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June 2011 Archive for Ask an Agronomist

RSS By: Farm Journal Agronomists, Farm Journal

Have your agronomic questions answered by a Farm Journal agronomist. E-mail us directly at, and we’ll respond on this blog to provide an interactive dialogue.

What Are Growing Degree Days?

Jun 30, 2011

Question: What are growing degree days, and how can I use them to determine how my corn crop is developing?

Answer: You can use heat measurement standards called growing degree days (GDDs), or growing degree units, to assess and project corn development stages.  Knowing the GDDs for your geography can also help equip you to select a package of hybrids for your farm that includes a variety of relative maturities.  Many seed corn companies readily provide such information for that purpose.
The amount of GDDs a specific corn hybrid requires to reach each development stage during the growing season remains constant from year to year. However, the amount of time that hybrid needs to accumulate those heat units can vary considerably from year to year due to planting date, weather conditions and temperature. Contact your local or state Extension service for specific information on GDDs for your geography. University Extension resources also often provide a GDD calculator to make tracking heat units an easier process.
GDDs are calculated for each day starting the day after planting. To calculate GDDs, review the basic equation, direction and examples provided here.
GDD= [(high °F + low °F)/2]-50
If the high is above 86°F, use 86°F in the equation.
If the low is below 50°F, use 50°F in the equation.
High: 81°F Low: 63°F
GDD= ((81+63)/2)-50= 22 GDD
High: 68°F Low: 44°F
GDD= ((68+50)/2)-50= 9 GDD
Be aware that while most companies rate hybrids based on the timing between planting and maturity, some rate them from emergence to maturity.  If the latter is what you find, add 150 to get the GDDs from planting.
This year’s class of Corn College graduates will leave the event with take-home-to-the-farm techniques to raise their corn yields higher.

What Impact Will Excess Moisture Have On My Corn Yields?

Jun 28, 2011

Question: My corn looked good but turned yellow around the v4 stage due to excess rain. It is starting to turn around and look better with improved weather. How much yield potential have I lost?

Answer:Just how much yield you can expect to lose is hard to say without seeing the crop in the field. When corn is in the 4-5 leaf stage with this kind of water, my experience has been that its survival and quality of survival will depend on your current air-soil temperatures.  If the temperatures are in the 45- to 60-degree range, the corn could actually survive being under water for about three days.  If the temperature is in the 65- to 80-degree range, however, the plant respiration is so high that after 24 hours there would be a lot of damage.  The other problem you have is that any time water goes over the top of corn there will be soil that gets down into the whorl of the corn.  The soil that gets into the whorl contributes to crazy top in the corn, so even if it survives there’ll be a percentage of that corn that will be negatively impacted. 
I’m also including a news release (see below) that was put out by North Dakota State University Extension recently on this topic. It may give you some additional food-for-thought to help you determine what to expect from your corn crop this season.
Impact of Excessive Rainfall on Corn Growth & Development
Excessive rainfall this spring, following an unusually wet winter, has resulted in extensive flooding in many regions of North Dakota. Even soils that are not visibly flooded quickly become saturated after a rain because there is little evapotranspiration occurring as a result of the low temperatures and lack of an established crop.
Waterlogging (flooded/ponded/saturated soils) affects a number of biological and chemical processes in plants and soils that can affect crop growth in the short and long term.
"The primary cause of waterlogging in crop plants is oxygen deprivation or anoxia because excess water does not react chemically with the plant," says Joel Ransom, North Dakota State University Extension Service agronomist. "Plants need oxygen for cell division, growth, and the uptake and transportation of nutrients. Since oxygen diffuses through undisturbed water much more slowly than well-drained soil, oxygen requirements rapidly exceed that which is available when soils are saturated."
The rate of oxygen depletion in saturated soil is affected by the temperature and rate of biological activity in the soil. Faster oxygen depletion occurs when temperatures are higher and when soils are actively metabolizing organic matter.
Cooler weather will delay the adverse effects of waterlogging on emerged crops. Generally, the oxygen level in a saturated soil reaches the point that is harmful to plant growth after about 48-96 hours. In an effort to survive, tissues growing under reduced oxygen levels use alternative metabolic pathways that produce byproducts. Some of the byproducts are toxic at elevated levels.
"Germinating seeds or emerging seedlings are very sensitive to waterlogging because their level of metabolism is high," Ransom says. "Crops, such as small grains and corn, tend to be more sensitive to waterlogging when their growing point is below the surface of the soil (before the five to six leaf stage)."
With the exception of winter wheat, all of the small-grain and corn crops in the state still are in these sensitive stages (if planted at all) and can be killed if soils are saturated beyond 48 hours and the soil temperature exceeds 65° F.
Crops can differ in their tolerance to waterlogging. Data from differing sources suggest a possible ranking of waterlogging tolerance. The most tolerant to most susceptible are rice, soybeans, oats, wheat, corn, barley, canola, peas, dry beans and lentils. Growth stage and variety can affect this ranking.
"Waterlogged conditions also reduce root growth and can predispose the plant to root rots, so the ultimate effect of excess moisture may not be known until late in the season," Ransom says. "It is common to observe plants that have experienced waterlogging to be especially sensitive to hot temperatures and to display nitrogen (N) and phosphorus deficiencies later in the season due to restricted root development. Yield losses can occur even if these obvious visible symptoms are not observed."
Waterlogging can impact cereal plant growth indirectly by affecting the availability of N in the soil. Excessive water can leach nitrate N beyond the rooting zone of the developing plant, particularly in well-drained, lighter-textured soils.
In heavier soils, nitrate N can be lost through denitrification. The amount of loss depends on the amount of nitrate in the soil, soil temperature and the length of time the soil is saturated.
"Research conducted in other states found losses from denitrification between 1% and 5% for each day that the soil remains saturated," Ransom says.

