The translocation pattern and distribution in rhizomes of 14C-glyphosate, 14C-quizalofop, and 14C-sucrose was examined in five quackgrass biotypes. Translocation of radioactivity in the different plant parts varied among biotypes. Translocation in the whole plant after treatment with the three 14C-chemicals varied among biotypes but was not correlated with their tolerance to herbicides. Detailed analysis of distribution of radioactivity in the primary rhizome showed that more 14C was found in the apical sections after treatment with 14C-sucrose. A similar pattern was observed after 14C-glyphosate application with all biotypes except one. Very low radioactivity was found in rhizomes after 14C-quizalofop application, but a preferential accumulation in apical sections of the primary rhizome was detected in two biotypes. The tolerance of one biotype to glyphosate was explained by the absence of radioactivity accumulation in the apical sections of the rhizome.

An experiment was conducted to evaluate the translocation pattern of 14C-glyphosate and 14C-quizalofop in five quackgrass biotypes at two stages of growth. Whole plant translocation differed between stages for each herbicide. The percentage of 14C-glyphosate and 14C-quizalofop in the leafwash and in the treated leaf did not differ among biotypes, but differences occurred for the translocation to the underground parts. The partitioning of the 14C-glyphosate and 14C-quizalofop in the treated crown, other crowns, propagating rhizomes, and other rhizomes differed among biotypes. On a disintegration per minute (DPM) mg-1 basis, differences among biotypes were found with 14C-glyphosate but much less with 14C-quizalofop. Metabolism of 14C-quizalofop in the treated leaf was similar in the two biotypes tested except for the percentage of polar metabolites 1 d after treatment. The percentage of 14C-quizalofop available for translocation was the same in both biotypes.

The effect of the isopropylamine salt of glyphosate and the ethyl ester of quizalofop on the rhizome bud viability of five quackgrass biotypes was tested. Two bioassays used were: direct measure of the growth of the bud on an agar medium and determination of the respiratory activity with tetrazolium chloride. The response to herbicide treatments differed among biotypes. The growth bioassay showed lower viability than the tetrazolium bioassay. Quizalofop reduced viability more than glyphosate. Viability of untreated buds increased with increasing distance from the base of the rhizome. The viability of herbicide-treated buds was more or less equal regardless of the position. It appeared that both herbicides were translocated to all the rhizome buds.

Space-planted johnsongrass [Sorghum halepense (L.) Pers. ♯ SORHA] was clipped back to a height of 26 cm after flowering and, when 45 to 61 cm in height, was treated with various rates of the sodium salt of dalapon (2,2-dichloropropionic acid), MSMA (monosodium methanearsonate), or the sodium salt of asulam (methylsulfanilylcarbamate). Treatments were reapplied as needed during the growing season whenever a majority of surviving plants in a plot had initiated new foliar growth. Most plants were not killed by a single application of any herbicide, although rhizome development, as measured by the number of rhizome buds, was inhibited by the initial herbicide treatments. MSMA at 4.5 kg ai/ha was more phytotoxic to johnsongrass than other herbicide treatments or MSMA at 2.3 kg ai/ha; a single application at the higher rate reduced the original number of rhizome buds on plants by 50 to 60%, and two applications killed 86 to 93% of the plants. Asulam at 4.5 and 6.7 kg ae/ha and dalapon at 6.7 and 9.0 kg ae/ha primarily controlled plants by inhibiting growth for an extended period. After two or three applications of asulam or dalapon at 6.7 kg ae/ha, only about 40% of the plants had been killed, but the original number of rhizome buds on surviving plants had been reduced by about 80%.

Soil microbial community structure and activity are linked to plant communities. Weeds may alter their soil environment, selecting for specific rhizosphere microbial communities. Rhizosphere modification occurs for many crop and horticultural plants. However, impacts of weeds in agroecosystems on soil biology and ecology have received less attention because effective weed management practices were developed to minimize their impacts on crop production. The recent development of herbicide resistance (HR) in several economically important weeds leading to widespread infestations in crop fields treated with a single herbicide has prompted a re-evaluation of the effects of weed growth on soil biology and ecology. The objective of this article is to review the potential impacts of herbicide-resistant weeds on soil biological and ecological properties based on reports for crops, weeds, and invasive plants. Persistent weed infestations likely establish extensive root systems and release various plant metabolites through root exudation. Many exudates are selective for specific soil microbial groups mediating biochemical and nutrient acquisition processes. Exudates may stimulate development of microbial groups beneficial to weed but detrimental to crop growth or beneficial to both. Changes in symbiotic and associative microbial interactions occur, especially for arbuscular mycorrhizal fungi (AMF) that are important in plant uptake of nutrients and water, and protecting from phytopathogens. Mechanisms used by weeds to disrupt symbioses in crops are not clearly described. Many herbicide-resistant weeds including Amaranthus and Chenopodium do not support AMF symbioses, potentially reducing AMF propagule density and establishment with crop plants. Herbicides applied to control HR weeds may compound effects of weeds on soil microorganisms. Systemic herbicides released through weed roots may select microbial groups that mediate detrimental processes such as nutrient immobilization or serve as opportunistic pathogens. Understanding complex interactions of weeds with soil microorganisms under extensive infestations is important in developing effective management of herbicide-resistant weeds.