In many areas, corn is beginning or has emerged with a pale yellow color. Sunlight and heat will help to “jump start” the crop into vigorous growth and once this occurs many of these problems will be remedied. A number or things, most of which is weather generated, cause discolored corn. The emerging corn not only is discolored (yellow to purple) but can be expected to show signs of insect feeding from cutworm, wireworm and white grub. If you haven’t or can't treat with an in-furrow insecticide there is some chance you have or will see feeding from the insects in your corn fields.
The weather has not only caused slow growth and discoloration to emerged crops but also provided conditions for diseases to develop. Cool and wet weather, especially after physical injury to the plant, are ideal conditions for seedling diseases such as damping off from phythium and rhizoctonia. Cultivation or aeration of the soil in the diseased areas help to limit the spread of the infestation, but, it is best to note the diagnosis and document the diseased fields and plant resistant varieties to these diseases.
Some areas which have experienced heavy rains, followed by the current cool temperatures have caused a number of concerns. Some crop damage has been severe enough to justify replanting. Things to remember if replanting corn 1) many herbicides restrict replant options and producers should use caution and consult the herbicide label prior to replanting. 2) Replanting corn at this time of the season can still maximize your total yield potential. It is not late enough in the season to think about changing maturities of hybrids or changing to soybeans entirely for the areas needing replanting. This conversation changes however, after May 16th. It is documented that yield potential in corn is reduced one percent for every day after May 15th that corn is planted. If your fields have stand losses of 30-40% or less after May 15th, it is still better to leave the field alone than replanting the entire field. If stand loss is greater than 50% across the entire field re-planting to corn or planting soybeans may be a better option. This is of course unless you intend to harvest and dry high moisture corn immediately after black layer development has occurred. Even then, you would be in a race with the average frost date of early October to get the corn to maturity. 3) Prior to re-planting, you may be provoked to re-apply pre-emergence herbicides and a supplemental dose of fertilizer in eroded areas where the soil particles which have bound to the herbicides/fertilizer has been physically moved through erosion from the storms. The important things to remember are that not all areas of the field had the treated soil layer removed through erosion. Re-application could promote increased herbicide carry-over potential for the rotation crop of next year in these areas. We don't know how much of the herbicide or fertilizer was lost unless a soil analysis of the area in question is preformed. I believe the most important thing to consider is getting the area re-planted and growing without further delay or possible added stress from additional applications. The best choice is to treat weed issues with post-emergence applications and if additional side dressed nitrogen if needed. .4) Soybeans can maintain much of their maximum yield potential even if stand is reduced 40 percent. 5) Soybeans maintain can be planted up to June 5th without a statistical yield reduction.
Corn Replant--Early Season--Example #1: Using the below table—(optimal planting date of April 30th): If you planted a 108 Relative Maturity hybrid at 30,000 on April 30th and have a viable stand now of
15,000, the yield potential is 82%. This table is useful for all replanting decisions (hail, soil crusting, insect damage, etc)
Planting Population (1000/ac)
Date 10 15 20 25 30 35
Apr. 1 62 76 86 92 94 93
Apr. 20 67 81 91 97 99 97
Apr. 30 68 82 92 98 100 98
May 9 65 79 89 95 97 96
May 19 59 73 83 89 91 89
May 29 49 63 73 79 81 79
% of maximum yield
from Nafziger,E.D. 1994. Journal of Production Agriculture. &:59-62
Based of research in the Central Corn Belt by the University of Illinois.
Corn Replant--Late Season--Example #2--The following table from Purdue University using 108-110 day hybrids demonstrates late season planting yield potentials.
Population (1000/ac)
Planting Date 16 18 20 22.5 25
May 16 86 90 93 96 98
May 21 83 87 91 94 95
May 26 80 84 87 90 92
May 31 75 79 82 85 87
June 5 69 73 77 80 81
June 10 63 67 70 73 75
% of maximum yield
At a population of 25,000 planted on June 10th a yield reduction of 25% has occurred before you have pulled out of the field. The chances of seeing a 15-20% yield reduction due to early frost/light test weight, etc. is also very real. Planting grain or forage sorghum has more yield and economic potential at this calendar date if your location and management practices would permit this to take place. I realize that previous corn herbicide applications or management practices may have you locked into replanting corn on certain acres. I would suggest that you don’t go more than 10 days earlier in relative maturity if you do decide to change the maturity of corn planted. Research and experience suggest that you are still better off using the same maturity hybrids you normally plant in your region due to their aggressive plant growth will allow the corn to “catch up faster”. Hope for a hot summer and a late frost.
