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Issue No. 5, Article 6/May 7, 2010

Early-Season Soybean Considerations, Scouting, and Counting

The NASS Illinois Weather and Crops Report for May 2 reported that 11% of soybean acres were planted, which was 6% progress over the previous week. Many producers were wondering if starting soybean planting was a good idea, and many more were thinking that the start to planting soybean was earlier than normal. This was true compared to recent years; however, we were also 11% planted on this same date in 2005 and 18% and 15% in 2001 and 2000, respectively. So the speed of soybean planting might be above average, but it has not set any records.

Much of the planting that occurred in April happened before sweeping rains across much of Illinois, ranging from a few tenths of an inch to multiple inches. Air temperatures have remained mostly above average by 2 to 5°F during the last 30 days in most locations, and enough sunshine has been available that soil temperatures have remained seasonally warm. Soybeans planted on April 22 for experiments on our South Farms in Champaign County have emerged nicely.

The conditions I've described should contribute to good emergence rates in most early-planted soybean fields. There were a few small areas with hard and heavy rains, however, and crusting might be a concern in some conventionally tilled fields. Some of the information regarding soybean emergence issues and early development is still relevant in a 2009 article of the Bulletin that Carl Bradley and I wrote (issue 10, May 29, 2009). I won't repeat any details, but I would like to expand on the importance of accurately recording your soybean plant stand establishments this spring.

Last week (issue 4, April 29, 2010), I discussed the effect of economic conditions on soybean seeding rates. Perhaps more important than simply reducing seeding rates as seed prices rise is accurately understanding the relationships that contribute to the percentage of plant stands established before making adjustments.

I am often asked my "recommendation" for soybean seeding rate, and I rarely give a precise answer because there is a lot of variability in seeding rates versus plant stand establishments. There are multiple reasons for this, including these: soybeans "compensate" for intraspecies competition well and thus can produce an equal yield for a wide range of established plant populations; there is much more variability in seedbed preparation for soybean than for corn (it is usually not as good); there is much more variability in the precision and accuracy of planting equipment used for soybean versus corn; and the quality of soybean seed (in terms of viability) can fluctuate more for soybean between years due to environmental conditions during seed production. All of these issues must be accounted for. The formula I presented in last week's article was seeding rate = desired plant stand establishment divided by the percentage of viable seeds planted, then divided again by the percentage of your expected emergence rate. The mathematics of this formula are simple, but the resulting accuracy is only as good as the assumption of expected emergence rate.

This assumption can only be improved by having the best knowledge possible of the variability associated with seedbed preparations, planting equipment, and overall planting conditions. This knowledge comes only with experience--including scouting and counting your soybean stand establishments. There are various methods for counting plant stands. The more area that is counted, the more accurate the count, and the more counts in a field, the more accurate average field estimates will be. How many counts to do in a field also depends on field variability and observed emergence variability. If a field appears to have uniform conditions and a uniform emergence pattern, then a handful of independent counts across the field are probably good enough. However, you certainly want to have independent counts for any major changes, such as differing varieties, tillage systems, and planter settings. You may also want independent counts for factors like "low" and "high" field areas, changes in soil type, and the like.

The area represented by each independent count can also vary. The "hula hoop" method is commonly recommended, particularly for narrow rows. To determine the plant population, divide the square feet in an acre (43,560) by the radius of the hoop (in inches) squared, then multiply by 3.14 and divide by 144. This provides a multiplying factor to convert the number of plants in the hoop to plants per acre.

Here is an example for a 36-inch hoop: 43,560 / (182 x 3.14) / 144 = 43,560 / 7.065 = 6,165. So for plants per acre, multiply the number of plants in the hoop by 6,165.

Another common method is to count several plants for a given feet per row. For instance, there are 17,424 linear feet of row per acre for rows 30 inches wide. So multiplying the average number of plants per foot by 17,424 in 30-inch rows will also give you plants per acre. This is not difficult, and many of you are well experienced with counting plant stands. I've gone through this, though, because the changing economies of seeding rates mean it is more important than ever to understand expected seedling emergence rates. Scouting and counting soybean seedlings in every field will be the best way to develop a good set of notes from various planting conditions specific to each operation, This in turn will provide the most accurate information for determining how to adjust seeding rates in the future.--Vince M. Davis

Vince Davis

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