Summary: We examined factors affecting conversion of light into dry matter of velvetleaf (Abutilon theophrasti) and soybean (Glycine max) grown in a replacement series near Ames, Iowa, U.S.A. in 1986 and 1987. Velvetleaf was taller than soybean over most of the growing season and had more branches, especially at the top of the canopy. Velvetleaf had greater light interception than soybean in the upper part of the canopy, particularly early and late in the day, although total light interception by canopies of the two species was similar on most sample dates. Velvetleaf had higher light utilization efficiency, or conversion of intercepted light into dry matter, in the middle and later part of the growing season. This combination of traits appears to give velvetleaf an advantage over soybean in competition for light.

Barnyardgrass was grown at densities of 0, 0.25, 0.5, 1, 2, 5, and more than 50 plants m−1 of tomato crop row in either a regular, random, or clumped pattern. Tomato was established at 0, 5, 10, or 20 plants m−1 of crop row in a regular pattern. Crop density and weed density or spatial arrangement had little effect on phenological development of barnyardgrass. In the absence of tomato, barnyardgrass was estimated to produce over 400,000 seeds plant−1 when not subjected to intraspecific competition (0.25 plants m−1 density), decreasing to about 10,000 seeds plant−1 when weed density exceeded 50 plants m−1 of row. Differences in seed production between plants in the regular and random spatial arrangements were minor, but the clumped distribution resulted in 30 to 50% reduction in seed production at weed densities between 1 and 5 plants m−1 of row. Tomato reduced barnyardgrass seed production. The magnitude of the reduction depended on both tomato density and barnyardgrass density. In the absence of tomato, barnyardgrass produced over 200,000 seeds m−2 in 1993 and over 500,000 seeds m−2 in 1994 at 5 plants m−1 of row. Production was almost 700,000 seeds m−2 when the weed density exceeded 50 plants m−1 of row. Barnyardgrass seed production at the single-season economic threshold density in tomato was sufficient to maintain the seedbank at a level that would mandate high levels of weed control in subsequent crops. Because of the high fecundity of barnyardgrass, our experiments suggest that stopping seed production is the best long-term management strategy for the weed. Spatial arrangement of the weed, at the scale used in these studies, would not be a factor in establishing long-term management guidelines based on weed population biology.

We examined relative competition abilities of velvetleaf {Abutilon theophrasti) and soybean {Glycine max) grown in a replacement series near Ames, Iowa, U.S. A. in 1986 and 1987. Relative above ground dry weight of soybean in mixtures was higher than expected from monoculture values early in the growing season but lower than expected later in the season, whereas velvetleaf had higher than expected values later in the season. Velvetleaf depressed seed yield of soybean in all mixtures; relative seed yield of velvetleaf was greater in all mixtures than in monoculture. Relative growth rate (RGR), leaf area ratio (LAR) and net assimilation rate (NAR) of soybean did not differ significantly among mixtures and generally decreased over the growing season. Velvetleaf exhibited no consistent differences among mixtures in RGR, LAR, and NAR. Net assimilation rate of velvetleaf was constant over most of the growing season. Relative growth rate and LAR of velvetleaf were highest early in the season and then progressively declined. Velvetleaf had higher NAR and higher RGR than soybean early in the season; LAR of velvetleaf was lower than LAR of soybean over most of the season. Velvetleaf and soybean did not significantly tTo whom correspondence should be addressed.interfere with each other early in the growing season, but velvetleaf gained resources at the expense of soybean during the middle and later parts of the season. Une Evaluation par des series de substitution de la competition entre I'abutilon fAbutilon theophrasti^ et te soja (Giycine maxN ous avons examine I'aptitude relative de la competition de I'abutilon {Abutilon theophrasti) et du soja {Glycine max) cultives dans des series de substitution pres d'Ames, Iowa en 1986 et 1987.Le poids sec relatif de la partie adrienne du soja en melange a ete plus eleve que ce qui etait attendu a partir des valeurs en monoculture en debut de saison, mais plus basse que ce qui dtait attendu en fin de saison; tandis que pour I'abutilon c'etait plus fort que ce qui dtait attendu en fin de saison.L'abutilon entraine une baisse du rendement en graine du soja dans tous les melanges; le poids relatif des graines d'abutilon etait plus eleve dans tous les melanges qu'en monoculture. Le taux de croissance relative (RGR), le taux de surface folaire (LAR) et le taux d'assimilation nette (NAR) du soja ne different pas entre les melanges et baissent de fa^on generale pendant la saison de croissance.

Field studies were conducted to determine how the spatial arrangement of weed populations influences interspecific competition. We studied the influence of regular, random, and clumped distributions of barnyardgrass on growth and yield of direct-seeded tomato planted at different densities. Increasing aggregation increased intraspecific competition in barnyardgrass. At the same time, interspecific competition experienced by tomato from barnyardgrass decreased. Differences in the amount of shading of the tomato canopy by barnyardgrass contributed to yield loss differences for the various spatial arrangements. Clumped barnyardgrass caused significantly less average shading than barnyardgrass in regular or random arrangements. At a typical planting density of 10 tomato plants m−1 of row, yield losses ranged from 10 to 35% (1993) or 8 to 50% (1994) when competing with a clumped arrangement of barnyardgrass. At the same tomato density, yields were reduced from 20 to 50% (1993) or 11 to 75% (1994) for the regular and random arrangements for the same barnyardgrass densities. Predicted single-season economic threshold densities for barnyardgrass at a typical tomato planting density of 10 plants m−1 would be one barnyardgrass plant per 25, 19, or 15 m of crop row, respectively, for regular, random, and clumped spatial distributions.

The effect of moderate moisture stress on the activity of the methyl ester of diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} applied to wild oat (Avena fatuaL.) and the influence of morphological and physiological factors on the activity of the herbicide on stressed plants were investigated in growth-room experiments. Wild oat was grown in very fine sandy loam brought to a gravimetric soil moisture content (SMC) of 20% (water potential of -0.3 bars) or 10% (-6.5 bars) prior to herbicide treatment. The activity of diclofop on wild oat was slightly, but significantly, reduced when the SMC was increased from 10 to 20% immediately after spraying. Maintaining the SMC at 10% after spraying further reduced the activity of the herbicide. Wild oat plants grown at 20% SMC had 22% greater leaf area and retained 23% more herbicide than plants grown at 10% SMC. Penetration of the methyl ester of14C-diclofop into wild oat grown at 20 or 10% SMC did not differ significantly 12, 24, or 48 h after application. Stressed and unstressed plants did not differ significantly in the metabolism of the methyl ester of14C-diclofop. However, growth at a low SMC prior to treatment decreased the proportion of the radiolabeled herbicide recovered in the apex, third leaf, and tillers of wild oat. This may partially account for the reduced activity of this herbicide on wild oat subjected to moisture stress.