Index

• Good Stands Critical to High Yielding Double-Crop Soybeans
• IPM Tip: Will Dry Weather Result in Soybean Thrips Problems?
• Micronutrients/Manganese Deficiencies
• Deer Damage
• Postemergence Herbicide Choices

Good Stands Critical to High Yielding Double-Crop Soybeans

With soybean prices now hovering around $4.50 per bushel with little hope for higher prices at harvest, obtaining the maximum potential yield for the soil and environmental conditions is a must. The first step to achieving this goal begins with planting and obtaining the necessary plant stand. In order for a soybean crop to achieve its maximum potential, it must intercept approximately 92 to 95% of all available light. In order to do this, the crop needs to build approximately 3.5 to 4.0 square feet of leaves for every square foot of land. In most cases, a full-season soybean crop can achieve this; however soybeans planted double-crop following the small grain crop is less likely to do this because of the shorter growing season. Steps that producers can follow to insure maximum canopy development for their double-crop soybeans include variety selection, narrow rows, higher seeding rates, and precision planting.

Selecting the latest maturing variety that can mature before frost is the first consideration. In Virginia, a late maturity group (MG) IV or V variety has been shown to give the best yields on average when double-cropping soybeans. MG VI varieties can be a little risky except in the southeastern corner of the state because they may be susceptible to an early frost before they mature. Likewise in the northern portion of the state, one may want to restrict their variety selection to an early MG V or less. Refer to the table under the variety selection discussion in Soybean Update Vol. 2, No. 2 or the Soybean Production Guide ("Variety Selection" chapter, Table 2) for double-crop long-term average yields of different maturity groups.

Row spacing and seeding rate is very important in obtaining the necessary canopy for maximum yields. Maximum double-crop soybean yields cannot be achieved in row spacing wider than 24 inches, therefore are not recommended. However, there is less benefit to narrowing the row spacing to less than 15 to 20 inches with a few exceptions. Research in Virginia has shown that early maturing varieties (MG III or early IV) benefited from narrowing row spacing from 18 to 9 inches (see figure below). Also on soils with low yield potentials that generally do not produce a large canopy, narrow rows (7 to 10 inches) can be beneficial. Similar to row spacing, increasing the final soybean plant population to 180,000 to 220,000 plants per acre should help meet canopy requirements. The lower range of the scale can be used for later MGs, narrow rows, and productive soils; the higher range for early MGs, wider rows, and less productive soils. Refer to the table under the soybean seeding rate section of Soybean Update Vol. 2, No. 2 or the Soybean Production Guide ("Cultural Practices" chapter, Table2) for conversion of plants/acre to seeds per row foot.

While selecting the appropriate variety, narrowing row spacing and increasing plant populations are important, maximum potential double-crop yields cannot be achieved without an evenly spaced stand that emerges uniformly. If 20% of the field contains 2 to 3-foot gaps, yield loses can range from 5 to 10%, even if the average plant population is adequate. See the Soybean Production Guide, "Cultural Practices" chapter, Table 5 for more information on the yield response of soybean to deficient stands. In many cases, soybean growers will increase their seeding rate in order to prevent gaps and hedge again poor emergence due to lack of soil to seed contact. Nearly all of Virginiašs double-crop acres are planted no-till. This is strongly encouraged to save time, conserve soil moisture, and prevent soil erosion. However, cutting through the straw residue can sometimes be a challenge and result in poor stands. Methods to help insure the best stand possible include:

1. Cut the small grain crop as high as possible. It is much easier to plant through standing straw that straw lying on the ground. Consider purchasing a stripper header.

2. Spread the residue uniformly. Straw choppers are an advantage. Make sure that the residue is spread the width of the combine head.

3. Don't plant soybeans in the morning hours unless absolutely necessary. Dew and moisture after a rain will cause the straw to be tough and hard to cut through.

4. Adjust the planter or drill to do the following:

• Cut the residue.
• Penetrate the soil to the appropriate depth of 1 to 1 _ inches.
• Obtain good soil to seed contact.

If the above criteria are met, a nearly perfect stand will occur. While the above three steps sound simple, they cannot be met without adequate weight and proper planter/drill adjustment. They also must proceed in order. Good soil to seed contact cannot be made if the residue is not cut and "hair pinning" is occurring. Neither can good soil to seed contact be made if the seed are planted too shallow or too deep. The double-crop system has allowed farmers to compete and be profitable. However, maximum profitability cannot be achieved without an adequate stand, regardless of other inputs. Unless one starts with a good foundation, the soybean crop's potential will not be realized.

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IPM Tip: Will Dry Weather Result in Soybean Thrips Problems?
D. Ames Herbert, Extension Entomologist

Background Last year, the spring and early summer were dry in many parts of Virginia and several soybean fields were inundated by thrips early in the season. Adults and immatures attacked young seedlings causing a lot of leaf crinkling and stunting. If dry weather continues, we might be faced with the same problem. This insect is normally not a problem, however, during dry weather thrips populations are often high and feeding, along with drought effects, can severely stunt plant growth or occasionally kill plants. It is difficult to find much information on soybean thrips, how they damage seedlings, or their impact on yield. Therefore, it is difficult to make a recommendation as to whether an insecticide should be applied. By way of review, here is what we know.

