C.O.R.N. Newsletter 2011-20

Dates Covered: 
June 27, 2011 - July 4, 2011
Editor: 
Jon Rausch

Western Bean Cutworm Flight Begins

Over the past few weeks, OSU-Extension Educators have placed 128 western bean cutworm traps all across the state to monitor for flight this year.  Last week, a few counties reported some catches (see our new map at: http://entomology.osu.edu/ag/) and this agrees with observations from other eastern Great Lakes regions who have also caught moths.  Most of our corn is very late planted, from V2 –V5, and is at risk for infestation (once corn tassels, it is a less preferred host).   Even in whorl stage, larvae can burrow through leaves and feed on the developing tassel. As we are likely at least a couple of weeks away, it is not too late to put out traps (see our article last week) to monitor the flight near your corn fields to know when to start scouting corn. Scouting for eggs and larvae should begin when adult catches occur on consecutive nights.  Inspect 20 plants in 5 random locations throughout a field.  Female moths prefer to lay eggs on the uppermost leaves which are still vertical in orientation, so those leaves should be inspected thoroughly.  If western bean cutworm eggs are found, please contact your County Extension Educator, or one of us (michel.70@osu.edu, hammond.5@osu.edu).   

Moderate levels of wheat head scab are present throughout the state

Moderate levels of wheat head scab are present throughout the state

Surveys of wheat fields for head scab began in Ohio during the 2nd week of June. Frequent rainfall accompanied by warm temperatures prompted the Wheat Scab Risk Assessment Tool (http://www.wheatscab.psu.edu) to show moderate risk of scab development across several areas of the state throughout the flowering period. So far, 140 fields in 27 counties have been surveyed, and the incidence of scab in untreated fields has ranged from 0.4% to 45%. An incidence of 45%, for example, means that 45 heads out of every 100 heads had scab symptoms. A few fields in all parts of the state (southern, central, and northern) had greater than 25% incidence, while other fields had very low incidence. This is consistent with the predictions given from the Risk Tool. Fields with low incidence levels may have escaped disease by flowering before or after the period of frequent rainfall and high humidity. Remember, scab develops best when wet, humid weather occurs when the wheat is flowering. On average, fields planted with a moderately resistant variety had lower levels of scab (6.5% incidence) than fields planted with susceptible or moderately susceptible varieties (8.2%). Also, fields planted with a moderately resistant variety and treated with a fungicide (e.g., Prosaro or Caramba applied at flowering) had lower levels of scab (5%) than fields planted with a susceptible or moderately susceptible variety and not treated with a fungicide (13.6%).

In fields with more than 10% incidence (49 of the 140 fields surveyed so far), vomitoxin may still be a concern. Grain with vomitoxin levels above 2 ppm may be docked or may receive a price discount at elevators. To minimize these discounts or even prevent dockage, harvest grain from scabby fields separately from more healthy fields, and turn up the combine fan to blow out scabby, lightweight grain. Have grain for fields with scab tested for vomitoxin before feeding it to livestock since several animals, particularly swine, may have serious health problems if fed grain with high levels of vomitoxin.

Three things are very clear from this year’s survey, emphasizing the importance of managing the wheat crop to minimize problems with scab and vomitoxin:

1-     Planting fields on different dates or with varieties with different maturities prevents scab from affecting entire fields or all fields at the same level. Some early- or late-flowering fields or sections of fields will escape scab-favorable conditions, reducing the total damage.

2-     We now have varieties with good scab resistance, plant them. These well reduce infection, spread on the fungus in the head and the accumulation of vomitoxin in the grain.

3-     A WELL-TIMED fungicide application does reduce scab (and vomitoxin). Combining a fungicide treatment with the best scab resistance provides the best control.       

Reduced corn stands: Some management considerations

Reduced corn stands: Some management considerations

I have been receiving questions about the productivity of corn fields with stands as low as 10-12,000 plants per A.  Causes of the stand loss in these fields  can be attributed to a wide range of factors, including saturated soil conditions and ponding, seedling disease and insect (esp. cutworm) injury, planter adjustment/malfunction,  herbicide damage, hail, and green snap). The reduced stands are often associated with mid-May plantings and the surviving corn plants are usually at or beyond the V7-8 stage. Despite the magnitude of the stand loss in these May planted fields; they may have the potential to produce yields comparable to June plantings that have much better stands.

Table 1 is from an article by Dr. Bob Nielsen (http://www.agry.purdue.edu/ext/corn/news/timeless/haildamageyoungcorn.html) at Purdue University that shows effects of planting date and plant population on final grain yield. It’s based on research conducted by Dr. Emerson Nafziger at the University of Illinois with yield responses to planting date extrapolated beyond May 25.  Although the table was used by Dr. Nielsen to discuss recovery from hail damage it may also have application in assessing yield loss in other reduced stand situations.

Let's assume that a farmer planted corn on May 10 and June 4 at a seeding rate sufficient to achieve a harvest population of 30,000 plants per acre. The farmer recently determined that his May stands were reduced to 12 -18,000 plants per acre by cutworm and saturated soil conditions, whereas his June stands were nearly 30,000 plant per acre.  According to Table 1, the expected yield for the existing stand planted May 10 would be 71-86% of the optimum, whereas the expected yield of the June 4 planting would be 75% of the optimum.  Of course there are other factors to consider…plant spacing may be highly variable in the field with reduced stands and plant development may be uneven. Growers need to be aware that weed pressure is likely to be much greater in such fields because a competitive corn canopy is largely absent. Weed size will need to be monitored closely to ensure that post emergent herbicides can be applied to optimize their efficacy. Another question concerning reduced stands is whether N rates should be cut. Research from OSU and neighboring states indicates that low and high plant populations (ranging from 20,000  to 40,000 plants per acre) show similar yield responses to N.

