Is the Future of Corn Production in Narrow Rows?

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Crop Insights written by Mark Jeschke, Ph.D., Pioneer Agronomy Manager

Summary

Adoption of narrow row corn to date has been extremely limited in most areas, yet it is widely believed that narrower rows will eventually be necessary to continue increasing yields.
Plant population and plant leaf architecture are two factors that could potentially affect optimal row spacing in corn.
Research suggests it is likely that optimum plant populations in corn will continue to increase in the future, but provides little evidence to indicate that narrow rows will be necessary to support higher populations, at least in the near term.
Leaf architecture in corn has changed greatly during the hybrid era, toward more upright leaves; however, it is not clear how much this has contributed to higher yields.
Research has not shown a relationship between hybrid leaf architecture and response to narrow rows by modern hybrids.
It is likely possible to develop hybrids optimized for narrow rows; however, it is not clear that this would necessarily lead to greater corn productivity.

Introduction

The vast majority of corn acres in the U.S. and Canada are currently planted in 30-inch rows, with narrow rows generally defined as any row spacing or configuration less than 30-inches. Narrow rows have proven beneficial in some scenarios, but generally have not shown a consistent yield advantage in the central Corn Belt region that makes up the bulk of North American corn production. Consequently, adoption of narrow rows has remained low.

Despite being used on less than 7% of corn acres, interest in narrow row production has persisted. This is largely due to the perception that evolving corn production practices will eventually favor a transition to narrower rows, similar to the past shift away from 36- to 40-inch rows into the current 30-inch standard. The purpose of this Crop Insights is to examine research that addresses 1) the question of whether changes in corn production will eventually favor narrow rows and 2) if a wide-scale shift into narrow rows will be necessary at some point to continue to drive gains in corn productivity.

Two factors that relate to row spacing, plant population and plant leaf architecture, will be examined in detail.

This is a photo of a corn field in mid-summer. A corn canopy needs to intercept 95% or more of photosynthetically active radiation at silking to maximize yield. — *A corn canopy needs to intercept 95% or more of photosynthetically active radiation at silking to maximize yield.*

Proven Benefits of Narrow Rows

To evaluate the potential benefit of narrow row corn in the future, it is worth examining scenarios where they have already proven beneficial. Research has shown a strong relationship between improved yields in narrow row corn and increased light interception (Andrade et al., 2002). To maximize yield, the crop canopy needs to capture 95% or more of photosynthetically active radiation (PAR) during the critical period immediately before and after silking (Figure 1). Corn at a given density can intercept a greater percentage of solar radiation when planted in narrow rows, which can increase yield in cases where corn in 30-inch rows does not meet this threshold (Andrade et al., 2002).

Chart showing percent of incident photosynthetically active radiation intercepted by a corn canopy in central Iowa planted in 30-inch rows. — **Figure 1.** Percent of incident PAR intercepted by a corn canopy in central Iowa planted in 30-inch rows. Adapted from Puntel, 2012.

Narrow rows can also improve nitrogen use efficiency of corn by increasing the ability of the crop to recover nitrogen from the soil (Barbieri et al., 2008). This can improve yield in nitrogen-deficient conditions; however, this advantage is reduced as nitrogen availability increases and may not result in increased yield when adequate nitrogen is available (Barbieri et al., 2000; Barbieri et al., 2008).

Yield benefits with narrow row corn have been observed more frequently in the northern portion of the Corn Belt in the area north of approximately 43°N latitude (a line running roughly through Mason City, IA; Madison, WI; and Grand Rapids, MI) (Lee, 2006). In a survey of several recent university studies comparing 15-, 20- or 22-inch rows to 30-inch rows, an average yield advantage of 2.8% with narrow or twin rows was observed in northern studies, compared to no advantage on average (-0.2%) in the central Corn Belt (Jeschke, 2018). The lack of a consistent yield benefit in the central Corn Belt is likely because light interception and nitrogen uptake are generally not yield limiting in this area. Several studies have shown that corn in 30-inch rows can routinely capture over 95% of PAR in Midwestern production (Figure 1) (Nafziger, 2006; Novacek et al., 2013; Robles et al., 2012; Sharratt and McWilliams, 2005; Tharp and Kells, 2001).

Plant Population

Long-Term Population Trends

It is generally assumed that optimum plant densities for corn will be significantly higher in the future than they are today. Examination of historical trends in plant population and corn yield show this assumption to be well-founded. Average corn yields have increased continually over the past 80 years as have average plant densities, increasing from around 12,000 plants/acre in the 1930s to over 31,000 plants/acre today (Duvick, 2005; Pioneer Survey 2017). However, research has shown that yield potential per plant has not greatly increased over that time period. At low plant densities, current hybrids do not yield substantially more than hybrids of past decades (Duvick et al., 2004).

