Soybean row spacing matters more than most Iowans think!
Palle Pedersen, Soybean Extension Agronomist
Originally published in the Iowa Soybean Review, February 2007

Row spacing is the third most important variable for maximizing soybean yield -  after variety selection and planting date. The benefit of narrow row soybean over wide row soybean is mainly a faster canopy development, which is extremely important for high yielding beans early in the growing season. Early in the growing season (at flowering), the soybean plant often lacks a canopy, and in turn, will abort flowers and pods simply because it does not have the resources available to support them.

Soybean canopy development, which is a function of row spacing, seeding rate and environmental conditions, is also an effective weed control tool. The canopy closure will occur earlier and will often close 15 days earlier in 15-inch rows, compared to 30-inch rows. The canopy provides greater shading of weed seedlings, which will decrease weed interference. As row spacing decreases, the number of weeds that emerge after herbicide application decreases linearly as a result of more light being intercepted by the soybean canopy.

Most research from the Midwest documents that narrow rows (less than 30-inches) yield greater than wide rows (30-inches or greater). Although research has documented the yield response to narrow row spacing, the majority of the acres in Iowa are still planted in wide rows.

Why? There are many reasons for this, but probably the biggest one is that very little research on row spacing has been conducted in Iowa over the last two decades to direct the farmers. As a result, research was initiated in 2004 through the soybean checkoff and the Iowa Soybean Association to investigate if we are losing yield in Iowa using predominately wide row spacing in our current production system. Throughout the 19 experiments we have conducted since 2004, we have seen a yield increase of 3.7 bu/acre from 15-inch rows over 30-inch rows.

In addition to the yield advantage from the narrow row spacing, we were also able to document that an increased seed cost is not necessary, as long as we use a planter. Many farmers have neglected to give up the 30-inch row spacing since the old recommendation suggests we need to increase our seeding rate when we decrease our row spacing.

That is not the case, as long as we use a planter, and we manage the weeds early (prior to V2). As long as you have a uniform stand of 100,000 plants per acre at harvest you will maximize your yield, and it will not matter what row spacing you are using. Of course if you use a drill it will take a higher seeding rate because you do not have the same soil to seed contact with a drill that you have with a planter.

Row spacing is influenced by the field environment and there are cases where narrow rows yield the same as wide rows. The reason is often associated with improper management decisions such as wrong variety selection. For example, if you plant a soybean cyst nematode susceptible variety in a soybean cyst nematode environment you are never going to get canopy closure – no matter what row spacing you are using. The nematodes will stunt your canopy development. White mold is a disease that many people associate with narrow rows, and it is the reason why many farmers have stayed with the wide rows.

An ultra low seeding rate should solve most problems with white mold, but if it is a year where white mold is going to thrive, it does not really matter what row spacing and seeding rate you use– it is all about host resistance.

An increase in planter costs is still a question we need to consider. With an average farmer in Iowa having at least 600 acres of soybean per year and with $7 per bushel it does not take more than two years before the money is returned. This is in line with data published from other universities in the Midwest. In general, based on an increased yield of 3.7 bu/acre and a price of $7 per bushel, it is an increase in gross income by more than $25 per acre.