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This research was initiated to determine how soybean [Glycine max(L.) Merr.] cultivars differ in competitive ability and also whether allelopathy functions to inhibit surrounding weed growth. Initially 280 soybean cultivars were screened in the field for competitive ability based on emergence, seedling vigor, quickness of canopy closure, and ability to inhibit growth of velvetleaf (Abutilon theophrastiMedic.) and foxtail millet [Setaria italica(L.) Beauv.]. Experiments were conducted at Lincoln, Nebr. on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudolls) during 1981 and 1982. Soybean cultivars of later maturity tended to compete more effectively with weeds. Competitive cultivars emerged quickly, rapidly formed a canopy, and were able to slow the growth of competing weed species. Twenty soybean cultivars of varying competitive ability were selected and grown in the greenhouse during 1981 and 1982 to determine the importance of allelopathy in competing with weeds. Exudates from roots of soybean cultivars grown in sand reduced the dry weight of 4‐week‐old velvetleaf plants an average of 15%, but foxtail millet was not inhibited. Incorporation of 1% ground soybean dry matter into Sharpsburg silty clay loam inhibited germination and dry weight of greenhouse grown velvetleaf an average of 46% each. Foxtail millet germination and dry weights were reduced an average of 82 and 65%, respectively. Undiluted soybean plant extracts of all cultivars tested slowed the germination and dry weight accumulation of 6‐day‐old velvetleaf and foxtail millet, but dilution of the extracts caused quite variable responses. Osmotic potential of the extract solution appeared to be the main cause of inhibition, however, extracts of some cultivars, diluted to decrease the osmotic potential, still inhibited growth. Allelopathy appears to be one mechanism for competition among soybeans and weeds. Soybean cultivars possessing the ability to chemically inhibit competing weed growth would be a great benefit to the soybean producer.

The weed seedbank in the soil makes weed control necessary every year in crop production. Research was initiated to determine if a farmer could reduce weed control practices after several years of excellent weed control. Experiments were conducted at five locations across Nebraska to determine the demise of weed seed in continuous corn (Zea maysL.) where weed seed production had been eliminated for 3 yr (1975–1977) and then four weed management levels were superimposed for another 3 yr (1978–1980). Soils were Cumulic Haplaquolls, Typic Argiudolls, Udic Agriustolls, Typic Haplustolls, and Aridic Haplustolls. Within 1 yr (1978) after weed eradication had stopped, the average number of weed seeds that germinated increased over the four weed management levels; and weed competition to corn was evident within 2 yr (1979). Weeds have the potential to rapidly increase and reinfest a field once there is a lack of vigilance in the weed control program. Thus, the weed seedbank assures a continuity to the weed problem that will require more than eliminating weed seed production for several years. If a farmer has achieved 3 yr of perfect weed control on a given field, he still must expect weed problems in subsequent crops because of seed longevity in soil.

The need for growers to conserve energy, labor, and money has stimulated the joint application of herbicides and liquid fertilizers. Chemical and physical reactions between herbicides and liquid fertilizers may affect the compatibility or herbicidal performance of a combination. The objective of this research was to determine the compatibility and phytotoxic characteristics of 10 common herbicides when individually mixed with each of six liquid fertilizers. Of the 60 herbicide‐liquid fertilizer combinations tested, 87% were compatible and 13% were incompatible. Attapulgite clay suspensions were the most compatible of the liquid fertilizers tested. Atrazine (6‐chloro‐N‐ethyl‐N'‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine] and metribuzin [4‐amino‐6‐(1,1‐dimethylethyl)‐3‐(methylthio)‐1,2,4‐triazin‐5(4H)‐one] were least compatible with the liquid fertilizers. Phytotoxicity of the herbicide, when combined with the liquid fertilizers, increased in 21%, decreased in 32%, and was equal to the herbicide‐water check in 47% of the combinations. Glyphosate [N‐(phosphonomethyl)glycine] and paraquat (1,1'‐dimethyl‐4,4'‐bipyridinium ion) showed the highest occurrence of decreased phytotoxicity, whereas acifluorfen {5‐[2‐chloro‐4‐(trifluoromethyl)phenoxyl‐2‐nitrobenzoic acid} and bentazon [3‐(1‐methylethyl)‐(1H)‐2,1,3‐benzothiadiazin‐4(3H)‐one 2,2‐dioxide] showed the highest occurrence of increased phytotoxicity when mixed with liquid fertilizers. Agitation was necessary to prevent separation in 52% of the compatible mixtures. Duration of contact between herbicide and liquid fertilizer did affect herbicidal phytotoxicity with certain combinations and especially with butylate [S‐ethyl bis(2‐methylpropyl) carbamothioate]‐liquid fertilizer mixtures.

Reduced tillage systems are being developed for the Great Plains that conserve limited moisture in order to improve soybean [Glycine max(L.) Merr.] cultivar response in this region. Fourteen soybean cultivars were grown in narrow rows using herbicides or tillage for weed control during the fallow period after oat (Avena sativaL.) harvest on an oat‐soybean rotation. Experiments were conducted at Concord, NE on a Colo silty clay loam (fine‐silty, mixed, mesic, Cumulic Haplaquolls) and at Lincoln on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic, Typic Argiudolls) during 1979 through 1982. The best broadleaf weed control occurred with ‘Gnome’, ‘Century’, ‘Weber’, and ‘Williams 79’ cultivars; also Gnome and Century had the best grass control as well as the lowest total weed yields. Early triazine injury was least (<30%) in Gnome, ‘Beeson 80’, ‘Elf’, ‘Wells II’, Weber, ‘Cumberland’, and Century. The highest yielding cultivars were ‘Mead’ and Century. Soybean cultivars generally yielded better on tilled vs. herbicide treated oat stubble, but the three cultivars showing the least difference in yields regardless of the prior fallow treatment were Cumberland, Beeson 80, and Weber. Cultivars selected for reduced tillage methods may differ from those used in conventional production systems.