C.O.R.N. Newsletter 2007-20

Understanding Corn Crop Water Needs: Available Soil Moisture

Authors: Peter Thomison, Robert Mullen

High temperatures and limited rainfall have created stress conditions in many corn fields across Ohio. Average water use by a corn crop during pollination and early grain fill is about 1/3 inch per day. Evapotranspiration rates in corn depend on temperature, humidity, wind, solar radiation and total leaf area of the crop. (Evaporation from the soil surface combined with transpiration from plants is evapotranspiration). When temperature is relatively low and humidity is high as on a calm, cloudy day, the evapotranspiration rate will be low. If temperature is high and humidity low as on a sunny, windy day, the rate will be high.

With drought-like conditions appearing in parts of the state, especially areas which have received negligible rainfall since April and early May, how much of the corn crop’s water needs can be met by subsoil moisture?

Soil textural characteristics dictate water holding capacity, intake rate and drainage rate. Soils may have available water capacity of as little as 4 inches or may exceed 8 inches in 4 feet of soil. This publication (http://www.extension.iastate.edu/Publications/NCH20.pdf) gives the available-water holding capacities of ten different soil types. Soils also differ as to depth adequate for active root development; some have underlying layers of gravel or hard pan that would restrict root growth. Information in this table might be used to estimate the number of days that moisture stored in the soil could “carry” a corn crop. For example, with a storage capacity of 1.8 in./ft, a fully charged silty clay loam soil might carry corn with a 3 foot rooting depth up to 18 days during silking and early grain fill stages (1.8 in/ft times 3 foot depth= 5.4 inches available water; 5.4 in. divided by 0.3 inch/A/day water requirement = 18 days). Although corn roots can grow as deep as 8 feet, when actively growing, corn obtains 90% of its water requirements from the top 3 feet of the soil profile.

A major factor determining the ability of corn to extract available soil water is soil compaction. In many Ohio corn fields, surface compaction, or later plantings into dry soil have resulted in corn root systems restricted to the top few inches of the soil profile. These flat, shallow root systems make the crop especially vulnerable to drought. In addition to soil compaction, when relating the information from Table 1 to various Ohio soil types, keep in mind that other factors may influence water availability. Differences in soil organic matter and texture often occur at different rooting depths, for example the top foot of soil may be a silty clay loam but underlying layers may be clay, sand, or gravel.

Growing Degree Days, How Does This Year Compare to the Average?

Authors: Peter Thomison, Robert Mullen

This spring our average temperatures have been a little warmer than normal, so how many growing degree days (GDDs) have our crops accumulated and are we likely to see early maturation of corn and soybeans?

The OARDC has weather stations located at sixteen locations across the state that record hourly weather information. Many of the stations have been in place since 1982 which gives a good amount of information to evaluate to determine what “average” is, and how far we are deviating from that within a given year. For this analysis only three locations have been evaluated to represent general geographic regions - Western Research Station (South Charleston, west/central Ohio), Northwest Research Station (Custar, northwest Ohio), and OARDC Main Campus (Wooster, north central/northeast Ohio). See the OARDC Weather site to calculate GDDs: http://www.oardc.ohio-state.edu/newweather/.

Table 1. Comparison of GDDs (86/50 cutoff method) for areas of Ohio.

location	April - Jun Avg GDDs*	2007 GDDs	May - Jun Avg GDDs	2007 GDDS
West central OH	1060	1250	964	1140
Northwest OH	988	1175	911	1087
North central OH	929	1034	850	957

*Dates for GDDs based on April 15 to June 30 in columns two and three and for May 1 and June 30 for columns four and five.

Based upon this year’s weather our crops have been exposed to more growing degree days than the historical average, and are likely maturing at a faster pace. We cannot overlook the rainfall pattern though especially for later planted crops. Lack of rainfall has hurt the development of some crops despite the above average temperatures, so your specific situation may not match our measured data perfectly.

