These studies were conducted to gain a better understanding of the differences in activity of two new herbicides with three commonly used herbicides with regard to their effects on growth, phytotoxicity and germination. This information is necessary for the further evaluation of the new compounds. New herbicides N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl ]-N,N′dimethylurea (tebuthiuton) and N-(phosphonomethyl)glycine (glyphosate) compared with 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat), alkanolamine formulation of 2,4-dichlorophenoxyacetic acid (2,4-D), and 7-oxabicyclo[2.2.1.] heptane-2,3 dicarboxylic acid (endothall). Growth and phytotoxic effects were evaluated on cowpea [Vigna unguiculata (L.) Walp.] and sorghum [Sorghum bicolor (L.) Moench] seedlings. Germination effects were studied on seed of sorghum, cowpea, and wheat (Triticum aestivum L.)
Greenhouse grown seedlings of cowpea (10 to 12-days old) and sorghum (25 to 28-days-old) were treated by applying 100 μ1 aqueous solution of the test compounds to one unifoliolate leaf of cowpea or to the partially unfurled true leaf of sorghum. Rates ranged from 1.4 to 1,434 μg/plant depending on the herbicide. Evaluation of phytotoxic and growth responses was made 10 to 12-days post-treatment. In cowpea, the growth response was determined by measuring the length of the first internode. In sorghum, fresh weight served as a measure of the growth response. Cowpea phytotoxicity was determined on percent dead plants per six-plant replicate. For sorghum, phytotoxicity was measured as percent desiccation/ plant. Absorption times were determined by washing treated seedlings at intervals for a 6-hour period. All experiments were repeated six to eight times with each treatment being replicated six and 10 times, cowpea and sorghum respectively.
The order of decreasing effectiveness of the five herbicides for growth and herbicidal effectiveness for cowpea and sorghum was paraquat, glyphosate, tebuthiuron, 2,4-D, and endothall. Endothall-glyphosate mixtures were synergistic in inhibiting growth of cowpea. Endothalltebuthiuron mixtures were generally synergistic in killing and inhibiting growth of sorghum.
Absorption of growth-inhibiting levels of all five herbicides occurred within the first 60 rain. of exposure. Phytotoxic levels of tebuthinron, paraquat, and endothall were absorbed within 5, 30, and 60 rain, respectively. Glyphosate showed rapid action and resistance to removal by washing.
Glyphosate and tebuthiuron had little inhibitory effect on germination of sorghum, wheat, or cowpea. Paraquat severely inhibited germination of all three species; endothall and 2,4-D inhibited only cowpea.