Many people do yield checks of corn and feel they can get relatively close (+/- 10 percent) to the actual yield. On the other hand, yield checks of soybeans seem to be much more difficult and getting within +/- 25 percent is doing fairly well. In my experience, most people over estimate the yield of soybeans. Generally, this is because they overestimate one or more of the yield factors—plant population, pod count, seed count or seed size. When you overestimate more than one component it can really magnify (increase) the theoretical yield.
The math required to estimate soybean yields is simple. Seeds per acre divided by seeds per pound divided by 60 pounds per bushel = yield per acre. Seeds per acre is a function of plants per acre and seeds per plant. Seeds per plant is a function of pods per plant and seeds per pod.
Estimating plant population is harder in soybeans than corn. Just knowing how many seeds you planted per acre will not tell you how many plants you have per acre. And knowing, on average, how many plants you have per acre does not tell you how many of those plants are actually contributing to yield. With soybeans there is more yield variation from plant to plant, as plant spacing within a row is often highly variable. A plant with five pods might be close to a plant with 80 pods. It can be very difficult to determine an accurate average population and seed count per plant.
Even if you know the average population and the average seed count per plant, you’ll still have no idea what the seed size will be. The number of seeds it takes to make a pound can vary from less than 2,000 to more than 4,000—depending on genetics and environmental conditions. That is a big unknown variable.
Some crop scouts, consultants or agronomists who look at enough fields can get pretty good at judging yields based on “looks” but I have never considered myself to be one of them. Things you normally see in high yielding soybeans are high pod counts per plant with a high frequency of three and four bean pods. The factor that influences yield the most in soybeans is pod count per plant.
I like to be conservative in my yield estimates which means not over estimating plant population, seeds per plant or seed size. If you planted 140,000 seeds you might only have 110,000 plants that actually set seed. If the average plant in the field has 100 seeds and it takes 3200 seeds to make a pound, then your yield is 110,000 x 100 ÷ 3200 ÷ 60 = 57 bu/acre.
In this example, if you over estimate population, seed count and seed size by 10 percent each, you would over estimate yield by a whopping 20 bu/acre! This is why so many growers prefer to estimate soybean yields with a combine yield monitor and scales. If you look at enough fields over time you can usually get pretty good at knowing if yields will be “good” or “bad.” But knowing if “good” means 70, 80, 90 or 100+ is difficult.
Lance Tarochione is a technical agronomist with Asgrow/DEKALB in west central Illinois. His work has focused on crop production, research and product development, and through his role at Monsanto® he currently supports the Asgrow® and DEKALB® brands in seven counties in western Illinois.