Even in Iowa, there is good evidence that narrow rows (less than 30 inch) will produce higher yield than wider rows (30 inches or greater). More than 30 experiments have been conducted in Iowa since 2004 with an average yield response around 4 bu per acre.
The data available for row spacing from the past twenty years is fairly consistent for Midwest production areas and indicates that narrower row spacing does improve yield. While a few studies have shown little to no response of yield to row spacing, no studies to my knowledge have shown a yield increase with wider row spacing. The move toward narrow row spacing is therefore increasing in popularity in Iowa. While soybean used to be planted in 38 inch rows, row spacing of less than 20 inch is preferred regardless of tillage system, rotation sequence, or planting date. In most cases, there is no difference between 7.5, 10, 15, or 20 inch row spacing and anything less than 30 inch is therefore consider narrow row spacing. Most of these studies have concluded that planting soybean in narrow rows will increase yields with the largest increases in yield occurring in the northern Corn Belt. The question surrounding row spacing is “Will the increase in yield pay for the cost of replacing or altering the planter to plant in narrow row spacing?”
Concerns when planting in narrow row spacing.
Farmers should take into account disease pressure and weed control options when planting in narrow row spacing. Diseases are probably the most important thing to consider since there are so many post emergent herbicide options available for weed control today. White mold is the largest concern for many growers in Iowa. This fungus has spread quickly due to the short rotations, decrease in tillage practices, and narrow row spacing. Once the fungus is present in a field, little can be done to remove the pathogen and management practices to lessen the impact of the fungus must be implemented. A more important factor determining the potential for white mold is environmental conditions. Cold and cloudy conditions in combination with high rainfall during flowering are optimal for pathogen infection. It is recommended not to drill beans and use a tolerant variety to white mold if a specific field has a history of white mold. Narrow rows have a yield advantage because they decrease the time to canopy closure and intercept more light throughout the season. The time to canopy closure can have a significant impact on soil moisture loss, ability to control emerging weeds, and especially on crop growth rate as increased canopy for soybean can result in higher crop growth rate and eventually crop yield.
Harvesting narrow row soybeans is easier.
Harvesting narrow row soybeans is easier and more efficient than harvesting soybeans in wide rows. Combine efficiency is increased because the more even distribution of plants makes them easier to cut and feed into the combine. Harvest losses are reduced because there are no cultivator ridges to interfere with cutting height.
Research is currently underway to help farmers in Iowa.
It is important to recognize that row spacing is influenced by the field environment. At high yielding sites narrow row spacing increased yields; however, when there is a lot of stress on the soybean plant and the yield potential is low, narrow rows do not always provide the same kind of a yield advantage. In 2004, an extensive research project funded through the check-off and the Iowa Soybean Association was initiated to investigate the advantage of using 15 inch row spacing versus 30 inch row spacing in Iowa. It should be a surprise, that the majority of the farmers in Iowa still use 30 inch row spacing despite research across the Midwest that has shown an advantage of narrow row spacing. Average across the last two years of research and 17 experiments we have seen 4.6 bu/ acre advantage of 15 inch over 30 inch row spacing. We will continue the research in 2006.
A fact sheet can be downloaded (pdf file) on soybean row spacing.