Chart showing average corn seeding rates reported by growers in North America, 1985 to 2017. — **Figure 2.** Average corn seeding rates reported by growers in North America, 1985 to 2017. Source: Pioneer Survey 2017.

Examination of more recent data supports these findings. Average corn yield in the U.S. has increased from 118 bu/acre in 1985 to 176.6 bu/acre in 2017 (USDA NASS, 2018). Average corn seeding rates have increased linearly over this period, from approximately 23,300 seeds/acre in 1985 to over 31,000 seeds/acre in 2017 (Figure 2). When corn yield and seeding rate data are used to calculate average grain yield/plant, the resulting trend line shows that average yield per plant has changed relatively little over the same time period, increasing from 0.29 lbs/plant in 1985 to 0.33 lbs/plant in 2017.

Chart showing average grain yield per plant in the U.S. from 1985 to 2017. — **Figure 3.** Average grain yield per plant in the U.S. from 1985 to 2017, based on average corn yields (USDA-NASS, 2018) and average corn seeding rates* (Pioneer Survey 2017).

*Assumes harvest stand = 95% of seeding rate.

Higher plant density is not the only path to greater corn yields. In fact, growers who produced corn yields greater between 300 and 350 bu/acre in the 2016 and 2017 NCGA National Corn Yield Contests did so over a wide range of plant populations. Harvest populations ranged from less than 30,000 plants/acre to over 50,000 plants/acre, with the majority between 32,000 and 42,000 plants/acre (Jeschke, 2018).

Graph showing harvest population and yield per plant for NCGA National Corn Yield Contest entries between 150 and 350 bu/acre, 2016-2017. — **Figure 4.** Harvest population and yield per plant for NCGA National Corn Yield Contest entries between 150 and 350 bu/acre, 2016-2017.

Yield per plant varied widely as well, ranging from 0.33 lbs/plant to 0.64 lbs/plant. Grain yield per plant for these entries averaged around 0.5 lbs/plant, well above the current U.S. average. These data show that it is possible to increase corn yields per acre by increasing individual plant yield as opposed to plant density; however, this has not been achieved on a wide scale.

There is no guarantee that current trends will continue into the future; however, it seems likely that optimum plant densities will continue to increase beyond their current levels and will continue to be the main driver of increased corn productivity. A recent survey of plant density tolerance in U.S. corn germplasm indicated strong potential for further increases in optimum plant density (Mansfield and Mumm, 2014). At the current rate of growth, average corn seeding rates would increase from around 31,000 seeds/acre today to approximately 37,000 seeds/acre in 2035 and 41,000 seeds/acre in 2050 (Table 1). Assuming that 7,000 seeds/acre above the average is representative of a “high-end” seeding rate on the most productive ground, this would correspond to high-end seeding rate of 44,000 seeds/acre in 2035 and 48,000 seeds/acre in 2050.

Table 1. Current average and high-end seeding rates and projected rates for 2035 and 2050 based on current trends.

Table listing current average and high-end seeding rates and projected rates for 2035 and 2050 based on current trends.

Why Might Narrow Rows be Favorable at Higher Populations?

The primary rationale for narrow corn row spacings is that reducing the crowding of plants within a row will reduce competition among individual plants and allow the crop to better utilize available light, water, and nutrients. As plant density increases, plants are closer together within a row and it seems reasonable to think that, at some point, this crowding could become yield-limiting (Table 2).

Table 2. Minimum distance between adjacent plants in 30-inch, 20-inch, 15-inch, and 30-inch twin row configurations over a range of plant densities.

Table showing minimum distance between adjacent corn plants in 30-inch, 20-inch, 15-inch, and 30-inch twin row configurations over a range of plant densities.

The lack of a consistent yield benefit to narrow rows observed in most areas thus far suggests that this theoretical yield-limiting point has not been reached with current management practices. However, if such a point is reached in the future, and higher corn yields continue to be driven by greater plant density, a wide-scale transition to narrower rows would then presumably be necessary to drive further gains. Several research studies may shed light on whether this theory is valid.

Narrow Row, High Population Research

A number of corn row spacing studies published during the last 20 years have included plant populations well above the current average (Table 3). If plant crowding within the row is indeed yield-limiting at high plant populations, then narrow rows would be expected to have a yield advantage over 30-inch rows in these studies.

