Crop tours and yield estimates are all the rage this time of year, with the corn and soybean crops rounding the corner toward harvest, one many say could yield record crop output.
But, no yield projection this early is perfect. Rarely does an estimate this time of year wind up being completely accurate. It's just too easy for Mother Nature to throw a wrench into things between now and harvest time.
If you want to get a feel for your own soybean yield potential instead of relying on others, just follow a few key steps. And now, you can use technology to get a better idea of how your soybean crop will fare this fall.
"You can start making soybean yield estimates as soon as end of the R4 stage, full pod, or at the onset of the R5 stage, beginning seed (seeds are 1/8 inch long on one of the top four nodes), knowing that the yield prediction is less precise at those early stages," says Kansas State University crop production specialist Ignacio Ciampitti. "Variability between plants relative to the final number of pods and seed size needs to be considered when trying to get an estimation of soybean yields. In addition, variability between areas within the same field needs also to be properly accounted for. Yield estimations should be made in different areas of the field, at least 6 to 12 different areas. It is important to properly recognize and identify the variation within the field, and then take enough samples from the different areas to fairly represent the entire field."
A "conventional" approach to estimating soybean yields this time of year is based on 4 variables, according to Ciampitti:
1 Total number of pods per acre (number of plants per acre x pods per plant)
2 Total number of seeds per pod
3 Number of seeds per pound
4 Total pounds per bushel, or test weight (for soybeans, usually around 60 pounds/bushel)
"The final equation for the estimation of the potential soybean yield is: 1 x 2 / 3 / 4 = Soybean yield in bushels/acre," Ciampitti says. "In the 'simplified' approach to estimating yields, sample 21-inch row lengths to equal 1/10,000th of an acre. The number of rows to sample will depend on the row spacing. With 30-inch row spacings, sample one row. With 15-inch row spacings, sample two rows. With 7.5-inch row spacings, sample four rows."
Seed size is another variable you need to watch to get a feel for your soybean yields, Ciampitti adds. There's a lot of variability in yield calculations with different seed sizes, and that needs to be taken into account when reaching a final calculation.
"Seed size can range from 2,500 (normal to large seed weight) to 3,500 (small seed size) seeds per pound. This season, conditions are mostly favorable in Kansas for promoting large seed sizes. In more stressful years, such as 2012 and 2011, seed size is normally smaller, meaning a larger number for the seeds per pound (e.g. 3,500 seeds per pound)," he says. "Let’s say that we have 120,000 plants/acre in a 30-inch rows. Then, we should have around 12 plants in 21 inches of row. In those 12 plants, we have measured on average 22 pods per plant, with a total number of 264 pods (22 x 12). If we assume a 'normal' growing season condition, then the final seeds per pod will be around 2.5, and for the seed size factor, we can assume large seeds, and will use a conversion factor of 15 units. For a 'droughty' (late reproductive, from R2 to R6 stages) growing season, the final seed number and size will be dramatically affected."
There's now a new tool for making the same calculations; the Manitoba Pulse Growers Association has a Soybean Yield Estimator web app in which you add a few variables -- plant population, pods per plant, seeds per pod and seed size -- to reach a yield calculation. The web-based tool is compatible with smartphones, tablets, laptops and desktop computers. The tool is best when "estimating soybean yield at on-farm scale before harvest time," Ciampitti says.