From emergence to V10 (ten visible leaf collars), leaf emergence occurs approximately every 85 GDDs. From V10 to tasseling, leaf emergence occurs more rapidly at approximately one leaf every 50 GDDs. (Some use a more conservative estimate of 60-65 GDDs associated with the appearance of new leaf collars during vegetative growth.). Many of our popular hybrids require 1300 to 1350 GDDs to reach 50% silking. Corn plants can develop up to 19 to 21 leaves. Presently many corn fields are at V12 or beyond. Given the current rate of heat unit accumulation, it’s likely some of our early April planted corn will be silking earlier than normal. Don’t be surprised if you see tasseling and silking in some fields in early July.

References:
Nielsen, R.L.. 2007. Determining Corn Leaf Stages. Corny News Articles,
Purdue Univ. [On-Line]. Available at
http://www.kingcorn.org/news/timeless/VStageMethods.html
Nielsen, R.L.. 2007. Use Thermal Time to Predict Leaf Stage Development in
Corn Corny News Articles, Purdue Univ. [On-Line]. Available at
http://www.kingcorn.org/news/timeless/VStagePrediction.html

 

Two-Spotted Spider Mites

Authors: Ron Hammond, Bruce Eisley

The past week has brought some much needed moisture to many locations in the state, along with slightly cooler temperatures. How much impact this will have on two spotted spider mites is unknown. This will be important because we did find our first field that had sufficient mite activity that warranted close monitoring and perhaps treatment. Hopefully the rainfall and cooler temperatures will lower mite populations. However, growers should continue to monitor for possible mite buildups in their fields in those locations that remain dry. If you have had a past history with mites in certain fields, now is a good time to check those areas. While checking for mites, remember to also monitor for soybean aphids.

Corn Silk Clipping

Authors: Ron Hammond, Bruce Eisley

Western corn rootworm adults are beginning to show up in corn fields. With corn silks not yet out, we expect to begin seeing feeding on leaves. The adult beetles scrape off the outer layer of the leaf resulting in a leaf that looks as if it has been burned. We normally are not concerned about this leaf feeding; however, when the silks begin to emerge the beetles will move to the silks and begin feeding. Additionally, Japanese beetles will add to the problem of silk clipping, especially around the edges of the fields.

Because of the potential for uneven silking with the variation in corn growth within fields, there may be only a small portion of the field that has fresh silks at any one time. In these cases, the beetles will congregate on those emerged silks and populations per plant may be larger than if the whole field had silked. As more plants silk, populations may become dispersed throughout the field and reduce the beetle numbers per plant and thus the need for treatment.

Rescue treatment for rootworm beetle silk clipping is warranted if 5 or more beetles are found per silk mass when 75% of the plants have silked and silk clipping to 1/4 inch or less is observed. For Japanese beetles, rescue treatment is warranted if there are 3 or more beetles per silk mass and pollination has not occurred.

Corn Lodging from Rootworms

Authors: Ron Hammond, Bruce Eisley

Now is the time to assess the effectiveness of your rootworm control measures. Before assuming that the lodging is because of rootworm larval feeding, growers should dig roots, wash them, and rate the roots for larval injury. There are many causes for plant lodging, rootworm feeding being only one of them. See the http://corn.osu.edu/story.php?setissueID=190&storyID=1142, from last week for instructions how to dig and rate rootworm injury.

Late-Season Postemergence Issues in Roundup Ready Soybeans

Authors: Bill Johnson, Mark Loux

Weeds continue to defy control in a number of fields throughout Ohio and Indiana, and lambsquarters, giant ragweed, and marestail appear to top the list of most problematic weeds. No surprises here. Failure to make initial postemergence glyphosate applications early enough in the season can be a primary reason for poor control. Dry weather caused many growers to delay postemergence applications, with the hope that an ensuing rain would improve conditions and the weeds’ response to herbicide. While the rain may have cooperated in some areas, the continued lack of rain in other areas resulted in older weeds that were generally more tolerant of herbicide. The effect of weather was evident in our research plots, where the effectiveness of the standard rate of glyphosate, 0.75 lbs ae/A, was reduced compared to wetter years, especially on lambsquarters and giant ragweed. Glyphosate was much more effective in our plots where it was applied at a rate of 1.5 lbs ae/A, or where the activity of preemergence herbicides resulted in smaller weeds at the time of postemergence application. Many growers continue to apply glyphosate at the rate of 0.75 lbs ae/A regardless of weed size and age orweather conditions, when applying a higher rate can be the better decision.

