Narrow row spacing (<76 cm) could improve crop competitiveness, suppress weeds and might provide yield advantage. Many studies have been conducted to evaluate the impact of narrow row spacing; however, no quantitative synthesis of these studies exists. The objectives of this meta-analysis were to (1) quantify the overall effect of narrow row spacing (<76 cm) on weed density, biomass, control, weed seed production, and yield in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] compared with 76-cm row spacing, and (2) assess the influence of agronomic management decisions (tillage type, weed management, herbicide application frequency and time) on effect of narrow row spacing on weed suppression and corn and soybean yield. We compiled 1,904 pair-wise observations from 35 studies conducted in 12 states in the United States during 1961 to 2018. Averaged across individual observations, narrow row spacing suppressed weed density by 34%, weed biomass by 55%, and weed seed production by 45%, while it improved weed control by 32% and crop yield by 11% compared with 76-cm row spacing. Narrow row spacing in soybean suppressed weed density by 42%, weed biomass by 71%, and increased crop yield by 12% compared with 76-cm row spacing. Although narrow row spacing had a nonsignificant effect on response variables in corn, the number of studies (n = 1 to 6) and observations (n = 1 to 59) addressing each response variable were limited. Tillage type (conventional and reduced) did not influence the response of weed density, control, and seed production in narrow row spacing; however, weed biomass and weed seed production were more greatly reduced with the sequential application of herbicides compared with a single application. Thus, narrow row spacing in soybean can be integrated with other options for management of herbicide-resistant weeds.

Weed control remains a major challenge for economically viable grain sorghum production in the southeastern United States due to crop sensitivity to weed competition during early growth stages. Field experiments were conducted in 2012 and 2013 to determine the effects of grain sorghum row spacing, population density, and herbicide programs on Palmer amaranth control, crop growth, and grain yield. Treatments included row spacings of 19, 38, and 76 cm; grain sorghum population densities of 99,000, 198,000, 297,000, and 396,000 plants ha−1; and three herbicide programs: (1) a nontreated control, (2) S-metolachlor at 1,410 g ai ha−1 plus atrazine at 1,820 g ha−1 PRE, and (3) S-metolachlor at 1,070 g ha−1 plus atrazine at 1,380 g ha−1 PRE followed by 2,4 D at 330 g ha−1 POST. Palmer amaranth control benefited from the addition of a POST herbicide and from crop density ≥297,000 plants ha−1. Under weedy conditions, Palmer amaranth density was not affected by narrower row spacing or increased crop density, whereas its dry biomass was reduced by 33% with 19 and 38 compared to 76 cm rows, and by 43% with ≥297,000 vs 99,000 plants ha−1. Row spacing had no effect on light interception by the crop canopy. However, crop density influenced canopy closure with maximum light interception occurring one and a half weeks earlier for density ≥297,000 plants ha−1. Yield increased by 18% for 19 vs 38 and 76 cm rows, whereas grain crop density had no effect. Overall, these results indicate that the combination of row spacing≤30 cm and crop density ≥297,000 plants ha−1 provided at least 97% Palmer amaranth control in the absence of POST application and reduced its biomass by 32% in nontreated plots compared to 76 cm row spacing and crop density≤198,000 plants ha−1.

Field experiments were conducted from 1995 to 1997 to test approaches to managing atrazine-resistant Datura stramonium. Eight weed management programs in a Zea mays and Glycine max rotation were evaluated for their effects on the dynamics of atrazine-resistant and -susceptible D. stramonium populations. Overall D. stramonium density and relative abundance of resistant (R) biotypes were greatly reduced in no-till Secale cereale L. (rye) cover crop management programs without triazine compared to conventional-tillage systems with the application of triazine herbicides. The negative effects of no-till on D. stramonium were greater under a G. max–Z. mays—G. max (SCS) rotation than under a Z. mays–G. max—Z. mays (CSC) rotation. A cropping system involving more G. max phases under no-till reduced both the resistant and susceptible D. stramonium populations. Results from this study support the use of soil management, crop rotation, and negative cross-resistant herbicides to manage atrazine-resistant biotypes.

Weed management strategies are needed for no-till soybean grown on clay soils. The effect of several weed management strategies on weed biomass, soybean yield, and gross return were evaluated in 1993, 1994, and 1995 on clay soils at two locations in southern Ontario. Soybean seeds were planted in narrow (19 cm) and wide (76 cm) rows with or without a rye cover crop. Herbicide treatments included glyphosate alone, glyphosate followed by imazethapyr + metribuzin applied PRE, and glyphosate followed by acifluorfen + bentazon applied POST. Two additional treatments with interrow cultivation were included in the wide-row soybean plots with glyphosate and glyphosate + PRE treatments. A nontreated check plot without rye was also included. Presence of a cover crop did not affect weed biomass or soybean yield. The glyphosate + broadcast PRE treatment provided the most consistent weed control both in narrow- and wide-row soybean. The weed control in this treatment ranged from 92 to 100%. The other treatments provided variable weed control across years and locations. The narrow- row plots with glyphosate + broadcast PRE treatment provided the most consistent soybean yields that were generally higher than the other treatments and ranged from 2,560 to 3,420 kg/ha. Soybean yields varied across locations and years in other treatments. Similar weed control and soybean yields were obtained with banded PRE herbicide + interrow cultivation and PRE treatments; however, herbicide use was 60% lower in banded PRE herbicide + interrow cultivation treatment. Narrow-row soybean averaged 27% higher gross returns than wide-row soybean for all broadcast herbicide treatments. Narrow-row soybean with PRE herbicide provided the highest gross returns. No-till soybean in narrow rows with preplant glyphosate and broadcast PRE treatment was the most risk-efficient weed management system on clay soils.

