Control of common milkweed (Asclepias syriaca L.) with postemergence herbicides was investigated in greenhouse and field experiments. In greenhouse experiments, amitrole (3-amino-s-triazole), dicamba (3,6-dichloro-o-anisic acid), glyphosate [N-(phosphonomethyl)glycine], and combinations of amitrole + dicamba, dicamba + 2,4-D [(2,4-dichlorophenoxy) acetic acid], glyphosate + 2,4-D, and glyphosate + dicamba reduced regrowth of 6-week-old common milkweed as compared with the untreated check. Postemergence herbicides commonly used in corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], and soybeans [Glycine max (L.) Merr.] had little effect on emerged common milkweed. Removal of common milkweed top-growth as early as 1 day after treatment did not effect subsequent growth reduction from glyphosate at 0.3 kg/ha or higher. In the field, satisfactory control (70% or better) of common milkweed was obtained with glyphosate at 2.2 kg/ha applied at the early or late bud growth stage. Plots plowed 1 day after application of glyphosate at 2.2 kg/ha still retained satisfactory control of common milkweed 1 yr after treatment. Herbicides applied with a herbicide glove or a roller applicator resulted in common milkweed control similar to that obtained from broadcast spray applications.

Canada thistle [Cirsium arvense [L.] Scop. # CIRAR] is a major weed problem in birdsfoot trefoil (Lotus corniculatus L. ‘Norcen’) seed production in northern Minnesota. Several systemic herbicides applied with selective applicators (roller and ropewick) were evaluated for Canada thistle control in birdsfoot trefoil. Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), clopyralid (3,6-dichloro-2-pyridinecarboxylic acid), and glyphosate [N-(phosphonomethyl)glycine] controlled Canada thistle better than MCPA [(4-chloro-2-methylphenoxy)acetic acid] and dicamba (3,6-dichloro-2-methoxybenzoic acid). Glyphosate and MCPA did not injure birdsfoot trefoil foliage or bloom, whereas, picloram, clopyralid, and dicamba injured both. Although selective applications of glyphosate controlled Canada thistle for a short term with the least birdsfoot trefoil injury, long-term Canada thistle control in birdsfoot trefoil does not appear feasible with selective herbicide applications.

The potential of preventing seed production and reducing seed viability of weeds that commonly grow taller than soybeans [Glycine max (L.) Merr. ‘Williams 82’] was evaluated. Chlorflurenol (2-chloro-9-hydroxy-9H-fluorene-9-carboxylic acid), chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide}, and glyphosate [N-(phosphonomethyl)glycine] were evaluated against sparse stands of velvetleaf (Abutilon theophrasti Medik. # ABUTH) and giant foxtail (Setaria faberi Herrm. # SETFA) growing in soybeans. Simulated-roller applications of all three herbicides significantly reduced seed production and germination of both weeds, although glyphosate was more effective than were the other two. Applications during early flowering of velvetleaf and early heading of giant foxtail reduced seed production more than later applications when some seeds were present. With the early application of glyphosate, 99% prevention of velvetleaf and 96% prevention of giant foxtail seed production were attained. With the early glyphosate application, germination of seeds produced was reduced by 50% in velvetleaf and by 95% in giant foxtail. Soybean yields were not reduced by either glyphosate or chlorflurenol but were drastically reduced by chlorsulfuron.

Three applications/yr of glyphosate [N-(phosphonomethyl)glycine] with a ropewick applicator (RWA), a roller applicator (RA), and a recirculating sprayer (RCS) were compared to hoeing and cultivation for the control of johnsongrass [Sorghum halepense (L.) Pers. ♯3 SORHA] in cotton (Gossypium hirsutum L. ‘Acala SJ-2′) in six field experiments from 1979 to 1981. Averaged over the 3 yr and dates of application, plots treated with glyphosate contained 78% fewer live johnsongrass shoots (15/m2) than cultivated-control plots (68/m2). Although applications of glyphosate with the RWA and RA improved or tended to improve yields of cotton, only plots treated with the RWA yielded significantly more seed cotton than cultivated control plots in all years. However, yields of plots treated with the RWA still averaged 38% less than hoed plots. Plots treated with the RA produced significantly more cotton than cultivated-control plots only in 1981.