Table 3. Yield advantage (%) of 15-inch, 20- or 22-inch, and twin rows compared to 30-inch rows observed in recent corn row spacing research studies in the Midwestern U.S. that included high plant populations (indicated in bold).

Table listing yield advantage percent of 15-inch, 20- or 22-inch, and twin rows compared to 30-inch rows observed in recent corn row spacing research studies. — Study 1: Coulter and Shanahan, 2012; Study 2: Novacek et al., 2013; Study 3: Pecinovsky et al., 2002; Study 4: Van Roekel and Coulter, 2012; Study 5: Robles et al., 2012; Studies 6,7: Haegele et al., 2014.

Studies 1 through 5 in Table 3 are university studies that included plant populations over 40,000 plants/acre. Four of these studies, conducted in Nebraska, Iowa, Minnesota, and Indiana, did not show any yield advantage to narrow or twin rows at high populations (Table 3). The one study that showed an advantage was conducted in northern Minnesota, where yield advantages with narrow rows have tended to be more consistent.

Studies 6 and 7 were conducted in Illinois and Indiana comparing 30-inch and twin rows at extremely high plant populations of up to 65,000 plants/acre. In both of these studies, corn yield was actually significantly reduced in twin rows at high populations (Table 3). In the Indiana study, at populations of 50,000, 55,000, and 65,000 plants/acre, yield in twin rows was 8% less than in 30-inch rows. In the Illinois study, at 45,000 and 55,000 plants/acre, twin rows had 5% lower yield but significantly greater yield at the lowest population tested (25,000 plants/acre). The researchers hypothesized that the yield reduction with twin rows at high populations may have been due to increased air and leaf temperatures in the middle stratum of the canopy, leading to accelerated leaf senescence.

Will Higher Populations Require Narrow Rows?

Because it is not supported by research, the theory that corn production at higher populations will need to transition to narrower rows for continued gains is called into question. Row spacing studies with high populations have not shown an advantage to narrow or twin rows outside of the northern Corn Belt, where narrower rows have historically had a more consistent yield advantage. In the 2017 NCGA National Corn Yield Contest, 90% of entries yielding above 300 bu/acre were planted in 30-inch rows, many of them at relatively high populations; clearly demonstrating the potential to achieve much greater yields at high populations without narrower row spacing.

Corn Leaf Architecture

The development of hybrids especially suited to a narrow-row, high-population environment is often cited as potentially favoring narrower rows in the future. The idea of optimizing hybrids for narrow-row production has most commonly focused on leaf architecture; specifically, that plants with narrower and more upright leaves may be more suited to narrow rows. Like plant population, plant architecture is another factor in corn production that has changed over the past several decades, so it’s not unrealistic to suggest that the future could bring further changes.

Changes in Leaf Architecture in the Hybrid Era

Continual selection for greater yield during the hybrid era has resulted in significant changes to many plant characteristics. For example, modern hybrids tend to be slightly shorter, with lower ear placement. Tassel size and number of branches has been significantly reduced compared to early hybrids. However, the difference in leaf architecture specifically, a trend toward upright leaves (Figure 5); is perhaps the most visually apparent contrast between early and modern hybrids (Duvick, 2005).

Figure 5. Pioneer^® hybrid 354 (introduced in 1953) and Pioneer P1365AMX™ brand corn (introduced in 2013) (Johnston, Iowa; July 16, 2013)

The shift toward more upright leaf architecture began with the introduction of Iowa State University’s B73 inbred into breeding programs during the 1970s (Figure 6). Subsequent hybrids tended to have a more upright leaf angle and a greater length to the leaf flagging point compared to their predecessors (Duvick, 2005; Meghji et al., 1984; Lauer et al., 2012). Today, nearly all North American hybrids could be characterized as having upright leaves compared to those of the past.

Photo showing Iowa State University inbred B73 corn plants in a Pioneer demonstration plot (Johnston, Iowa; July 16, 2013.) — **Figure 6.** Iowa State University inbred B73 in a Pioneer demonstration plot (Johnston, Iowa; July 16, 2013)

This industry-wide transition to more upright leaves is commonly considered to be an important factor that has enabled corn performance at higher plant densities. Upright leaves increase the distribution of light in the canopy; less light is captured by the uppermost leaves and more light penetrates further down where it is captured by lower leaves, thereby increasing photosynthetic efficiency. This improvement is greatest in corn canopies with a high leaf area index, generally associated with high populations. The canopy of a typical corn crop has greatly increased in leaf area index over the years, from approximately 2.4 m²/m² in the 1930s to 4.8 m²/m² or greater today (Lee and Tollenaar, 2007).

