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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2010-09

Dates Covered: 
April 19, 2010 - April 26, 2010
Steve Prochaska

Seeding Rates for Corn: 2009 Research Update

According to the National Agricultural Statistics Service, corn populations in Ohio have increased 16% during the past 10 years. Final stands for corn in Ohio averaged 28,200 plants/A in 2009 and final stands on nearly a third of state’s corn acreage was 30,000 plant/A or higher. Three years ago, final stands were 30,000 plant/A or higher on only 14% of the corn acreage. Newer hybrids are more stable under stress with improved resistance to lodging, diseases and insect pests. Studies have demonstrated that the superiority of modern hybrids is fully expressed only at higher plant populations. 

Most corn agronomists recommend adjusting seeding rates by using the yield potential of a site as a major criterion for determining the appropriate plant population. Some seed companies recommend final stands for hybrids as high as 35,000-36,000 plants/A. In 2006, we initiated an evaluation of corn response to plant population across a range of production environments. Our tests were conducted at eight to nine locations each year in fields with high yield potential (200 bu/A + under favorable conditions). We considered final stands of 24, 30, 36, and 42,000 plants/A. Results have varied over the past four years, with yields averaging 191, 191, 177, and 228 bu/A for 2006, 2007, 2008, and 2009, respectively. In 2006 and 2009 results suggested that final stands of 36,000/A were required for optimal yields. In 2007 and 2008, there was no yield response to plant populations above 30,000 plants/A. Higher plant populations were usually associated with more stalk lodging (stalks broken below the ear). The most severe lodging occurred in 2008 when high winds associated with Hurricane Ike contributed to severe lodging (especially at 42,000 plants/A). Averaged across locations, lodging in 2008 was 10, 16, 33, and 52% at 24, 30, 36, and 42,000 plants/A, respectively and the extensive stalk damage may account in part for the absence of yield increases above 30,000 plants/A. However, in 2007 lodging was negligible at all populations yet there was no yield response above 30,000 plants/A. On average, increasing plant population from 24,000 plant/A to 42,000 plants /A reduced grain moisture but effects were highly variable form year to year ranging from 0 in 2007 to 1.1% in 2009. 

We are also performing research to determine if Bt traits for corn rootworm influence hybrid response to varying seeding rates. In 2008 and 2009, we conducted evaluations of two VT3 triple stack and their RR isolines at the OSU research stations near Hoytville and S. Charleston to determine if the optimal seeding rates for non-Bt hybrids were the same as those for Bt CRW hybrids. Preliminary Ohio results indicate that the yield response of triple stack and RR hybrids to plant population was similar. However, insect pressure was low and differences in root and stalk lodging between triple stack and RR hybrids were not evident. 

Based on studies to date, a seeding rate of 31 - 33,000 seeds/A will be adequate for optimal yields in most production environments in late April and early May. For fields with low yield potential, seeding rates of 24 - 26,000 seeds/A will probably be sufficient. For fields with very productive soils and high yield (180-190 bu/A levels, seeding rates of 36 - 37,000 seeds/A may be necessary. (These seeding rates estimates are based on 10% mortality). Planting rate or population can be cut to lower seed costs but this approach typically costs more than it saves. Most research suggests that planting a hybrid at suboptimal seeding rates is usually more likely to cause yield loss than planting above recommended rates (unless lodging becomes more severe at higher population levels). Follow seed company recommendations to adjust the population for specific hybrids. 

To find out what other university extension specialists in neighboring states are saying about seeding rates for corn in 2010 check out the following: Roth, G. 2010. Corn Seeding Rate: Final Comments Penn State University Field Crop News April 6, 2010. Vol. 10:05 Lee, C. 2010. Corn Seeding Rates. University of Kentucky Grain Crops Update. Wednesday, March 31, 2010.


Reminders About Dry weather and Preemergence Corn Herbicide Performance

We are in the middle of a dry weather period that is enabling an early and rapid start to corn and soybean planting. Our concern in this type of situation is that continued dry weather can result in at least partial failure of preemergence herbicides, which are currently being applied at a rapid rate also. In a typical spring planting season, we receive rain often enough to ensure preemergence herbicide effectiveness, and it’s possible that the rain forecast for the end of the week will ensure that it occurs this year also. Preemergence herbicides reach maximum effectiveness only if rain moves them down into soil before weed shoots emerge. Once the shoots have emerged, herbicides that act strictly through residual activity have lost most of their effectiveness, unless you are a big believer in “reachback activity” (see C.O.R.N. article from April 21, 2009). 

