|
|
|
POOR DOUBLE-CROP STANDS
Wheat harvest and soybean planting has been rather frustrating during the month of June. Weather records at the TAREC indicated that from June 10-20, seven rainfall events occurred. None were over 0.5 inches, two were between 0.2 and 0.3 inches, and three were less than 0.1 inches. Although the rains prevented wheat harvest and kept the straw wet, they added little to the soil. With this in mind, I've noticed that many small grain fields planted to double-crop soybean are experiencing poor stands and slow emergence. In my view, the main culprit seems to be the inability to cut through tough wheat straw. The almost daily light rains that have prevented timely wheat harvest have also caused the residue to stay wet and remain tough after harvest. The result is the "hair-pinning" of residue into the soil, therefore not allowing good soil-to-seed contact. The seed is largely contacted with wheat straw that dries out rapidly. Without good soil-to-seed contact, the soybean seed is not able to imbibe water properly. This results in somewhat of a drought-stressed scenario for the germinating seed, especially in those areas that have received only trace amounts of rainfall at a time.
Refer back to the June issue of Soybean Update to determine whether or not re-planting is warranted. I'll remind you that after mid-June, soybean yields will decline approximately 0.5 bushels per day. Therefore, carefully evaluate the stand and costs associated with replanting before making that decision.
Return to the index.
Every year manganese (Mn) deficiencies occur throughout Virginia, especially on soils where the pH is above 6.2. Where the deficiency occurs, a foliar application of _ lb chelated (elemental basis) or 1 lb inorganic Mn is warranted to insure normal growth and yield. Symptoms are characterized by interveinal chlorosis on the top leaves of the plant. It is important to apply a foliar application as soon as possible after the appearance of the symptoms and before flowering. More than one application may be needed to correct the deficiency.
Return to the index.
Diagnosis of nutrient deficiencies can be aided by the use of plant tissue analysis. Samples taken prior to or at initial flowering from good and bad portions of the field in conjunction with soil test results from each area can provide more information and permit accurate nutrient diagnosis. Approximately 25 soybean leaves (trifoliate leaf sets composed of 3 leaflets with petiole removed) per field should be taken and air-dried for shipment to a private laboratory offering plant tissue analysis services. For samples collected at initial bloom, adequate nutritional status is usually indicated when leaf concentrations are within the following sufficiency ranges:
| N | P | K | Ca | Mg | S | Mn | Fe | B | Cu | Zn | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| % | ppm | |||||||||||
| Low | 4.25 | 0.30 | 1.75 | 0.50 | 0.25 | 0.20 | 20 | 50 | 25 | 6 | 20 | |
| High | 5.00 | 0.50 | 2.50 | 1.50 | 0.80 | 0.60 | 200 | 300 | 60 | 30 | 50 | |
Return to the index.
LATE-SEASON APPLICATIONS OF BORON AND NITROGEN
There has been considerable interest in late-season applications of boron and nitrogen to soybean. Research is currently underway to determine if these practices could consistently increase yield. At this point, I am not recommending that such applications take place except under certain situations or on a trial or test plot basis. Regardless, I will attempt to give a brief overview of past successes and the rational for such applications.
Boron. First, lets take a look at boron (B). B is an essential micronutrient that is needed for many physiological activities in the plant, including cell wall development, sugar transport, cell division, hormone regulation, and reproductive structure retention. Although soil applications or applications to soybean in the vegetative stages do not usually result in yield benefits, applications made to the crop at the R3 development stage (defined as a pod 3/16 " long at one of the four uppermost nodes on the main stem with a fully developed leaf) have occasionally resulted in average yield increases of _ to 3 bushels per acre. However, results have been mixed. In Georgia, yield responses have been found when applied at the R3 stage to soybean grown in sandy soils. Georgia Cooperative Extension is currently recommending applying B in combination with Dimilin, an insecticide, when needed for control of defoliating insect pests. A summary of 29 midwestern experiments revealed increased average soybean yields of 1.4 bu/A or 3% on soils not considered to be B deficient. However, inconsistency in the results did not lead the researchers to recommend B applications. Small plot research conducted in 1997 or on-farm research conducted by county agents in Virginia did not reveal significant yield increases.
