Effects of Narrow Row Corn on Weed Management

Chris Boerboom

A successful weed management program in corn may depend upon many variables. Typically, initial weed control depends on proper herbicide selection and application, rotary hoeing, or cultivation. However, crop competition has a large role on the final outcome of a weed management program. Without an adequate shading from a fully developed crop canopy, weed control will suffer. The benefits of narrow row spacing on weed management has been well documented in soybeans. It would be logical to expect that weed management should improve if the row spacing of corn was narrowed. However, the planting density is much lower in corn than in soybean. Therefore, it is possible that narrowing corn row spacing may not provide a dramatic improvement in weed control as compared to soybeans. To determine if there is a benefit to be gained, recent research on narrow row corn will be reviewed.

Study 1 : Interactions of row spacing, population, and weed level

At Arlington, Wisconsin in 1997, Harvey and others (1997) planted corn in 15-inch and 30-inch rows at populations of 24,000 and 32,000 plants/a to determine its effect on weed control. Four herbicide programs were also applied to give a range of weed suppression. Only the results of Lasso plus Bladex at full and half rates will be shown.

Harvey's results were from a single year. Weed control, based on weed biomass, was better when corn was planted in 15-inch rows than 30-inch rows at the higher corn population with the full herbicide rate and the low population with the half herbicide rate (Table 1). Weed control was the same between row spacings in the other two comparisons. Visual ratings of weed pressure in mid summer did not show statistically greater weed control with 15-inch rows compared to 30-inch rows. The results show the potential that planting corn in narrower rows may aid weed control, but no consistent or major improvement was measured.

Study 2: Effect of row width on herbicide and cultivation
requirements in row crops

Forcella and others (1992) studied weed control in 15- and 30-inch row corn when planted at 20,000 and 30,000/a in 1986 and at 25,000 and 35,000/a in 1988 and 1989 in west central Minnesota. Herbicide treatments included Lasso and Bladex at full and one-third rates. Wide row corn was cultivated once, but narrow row corn was not cultivated.

In this study, the high or low corn seeding rates did not affect weed control. Full herbicide rates gave 95% or greater weed control in both the narrow row corn and the cultivated, wide row corn (Table 2). Unfortunately, this result does not show any benefit for narrow row corn because the weed control was excellent in both row spacings. At the one-third herbicide rate, control was statistically the same for the two row spacings within a year. However, weed control tended to be better in narrow rows in one year and control tended to be better in the cultivated, wide rows in the other two years. The authors concluded that the narrow rows increased crop competition enough to replace cultivation, but I don't think the results are highly conclusive on this point.

Study 3: Influence of narrow row/high population corn on
weed control and light transmittance

Teasdale (1995) studied weed control in corn planted at 23,000/a in 30-inch rows and 44,000/a in 15-inch rows in Maryland. Dual plus atrazine were either applied at full or quarter rates and the plots were not cultivated.

In this study, weed control was good to excellent at the full herbicide rates so we need to look at the quarter herbicide rates to see if there was a benefit with the narrow row/high population corn. Weed control was statistically better in two of the four years with the narrow row/high population corn, but there was a trend for better weed control in each year (Table 4). Teasdale also measured light intercepted by the corn canopy and found the narrow row/high population corn canopy was closing 7 days earlier than the wide row corn at normal populations. Corn yields were equal in the narrow and wide rows with full herbicide rates. This study shows a definite advantage towards narrow rows when combined with higher corn populations for weed control. However, the study did not directly compare 30-inch rows to 15-inch rows at the same populations. So, we still do not have a conclusive answer to our question on the benefits of narrow row corn on weed management.

Study 4: Influence of corn population and row spacing
on corn and velvetleaf yield

Teasdale (1998) also conducted a study in Maryland on the effect of planting corn at densities of 26,000, 39,000, and 52,000 plants/a in both 15- and 30-inch row spacings on velvetleaf. Teasdale did not measure any difference in the effect of row spacing on velvetleaf survival or growth. However, velvetleaf survival, growth, and seed production were reduced as corn density increased (Table 5). Velvetleaf survival was less in two of three years when the planting density was higher and velvetleaf size (biomass) and seed production were always less at higher corn densities than the density of 26,000 plants/a. Differences in survival, biomass and seed production among years were mostly due to differences in rainfall among the three years. Tollenaar and others (1994) also reported that higher corn densities reduced weed biomass compared to lower densities. The improved weed control in Tollenaar's study was due to less light penetrating the corn canopy.

