We conducted a previous systematic and meta‐analysis review that showed differences in results from studies that evaluated the effectiveness of cover crops for weed suppression in cropping systems; these differences were largely due to management approaches used in growing the cover crop and main crop. The current meta‐analysis provides a quantitative review on how cover crop and main crop management practices influence the impact of cover crops on weed suppression. The meta‐analysis used observations from 53 studies published from 1990 to 2018. Cover crop biomass was inversely related to the amount of weed biomass (r2 = 0.67) and weed density (r2 = 0.64). In general, the meta‐analysis shows that cover crops provided a range of weed suppression depending on management decisions such as choice of cover crop species, cover crop sowing season (fall or spring), sowing dates within seasons, seeding rate, termination date, delay in main crop planting date after cover crop termination, tillage system under which the cover crop was produced, and integrating the cover crop with other weed control inputs. For example, grass cover crop species provided greater weed suppression than broadleaf species. Fall‐sown cover crops provided greater weed suppression (weighted mean of response ratio [R*] = 0.19) than spring‐sown cover crops (R* = 0.48) by the summer. Weed suppression increased by increasing seeding rate of cover crops from 1× (R* = 0.50) to 2× (R* = 0.27) or 3× (R* = 0.10). In addition, cover crops provided greater weed suppression in reduced tillage systems (R* = 0.19) than no tillage (R* = 0.29). The differential weed suppression provided by these management approaches suggests that a cover crop management approach should be rightly selected for weed suppression benefits.

Field studies were conducted under full-season conventional tillage in Delaware and New Jersey to determine the critical time to apply glyphosate with or without residual herbicides for optimum weed control in glyphosate-resistant soybean (GRS). The residual herbicides tank-mixed with glyphosate (0.84 kg/ha) were clomazone (0.55 kg/ha) and imazethapyr (0.063 kg/ha). Herbicide application was made at cracking, unifoliate, and one- to six-trifoliate stages of GRS. Weeds varied in growth stages from preemergence (PRE) at cracking to an average height of 30 cm at the six-trifoliate stage of GRS. Herbicide activity varied by year and weed species. Herbicidal action was better under high (>125 mm/mo) than low (<100 mm/mo) rainfall regime. Glyphosate application without residual herbicides was less effective at cracking and unifoliate than at one- to three-trifoliate leaf stages. Mixing residual herbicides with glyphosate at cracking and unifoliate stages enhanced weed control but made no difference when application was delayed until one- to three-trifoliate stages. For optimum weed control in GRS, the window of application for glyphosate alone was between the one- and three-trifoliate leaf stages, approximately 18 to 28 days after planting (DAP). If glyphosate was tank-mixed with residual herbicides, the window of application extended from cracking until the four-trifoliate stage; and weed interference until the four-trifoliate stage (approximately 32 DAP) did not depress GRS yield.

Field experiments were conducted in 1992 to 1993 and in 1995 to 1996 in Ibadan, Nigeria, to assess the effect of velvetbean and herbicides on maize (corn) and cogongrass growth and to assess regrowth of the weed 1 yr after treatment. In 1992 and 1995 cover cropping with velvetbean and imazapyr and glyphosate application reduced cogongrass density as much as the handweeded control. The smothering effect of velvetbean was equivalent to that of glyphosate at 1.8 kg/ha but was less than imazapyr even at the lowest rate of 0.5 kg/ha. Addition of adjuvant did not improve the efficacy of either herbicide. Maize grain yield was higher in velvetbean plots than in fallow plots dominated by cogongrass. Velvetbean and herbicide effects on cogongrass 1 yr later (1993 and 1996) followed a similar trend as observed in the year of application. Annual weed density was highest in glyphosate plots, followed by imazapyr, and least in plots previously seeded to velvetbean. Maize grain yield was higher in herbicide plots (average yield of 3,170 and 1,920 kg/ha in 1993 and 1996, respectively) than in velvetbean plots (2,800 to 1,180 kg/ha in 1993 and 1996, respectively) and handweeded plots (2,890 and 723 kg/ha in 1993 and 1996, respectively). In 1996 the lowest maize yield was in handweeded plots without velvetbean, suggesting that weeding four times suppressed cogongrass density and biomass, but was not sufficient to minimize the subsequent competition from annual weeds. Uncontrolled cogongrass reduced maize yield to zero. These studies suggest that planting velvetbean for cogongrass control may be a better alternative for farmers without the resources to purchase herbicides.

The efficacy of glyphosate applied alone or in combination with residual herbicides in full-season no-till glyphosate-resistant soybean (GRS) was investigated in New Jersey and Delaware on sandy drought-prone soils. Treatments were in a two- by two- by five-factorial arrangement laid out in three or four randomized complete blocks. The factors investigated were—two preplant glyphosate applications: preplant glyphosate applications or no preplant glyphosate applications; two herbicide treatments: 0.8 kg ae/ha glyphosate alone or 0.8 kg/ha glyphosate tank-mixed with 0.6 kg ai/ha clomazone plus 0.07 kg ai/ha imazethapyr; and herbicide application at five GRS growth stages: at cracking or one of the four times between the V1 and V7 stages. Preplant glyphosate application for the control of emerged weeds was essential for satisfactory control of common annual weeds with glyphosate alone or glyphosate combined with residual herbicides when rainfall was high (avg. 120 mm/mo), but less important when rainfall was low (avg. 72 mm/mo). Compared to glyphosate alone, glyphosate plus residual herbicides improved the control of common lambsquarters, fall panicum, and common ragweed, when applied at cracking or at the V1 stage and preceded by preplant glyphosate applications. At all stages of application, satisfactory full-season control of ivyleaf morningglory was achieved only with glyphosate plus residual herbicides. Horseweed, large crabgrass, giant foxtail, or smooth pigweed control varied from good to excellent (80 to 100%) at all stages of application of glyphosate alone or with residual herbicides. Glyphosate applied alone or with residual herbicides was safe on GRS regardless of time of application up to the V7 stage. The highest soybean yield was consistently achieved with preplant glyphosate applications followed by glyphosate alone at the V2 to V4 stages or a preplant glyphosate application followed by glyphosate plus residual herbicides applied from crop emergence to the V4 stage.

Summary A reconnaissance survey of the occurrence of Tithonia diversifolia (Hemsl.) A, Gray in south western Nigeria was carried out along the major highways linking Ibadan to major towns be‐tween April and August in 1991 and 1992. The survey revealed that T, diversifolia is prominent along the Ibadan—Oyo—Ogbomoshollorin, Iba‐dan—Abeokuta and Oyo—lseyin—Saki roads, and common along the Ibadan—lwo—Osogbo—Offa, Ibadan—Ife—Akure, Ife—Ondo and Ibadan—Ijebu Ode roads It is rare along Ibadan—Ikeja, Akure‐Ado Ekiti—Kabba, Ilorin—Jebba, and Ijebu Ode—Benin City—Asaba roads. T. diversifolia is a shallow‐rooted annual/perennial broad‐leaved plant that grows to a height of 5 m or more and varies from highly branched at low populations (< 5 plants m‐2) to practically unbranched at high populations (> 30 plants m‐2). Under natural conditions in the Ibadan area (7°25′N, 3°54′E), T. diversifolia is established from seed in March—April, flowers between September and November and produces mature seed by Decemher—January. Seed is disseminated between January and March by shedding. Under frequent slashing or in a valley bottom with a high water table, or on river banks T. diversifolia behaves as a perennial shrub and flowering under this condition occurs sporadically throughout the year.