Row Spacing Decisions

Plants of these varieties were generally tall with lateral branches that resulted in the formation of a complete canopy when grown in wide rows. In fact, these varieties grew to a height that often led to lodging that completely obscured the wide middles between rows.

My first research projects usually included a row spacing component where wide and narrow rows were compared in various production systems. Results from these studies showed little to no advantage to planting the above varieties in narrower rows.

Change

In the late 80s/early 90s, things changed. Herbicides that controlled most commonly occurring weeds in soybean production systems became prominent weed control tools. Thus, post-planting cultivation was no longer a necessity.

The Early Soybean Production System (ESPS) with its use of indeterminate, non-branching, short-statured, early-maturing varieties proved to be a better production system that subsequently became the preferred system on over half the soybean acreage in the Midsouth.

Today, essentially all soybeans in the midsouthern US are grown in narrow rows because research has shown that increased yields and better weed management (Bradley 2006 & Knezevic et al. 2003) will result when compared to wide rows.

What is a narrow row?

The definition of a narrow row has become a point for debate. In some situations, a 30-in.-wide row may be considered narrow. For those who broadcast soybeans, a 7- or 8-in.-wide row may be the definition of narrow. Anything in between these two extremes, then, would also be considered narrow.

A hybrid row spacing system involves having a wide space (>30 in.) between the rows that are adjacent to the tractor wheel tracks. Rows between and outside the wheel tracks are narrowly spaced (usually 15 to 20 in.). This system can be set up to accommodate either single- or dual-wheel tractors (Poston 2007, slide 9).

Below are a few points to consider when deciding what “narrow” row spacing to use for soybeans in the midsouthern US.

Soil texture and row spacing

With adequate rainfall and/or irrigation to replenish depleted soil water, loamy soils (silty clay loam, silt loam, and fine sandy loam) promote rapid soybean growth and development because of relatively easy soil water availability. Conversely, clayey soils (generally >50% clay particles) with adequate rainfall and/or irrigation to replenish depleted soil water will promote relatively slower growth because of their slow release of water.

Thus, a variety planted on a particular early date on most loamy soils will produce a larger plant than will that same variety planted on the same date on most clayey soils. Therefore, ESPS plantings grown in 30- in.-wide rows on loamy soils will probably form a complete canopy, while the smaller-statured plants that will be produced on clayey soils will require a narrower row, say 20 in. or less, to achieve complete canopy closure.

Thus, the definition of “narrow” should be considered in concert with the soil texture of the field.

Beds

I include this discussion here because it ties in with the discussion in the following section.

• Regardless of soil texture, risks associated with early planting of soybeans in the midsouthern US will decrease when beds are used.


• A bed planting system will reduce potential stresses associated with early-season surface water accumulation.


• Beds, or rather the resulting furrows, will facilitate surface irrigation during the growing season.


• The attributes of beds will benefit corn and grain sorghum that may be grown in rotation with soybeans.

Experience and observation have shown that beds constructed on 30 in. or narrower centers are not practical. They do not keep their integrity during the months between construction and planting, they are not easily planted, and they do not provide enough furrow and bed height to facilitate drainage and irrigation.

Conversely, beds that are 38 and 40 inches wide can be constructed effectively. They will most likely maintain themselves during the off-season, especially if rolled flat following formation to minimize bed degradation during high-rainfall winter months.

A wide-bed planting system should also be considered. These beds can be constructed in the fall soon after harvest using an implement that combines sweeps to dig furrows with wings to distribute the resulting loose soil over a wide area, say 80 inches (MSU Bulletin 1072).

A wide bed will accommodate various narrow row spacings, and will be suitable for different crops that may have varying row spacing requirements. This is probably the most manageable bed system for clayey soils. This system will also facilitate furrow and flood irrigation on clayey soils that crack and allow lateral movement of irrigation water between the widely-spaced furrows.

Soybeans grown in ESPS plantings will be harvested from late July through early September. Since this is normally the driest time of year in the midsouthern US, there should be no ruts left by the combine. Thus, a bed system can remain in place from year to year if there are no harvest ruts, and can be refurbished without destructive primary tillage in the fall when necessary.

See the video on this website that shows the various row spacing configurations that can be used with beds.

Twin rows

A twin-row planting system is being used with increasing frequency (DFP 2005, LSU 2008, TAM 2007, Bruns 2011& Bruns 2011). Research in the midsouthern US has generally shown that yields from a twin-row system will be greater than those from 38- and 40-in.-wide rows.

This system allows planting on 38- or 40-in.-wide beds but at row spacings that are less than the bed width. For example, twin rows spaced 8 inches apart on a 38-inch-wide bed give the same spacing between rows on adjacent beds as a single row planted on a 30-inch-wide bed. This row spacing is preferred for corn and grain sorghum, and is adequate for soybeans planted on loamy soils.

The 30-in.-wide row spacing in the twin-row system may be too wide for ESPS plantings made on clayey soils because of the growth differences between loamy and clayey soils described above. The problem, then, is how to construct and maintain a bedded planting system on clayey soils that require narrow row spacings to ensure canopy closure.

The above-described wide-bed system with its flexibility for various narrow row spacings for a majority of the rows may be the answer. This system likely will result in better canopy closure, and subsequently, better weed control than soybeans grown in both twin rows and wide rows.

Summary

• Today’s soybeans should be grown in “narrow” rows in the Midsouth. This is now a foregone conclusion.


• The determination of the proper “narrow” row spacing should be made with soil texture in mind. Generally, a wider “narrow” row spacing can be used on loamy soils than on clayey soils.


• A twin-row system is a variation of a narrow-row system and is best suited for loamy soils vs. clayey soils.


• The combination of a bed system with a narrow-row configuration matched to a site’s soil series offers the best management option for soybean production in the midsouthern US.

larryheatherly@bellsouth.net