Fertilizing/Re-fertilizing, Nitrogen Loss to corn, sorghum and soybeans should also be avoided until the crops have a chance to resume active growth. Increasing fertility to injured plants can amplify the injury and delay the recover period. Nitrogen leaching is always a question in cool, wet springs. The amount of nitrogen lost depends on the form of nitrogen used. Solution nitrogen (28% or 32%), or ammonium nitrate are more subject to leaching because they contain nitrate-N which is not readily held in the soil and moves with the soil water. Ammonia N (anhydrous) has to go through nitrification (conversion to nitrate), in order to be leachable and plant available. With the “rollercoaster” temperature swings, cooler soil temperatures, nitrification has been slow and most ammonia N is still present as ammonium and not subject to soil leaching. The result of this might be that corn has emerged and is yellow. One reason for this is obviously lack of sunlight, but usually it is a result of the nitrogen being unavailable for plant uptake or it has been leached out of the germination zone. The only way to tell is to soil test for residual nitrate in the top two feet after the corn has reached to V10 (8-leaf) stage. At that time, if needed, additional nitrogen can be applied through side-dress or fertigation applications. In-season uses of chlorophyll meters are generally the best approach to determining fertility needs.
Soybean diseases such damping off from Phythium and Phytophthora are and will continue to be prevalent in water logged soils. Stand reduction has and will continue to occur in these areas. “Damping off” is a cover-all term used to describe symptomology of the disease. The rule is if damping-off occurs in warm soil (70 to 80 degrees) it is more likely to be caused by Phytophthora. If it occurs in cool soils (50-60 degrees which we have right now) it is more likely to be caused by Phythium. Both diseases cause the root and lower stem tissue to turn brown/purple with a slimy texture which can lead to seedling death. The best line of defense is to purchase genetic packages with strong tolerances to these diseases. You can increase your stand establishment percentages with the use of Seed Treatments. If you haven’t done so already, consider the investment. In a spring like this one it will likely pay you bring dividends.
Preemergence Herbicides Applied Postemergence--Many cornfields will be wet enough for a number of days following this weeks rains to delay pre-emergence herbicides to cornfields. If you were shut out and are now wondering whether the chemicals can be applied post-emergence, the answer for many such herbicides is yes. Most chloroacetamide herbicides may be applied postemergence to corn, but if weeds have emerged, the addition of POST-active herbicides is necessary. Many of the chloroacetamide herbicides are pre-packed with atrazine. Atrazine is commonly used as a pre and post-emergence treatment however, the addition of crop oil concentrate as an tank-mix additive increases the herbicide activity on grasses up to 2" in height.
The following table lists corn herbicides that are commonly used in Nebraska and can be used both preemergence and early postemergence. The table depicts some of the more common treatments but is not all-inclusive. Label limitations prevent certain soil-applied compounds from being used after crop emergence. Only herbicides that can be used after crop emergence are listed below.
TREATMENT CROP STAGE WEED STAGE
Corn
Aatrex/Atrazine 0-12" 1.5" grass
Axiom 0-5” unemerged
Bicep II Magnum/Cinch ATZ 0-5" 2-leaf
Callisto 0-30"--8 leaf corn broadleaves <5"
Dual II Magnum/Cinch 0-5* unemerged
Dual + Aatrex 0-5" 2-leaf
Guardsman MAX 0-8" 1.5"
Harness/Degree 0-11" 2-leaf
Harness Xtra/Degree Xtra 0-11" 2-leaf
Hornet 0-24” 2-8” broadleaf
Ignite 0-24” 0-4”
Laudis 0-24” 0-6”
Lumax 0-5" broadleaves <5"
Marksman 0-5" 0-4" broadleaf
Outlook 0-8" unemerged
LIQUID NITROGEN--HERBICIDE COMBINATIONS AFTER CORN EMERGENCE
Corn and sorghum in fields intended for liquid nitrogen--preemergence herbicide combinations sometimes emerge before herbicide application. While certain preemergence herbicides can be applied after crop emergence, they are not labeled for application with fluid fertilizer after emergence, due to risk of injury. Emerged corn is more tolerant than sorghum of fertilizer-herbicide mixtures. Some growers may accept the injury in return for accomplishing two jobs at once.
