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Issue No. 9, Article 9/May 22, 2009

Should I Switch from Corn to Soybean? (Continued)

It seems we just can't catch a break from Mother Nature this spring. The May 18 USDA NASS weather and crops report indicated Illinois has planted 20% of corn and 1% of soybean acres. The five-year averages are 70% for corn and 50% for soybean, so we are extremely behind the typical planting schedules. Moreover, many of the acres that were planted last week received excessive rainfall. The decision to keep a low stand or replant some areas or entire fields will likely be a consideration many farmers will face on the few acres planted in the beginning of last week. The only consolation is we seem to be in a 7- to 10-day stretch of much-needed warm and dry days. Here are some of my thoughts regarding the question many producers may be asking.

So should I switch? This is a difficult decision for farmers to make when much winter effort has been spent planning and preparing for a particular crop, with crop management practices for particular fields. Emerson Nafziger wrote about economic considerations of this decision based on yield expectations in issue 7 of the Bulletin (May 8), and I encourage you to read his comments. If you are thinking of switching to soybean in fields where you grew soybean in 2008, additional considerations are the potential levels of soilborne diseases and whether you have already applied a preemergence herbicide and/or nitrogen in preparation for corn. If the previous crop was soybean but the field hasn't been in soybean for longer than 3 years, you can plant soybean again and achieve full yield. However, variety selection will be particularly important. I wrote about the value of variety selection in issue 4 (April 17). Most important in that article is the suggestion to use the University of Illinois Variety Testing program and the Varietal Information Programs for Soybeans in addition to information from your seed supplier to select varieties.

First, examine the infestation level of soybean cyst nematode (SCN) and choose a variety with an appropriate type of SCN resistance. It is best to choose a variety with a different genetic source of SCN resistance than you planted last year. Because PI88788 is the predominant type of SCN resistance available among commercial soybean varieties, this will require some attention and careful selection. If switching sources of SCN resistance is not possible, at least use a different variety. Another consideration is the level of infestation from Pythium, Phytopthora, Rhizoctonia, and Fusarium in your previous soybean seedlings. This is where your early-season scouting notes from last year are needed. Phytopthora and Rhizoctonia are more problematic in warmer soils, so they may be a bigger concern than Pythium as the date progresses. Again, choose varieties with appropriate resistance to potential diseases, particularly for fields with a history of previous problems.

If you have already applied nitrogen in preparation for planting corn, this does not prevent you from planting soybean. In fact, a high-yielding soybean crop requires more nitrogen than corn. However, soybean produces half to three-quarters of its own nitrogen through fixation from symbiotic bacteria (Bradyrhizobia japonicum). Nitrogen fixation (root nodulation) will either not occur or will occur at a low rate where nitrogen was applied, so the extra nitrogen will typically not increase soybean yield. Do not expect any benefit to your soybean crop from the nitrogen you applied for corn. With the high price of nitrogen fertilizers, this is a major economic consideration in switching from corn to soybean. Lastly, having already applied a preemergence corn herbicide is the one thing that may put a halt to switching to soybean. There are legal restrictions regarding planting soybean in the same season following some corn herbicides (e.g., atrazine), though other herbicides may allow it. You will need to check the herbicide label for rotation restrictions.

My closing thought is not directly about agronomic decision-making but on our actions that can influence it. I know that when the weather allows, the pressure to till, fertilize, spray, and plant will push many of you to work long hours on low sleep. Please work deliberately and safely to protect yourself, your workers, and your loved ones. Sleep deprivation could affect agronomic concerns like crop damage and economic losses, for instance, should one hastily grab the wrong seed, or the wrong jug of herbicide because it has a name similar to the right one (pointed out by Aaron Hager in issue 5 on April 24). No one wants to begin with potential injuries from hasty actions, so be careful. I wish for everyone to have a safe spring.--Vince M. Davis

Author:
Vince Davis

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