There is some physiological rational for the yield increases with B, therefore I am not so quick to dismiss the possibility of applying B to improve soybean yield. However, until a better understanding of when and under what conditions a yield increase can be realized, I am hesitant to recommend this application. Only in situations that another unrelated application is being made at this time would I recommend adding B to the tank mixture. I will remind the reader that B can be toxic to the plant at high rates and leaf burn can occur. Therefore, be careful when mixing the nutrient with other inputs. Insecticides or certain herbicides containing surfactants could increase B absorption and cause toxicity problems. For those who would like to apply B in a test plot setting, recommended application rates are 0.2 to 0.25 lbs/A of elemental B. Nitrogen. Nitrogen (N) is the nutrient required in the highest amount by soybean. Because soybean is a nitrogen-fixing plant, N applications are not recommended. Total N needs for the soybean plant is approximately 5.5 pounds per bushel of seed yield. Therefore a 30 bu/A crop will require approximately 165 lbs of N. On the other hand, a 70 bu/A crop will require over 350 lbs. With this in mind, it becomes questionable whether or not a soybean plant can fix the needed N under high-yield situations. Soybean need N in the highest amount during the critical pod development and seed filling stages (R3 through R6). Taking into account these conditions, will soybean respond to a late-season application of N?
Research over the past 10-15 years at various locations have occasionally shown significant increases in soybean yield due to late-season N applications. In nearly all cases, these increases have occurred under irrigated or high yield conditions. Results were similar from foliar or soil applications. Research conducted in Virginia in the early 90's showed no advantage of late-season N applications. Last year, we found a slight, but not significant increase in soybean yield from increasing rates of late-season N applications to the soil in one experiment, but not another. Again, much like B, yield increases due to late-season N applications are inconsistent. Future research will attempt to get a better handle on the environmental conditions in which a yield increase can be expected.
In addition to the inconsistent yield responses, there are logistic problems to applying N to soybean late in the season. Foliar applications should probably be limited to 20 lbs of N per application to avoid leaf burn, but higher rates are likely needed to get a yield response. Therefore, multiple applications may need to be made. One could dribble 30% UAN to the soil, but narrow row spacing makes this difficult. Another, but more expensive option is to apply dry fertilizer. Injecting N through the irrigation system would be the least troublesome option, but few Virignia farmers are irrigating soybean. Regardless, responses to late-season N application would have to be large and consistent to justify the cost and application problems.
Return to the index.
Potato leafhopper populations are high in soybean fields very early this year. I have received several calls reporting severe damage to soybean seedlings. This is the first time I have seen such severe damage to soybean seedlings and I am at a bit of a loss as to what to recommend in terms of management. By way of background, the potato leafhopper is a common pest of forage legumes, beans and other vegetables. It also attacks soybeans but rarely reaches population levels that affect yields. The most recent information suggests that leafhoppers overwinter in most southeastern states including the warmer areas of Virginia. They move onto spring host plants as temperatures warm. They showed up very early this year in alfalfa and potatoes, and early planted soybeans. Adults are highly mobile, spindle-shaped, yellow green and about 1/8 inch (3mm) long. Several nymphal stages exist, each of which is wingless, colored similarly to, and smaller than the adult. Nymphs and adults feed on plant sap from the under-sides of leaves. While feeding, they also inject a toxic substance, which interferes with normal plant growth. The symptoms of leafhopper injury include localized stippling, curling and yellowing of leaf margins (hopperburn). Dense pubescent varieties are less susceptible. The presence of leafhoppers can be determined by a sweep net.
Treatment is suggested if populations exceed 4 adults/nymphs per sweep in stressed beans or 8 per sweep in normal growing fields. Whole plant counts can also be made to determine population levels. To sample, slowly walk up to the plant and watch for and count any small yellowish adult leafhoppers that jump from the plant. Then examine the plant thoroughly, counting both adults and lime-colored nymphs. Sample 5 plants in each of 5 areas of a field. Determine the average number per plant and the stage of plant development. In soybean fields with less than 4 trifoliates per plant, control may be needed if leafhoppers average more than 2 per plant; on plants in bloom, control may be needed if 6 leafhoppers are counted per plant; during pod development, 14 leafhoppers or more per plant may cause economic damage and control should be considered. Damage to seedlings is most important and should be managed, especially if plants are under moisture stress.
Below are some insecticide recommendations. These products could be tank-mixed with herbicide applications to reduce application costs. Otherwise, they could be broadcast. Leafhopper is not a difficult insect to control so low rates are effective. Higher rates would provide more residual activity.
| Asana XL | 3 oz/A |
| Karate 1EC | 2 oz/A |
| Scout X-Tra | 1.7 oz/A |
| Sevin XLR | 2 pt/A |
Return to the index.
Help us to continue to strengthen our industry and Association by joining today. In the Virginia Soybean Association, membership does make a difference!
Sincerely,
David L. Holshouser
Extension Soybean Specialist
Return to the index.
Return to David Holshouser