We are also hearing reports that many of the weeds not controlled by postemergence glyphosate applications have been infested with stem-boring insects. University research has shown some potential for a reduction in control when the stems have been infested with borers and that increasing glyphosate rate can partially compensate for the reduction in control. This same research shows that borers are a problem primarily when the weeds were initially treated when they were too large or too old, which allows enough time for borers to infest the weeds. Applying glyphosate to small weeds, prior to the appearance of stem-boring insects, is the most effective means of avoiding this problem. We suggest that glyphosate be included in late-season applications even where stem borer is occurring, on the assumption that it will still contribute to the control of borer-infested weeds.

We can offer the following information on late-season control of weeds, especially those weeds that have escaped prior treatment.

Timing of late postemergence applications?
Our research generally shows that an interval of no more than 3 to 4 weeks should occur between postemergence glyphosate applications, which means that postemergence treatments should be largely finished by the second week of July. This allows enough time for surviving weeds to show some regrowth, but weeds have typically not regrown to a large size. It’s not necessary to allow the weeds to regrow to the point that they are taller than the soybeans. Weeds that are about the same height or slightly smaller than the soybeans can receive adequate amounts of herbicide spray. Soybeans treated in late July or later in the summer are more likely to be injured by glyphosate. Labels for Monsanto brand glyphosate products state that they can be applied to soybeans through the R2 growth stage (flowering), but not after the R3 growth stage begins. The R3 growth stage begins when one of the uppermost four nodes with a fully developed leaf has a pod that is at least 3/16 inches long. Application of glyphosate to soybeans that have started the R3 stage is not supported by the labels (i.e. not legal). We assume that the labels for most other glyphosate products contain similar guidelines, but we have not conducted a thorough search of labels to verifywhether this is the case.

What to spray?
It is important to keep in mind that the amount of glyphosate that can be applied to Roundup Ready soybeans between emergence and the end of R2 growth stage is 2.25 lb ae/A. So, if you applied 0.75 lb ae/A in the first postemergence treatment (22 oz/A of Roundup Weathermax or 24 oz/A of Touchdown, or 32 oz/A of a generic glyphosate), you can apply up to 1.5 lb ae/A in the second treatment. If you applied 1.5 lb ae/A in the first treatment, you are limited to 0.75 lb ae/A in the second treatment.

Marestail. Glyphosate-resistant marestail is widespread in southern Indiana and Ohio. However, most populations are somewhat suppressed by glyphosate, so the best strategy will be to apply the maximum amount of glyphosate allowed (0.75 to 1.5 lb ae/A) in a mixture with either FirstRate or Classic. There are a few populations of marestail that are resistant to both glyphosate and Classic/FirstRate, but Purdue surveys suggests that less than 10% of the populations in Indiana are resistant to both glyphosate and ALS inhibitors. FirstRate typically provides 5 to 10% better control of marestail than Classic. If you don’t know if whether your population is resistant to ALS inhibitors, consider past herbicide use in that field. Repeated use of FirstRate, Classic, or Synchrony in a field over the past 5 to 7 years increases the probability of ALS resistance.

Common lambsquarter. Under dry weather conditions, lambsquarter becomes more difficult to control with glyphosate or any other systemic herbicide. As mentioned above, the best strategy at this point in the season would be to increase the glyphosate rate if you have the ability to do so based on the rate used in earlier treatments. Including additional nonionic surfactant may result in slight improvements in lambsquarter control, but is not a substitute for increasing glyphosate rate where it is possible to do so. If the 1.5 lb ae/A rate of glyphosate was used in the first postemergence treatment, and you are limited to 0.75 lb ae/A, the addition of Harmony GT may improve activity on lambsquarter.

