Every farmer has a feel for it. Some can guess fairly well. Some can’t come close at all. It seems to be a lot easier with corn. But most would agree that it is difficult to estimate soybean yields with any degree of certainty. And with that, let’s have a go at the toughest job on the farm.
Soybeans this year had the benefit of being planted early in many regions of the Cornbelt, and have not had the stresses of some years, particularly in the way of insects and disease. Sure, there has been a good sized splash of Sudden Death Syndrome, and that will hurt yields in some areas. But crop maturity has kept ahead of average, and may allow most yields to hold despite the stress of dryness and heat last month.
Purdue soybean agronomist Shaun Casteel offers his method of estimating yields. A couple early indicators of soybean maturity are the first noticeable drop of leaves which is right before the R-7 stage that is physiological maturity. That stage is confirmed with the turning of one pod to a mature tan or brown color. That will cause soybean moisture content to decline from 65% in the green pods to 35% in the tan pods. Casteel says yield estimates improve the closer to harvest, since yields are based on the number of plants per acre, pods per plant, seeds per pod, and seed size. Stand counts are needed for yield estimates. Plant stresses influence pod counts, along with seed development and pod fill. As beans reach mature size, there is more accuracy in determining how many seeds it will take to make a pound.
One key to estimating bean yields is to measure 1/1000 of an acre, and the length of the soybean row will depend on the row width of your planter. A narrow 7.5 in. row width will require 4 rows of 17.5 feet to equal 1/1000 of an acre. If your rows are 30 inches apart, it only takes one row of 17.5 feet. The purpose in finding 1/1000 of an acre is to count the plants with beans. If you have plants showing the impact of SDS, Casteel says they should be counted, unless the pods are lost or seed growth has stopped.
Once the number of plants in 1/1000 of an acre is established, count the number of pods on 10 randomly selected plants, including those with disease problems. Average the number of pods for the 10 plants.
Compute the number of seeds per acre by multiplying the pod average by 2.5 seeds per pod. That should be expanded to the number of plants in that 1/1000 of an acre, since your count was only for 10 random plants. If you have to capacity to determine a more exact number than 2.5, use that, but Casteel says 2.5 has worked well.
Calculate the number of pounds of seeds per acre by dividing the number of seeds per acre by 2,900 per pound. Seed size will vary because of planting date, seeding rate, heat stress, water stress, disease stress and other factors. Once you have the number of pounds per acre, divide that by 60 pounds per bushel to obtain a yield estimate.
Keep in mind that you have determined your yield for those 17.5 feet of row in a large field. Casteel says it should be replicated in 10 randomly selected areas of the field, and averaged.
Summary:
The closer you are to soybean harvest, the better the chance at estimating the yield of your bean field. As pods begin to turn brown, the beans lose moisture and may only need another 5-10 days of good drying weather. To estimate a yield, count the number of plants in 1/1000 of an acre, and the pods on 10 randomly selected plants and multiply that number by the number of plants in your sample. Multiply the pods by 2.5 beans per pod to get seeds per acre, then divide the total by 2,900 beans per pound to get pounds in an acre. A last calculation to obtain yield is dividing the pounds per acre by 60 pounds in a bushel.