© 2006 Plant Management Network.
Reducing Row Widths to Increase Yield: Why It Does Not Always Work
Chad D. Lee, 1405 Veterans Drive, Department of Plant and Soil Sciences, University of Kentucky, Lexington 40546-0312
Corresponding author: Chad D. Lee. Chad.Lee@uky.edu
Lee, C. D. 2006. Reducing row widths to increase yield: Why it does not always work. Online. Crop Management doi:10.1094/CM-2006-0227-04-RV.
Numerous studies have been conducted to determine if decreasing row widths will increase yields of corn and soybean. Full-season corn (Zea mays) yields typically do not increase as row widths decrease south of 43°N latitude (about the Wisconsion-Iowa border extended east and west) in the United States. Soybean (Glycine max) yields follow a similar but less consistent trend. A corn or soybean crop must produce sufficient leaf area for maximum radiation interception just prior to or during flowering and seed set to achieve maximum yields. Full-season corn hybrids and soybean varieties at recommended plant densities in wide rows in the central and southern United States have ample time and heat units for crop growth to intercept maximum radiation prior to flowering, making narrow row widths unnecessary for maximum yields. Conversely, full-season corn and soybean in the northern United States and short-season corn and soybean in central and southern United States have limited time and heat units to reach maximum radiation interception prior to flowering. Based on these observations, wide rows for full-season corn and soybean in the central and southern United States are usually sufficient for maximum yields.
Wide versus Narrow Rows
In the context of this paper, narrow row widths are less than 30 inches for both crops. The most common narrow rows for corn range from 22 to 15 inches and for soybean are 15 inches or less.
The central argument presented here is that, in general, narrow rows south of 43°N latitude (about the Wisconsin-Iowa border extended west and east) in the United States do not increase yields, while narrow rows north of 43°N may increase yields. This paper focuses on supporting this argument with data from the literature and an explanation for the crop responses.
Observations: Yield Response to Narrow Rows
Corn row width. Corn studies under rain-fed conditions from Texas to northern Iowa have shown no consistent yield increase from narrow rows (Fig. 1). In several studies, yields were reduced by 3 to 13% in narrow rows in Kentucky (7), southern Illinois (14) and central Iowa (18). Yield increases from narrow rows occurred in some field studies near or north of latitude 43°N (Fig. 1). In four of six locations in Iowa (41.2 to 43°N), average corn yields across plant densities in 30-inch rows were higher than yields in 15-inch rows (18). Yields were similar at one location and yields in the 15-inch rows were 3% higher than yields in 30-inch rows at Sutherland, IA (43°N), the northernmost location (18). The data in Figure 1 demonstrate that corn yields tend to increase in narrow rows north of 43°N, but these increases did not occur in every comparison. While corn yields averaged 5.6% more in narrow rows across Minnesota (44.1 and 44.2°N) , yield advantages for narrow rows occurred in only three out of the six comparisons (30). No yield differences were observed in the remaining three comparisons. No yield differences were observed across row widths in other studies north of 43°N (29,40).
Soybean row width. Soybean yields typically did not increase in narrow rows in the southern United States when full-season cultivars were seeded at optimum dates, but the trends in yield increase from narrow rows from south to north are not as clear as in corn (Fig. 2). No yield increases occurred in narrow rows in some studies from Louisiana to Wisconsin on full-season soybean seeded at optimum dates, while yield increases occurred in other studies, particularly in Ontario (1), Wisconsin (28), and Minnesota (23). Early maturing soybean varieties planted at optimum seeding dates in Kentucky did not increase yields in narrow rows (16). Early maturing and full-season soybean varieties seeded late in the southern United States consistently had higher yields in narrow rows (Fig. 2).
Explanation: Why No Yield Increase When Row Widths Decrease
The explanation for no yield increase to narrow rows in the central and southern United States is likely the result of the interaction between light interception and canopy development at early reproductive stages. Canopy development is affected by factors such as time from crop emergence to flowering, temperature, plant density, phenotypes, and water availability.
Light interception. Crop canopy development resulting in maximum light interception is critical for maximum yield of both corn and soybean. For corn, seeds per plant (22) and seeds per area (3) increased linearly as intercepted photosynthetically active radiation (PAR) increased. Kernel number per area was usually maximized when PAR interception exceeded 90% at flowering (3). Corn yield was also maximized as light interception approached 95% at flowering regardless of row width (2,40). Similar to corn, soybean crop growth rate increased linearly as light interception increased, with maximum growth rates occurring at interception levels above 90% (34). Soybean yield was maximized when light interception approached 90% during flowering (9) and 95% during seed filling (8).
Andrade et al. (2) concluded that corn yield increases from narrow rows were due to increases in light interception. However, narrow rows do not always increase light interception. Light interception was similar for corn in 19- and 38-inch rows in South Carolina (34.6°N) (27); for 15-, 22-, and 30-inch rows at populations above 27,800 plants per acre in Michigan (42.7°N) (37); for 15- and 30-inch rows widths in Michigan (42.7°N) (11); and in 15- and 30-inch row widths in Minnesota (45.58°N) (40). In each of these studies where light interception was similar across row widths, yield also was similar across row widths.
For full-season soybean varieties seeded at an optimum date in Louisiana (30°N), light interception was greater than 80% in both 20- and 40-inch rows. For late seedings, at least 80% light interception was achieved only in the 20-inch rows (9). Yield increases in narrow row widths occurred only in the late-seeded soybean. In Michigan, full-season soybean in 30-inch rows reached a maximum of 80% light interception in three of four years while 7.5-inch rows reached 95% light interception all four years (11). In addition to higher levels of light interception, seed yield was greater in the narrow rows
Canopy development. Canopy development necessary for maximum light interception is critical to yield. In addition, the rate at which canopy development occurs can be critical to yield. One of the factors affecting crop growth and canopy development is time from seedling emergence to flowering.
