When you start to figure yield it may seem pretty good but remember that these are only estimates and the true test comes when we put the combines in the field.
You can start checking when the corn is dented and the soybeans are filling or have full pods. The first question is what type of yield this year is going to have. There are several ways to calculate estimated yield this time of year. However, these estimates are based on average growing conditions and good grain fill. Corn is the easiest and most reliable estimate. For corn, figure rows per ear and kernels per row to give kernels per ear. Now figure the plant stand. One easy way is to count stalks in 1000th of an acre on 30” rows. his is 17.5 feet. In 36” rows this is 14.5 feet. Now take the stand count from a given area and multiply by 1000 to get the stand count per acre. Now take the stand count per acre by kernels per ear per plant to give kernels per acre. Then divide by 90,000 which is the average kernel count in one bushel of corn. Now you have an estimated yield. Remember this is only an estimate.
For soybeans, yield estimates are tough to do. Soybean seed will vary greatly due to germ plasma. Soybean seed on a normal year will vary from 2400 to 3200 seeds per pound. This, when figuring yield by seed, will vary estimates by 25 percent. On a dry year it would not be uncommon to see seed counts at 4100 or higher seeds per pound. This then could turn into a 50 percent yield variation. So keep this in mind when figuring yield. To show how to figure the estimated yield, first we need stand count per acre like before. Count plants in 1000th of an acre. This is 17.5’ on 30” rows or 35’ on 15” rows. Now multiply this figure by 1000 to get stand count. Now figure average pods per bean and multiply by beans per pod and multiply by stand count to get beans per acre. Then divide by 180,000 to get yield. The yield based on 3000 seeds per pound of beans. If you figure your beans at 4000 seeds per pound then divide by 240,000 instead of 180,000. For better estimates ask your seed dealer for average seeds per pound of the variety planted then multiply by 60 to get seeds per bushel. Again, remember bean estimates can vary a lot so do not count on estimates.
Source: Paul O. Johnson