In a situation where it is dry for the first several weeks after planting, we assume a subsequent rain will eventually “activate” preemergence herbicides, so that they will provide weed control, even if they fail to control the weeds that emerge soon after planting. One solution here is to apply a low rate of glyphosate or other postemrgence herbicides to control the small weeds that emerge initially, with the hope that the preemergence herbicides will control the later-emerging weeds. 

It may be worth reconsidering an application of preemergence herbicides when there is no rain in the foreseeable (week or more) forecast, and switching to an early postemergence application. An early postemergence application of foliar plus residual herbicides can be just as effective at preventing yield loss due to weed interference, compared with a program of consisting sequential applications of preemergence and postemergence herbicides. Most preemergence corn herbicides can be applied to emerged corn, and some of them have enough foliar activity to control small, emerged weeds without the need to add postemergence herbicides. In addition, the majority of the corn planted in 2010 is resistant to glyphosate and/or glufosinate (Ignite), and these can be combined with preemergence herbicides to control weeds emerged at the time of application. 
For more information on this subject, see the article in the May 4, 2009 issue of CORN, “Corn planted with no herbicide applied? Don’t panic”. 


Is Your Wheat Field at Feekes Growth Stage 6?

The above average temperatures over the past few weeks may have accelerated the development of the wheat crop. It is important to know when the crop has matured to Feekes Growth Stage 6 (initial stem elongation or jointing). Basically the crop switches from vegetative growth to reproductive at this time and can be visually determined when the first node is observed above the soil surface. To check for this node, dig up plants from multiple locations in the field, remove the secondary (smaller) tillers and observe the main stem (may appear as large tillers). Gently pull down the lowest two leaves (including the sheath around the stem) one at a time to the base of the tiller. If there is a swollen area, or an area that is slightly a different shade of green than the rest of the stem, then the first node is present. It is generally located about an inch above the soil surface. At early Growth Stage 6, the node is only detected by pulling down the lower leaves, as the plant matures, the node (a bump) can be detected by running the fingers along the base of the stem. If two nodes are present, the field is at the next growth stage, Feekes 7. 

Growth Stage 6 is an important developmental time for the crop which should be reflected in management practices. Top dressing of N needs to be completed by or soon after this stage since the crop will rapidly uptake N from this point until flowering. OSU research has shown a 10 – 15 % yield reduction when the first spring application of N did not occur until late stem elongation (Feekes Growth Stage 9). Also, labels for several wheat herbicides indicate that the product should no longer be applied after jointing due to the risk of yield loss. Dicamba is the primary herbicide of concern here, but the labels for some 2,4-D products also state that application should occur prior to jointing. OSU research has generally shown that 2,4-D can be applied past jointing and prior to joint with minimal risk of injury, but it’s important to use a 2,4-D product that supports this use. 

In recent years, some producers have applied or shown an interest in applying foliar fungicides at Growth Stage 6 without disease pressure. This questionable practice generally makes a half-rate application at Feekes 5-6 followed by a second half-rate or full rate at or after Feekes 8 (flag leaf emergence). OSU research to date has shown that yield response to these early applications of fungicides is highly variable and depends on how early and how much disease develops. For instance the results from six fungicide trials showed yield responses ranging from -1.1 to 7 bushels/acre. This 7-bushel yield increase was observed in an inoculated trial in which final disease severity on the flag leaf was more than 90% in the check compared to 68% in the fungicide treated plot. Inoculations had been done in the fall which led to an increased early disease development in the spring. Unnecessary use of fungicides and applications at half-rates increase the risk of fungi developing resistance to these products, making them ineffective when they are really needed to manage diseases. So even if early half-rate fungicide use occasionally leads to a marginal yield increase, this practice may result in the loss of efficacy of valuable fungicides. 

Weather Update

Expect a mainly dry week with near normal temperatures but quite low relative humidity. The threat for any signficant freezes appears over. 

We do have rain chances in our future from this weekend into next week and into the first few days of May. However, there is a large amount of uncertainty in location of rainfall with the best chances of the heaviest rain passing east, south and west of Ohio. Rain totals by our computer models range from less than 0.25 inches to over 2 inches. I expect most places will see a 0.50 to 1 inch rain in the 2 week outlook. 

The longer-term trend remains in place for May and beyond of warmer and drier than normal conditions, but there is enough uncertainty in this forecast that the official climate outlooks continue to give us equal chances or near normal conditions from May into summer.

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Crop Observation and Recommendation Network

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.