Time to flowering. Full-season corn and soybean varieties grown in the southern United States typically require more time to reach flowering than full-season corn and soybean varieties grown in the northern United States. The longer time is a result of more days available for corn and soybean growth in the south and the maturity of southern corn and soybean varieties.
Heat units or growing degree days for corn has been calculated as the difference between the average daily temperature and a base temperature of 10°C (50°F), where the minimum daily temperature used in the calculation is no less than 10°C (50°F) and the maximum daily temperature in the calculation is no more than 30°C (86°F) (24). Annual accumulation of heat units or growing degree days range from 4,000 in central Arkansas to 2,400 in central Minnesota (24). In general, early maturing corn hybrids and soybean varieties in the north require fewer calendar days or heat units to reach flowering than later maturing hybrids and varieties in the south. In Texas (35.18°N), Pioneer 3737 flowered nine days before Pioneer 3245 (19). A corn hybrid with a 73-day maturity reached silking 12 calendar days and 148 heat units sooner than a 110-day maturity hybrid in Arkansas (15). Similarly, Maturity Group 00 soybean varieties flowered 11 days earlier than Maturity Group III varieties grown in Arkansas (36.08°N) (31). A Maturity Group 3.4 variety seeded on 25 April in Mississippi (33.4°N) reached flowering 23 days earlier than a Maturity Group 5.9 variety (42).
Narrow rows help offset the reduction in time by facilitating crop canopies that either reach maximum light interception by flowering or sooner than wider rows. In studies where narrow rows increased corn yields, narrow rows reached a higher maximum light interception than wide rows during flowering (2). In soybean, narrow rows shortened the time to reach 95% light interception (4,11,35).
Cooler temperatures. Moving from south to north in the continental United States not only reduces the amount of time a crop has to reach maximum light interception, but also reduces the amount of heat units available for crop growth and development (24). Cooler temperatures slow crop growth rate and may favor narrow rows (40). In Argentina (27.87°S) at 3,660 ft above sea level where heat units are limited, decreasing row widths from 40 to 20 inches increased corn yield (33). Barbeieri et al. (5) observed similar yield advantages when decreasing row widths from 28 to 14 inches in a cooler climate (37.83°S).
Plant density. In environments with limited heat units, positive yield responses were attributed to decreased row widths and increased populations. Ball et al. (4) concluded that higher plant densities, in addition to narrow rows, were required to reduce the time needed to reach maximum light interception and thereby maximize yields. Westgate et al. (40) found that higher plant densities increased maximum light interception and reduced thermal time required to reach one-half of maximum light interception. Edwards et al. (15) predicted that a density increase of 40,500 corn plants per acre was needed to overcome a reduction of 300 heat units. The increased plant density added to the normal population of 24,000 plants per acre brought the total plant population up to 64,500 plants per acre. While this plant density is too high for modern corn hybrids, the model does illustrate the importance of plant density in reduced temperature climates. In addition, yield improvements in corn can be attributed primarily to increased stress tolerances allowing for higher plant densities (13,38). If this trend continues, then at some point, narrow rows may be needed to facilitate denser populations, especially in environments with lower temperatures and lower heat unit accumulations.
Phenotypes. Phenotypic differences in cultivars, such as plant height, affect yield increases from narrow rows. A taller corn hybrid reached one-half of maximum light interception sooner than a shorter hybrid (40). Similarly in soybean, taller-stature genotypes intercepted more PAR during vegetative development than shorter-stature genoptypes regardless of row width and plant density (39). In 19-inch rows and the high plant density, intercepted PAR was similar for all genotypes during reproductive development, while in the 38-inch rows and low plant density the three tallest genotypes intercepted the most PAR and two of the three tallest genotypes had the highest yield (37).
Water availability. Water availability is critical for crop growth and development. In water-limiting conditions in Georgia and South Carolina, corn yields in narrow rows were less than corn yields in wide row widths; however, under irrigated conditions, 15-inch rows increased yields higher than 30-inch rows (D. Lee and J. Frederick, personal communications). Late-seeded soybean in narrower rows in Arkansas had a greater yield increase under irrigated than rain-fed conditions (5). Soybean yield increases in narrow rows occurred under irrigated conditions in Kansas and Nebraska, but rarely under rain-fed conditions (12,17).
Full-season corn hybrids south of 43°N in row widths less than 30 inches typically do not yield more than hybrids in 30-inch rows. Soybean varieties follow a similar, but less consistent pattern. Light interception at the time of flowering is critical to achieving maximum yield of corn and soybean. Most row width and plant density combinations are designed for crop growth and canopy development that intercepts approximately 90% of the light at or during flowering in corn and soybean (31). Full-season corn hybrids and full-season soybean varieties in the central and southern United States have more time and heat units from emergence to flowering than full-season crops in the northern United States. The additional time and heat units allows full canopy development in wider rows and lower plant densities than in the northern United States. This relationship between light interception and canopy development is supported by many field studies, but in particular the studies demonstrate that narrow rows improve yields in short-season soybean crops in the southern United States similarly to full-season crops in the north. Conversely, full-season crops in the central and southern United States do not need narrow rows for maximum yields under current agronomic and climatic conditions.
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