Study 5: Effect of planting patterns and inter-row cultivation
on competition between corn and late emerging weeds

Murphy and others (1996) studied the effects of corn planted in 20- and 30-inch rows at either 28,000 or 40,000 plants/a on control of late emerging weeds in Ontario from 1990 to 1992. Late emerging weeds were those emerging after the 3-leaf stage of corn.

In the three years of this study, 20-inch row corn gave 16 to 21% greater suppression of late emerging weeds than 30-inch row corn (Table 6). The higher corn density also gave 30 to 41% greater suppression of late emerging weeds than the normal corn density. The greater suppression of weeds occurred because of greater light interception by the canopies of both narrow rows or high populations. Narrow rows intercepted about 8% more light at silking than wide rows (Table 6) and the higher corn density intercepted about 11% more light at silking than the normal corn density. This study from southern Ontario shows a consistent weed control advantage to both narrower rows and higher corn densities.

Study 6: Rapid canopy closure for maize production in the
northern US corn belt: Radiation-use efficiency and grain yield

Since several studies have measured differences in the amount of light intercepted between different corn densities or row spacings, it may be worthwhile reviewing the study by Westgate and others (1997). This study measured the development of corn leaves and the canopy when the corn was planted in 15- and 30-inch rows and at densities from 20,000 to 50,000 plants/a. The corn hybrid was Pioneer 3790 and the study was done in west central Minnesota. All weeds were controlled in this study so there are no weed control results.

Westgate did not measure any differences in the amount of leaf area produced, the maximum light intercepted, or the time of canopy closure (based on light interception) when corn was grown in either 15- or 30-inch rows in either year of the study (Table 7). This differs from some of the results in the previously discussed studies. However, Westgate found that increasing the corn's density increased the total amount of light intercepted by the canopy and caused the canopy to close sooner (Table 7). Although row spacing may not affect the total amount of light intercepted by corn, Westgate did note that changes in row spacing may affect how uniformly the corn canopy captures light. In wider rows, the corn may give more shading in the rows than narrower rows, but wider rows may have less shading between rows than narrower rows.

Study 7: Integrated weed management using narrow row
corn spacing, herbicides, and cultivation

Johnson and others (1998) studied weed control in 20-and 30-inch row corn when planted at 32,000/a in south central Minnesota in 1995 and 1996. Some of the main herbicide treatments were full vs. half rates of acetochlor (Harness or Surpass) alone or followed by postemergence dicamba. Each herbicide treatment was split so half was cultivated and the other half was not.

The two main conclusions from this study were that row spacing had little effect on giant foxtail or common ragweed control and that cultivating was the most important factor for successful use of reduced herbicide rates. Specifically, giant foxtail control was similar in both row spacings for both years except in 1996 when control in narrow rows (67%) was slightly greater than in wide rows (60%) without cultivation. Common ragweed control was also greater in narrow rows than wide rows when treated with half rates of acetochlor in 1995, but control was the same at full rates in 1995 and in both row spacings in 1996. The authors also felt that increasing corn population to the recommended level would be the first approach to increase corn's competitive ability against weeds before using narrow rows, which appears to have less potential for improving weed control. In this study, wide row corn tended to out yield the narrow row corn, which the authors could not explain.

Summary

Overall, it appears that narrow row corn should be developing a canopy faster than wide rows, which should increase weed suppression. However, two of these studies do not show a consistent advantage of narrow row corn for suppressing weeds. Perhaps this is because the corn could not canopy early enough to out compete the early emerging weeds that escaped reduced herbicide rates. Ideally, the benefits of rapid canopy closure would have the greatest effect on later emerging weeds after a good initial weed control treatment. This result was consistently shown by Murphy in study 4. Higher corn populations should also improve weed control because of faster canopy development. The benefit of higher populations was seen in Murphy's and Teasdale's studies, but not in Forcella's study. This may have been due to a difference in time of weed emergence.

The other consideration for weed control in narrow row corn is whether or not the corn can be cultivated. The research by Johnson clearly showed that cultivation was very beneficial if herbicides did not provide adequate initial control. Greater crop competition from narrow rows may replace a portion of the weed control traditionally provided by cultivation, but there is not much evidence that narrow rows can completely replace cultivation. Until more consistent and favorable results exist for weed suppression by narrow row corn, it seems wise to either be prepared with a narrow row cultivator or be prepared to repeat herbicide applications in years when initial herbicide treatments do not give adequate control.

Literature Cited