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Our objectives were to evaluate the seed yield of soybean, and Duncan's model with respect to relationship between seed yield and plant population, and analyze the response of soybean seed yield components to light enrichment initiated at different growth stages. Duncan coined the term crowding to include the effects of density and planting pattern. Duncan's model states that there is a linear relationship between natural logarithm of yield plant−1 and crowding. Results of the studies fitting Duncan's model to the data obtained from different soybean cultivars planted at different densities and planting patterns indicated that the model can predict the changes in yield with changing densities and planting pattern especially if the variability in the data is low. In order to analyze the response of soybean seed yield components to non-destructive light enrichment initiated at different growth stages, light enrichment was imposed on the indeterminate soybean cultivar Evans by installing wire mesh fencing on either side of the center row to push the adjacent rows aside at different growth stages. Fences prevented plants in the neighboring rows from encroaching on the growing space of the center row plants. Pod number per plant and to a lesser extent seed size accounted for variation in seed yield. Light enrichment initiated at late vegetative or early flowering stages increased seed yield 217%, mainly by increasing pod number, while light enrichment beginning at early pod formation increased seed size 23%, resulting in a 115% increase in seed yield. Responses to light enrichment occurred proportionately across all node positions despite the differences in the time (15 to 20 days) of development of yield components at the different node positions. Although maximum seed size may be under genetic control in soybean plants, our results suggested seed size can still be modified by the environment with some internal control moderating the final size of most seeds in all pods. It indicates that plants are able to redistribute the available resources to components not yet determined, in an attempt to maintain or improve yield.

Laboratory and greenhouse studies were conducted to examine the leaf surface, epicuticular wax content, spray droplet behavior, and primisulfuron activity (with and without surfactants) on common lambsquarters, common purslane, velvetleaf, barnyardgrass, and green foxtail. Adaxial and abaxial leaf surfaces were examined using scanning electron microscopy. Leaf wax was extracted and quantified. The spread of 1μl droplets of distilled water, primisulfuron solution (without surfactant), primisulfuron solution with a nonionic surfactant and with an organosilicone surfactant was determined on the adaxial leaf surfaces of each of the weed species. The activity of primisulfuron without or with surfactants was assessed 3 weeks after treatment in terms of percent injury and plant fresh weight. Greenhouse studies were also conducted to investigate the bioherbicidal activity of Alternaria helianthi (Hansf.) Tubaki & Nishih. on multiple-seeded cocklebur as affected by various surfactants. The number of stomata per unit area on abaxial surface was more than on adaxial leaf surface of barnyardgrass, common lambsquarters and velvetleaf, whereas, common purslane and green foxtail had more stomata on adaxial surface than abaxial. Common lambsquarters had the highest wax content per unit of leaf area (274.5 μg cm-2) and velvetleaf had the lowest (7.4 μg cm-2). Wax content of common purslane was 153.4 μg cm-2. The mean values of the wax content per unit of leaf area in barnyardgrass and green foxtail were 35.91 μg cm-2 and 19.14 μg cm-2, respectively. Surfactants increased primisulfuron activity on common lambsquarters, common purslane, velvetleaf, and green foxtail. In general, organosilicone surfactant reduced the contact angle with increased spread area of the primisulfuron droplets more than the nonionic surfactant treatments and resulted in enhanced activity of primisulfuron. Alternaria helianthi resulted in significant reduction of fresh weight of multiple-seeded cockleburs when followed by a 12 h dew period as compared to a 6 h dew. Under short dew period (6 h), greater control of multiple-seeded cocklebur was achieved using higher rates of Activator 90 and Silwet L-77 and may have great potentials for achieving effective biological control.

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