Dual applied with liquid nitrogen on emerged corn and sorghum would result in injury similar to the fertilizer alone. Injury is more serious with atrazine combinations. The tolerance of emerged corn and sorghum to liquid nitrogen and these herbicides is greatest in the spike stage and decreases as the plants grow. Minnesota experiments have shown that atrazine applied to 4-leaf corn with 60 lbs N/acre caused heavy burning with widespread necrosis on the 2nd, 3rd and 4th leaves. This injury, from which the corn recovered, was similar to injury caused by 150 lbs N/acre applied by itself. Injury to emerged corn is usually more severe with herbicide combinations in liquid N than fertilizer alone. If nitrogen must be applied as liquid N to emerged corn, early post herbicides should be applied in water several days prior to the application of liquid nitrogen. Cool, wet weather can be expected to increase the injury caused by such treatment.
Soil Compaction during planting is certainly more an issue now than it was 2 weeks ago. As the calendar approaches May 15th this week, producers may/will plant into wetter than normal conditions in order to complete corn planting in a timely fashion. Surface and sidewall compaction from tillage and planter passes across wet fields certainly increase the chances of having restricted root growth in the seed slot and added discoloration to the corn plant. This is most common in fine-textured soils and aggravated by continued cool and wet conditions. Sidewall compaction is a product of the force of the disk openers of the planter smearing the sides of the seed slot when the soil conditions are too moist at the seeding depth. Since early root growth is restricted laterally to the width of the seed slot, the plant's ability to extract water and nutrients from the soil is compromised and the plant will fall under stress.
Some growers believe that as long as the press/closing wheels aren't picking up mud on the tread that it is dry enough to plant. That may be true however, digging to seeding depth and making sure that the soil is friable--crumbles in your hand with little to no soil sticking in you palm, will insure that the soil is not too wet to plant to promote added compaction. Waiting the extra day is hard to do but, can pay you in the end!
Crop competition with weed pressure this spring has been virtually non-existent. Winter annual weeds have been slow to emerge and their lack of establishment may be a lingering product of last years cool and wet weather. Recent soaking rains, combining with consistent warmer temperatures, will increase weed germination and development. Soon summer annuals will emerge and actively grow in you fields. While your method of control has likely changed our the years with the development of Roundup Ready Technology, crop and weed competition has not changed for some time. This year it is possible that you may be finishing planting activities while first planted fields need post-emergence applications for weed control. Potential for yield reduction involves many things such as weed height at application, crop height at application, weed type, weed density and weed distribution throughout the field. Planning when to make your applications can be determined mainly by the type of weeds you have present along with their density and in what crop they have occurred.
Crops compete with weeds for water, sunlight, and nutrients needed to sustain active growth. Corn competes differently than soybeans, sorghum or wheat can against different species of weeds. The following is some work done from 1996 to present through the University of Nebraska Weed Science Department that addresses just this issue. Note the number of broadleaves compared to the number of grasses it takes to cause the same percentage of potential yield reduction. Table A. represents the number of weeds per 100 ft2 that will cause a given yield reduction in corn. This assumes that weeds emerged with corn and no herbicide has been applied. These numbers only consider the effect of the weed in the current year and do not take into account future weed problems due to weed seed production. This should be considered if these numbers are used to make long-term decisions.
Table A. Number of weeds per 100 ft2 causing a 1.0, 2.5, 5.0, and 10.0% yield loss in corn.
% Yield Loss___________________
Weed 1.0 2.5 5.0 10.0
Barnyardgrass 24 62 129 284
Sandbur 18 46 97 213
Fall Panicam 18 46 97 213
Crabgrass 15 37 78 170
Foxtail 7 19 39 85
Lambsquarters 5 12 26 57
Common ragweed 5 12 26 57
PA Smartweed 5 12 26 57
Russian thistle 3 8 17 37
Kochia 3 7 16 34
Pigweed 3 7 16 34
Waterhemp 3 7 16 34
Shattercane 2 5 11 24
Black nightshade 2 5 11 24
Velvetleaf 2 4 9 20
Cocklebur 1 3 7 16
Sunflower <1 2 4 9
(University of Nebraska-Lincoln Weed Science Department. 1996)