Giant ragweed. Populations that have evolved a low level of resistance to glyphosate can be found in at least 10 counties in Indiana and 11 counties in Ohio. The populations in Indiana are randomly scattered all over the state. The best strategy at this point would be to increase the glyphosate rate if you have the ability to do so based on the rate used in earlier treatments. If the 1.5 lb ae/A rate of glyphosate was used in the first postemergence treatment, and you are limited to 0.75 lb ae/A, the addition of FirstRate, Flexstar, Cobra, or Phoenix may improve activity. Important points to keep in mind as you think about tankmix partners for giant ragweed include:

1) Although we haven't discovered a large number of populations resistant to both glyphosate and FirstRate, keep in mind that if you have previously observed poor performance by ALS inhibitors on giant ragweed in the field of concern, that the population may be ALS resistant. This could result in inadequate control with tank mixes of FirstRate and glyphosate.

2) Flexstar carryover potential is greater when it is used late in the growing season. You may want to use Cobra or Phoenix instead, to reduce the potential for Flexstar carryover. Also, keep in mind that the use of PPO (protoporphyrinogen oxidase) herbicides such as Flexstar or Cobra can antagonize the activity of glyphosate on some weeds. We have conducted trials to evaluate the use of different adjuvants in mixtures of glyphosate and Flexstar or Cobra, and our preliminary results suggest that we should stay with nonionic surfactant when mixing Flexstar and glyphosate. Our current research seems to indicate that while the use of crop oil concentrate reduces glyphosate activity, the mixture of Cobra plus glyphosate may be more effective when crop oil concentrate is added.

3) We have research in progress to resolve the rates that should be used in mixtures of glyphosate plus Cobra. While our information is incomplete at this time, this research seems to suggest that where the glyphosate rate is limited to 0.75 lbs ae/A in a late application, the Cobra rate should be 12.5 oz product/A. The Cobra rate can be reduced where it’s possible to use higher glyphosate rates.

Manganese and glyphosate?
The activity of glyphosate can be reduced when applied in mixtures with manganese. This reduction in activity is of greater concern when trying to control weeds that have escaped prior herbicide treatment and under drought conditions. Control of velvetleaf can be especially problematic when using these mixtures. Our best suggestion is to apply glyphosate and manganese separately when trying to clean up weedy fields this late in the season. Ideally, the glyphosate is applied first, followed by application of manganese at least one day later. When it’s impossible to avoid mixing glyphosate and managanese, follow these guidelines:

1. Use an EDTA-chelated form of managanese, which has less potential to reduce glyphosate’s activity in comparison with other forms of manganese.
2. Include ammonium sulfate in the spray mixture at the rate of 17 lbs/100 gallons. Add the ammonium sulfate to the spray tank first, followed by the manganese, followed by the glyphosate.
3. Use the highest glyphosate rate that can legally be applied, based on the amount used in previous postemergence applications (see comments on rate earlier in article).

Sentinel Plot Report July 2

Authors: Anne Dorrance

For Ohio, Frogeye leafspot was reported from only three locations last week.
Reports are still very low for this disease and the continued dry weather does not support spread. Be careful though of areas with heavy dews. Last Saturday, dews and fogs were quite heavy on the eastern side of the state. This does provide a great environment for foliar diseases. On the insect end spider mites were found in three locations, please see the entomologist report or more information on this aspect.

Nationally soybean rust was reported in a sentinel plot in Alabama. The southeast is finally receiving some much needed rains. We will see what this brings as far as rust reports. I am very curious to see if 4 months of dry weather can knock this out of some parts of the southeast. If that is the case it will take at least a month for the inoculum to build-up. For Ohio, the risk of rust is very, very, very low.