Weed competition in soybeans [Glycine max(L.) Merr. ‘Clark 63′] with planted stands of velvetleaf (Abutilon theophrastiMedic.), prickly sida (Sida spinosaL.), and Venice mallow (Hibiscus trionumL.) is reported. Weeds were planted 0, 10, 20, 30, and 40 days after soybeans and competed until soybeans matured. Soybean seed yields were reduced 720, 250, and 230 kg/ha by velvetleaf, Venice mallow, and prickly sida, respectively. Weeds that emerged with soybeans reduced yields 1,010 kg/ha. Weeds planted 10 days later reduced yields 480 kg/ha, but weeds planted 20 to 40 days after soybeans did not significantly reduced yield. Soybean yields were reduced regardless of weed placement in or between soybean rows. Weed competition reduced numbers of soybean pods per plant more than other soybean-yield components.
Competition studies were conducted with soybeansGlycine max.(L.) Merr. ‘Clark 63’ and Venice mallow (Hibiscus trionumL.). One Venice mallow per 7.5 cm of soybean row reduced soybean seed yield 632 kg/ha after 85 days competition. Thirty to 40-cm weed bands in and between soybean rows reduced yields 270 to 651 kg/ha with 35 to 40 days of competition. A natural stand of 215 Venice mallow plants per square meter reduced soybean yield 454 kg/ha after 30 days of competition, and competition up to 110 days reduced yield as much as 1490 kg/ha. Weed competition affected the number of pods per soybean plant more than any other seed-yield component, and soybean height was reduced. Soybean yields were reduced more when soil moisture was abundant early in the growing season and limited in late summer than when moisture was limited early in the growing season and above average until soybeans matured.
Grain sorghum (Sorghum vulgare Pers.) was grown in combination with varying densities and durations of tall waterhemp (Acnida altissima Riddell) during 1964 and 1965 at Manhattan, Kansas. As waterhemp density and duration increased, sorghum fodder and grain yields were reduced. Yields of both species and competitive influence of waterhemp were highest during the year of high rainfall and supplemental N fertility. The net competitive influence of waterhemp on sorghum grain was accounted for principally by reduced seed size. Competition for soil moisture in 1965 was greatest when weeds remained full-season, and was most pronounced early in the season at soil depths below 20 inches.
Grain sorghum (Sorghum bicolor(L.) Moench ‘RS 671’) was grown during 1969 and 1970 in competition with two other members of the same species, wild cane and Kansas Orange forage sorghum, transplanted at various densities along grain sorghum rows. All densities of wild cane and Kansas Orange significantly reduced both grain and fodder yields of grain sorghum. Grain yield was highly and positively correlated with leaf area, culm length, culms per plant, panicles per culm, seeds per panicle, and amount of light received at the grain sorghum canopy; and grain yield was negatively correlated with plant factors that benefited wild cane and Kansas Orange. Grain yield did not correlate with either plants per hectare or seed weight. Panicle size was most important in determining grain yield. Kansas Orange reduced grain yields more than did wild cane in 1969. During 1970, with less favorable rainfall, grain sorghum responded similarly to both transplanted weed types.
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