CORN
Crop Observation and Recommendation Network

In This Issue:

A) Additional Considerations for Double-Crop Soybeans
B) Leaf Cupping and Puckering Symptoms on Soybeans
C) Potato Leafhopper in Alfalfa
D) Check for Two-spotted Spider Mites Before Spraying
E) Drought Effects on Corn Yield Potential
F) Harvesting Drought Stressed Forages

A) Additional Considerations for Double-Crop Soybeans - Ed Lentz CORN Questions

B) Leaf Cupping and Puckering Symptoms on Soybeans - Mark Loux CORN Questions

One of the first herbicides to get blamed in many fields is Banvel, which may have been applied in a nearby corn field. Spray drift or volatilization of Banvel from a neighboring field can occur, and may be the culprit in some fields. However, many of the affected fields seem to be far enough away from treated corn fields, or Banvel was not used in the area, and this possibility can be ruled out. The most typical symptoms from exposure of soybeans to Banvel or Clarity are puckering of the new leaves that are emerging 7 to 10 days after exposure. This may be accompanied by stunting of the plant. Soybeans may show these symptoms on several trifoliates, and then recover completely. Research indicates that soybean yield is not generally reduced when minor symptoms occur, and yield loss is more likely if soybeans are in the reproductive stage at the time of exposure (although still unlikely unless symptoms are severe).

We have heard reports of and observed fields where leaf puckering or cupping was uniform over the entire field. Other fields have shown symptoms only in some areas. In OSU research plots, we have occasionally observed puckering in Roundup Ready soybeans following application of Roundup. Spider mites and leafhopper have been known to cause cupping and wrinkling of soybean leaves. Many of the fields with puckering were previously treated with a postemergence herbicide other than Roundup. Pursuit, Classic, and Pinnacle seem to be the herbicides most often used in fields with the symptoms, but other herbicides have also been used. One working theory about these symptoms when the postemergence herbicide causes injury to the terminal buds on soybeans, apical dominance is altered, and plant hormones are redistributed within the plant. The result is the appearance of injury that is similar to that from plant growth regulator herbicides (dicamba, 2,4-D). New shoots may occur at nodes below the injured zone, the plant may take on more of a bushy appearance, and leaves may be wrinkled and cupped. However, most of the fields have not exhibited the increased "bushiness" such as might occur if apical dominance was lost.

While herbicides may be responsible for some of the puckering, cupping, and wrinkling that has been observed, we suspect that environmental conditions and soybean variety may have a significant role. This is based on the observation of uniform cupping in fields where no postemergence herbicide was used. Some varieties may be more likely to show symptoms than others. We have not been able to come up with a good explanation for this phenomenon. However, the good news is that leaf cupping and wrinkling should not affect yields, and soybeans generally compensate well for other herbicide-related problems given enough time and moisture.

Reprinted from C.O.R.N. Newsletter 98-19

C) Potato Leafhopper in Alfalfa - Bruce Eisley CORN Questions

Treatment thresholds are based on the number of leafhoppers collected in a sweep net and the height of the alfalfa. To require treatment the number of leafhoppers collected in 10 sweeps should be equal to or exceed the height of the alfalfa (ex10 sweeps in 6 inch alfalfa collecting 7 leafhoppers would be just above the threshold and treatment would be justified). This threshold is based on alfalfa that is growing normally with adequate moisture. Areas of the state that are very dry may want to lower the leafhopper treatment threshold and apply treatments earlier.

Refer to Table 6 in OSU Extension Bulletin 545 - "Insect Pests of Field Crops" and consider using the numbers in the Low Tolerance column when deciding treatment thresholds in areas under drought conditions.
 

Potato leafhopper damage to alfalfa.

D) Check for Two-spotted Spider Mites Before Spraying - Bruce Eisley CORN Questions

Areas that received rain over the weekend may not see a decrease in mite numbers but the soybeans may put on enough new foliage to slow the damage.
 

Spider mite damage to soybean leaves

Refer to  (C.0.R.N. 99-13) for guidelines on when to treat for spider mites.

E) Drought Effects on Corn Yield Potential  - Peter Thomison CORN Questions

To estimate what impact this year's drought will have on corn yield potential, let's review the effects of moisture stress on corn growth and development from the late vegetative stages,  prior to pollination, to the dent stage of kernel development. (Some of this information was discussed previously in my C.O.R.N. article of June 14, issue 99-13).

Yield losses to moisture stress can be directly related to the number of days that the crop shows stress symptoms during different growth periods. Table 1 summarizes findings of Iowa research by Classen and Shaw on effects of drought on grain yields in corn. This Iowa data is widely used in estimating the potential impact of water stress on yield potential.

