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.

Field experiments were conducted over 2 yr (2019 to 2020) at two locations in Iowa to evaluate multi-tactic strategies for managing multiple herbicide–resistant (MHR) waterhemp [Amaranthus tuberculatus (Moq.) Sauer] in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation. The effect of three herbicide programs on A. tuberculatus control was tested in corn (2019). The effects of the prior year’s corn weed control, a cereal rye (Secale cereale L.) cover crop, and soybean row spacing (38-cm vs. 76-cm wide) on A. tuberculatus density, biomass, and seed production were tested in soybean (2020). A herbicide program used in corn with two sites of action provided only 35% control of MHR A. tuberculatus compared with ≥97% control by a herbicide program with three sites of action. In soybean, adequate control of A. tuberculatus (≥90%) in the prior year’s corn crop and use of a cover crop or narrow rows reduced A. tuberculatus density by more than 60% at 3 and 9 wk after planting (WAP) compared with inadequate control (30%) in the prior year’s corn and no cover crop. Cover crop and narrow-row soybean reduced A. tuberculatus density by 44% at 3 WAP compared with no cover crop and wide-row soybean. Inclusion of a single control tactic, adequate control (≥90%) with multiple herbicides in the prior year’s corn, use of a cover crop, or narrow-row soybean reduced A. tuberculatus biomass and seed production at soybean harvest by at least 24% compared with inadequate control (30%) in the prior year’s corn, no cover crop, and wide-row soybean. The combination of all three control tactics reduced A. tuberculatus biomass and seed production at soybean harvest by at least 80%. In conclusion, diverse control tactics targeting A. tuberculatus at multiple life-cycle stages can make substantial contributions to the management of MHR populations.

Field studies were conducted in 2010 and 2011 at two locations in Michigan to examine the effect of row width and herbicide combination on weed suppression and yield in the new Type II black bean variety ‘Zorro.' Black bean was planted in 38- and 76-cm rows. Six weed control strategies were examined: S-metolachlor + halosulfuron (PRE), S-metolachlor (PRE) followed by (fb) bentazon + fomesafen (POST), halosulfuron (PRE) fb clethodim (+ fomesafen at one site in one year) (POST), imazamox + bentazon (POST), a weed-free control, and a nontreated control. Weed control and crop injury were evaluated throughout the growing season. In addition, weeds were counted by species in late July, and weed biomass was harvested and weighed at the end of the season. Black bean yield was obtained by direct harvest. Narrow rows reduced weed populations in two of the four site–year combinations (referred to hereafter as site–years), reduced weed biomass in three of the four site–years, and often improved control of upright broadleaf weeds. All herbicide combinations generally reduced weed populations and biomass, but control of specific weeds was variable. Crop injury was generally slight and transient. Yield was greater in narrow rows in two of the four site–years. All herbicide combinations increased yield compared with the nontreated control and resulted in similar yields to one another. Yield and weed suppression was often maximized in narrow rows, while herbicide performance varied by year and weed spectrum.