Evaluating Soybean Stands and Determining Replant

Rain has been abundant in Illinois during the month of June. With the large amounts of rain over the last 2 weeks it is important to evaluate your soybean plant stands.

Once a soybean crop is planted, there are many things that can affect your crop’s emergence and plant stand. These include: field conditions at time of planting, planting depth, soil type, fertility, herbicide crop response, rainfall, temperature, crusting, hail, insects and disease just to name a few. Unfortunately, this year we have experienced all of these conditions.

Earlier planted soybeans experienced excellent planting conditions, but then the weather turned cold and wet followed by crusting at a critical emergence time. The crusting impeded soybeans plants from necking out.

This year I have also observed increased herbicide effects on soybeans, due to the cool and wet conditions in relation to the timing of planting and herbicide applications. When herbicides are applied to the soil or over the tops of plants, even when soybeans are tolerant or immune, the bean plant has to metabolize this product. When conditions are cool, wet and cloudy the plants metabolism slows down and the soybean plant doesn’t metabolize the product as fast—thus some damage to foliage. These effects have largely been cosmetic, but I have noticed stand losses. The larger issue at this point is the saturated or ponded soils.

There are several factors to consider when evaluating your soybean fields for potential replant. Considerations include current plant stands, anticipated plant stands, planting date, input costs, field conditions and weed control. First, an evaluation of the current stand must be completed. There are two methods for accurately determining your plant stands. The first is the linear row method. To use this method count the number of viable plants in 17.5’ of row if you have 30-inch rows or if you planted in 15-inch rows count the number of viable plants in 34’10” of row. Once you have your counts multiply by 1,000.

The other method is the hula hoop method. There are several sizes of hula hoops and you can find charts on the Internet that will help you calculate your populations in correlation to their size, but the easiest thing to do is to cut down a hula hoop to 28 ¼” diameter. If you utilize a hoop this size you simply take your plant count within the circle of the hoop and multiply by 10,000 to get your population.

Regardless of what method you use, be sure to complete this in multiple areas of the field for an accurate representation. Another consideration when you are reviewing your plant stands—be sure to evaluate the health of the soybean plants. Only count those plants that you believe will continue to be viable. With saturated soils, we can continue to lose soybeans. Another key effect on soybean yields is the plant spacing. Even with adequate plant population if you have areas of skips or barren areas, soybean yields will be negatively affected.

Once you have an understanding of your current stand you can make an informed decision about replanting. Again, this is not an easy decision. Below is a chart from Purdue University that indicates the yield effects from reduced plant populations. As you can see, it takes a significant stand loss to have a large impact on yield.

Source: Purdue University Corn and Soybean Field Guide, 2013 Edition

If you determine that the current population justifies replanting, you must then determine the effects and economics of the replant timing. Below is another chart from Purdue University addressing replant dates and the expected impact on yield.

Source: Purdue University Corn and Soybean Field Guide, 2013 Edition

If you have the ability to utilize smartphone or tablet apps, The Corn & Soybean Field Guide App has a soybean replant calculator. This will allow you to utilize market price, yield data, stand counts and input costs to determine if replanting your soybean crop will pay. There are also spreadsheets on the Internet from Iowa State University, Purdue University and the University of Illinois that will help you calculate the financial aspects of replanting.

If you calculate that replanting is justified based on the stand, then you have to decide whether to tear out the old crop and the cost to do so, what it will cost to replant and what the potential yield may be with the later planting. In the end, the yield from the reduced stand maybe be equal to the yield from the replant so replanting isn’t justified.