Isn't It Too Early For Soybean Aphids?

Jun 24, 2011

Question: I’m hearing reports about soybean aphids already. Isn’t it too early for them to be in fields?

 Answer: We’re hearing the same thing, though it is a bit early in the season for them to be showing up in large numbers.That said, we would encourage you to touch base with your local county extension or consulting agronomist to determine what the outlook for aphids is in your area this season. Soybean aphids weren’t a widespread problem in 2010, but late-season build-ups last summer resulted in a number of predictions that they would be more of a problem this season. They are a pest you want to stay on top of. They transmit viruses such as the soybean dwarf virus and soybean mosaic. The economic threshold for aphids is 250 aphids per plant. We use this threshold up until the later reproductive stages, around R4. Some research we’ve seen indicates there could be an economic benefit from using this threshold through R5. You can use a simple speed counting process to determine whether you’ve reached threshold levels—it doesn’t have to take a ton of time and is certainly well worth your effort. The protocol for this process was developed by researchers at the University of Minnesota. For more information on the procedure, go to: soybean aphid counts.

What Can I Do To Prevent Frogeye Leaf Spot?

Jun 20, 2011

Question:I had a problem with frogeye leaf spot in soybeans last year and want to be proactive this season to prevent it.   What would you advise?

Answer:Cool, wet conditions and early planting can set the stage for seedling diseases and for frogeye leaf spot. Fortunately, it’s becoming less of a threat in the Midwest as growers adopt resistant varieties, and hopefully that’s what you’ve done this year. Carl Bradley at the University of Illinois has detected fungicide-resistant strains of Cercospora sojina, the causal agent of frogeye leaf spot, which suggests the disease should not be taken for granted. Bradley found the strobilurin fungicide–resistant strains in plant samples from a Tennessee soybean field. The active ingredient strobilurin is used in several popular fungicide products. So far, the fungicide-resistant strains have not been found to be widespread. But their existence reaffirms the importance of planting varieties resistant to frogeye leaf spot. If you planted a susceptible variety and are considering applying a fungicide, choose a triazole fungicide rather than a strobilurin product. If you need to spray a strobilurin fungicide for other foliar diseases as well as frogeye leaf spot, apply a product or a tank mix containing both classes of fungicides.
Know what conditions can foster each disease, select resistant or tolerant varieties and be prepared to apply a fungicide if necessary.

What Causes Poor Corn Spacing?

Jun 17, 2011

Question:I heard something recently about how to tell the causes of poor corn spacing but didn’t get the entire story. Please pass that along when you have a chance. Thanks.