The extent to which changes in leaf architecture have actually directly contributed to increased corn yield is unclear, however. Several experiments on corn leaf angle conducted during the 1960s and 1970s produced variable results; some showed an advantage with upright leaves at higher plant densities (Lambert and Johnson, 1978; Pendleton et al., 1968; Pepper et al., 1977) and some did not (Hicks and Stucker, 1972; Russell, 1972; Whigham and Woolley, 1974). It is possible that increased light penetration in the canopy associated with upright leaves may provide indirect benefits via increased carbohydrate partitioning to the ear and delayed leaf senescence (Hammer et al., 2009).

Research Comparing Hybrid Response to Narrow Rows

Most research studies conducted during the past 25 years have not found consistent differences in hybrid response to narrow rows. Out of 15 university row spacing studies published between 1997 and 2013 that included more than one hybrid, only one reported a significant hybrid by row spacing interaction (Farnham, 2001). Furthermore, none of these studies showed a significant difference in hybrid performance in narrow rows that was specifically associated with a difference in leaf architecture.

Research conducted in Michigan compared performance of six hybrids in narrow rows (Widdicombe and Thelen, 2002). Of these hybrids, two were characterized as having erect leaf orientation, three with semi-upright leaves and one with wide leaves. Average corn yield was significantly higher in narrow rows, but performance did not differ among hybrids. A study in Minnesota comparing two hybrids of differing leaf architecture also found no difference in yield response to narrow rows (Sharratt and McWilliams, 2005).

A three-year Pioneer/University of Missouri study compared 11 hybrids in 15- and 30-inch rows. This study found a significant hybrid by row spacing interaction; however, hybrids with more upright leaves did not perform any better than other hybrids in narrow rows.

Can Hybrids be Designed for Narrow Rows?

The fact that most recent research studies have not found a significant difference in hybrid response to row spacing indicates that there is likely little variation among modern hybrids in their suitability to narrow rows, although the few studies that have found such a difference show that some variation does exist. Whether or not this variation could be exploited to design future hybrids for narrow rows, and whether or not this would significantly increase corn productivity, is unclear. The transition to more upright leaves in modern hybrids has likely contributed to improvement in corn yield associated with higher plant densities to some extent; however, research suggests it is unlikely that further changes in leaf angle offer a meaningful opportunity for yield improvement in the future (Lee and Tollenaar, 2007).

Past research on hybrids with extremely upright leaves has shown that narrow rows may increase productivity for hybrids that are unable to capture 95% of PAR in 30-inch rows. Extremely upright leaves that remain close to the stalk can have the negative effect of allowing light in the interrow to penetrate to the soil surface; an effect that narrower rows would tend to help mitigate. A research study including a Chinese hybrid with extremely upright leaves noted this effect (Stewart et al., 2003). A canopy photosynthesis model predicted that changing from 30-inch to 15-inch rows would significantly increase photosynthetic production with this hybrid; whereas minimal benefit was predicted for a comparative hybrid at a similar leaf area index.

Research has examined the potential of developing semidwarf hybrids for corn production in the far northern Corn Belt, the primary advantage of which would be earlier maturity than conventional hybrids (Schaefer et al., 2011; Combs and Bernardo, 2013). Such hybrids would require narrow rows and extremely high plant populations, similar to small grain production, to maximize productivity. Semidwarf hybrids could also potentially be advantageous in arid climates or in double crop rotations, although their overall value for improving corn productivity remains yet to be determined.

Photo of young corn plants - early spring.

Is the Future of Corn Production in Narrow Rows?

It is possible that changes in corn production practices may eventually favor a transition away from the current 30-inch row spacing standard to narrower rows; however, research provides little evidence to suggest such a transition will be necessary or justified in the near future. Future yield gains will likely continue to be driven by higher plant populations, but research that has compared row spacing at populations from 40,000 to 65,000 plants/acre has generally not shown a yield advantage to narrow rows outside of the northern Corn Belt.

Modern hybrids typically have not differed in their response to narrow rows. When yield differences have been observed, they have not been associated with any particular characteristic of leaf architecture. Research with extremely upright-leaf hybrids and semidwarf hybrids has shown that narrow rows can be beneficial when 30-inch rows do not allow complete capture of PAR at silking. These studies indicate that development of hybrids optimized for narrow rows is possible; however, it is not clear if such hybrids could lead to greater productivity on a wide scale.

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