both pests. Currently, Dimethoate and Lorsban 4E are the only products labeled for mite control in soybeans (that work at all), and they are only partially effective. Of these, Lorsban does the best job of controlling worms. Much better worm control products are available (examples: Larvin, Asana, Pounce, Ambush, Karate, Scout), but none control mites. Although karate lists "mite suppression" on the label, that's what it means. At high rates (5.1 oz/A), well over the rates needed for worm control, it gives some suppression of mites. Don't expect much mite control at the lower worm control rates. Listed on the following pages are a summary of moth activity through August 6 and a table of corn earworm thresholds for soybean. If threshold levels are reached, several insecticides are available for control (Larvin, Lorsban 4E, Asana, Pounce, Ambush, Karate, Scout). Refer to the 1998 Soybean Production Guide, Plant Protection Guide, or the label for correct rates.

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1998 CEW Blacklight Trap Data

n/a = report not available

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Estimating Soybean Yields Prior to Harvest

In most areas of Virginia, soybean are grown for grain harvest. But, in dry years when high seed yields are questionable, an alternative is to utilize the crop for hay or silage. Before making the decision to utilize the crop for hay, the producer should reasonably estimate the soybean seed yield. In many cases, the value of the seed may outweigh that of a hay or silage crop. Below are methods to estimate the seed yield potential of a soybean crop. Caution must be taken however when using the approach below. Although high number of pods can be found on the plant, these pods can abort due to dry weather, leaving the plant with much lower yield potential. Furthermore, stress during the R5-R6 stages (seed-fill), can cause large yield reductions due to inadequate filling of the seed. Only until the seeds have completely filled the pod cavity at the top four nodes and the lower leaves begin to turn yellow and senesce (R6.5-R6.75) has most (50-80%) of the dry matter been accumulated and can accurate yield estimations be made. If estimating yields at earlier stages, a conservative approach of taking 50 to 60% of the estimate is advised.

1. Determine the number of row feet needed to make 1/1000th of an acre from the table below. In narrow rows, one may use 3 or 4 side-by-side rows instead of one long row.

2. Determine the plant population per acre. Count the number of plants for the row feet determined above in 6 to 10 randomly chosen areas of the field. Multiply this number by 1000. Average the number of sampling areas.

3. Determine the number of pods per plant. Count the number of pods on 5 to 10 randomly selected plants within the sample area and average.

4. Determine the pods per acre. Multiply the pod average by the plant population (step 2).

5. Determine the number of seeds per acre. Multiply the number of pods (step 4) by 2.5 seeds/pod.

6. Determine pounds of seed per acre. Divide the seeds/acre (step 5) by 2800 seeds/pound. The assumption of 2800 seed/pound is average for Virginia. A better estimate may be obtained from the tag of the seed planted. Higher numbers represent smaller seed which are more likely during drought.

7. Determine the yield. Divide the pounds/acre (step 6) by 60 pounds/bushel.

   Row Width	Length of a Single Row to Equal 1/1000th Acre
   6	87'1"
   7.5	74'8"
   10	52'3"
   15	34'10"
   20	26'2"
   30	17'5"
   36	14'6"

   Sincerely,

   David L. Holshouser
   Extension Soybean Specialist

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