Answer:After the corn emerges, there are simple ways to determine the causes of problematic plant spacings. For instance, a double occurs when the planter meter picks up two seeds and drops two at the same time. In the field, you can tell whether this is what occurred when it looks like you have good uniform spacing on either side of the two plants that are too close together. However, a misplaced seed looks different from a double. With a misplaced seed, while the spacing is still real close together, you will see a gap from where the seed should have been on one side of the grouping. In this case the meter functioned properly in releasing one seed at a time, but there was ricochet in the seed tube, which caused the seed to bounce and the timing to be off. You can learn more about corn spacing guidelines in Episode 3 of Corn College TV. 

Is It Too Late To Replant Corn?

Jun 12, 2011

Question: Is it too late to replant corn?

Answer: As you probably already know, it’s really questionable whether you should replant corn at this point. That said, here are some thoughts that Bob Nielsen, Purdue University corn specialist, shared with Farm Journal editors about replanting corn at this time in the season.
1.                         Calendar date. The date is getting awfully late, he says. Historical yield potential drops off so substantially that you have to have a really low stand of corn at this point to even think about replanting.
2.                         Stand assessment. Would you be planting the entire field or just part of the field?  Nielsen suggests taking a careful assessment of the surviving population of a field and weighing that against the potential for lower yields of something planted this late. "This is not a straightforward estimate to make because there are so many factors,” he says. "But, this kind of assessment is important.”
3.                         Herbicides. Have you already put down corn herbicides on this field? If you have, he says, you are probably restricted from planting soybeans in that field. So, you can either try your hand at replanting or hope for the best.
If you do decide to replant some corn, Nielsen suggests trying to find a hybrid with strong disease resistance: "Late planted corn can have the potential for more disease problems, as a disease can attack a corn plant when it is younger, letting the disease have more time to develop.”
For More Information

How Can I Tell If I Have Down Pressure Issues With My Planter?

Jun 07, 2011

Question: If I had some down pressure issues with my planter this spring, how could I tell? 

Answer: Corn roots can tell you whether you had the correct amount of down pressure on your planter units. Instead of digging right on top of the plants, stay back outside of where the depth wheels ran. Dig around the plant and pull back on it. If the slot opens up that suggests that the slot was smeared. Next, examine the roots. You want to see a system where the roots come off the crown in a circular pattern and move down through the soil at a 30-degree to 35-degree angle. Dig the plant out of the ground and carefully remove the soil from the root system. Pay close attention to the root system in the area where the depth wheels ran. If there is too much depth-wheel compaction, the roots will be forced to find a way to grow around it, instead of randomly growing down through it. In that case, you’ll see lobes underneath the path where the planter wheels ran, and you’re going to have root systems that are not moving down through the soil as freely as they want to. One side note: Although it’s not always the case, down pressure is more likely to be a problem in conventional tillage than in no-till. 
Take this quiz to test your down pressure know-how. Setting down pressure is challenging because it is dependent on many factors that are unique to your fields and that vary on the days you're planting.

How Do I Address Sulfur Needs In Corn?

Jun 03, 2011

Question: I'm very confused with everything you read about S needs, removal, uptake and recommendations. One expert says for 200-bushel corn, uptake is 30 lbs and removal is 15 lbs. I don’t understand why the numbers are so different. Another expert says that S removal is .07 pounds per bushel, which is close to agreement with the above statement. Do I need 15 or 30 pounds for 200-bushel corn?

Answer: Nutrient removal values are different that nutrient uptake values. Nutrient uptake refers to the total amount of nutrients accumulated in the crop biomass, some of which, depending on the crop, is recycled in the field. Nutrient removal values refer only to that fraction of the crop that is harvested and removed from the field. So, in other words, when residue is left in the field it contains nutrient value. Removal rates are typically based on pounds per bushel. As long as your cropping history has residue left in the field, you should be fine applying just removal rates of Sulfur. Keep in mind that your soil test can also give you an indication of residual levels of sulfur in your soil. For a 200 bu/A corn crop, consider applying 15 lb Sulfur per acre. 

This farmer addressed sulfur losses in his corn crop.
A 2010 survey of alfalfa stands in Wisconsin lends still more credence to the notion that growers in parts of the country need to be on the lookout for signs of sulfur deficiency in their crops.
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