 Reference

Nielsen, R.L. 2011.Recovery from hail damage. Corny News Network, Purdue Univ. [On-Line]. Available at (http://www.agry.purdue.edu/ext/corn/news/timeless/haildamageyoungcorn.html) [URL accessed June 27 2011].

The Importance of Sunlight

Warm temperatures have helped bring the corn crop along nicely in Illinois, at least in fields and parts of fields where the plants have stayed above water. As we pass the longest day of the year in the northern hemisphere (June 21), with its maximum amount of sunlight, it's a good time to think about sunlight and its effect on the crop.

If we think of the crop as a yield-producing factory, sunlight is the fuel that powers it. But this is a factory that has to build itself. As leaves emerge from the whorls and green up as they open to the sunlight, they start to photosynthesize, producing the sugars that fuel all crop growth and yield. This in turn helps leaves and the rest of the plant to grow (add dry matter), helping the factory pick up speed. The factory needs to be at full speed, with a full light-gathering "roof" (crop canopy) by pollination time in order to produce maximum grain yields.

So the development of leaf area, collectively called the crop canopy, is critical to the growth process. We often say that the canopy "closes" at the time when we can't see soil anymore as we look down the rows from the end; the canopy seems to form a solid surface that covers the field. In 30-inch rows, this happens when the crop is about waist-high, generally at about stage V10. Fields planted in April or early May are at or nearing this stage now.

Though canopy closure would seem to be the point at which the crop begins to intercept all of the sunlight, that is not the case: a single leaf lets some of the light pass through, and leaves are not evenly distributed, so some amount sunlight is still reaching the ground. Plants need to develop a "leaf area index"--square feet of leaf per square foot of ground area--of 3 or more before the crop intercepts more than 95% of sunlight. In corn, this happens only when the crop is tasseled, at which time its exposed leaf area is close to the maximum.

Because the crop is growing so rapidly by stage V8 to V9, corn in narrow or twin rows does not have a very large or lasting advantage over 30-inch rows in the rate at which they increase leaf area and sunlight interception. While any such advantage is often positive, the difference in total sunlight intercepted by wide and narrow by the time plants reach full canopy is relatively minor--less than 10 percent. Pollination success and yield are determined after full canopy, and by then there is often no difference in light interception between wide-row and narrow-row corn.

Because sunlight is the driving force for all crop growth, there is considerable concern about the amount of sunlight and whether it may limit yields. The Water and Atmospheric Monitoring (WARM) program of the Illinois State Water Survey publishes monthly data on sunlight at a number of Illinois locations. The sunlight data are in a column headed "Total Solar Rad" with units of megajoules per square meter.

A megajoule (MJ) is a unit of energy equal to about 240 kilocalories or 0.28 kilowatt-hours. A square meter is about 10.7 square feet. The maximum sunlight received during a summer is about 32 MJ/square meter, and daily averages are typically about 3/4 of the maximum. In more familiar units, 30 MJ/square meter received on a sunny day is about 670 kcal/square foot or about 3/4 of a kilowatt-hour per square foot. That amount of sunlight energy is the equivalent of 33 megawatt-hours of electrical power per acre; in terms of chemical energy, it is the equivalent of some 14 tons of sugar per acre.

Only about half the energy in sunlight is in the visible wavelengths; most of the rest is infrared (heat) or ultraviolet and is "invisible" to both our eye and to plants. Plants intercept more red and blue light than green light; they reflect or transmit much of the green light and so appear green. For a variety of reasons, the plant cannot convert sunlight to sugars with high efficiency. In fact, on a good day the plant will typically convert only about 2% of the sunlight energy into dry matter. But with so much energy falling on an acre in a season, very high yields are still possible.

Of the three most recent crop seasons, 2008 and 2010 had high sunlight and 2009 had low (Table 1), with the total over three months in 2009 about 10% less than the totals in the two high-sunlight years. In these three years, yields were highest in 2008 and lowest in 2010. In 2004, the highest average yield on record in Illinois came with only 1,987 MJ/square meters of sunlight over these three months, about the same as in 2009. So it's clear that while sunlight has an effect on productivity, it appears to be less important than temperature and rainfall. These factors are all correlated to some extent, making it difficult to single out the most important factor in determining yield.

Table 1. Summer monthly totals of sunlight received at Champaign, Illinois, 2008-2010.

Month

MJ/m*m

2008

2009

2010

June

753

680

720

July

741

667

730

August

684

617

731

Total

2,178

1,964

2,181

Source: www.isws.illinois.edu/warm/datalist.asp.

Corn planted this year at the end of March at Urbana is at stage V12 and about chin-high now. The late-May planting is at V4 and less than a foot tall. Clearly, the amounts of sunlight that these two canopies will intercept during these longest days of the year differ vastly. But if the season lasts long enough and there is little water stress, the late-planted crop can still intercept enough sunlight to yield 80% or more of the early-planted crop. Both need careful attention to limit stresses and to keep the canopy in top shape in order to reach their potential.

 

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About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.