Field experiments were conducted at the Elora Research Station, Elora, Ontario, in 1988 and 1989 to quantify the inherent competitive ability of three white bean cultivars: OAC Gryphon, OAC Sprint, and OAC Laser growing with a naturally occurring population of annual weeds. An attempt was made to increase the competitive ability of these cultivars by altering row spacing and seeding density. Uncontrolled populations of weeds reduced white bean yields by 70%. OAC Gryphon and OAC Laser reduced weed biomass by 10 to 35% compared to OAC Sprint. The ability of cultivars to reduce weed biomass was further enhanced in medium and narrow rows compared to traditional wide rows. Cultivar, row spacing, and seeding density combinations which maximized leaf area index when grown under weedy conditions also had significantly less weed biomass. However, cultivar selection, row spacing, and seeding density did not reduce weed density. A significant negative correlation was observed between weed biomass accumulation and final yield of white bean. For each kg ha−1 increase in weed biomass the corresponding white bean yield loss averaged 0.380 kg ha−1. Season-long weed competition significantly reduced total number of pods per plant, number of seeds per pod, and 100-seed weight.

Planting glyphosate-resistant sugarbeet in narrow rows could improve weed control with fewer herbicide applications and cultivations. Field studies were conducted in 2007 and 2008 at multiple locations in Michigan to compare weed management and sugarbeet yield and quality in glyphosate-resistant sugarbeet planted in 38-, 51-, and 76-cm rows. At all locations, weed densities and biomass were less after glyphosate treatments than after conventional herbicide treatments. Weed densities and biomass also were less in 38- and 51-cm rows compared with 76-cm rows following a single glyphosate application when weeds were 10 cm tall. Averaged over row width, sugarbeet treated with glyphosate when weeds first reached 2 cm in height and again as needed thereafter yielded similarly to sugarbeet treated when weeds were 5 to 10 cm tall. However, root yields were reduced when glyphosate application was delayed until weeds averaged 15 cm in height. Sugarbeet root and sugar yields were greater from 38- and 51-cm row widths than from the 76-cm row widths, averaged over all herbicide treatments. Regardless of row width, initial glyphosate applications should be made before weeds reach 10 cm in height to maximize yield and minimize weed competition with sugarbeet.

Living mulches are vegetative covers that can be grown in association with row crops to reduce soil erosion, improve trafficability and suppress weeds. Interference by the living mulch can reduce yields of an associated crop. The interference between a white clover (Trifolium repens L. ‘New Zealand’) living mulch and sweet corn (Zea mays L. ‘Golden Jubilee’) was studied using an established clover sward that was mowed and then sprayed with 1 to 1.5 kg ai/ha of atrazine. Corn was planted at different densities and planting arrangements into a narrow band tilled in the clover. Interference by clover reduced corn yields by 12 to 39%. However, when corn row width was reduced from 0.76 to 0.38 m, competition among corn plants declined; they became more vigorous and clover- suppress ive and reached even higher yields than conventional (no mulch) corn in 0.76 m rows. Similarly, sweet corn planted at a range of densities into a clover mulch killed by atrazine yielded more in equidistant planting than in wide (0.76 m) rows. A near equidistant corn planting arrangement can be a low-input alternative to achieve season-long clover suppression and thus minimize clover's competition with the intercropped corn.

Crops grown in narrow rows (NR, 0.25 to 0.38 m) shade weed seedlings more than do those grown in traditional wide rows (WR, 0.76 m). NR crops may require less herbicide and interrow cultivation than WR crops for equally effective weed control. This hypothesis was tested by comparing weed control and crop yield in NR and WR crops when the following percentages of recommended application rates (RAR) of standard herbicides were applied: soybean, 0, 50 and 100%; sunflower, 0, 25, 50, and 100%; and corn, 0, 33, and 100% in three separate sets of experiments conducted over 2, 3, and 4 years, respectively. In all treatments with 100% RAR, excellent weed control prevented reductions in crop yield. When only 25 to 50% RAR was applied, weed control was consistently high in NR (82 to 99% control), but variable in WR (42 to 99% control). Weed control and crop yields typically were lowest in NR without herbicides. Interrow cultivation controlled 0 to 81% of weeds in WR crops. In reduced herbicide treatments (25 to 50% RAR), yields of NR soybean and sunflower typically were about equal to those in WR with 100% RAR, but NR corn yields were about 10% less. Considering the reduced herbicide use and lower weed control costs, planting corn, soybean, and sunflower in narrow rows may represent a practical form of low-input production of these important crops.