Vegetative Stages: Drought stress during early vegetative growth usually has a negligible impact on grain yield. However, during later vegetative stages, when kernel numbers per ear are determined, plants are more sensitive to stress. Kernel row numbers on the ear are determined by the 12th collared leaf stage and the potential number of kernels per row is complete about one week before silking (about the 17th collared leaf stage). According to Classen and Shaw's findings, four days of stress (i.e. corn wilted for four consecutive days) at the 12th-14th leaf stage has the potential of reducing yields by 5-10 percent. Since most stressed corn in Ohio is not far beyond the 12th collared leaf stage, the impact of early drought on yield may be limited if favorable growing conditions return shortly.

Tassel Emergence: As the tip of the tassel begins to emerge from the whorl, the upper stalk internodes rapidly elongate and the ears begin to expand. Silks from the base of the ears are also rapidly elongating. Four days of moisture stress at this stage has the potential to reduce yields 10 to 25%.

Silk Emergence to Pollen Shed: At this stage, leaves and tassels are fully emerged and the cobs and silks are growing rapidly. This is the most critical period in terms of moisture use by the plant. Four days of moisture stress at this stage has the potential to reduce yields 40-50%.

Blister Through Dent Stage of Kernel Development: About 12 to 36 days after silking, the cobs, husks and shanks are fully developed and the kernels are increasing in dry weight. Moisture stress will reduce kernel fill from the ear tip down. Four days of drought at the blister stage has the potential of reducing yields 30-40%, and at dough stage, 20 to 30%.

Severe moisture stress has caused stand loss in some fields and this will have a direct effect on corn yields. Where dry weather has resulted in corn stands with uneven germination, emergence, and development, yield loss may range from 5 to 20% depending on various factors such as the length of emergence delays and the percentage and distribution of later emerging plants. Where there is considerable variability in plant size, smaller, stunted plants will be at a competitive disadvantage with larger plants for nutrients, water and sunlight. If drought stress has enhanced plant injury from herbicides, fertilizers and insects, then some permanent  reduction in yield potential is possible. Limited soil moisture has contributed to problems involving root injury from pop-up starter fertilizer and anhydrous ammonia which in some cases have caused significant stand loss.
 

TABLE 1.  Effects of Drought on Corn Yield

Stage of development	Percent  yield reduction from four  consecutive days of visible wilting
Early Vegetative	5-10
Tassel emergence	10-25
Silk emergence, pollen shedding	40-50
Blister	30-40
Dough	20-30
Source:	Classen, M.M., and R.H. Shaw, 1970. Water deficit effects on corn II - Grain components.  Agron. J. 62: 652-655.

F) Harvesting Drought Stressed Forages - Mark Sulc CORN Questions

In established grass stands, growth has been slow at best. Established grass stands can survive through severe drought conditions and regrow once rains return. One notable exception to this rule is perennial ryegrass. We've seen that species lose alot of stand during droughts.

Deep-rooted established legumes such as alfalfa have been the "green" forage this year, maintaining very good growth up to this point on most soils. If the dry weather continues though, even alfalfa stands will begin to suffer. In fact, on lighter soils, established alfalfa already shows limited growth from lack of moisture. Alfalfa and birdsfoot trefoil have good drought tolerance, while red clover stands can be reduced during severe drought and heat stress. Therefore, harvest management strategies are different for these legumes.

Alfalfa, unlike birdsfoot trefoil and red clover, can actually maintain production during short periods of dry weather (provided the soils allow development of a deep taproot). But with extended periods of drought and high temperatures, alfalfa growth will be reduced and flowering will occur on short, stunted plants. Cutting during these stressful periods DOES NOT WEAKEN ALFALFA PLANTS or cause stand reductions. If moisture is lacking after cutting, alfalfa plants go dormant until sufficient moisture is again available for regrowth.

During the initial phases of moisture stress, alfalfa stems stop elongating, but the plant continues to manufacture carbohydrates. These carbohydrates are stored in the root system since they are not being used for topgrowth. These energy reserves are available for regrowth after cutting and when soil moisture is recharged. So if there is enough harvestable forage to economically justify a hay cutting, then go ahead and harvest it. You still should maintain about a 30- to 35-day interval from the last harvest. This is especially important if an early first harvest was made. Even though the plants may be in full flower, the forage will probably be higher in quality than normal growth in full bloom. If fencing is available, controlled grazing of drought-stressed alfalfa stands is a very economical way to utilize the forage that is present.

Red clover is not as tolerant to the combined effects of drought and heat stress as alfalfa. Cutting during periods of hot and dry weather CAN WEAKEN RED CLOVER PLANTS and may cause stand reductions. If feed is badly needed, red clover stands can be lightly grazed during drought stress.

When cutting or grazing birdsfoot trefoil during periods of heat and drought stress, be extra careful to harvest when plants are at least in mid-bloom stage and leave a 3-inch stubble. During the growing season, birdsfoot trefoil maintains relatively low levels of reserve carbohydrates in the roots and crowns. Cutting or grazing when plants are well into bloom stage and leaving sufficient leaf area will improve the regrowth potential of birdsfoot trefoil.