Many broadleaf weeds in soybeans [Glycine max (L.)] are not adequate-ly controlled by currently available preemergence herbicides. Recent studies have shown that several postemergence herbicides, such as bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide], chloroxuron [3-[p-(p-chlorophenoxy) phenyl]-1,1-dimethylurea], dinoseb [2-sec-butyl- 4,6-dinitrophenol] plus naptalam [N-1-naphthylphthalamic acid], 2,4-DB [4- (2,4-dichlorophenoxy) butyric acid], and RH-6201 [sodium 5-2-chloro-4- (trifluoromethyl)-phenoxy-2-nitrobenzoate], can be applied overtop of soybeans without causing severe injury. The objective of this 1977 study was to determine the efficacy of these herbicides, when applied at different stages of growth, for control of tall morningglory [Ipomoea purpurea (L.) Roth], jimsonweed [Datura stramonium (L.)], giant ragweed [Ambrosia trifida (L.)], common lambsquarters [Chenopodium album (L.)], and velvetleaf [Abutilon theophrasti (Medic.)] in soybeans. The experiment was conducted on sites where natural infestations of jimsonweed, tall morningglory, common lambsquarters, and velvetleaf occurred. The giant ragweed experiment was seeded. Alachlor [2,-chloro- 2\u27,6\u27-diethyl-N-(methoxymethyl) acetanilide] was applied as a preemergence herbicide over each experimental area to control annual grasses. Herbicides were applied overtop of soybeans in late spring and early summer of 1977 as the weeds reached various plant heights, i.e., 3 to 5, 10 to 15, 20 to 25, and 38+ cm. tall. Bentazon effectively controlled giant ragweed less than 15 cm, tall, common lambsquarters less than 10 cm. tall, and velvetleaf less than 30 cm. tall. It did not control tall morningglory but controlled jimsonweed through the 38+ cm. height. Chloroxuron effectively controlled common lambsquarters less than 10 cm. tall, velvetleaf less than 15 cm. tall, jimsonweed less than 15 cm. tall, and tall morningglory less than 5 cm. long. Dinoseb plus naptalam effectively controlled giant ragweed less than 15 cm. tall, tall morningglory less than 5 cm. long, velvetleaf less than 15 cm. tall, and jimsonweed at all stages of growth. It did not control common lambsquarters. Giant ragweed less than 30 cm. tall was effectively controlled by 2,4-DB. Common lambsquarters (less than 5 cm. tall) and velvetleaf (less than 15 cm. tall) were also controlled by 2,4-DB. Jimsonweed and tall morningglory (Spring Hill experiment only) were susceptible to 2,4-DB at all stages of growth used in this study, and tall morningglory (Knoxville experiment only) less than 15 cm. in length was also susceptible. The herbicide RH-6201 effectively controlled giant ragweed less than 15 cm. tall, common lambsquarters less than 5 cm. tall, tall morningglory less than 5 cm. in length, velvetleaf less than 5 cm. tall and jimsonweed at all stages of growth