General Description Thrips are tiny, slender insects that feed by rasping off the outer layer of leaves and sucking out the juices. Immatures are straw colored to yellowish and adults are often black or black and white banded, depending on the species. They are difficult to see but can be seen with the naked eye when they are crawling on plants or if shaken onto light colored surfaces (index card, vehicle hood). On seedling plants thrips may be found on the upper and lower leaf surfaces as well as in unfolded leaves. Crinkled, deformed leaves that often have a silvery appearance always accompany thrips problems. Leaf drop can also occur as a result of extensive feeding damage.

Scouting Procedure Sample only if damage is readily noticeable. At each sample site, randomly pick 10 leaves (a leaf consists of 3 leaflets) and examine them for thrips damage (substantial silvery scaring and/or deformed leaves). Record the number of leaves damaged. Sample a minimum of three sites (small fields) or maximum of 10 sites (large fields). After sampling the field calculate the percentage of leaves damaged. If the plants are stressed for any reason (e.g., chemical injury or drought) take special notice of thrips activity since thrips damage and other plant stress factors are additive. This is important and we see the same situation in cotton and peanuts. With no additional stress, peanut and cotton plants can overcome early season thrips injury and stunting. However, if stresses are combined (thrips and drought), yields are often significantly reduced. Thrips numbers can be checked by careful leaf examination or by picking leaves and slapping them over a horizontally held white card.

Action Threshold Thrips rarely require treatment; however, early season injury to drought-stressed plants may occasionally reduce yields in soybeans. Treatment can be considered if 75% of the leaflets are damaged, the plants are under stress, and numerous thrips are present (more than 8 per leaflet). All three conditions should be met before treatment is applied. If the plants are growing vigorously, it is likely that they will outgrow the damage. Most states refer to data indicating no effects on yield unless thrips damage is combined with drought stress.

Of the products recommended for thrips control (Soybean Production Guide, " Insect Control in Soybeans" chapter, Table 6), Karate Z (or Warrior T) would probably be the most effective. Recommended rates are 0.96 - 1.6 oz/acre for Karate Z, or 1.92 - 3.2 oz/ acre for Warrior T. The low rate could be used if the spray can be applied as a band to rows that are spaced 30 - 36 inches apart. Band spraying low rates is very effective in peanuts and cotton. If soybeans are broadcast or drill-planted, a broadcast spray application with the higher rate should be considered.

A final word: if dry weather persists, soybean fields should be checked for thrips. Treat if the recommended criteria are met.

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Micronutrients/Manganese Deficiencies Unless soils have been excessively limed, application of micronutrients (Mn, Fe, Cu, Zn) are not usually needed. Soils in the pH range of 5.6 ­ 6.3 can usually supply enough nutrients to meet soybean requirements. However, over-liming (pH > 6.5) can induce deficiencies. Manganese (Mn) deficiencies are the most common. Mn deficiencies are characterized by interveinal chlorosis occurring in the new growth and are generally restricted to the top of the plant. To overcome Mn deficiency, apply 1/2 lb. chelated Mn (elemental basis) or 1 lb. inorganic Mn (elemental basis) per acre foliarly upon appearance of symptoms and prior to bloom. More than one application may be needed.

A word of caution is needed regarding tank mixtures of Mn with Roundup® herbicide in Roundup-Ready soybean. Henry Wilson, Weed Scientist at the Eastern Shore AREC has found that tank mixtures of Roundup and Mn lower the effectiveness of the herbicide on several weed species. Results from his 1998 experiments are shown below. Therefore, until further information is developed, do not tank mix Roundup with Mn; make separate applications.

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Deer Damage

Deer continue to plague Virginia soybean growers, resulting in sometimes-heavy losses. Research in Virginia conducted from 1992 to 1994 showed yield losses ranging from 12 to 97%, depending on the intensity of depredation. This research also showed that varieties with insect resistance (e.g. Lamar) or dense pubescence resisted deer damage most. However, these varieties had much lower yield potential than our better varieties. The table below shows the influence of clipping (to simulate deer depredation) at various growth stages on yield and height of 'Hagood' soybean. Clemson University conducted the research in Pendleton, SC.

The clipping treatments removed the upper one-quarter to one-third of each plant at the shown soybean development stages. Yields were severely reduced by any clippings in the late pod stage (R4). The above 2-year study was conducted under good crop growth and weed-free conditions. The lack of canopy closure due to deer damage could lead to further losses if late-season weed flushes occurred or under droughty conditions.

In fields with perennial histories of heavy deer depredation, it is recommended not to grow soybeans. The only practices that were found to lead to profitable soybean yields with heavy deer depredation are to reduce the deer population through hunting and kill permits or with electric fencing, which can be rather expensive. With moderate levels of deer depredation, later maturing varieties planted in narrow rows at high populations will help alleviate the damage.

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Postemergence Herbicide Choices

Listed below is the relative effectiveness of postemergence soybean herbicides. Remember to follow label instructions and be aware of weed size limitations.

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