Low recoveries of total applied14C in translocation studies and erratic control of hemp dogbane (Apocynum cannabinumL.) in the field showed a need for a balance-sheet study of absorption, translocation, and metabolism of14C-2,4-D [(2,4-dichlorophenoxy) acetic acid] and14C-glyphosate [N-(phosphonomethyl)glycine]. Total recovery of14C-herbicides applied to hemp dogbane in the laboratory was 97% for 2,4-D and 105% for glyphosate. Of the14C recovered after 12 days in the hemp dogbane, 34 to 55% was parent-2,4-D after 2,4-D treatment, and 93 to 96% was parent glyphosate after glyphosate treatment. Only negligible amounts of14C were lost via volatilization or evolution as14CO2. A broadcast treatment with unlabeled herbicide did not significantly affect subsequent absorption, translocation, or metabolism of either herbicide. Total herbicide absorbed and translocated out of the treated area of the leaf generally increased during the subsequent 12 days for 2,4-D but only 3 days for glyphosate. A greater percentage of the total applied 2,4-D (31 vs. 14%) and glyphosate (14 vs. 8%) was translocated from upper rather than lower leaves of hemp dogbane, respectively. Higher temperatures (30 vs. 25 C) resulted in greater translocation of glyphosate (39 vs. 18%) but not 2,4-D (35 vs. 39%). Higher light intensities resulted in greater accumulations of 2,4-D into roots and of glyphosate into untreated areas of the treated leaf. Autoradiographs showed that both herbicides moved through hemp dogbane in a typical symplastic pattern and accumulated in roots and new leaves.
Herbicides used in corn(Zea maysL.), sorghum [Sorghum bicolor(L.) Moench], and soybeans [Glycinemax (L.) Merr.] were applied in the spring and their persistence into late summer was determined during 1974 to 1976. Composite soil samples from the top 5 cm of field plots were taken each August and bioassayed in the greenhouse. Bioassay species used were winter wheat [Triticum aestivumL.) and soybeans for herbicides used in corn and sorghum, and winter wheat and white mustard(Brassica hirtaMoench) for herbicides used in soybeans. Soil persistence of triazine herbicides caused more injury to winter wheat, soybeans, and white mustard than any other class of herbicides tested. Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] showed the most soil persistence of the five triazines evaluated. At normal field application rates, herbicides other than the triazines showed little injury to the bioassay plants. Soil persistence of herbicides was further reduced when combinations of reduced rates of each herbicide were utilized. Herbicides used for spring applications in corn showed more soil persistence in August than did the herbicides for sorghum, while herbicides for soybeans generally were least persistent. Postemergence herbicide applications resulted in more injury in bioassay species than preplant incorporated or preemergence applications. Persistence of some herbicides will restrict certain options to the grower such as changing crops in case of crop failure, fall planting of winter wheat, double cropping, or certain crop rotations.
All land uses in eastern and southeastern Nebraska were infested to some extent with hemp dogbane (Apocynum cannabinumL.). The highest infestations were observed in oats (Avena sativaL.) and soybeans [Glycine max(L.) Merr.] and the lowest infestations were in alfalfa (Medicago sativaL.), pastures, and winter wheat (Triticum aestivumL.). Yield reductions from hemp dogbane infestations ranged from 0 to 10% in corn (Zea maysL.), 28 to 41% in soybeans, and 37 to 45% in sorghum [Sorghum bicolor(L.) Moench]. Emergence of hemp dogbane from crown roots occurred when the soil temperature was 17 to 19 C, during April in 1977 and 1978. Plants attained the bud stage within 4 to 7 weeks after emergence. Early flower, full bloom, and pod initiation occurred subsequently at about 1 week intervals. Seeds produced were first viable 10 weeks after full bloom. Root activity or regenerative capacity as measured by length and number of new shoots and roots produced at monthly intervals in the germinator showed a cyclic pattern. The highest activity occurred in the spring and late fall and lowest activity in summer and early fall. Protein levels in the roots ranged from 7 to 9% in the fall and spring to 4 to 5% during the summer. Percentage total nonstructural carbohydrates (TNC) ranged from 20 to 31% in lateral roots and 32 to 53% in crown roots, but there was not a consistent cyclic pattern of percentage TNC during the growing season.
Hemp dogbane (Apocynum cannabinum L.) has become a serious weed problem in eastern and southcentral Nebraska. Greenhouse and field studies were conducted to evaluate control of hemp dogbane with herbicides. In greenhouse studies, postemergence applications of herbicide combinations which included 2,4‐D [(2,4‐dichlorophenoxy)acetic acid] ester, 2,4‐D amine, or glyphosate [N‐phosphonomethyl)glycine] controlled hemp dogbane more effectively than those containing dicamba (3,6‐dichloro‐o‐anisic acid) or amitrole (3‐amino‐s‐triazole). Control was greater with equivalent rates of 2,4‐D ester than with 2,4‐D amine. Control with 2,4‐D increased with longer slide‐chain lengths of both ester and amine formulations. Postemergence control of hemp dogbane with herbicides was erratic in field studies over years. Glyphosate generally provided better control than 2,4‐D, and amitrole consistently gave poor control. A preliminary study showed that growth of hemp dogbane from crown and lateral roots could be suppressed or controlled with preplant treatments of thiocarbamate herbicides. Increasing order of effectiveness for controlling hemp dogbane was vernolate (S‐propyl dipropylthiocarbamate), EPTC (S‐ethyl dipropylthiocarbamate), and butylate (S‐ethyl diisobutylthiocarbamate). In the field, butylate + R‐25788 (N,N‐diallyl‐2,2‐dichloroacetamide) and EPTC + R‐25788 at 4.5 + 0.2 kg/ha provided good control (60 to 98%) of hemp dogbane during the first 2 months after corn (Zea mays L.) planting, but decreased to 0 to 40% at harvest. Higher rates of butylate + R‐25788 controlled hemp dogbane slightly longer. Thiocarbamate herbicides inhibited deposition of epicuticular wax on the leaves of hemp dogbane in 1977 but not in 1978. It appears that preplant applications of the thiocarbamate herbicides may aid in controlling hemp dogbane via delayed emergence, reduced plant vigor, and possibly greater adsorption of 2,4‐D applied subsequently as a foliar application.
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