Glyphosate desiccation

Abstract This chapter describes the underlying criteria and assumption in the development and execution of field and associated laboratory bioassays. It provides details and commentary regarding the materials and methods used. More specifically, describes how glyphosate-desiccated wheat and other cover crops (crimson clover, subterranean clover, and rye) in no-till systems can directly and indirectly influence morningglory, pigweed, and prickly sida seedling emergence, with emphasis on the role of phenolic acids in plant residues and soil. 

A variety of glyphosate-desiccated cover crops (crimson clover Trifolium incar-natum L. Tibbee ), subterranean clover (T. subterranean L. Mount Barker ), rye (Secale cereale L. Abruzzi ) and wheat (Triticum aestivum L. Cocker 983 )) were... 

Fig. 3.12 The number of pigweed seedlings in cover crop and reference plots for the 1993 experimental period in no-till Cecil A soil. Glyphosate desiccation of cover crops occurred on April 29 (a) and May 10 (b). Where C equals crimson, S equals subterranean and reference equals no-cover crop plots. Figures reproduced from Blum et al. (1997). Henry A Wallace Institute for Alternative Agriculture Inc, figures used with permission of Cambridge University Press...

When determining the potential influence of cover crops on the physicochemical state of Cecil A no-till soils or tilled soils, there are a variety of factors that are of interest. Among them are above- and below-ground cover crop biomass, morphology of cover crops (as related to shading and root distribution), soil temperature, soil moisture, soil compaction, soil pH, chemical composition of cover crops before and after glyphosate desiccation (e.g., C/N ratio and phenolic acid content), soil C/N ratio, soil nitrate-N, and soil phenolic acid content. 

Crimson and subterranean clovers are composed primarily of leaves, petioles, and runners (stolons) while wheat and rye are composed primarily of leaves and stems. The clovers have a taproot and primary, secondary, and tertiary roots while the grasses have primarily adventitious roots. Upon desiccation, crimson clover aboveground residue either remained erect or formed dense tangles of petioles and runners resulting in a mosaic of open and covered soil surfaces. Subterranean clover residue formed a very thin dark brown mat that covered the surface. Rye residue quickly formed a thick dense mat that covered most of the soil surface. Most of the wheat residue remained initially erect resulting in considerable open soil surface area. The amount of open soil surface area, however, varied with wheat density and time of glyphosate desiccation. As wheat stalks fell over and/or shattered after glyphosate desiccation, the open soil surface area was reduced with time and eventually eliminated. 

Total phenolic acid content of the cover crops taken at monthly intervals from the field after glyphosate desiccation declined over time. However, the decline of total phenolic acid content for rye and wheat residues was not evident until 2 months after glyphosate desiccation (Lehman 1993). After 4 months the total phenolic acid content of the cover crops had declined by 31, 36, 38, and 56% for wheat, crimson clover, rye, and subterranean clover, respectively (Lehman 1993). Estimates of ferulic acid for sterilized (autoclaved) cover crop residues extracted with 0.25 M... 

All total phenolic acid values in ferulic acid equivalence were determined by water-autoclave extraction and the Folin Cicoltaeu s phenol reagent approximately biweekly during the 1992 and 1993 experimental periods. Mean total phenolic acid for the upper 2.5 cm of the soil in 1992 were as follows plots without cover crops (reference plots) 41 1 rg/g soil (mean standard error), crimson clover plots 55 1 JLg/g soil, subterranean clover plots 58 1 p,g/g soil, rye plots 50 1 irg/g soil, and wheat plots 45 1 rg/g soil (Fig. 3.14 Blum et al. 1997). In 1993 soil total phenolic acid values for the upper 2.5 cm of the soil were higher than in 1992 (Fig. 3.14 Blum et al. 1997). The values for the April glyphosate desiccated plots were plots without cover crops (reference plots) 62 2 t,g/g soil, crimson clover plots 83 4 jtg/g soil, subterranean clover plots 83 3 t,g/g soil. 

Blum et al. (1997) found that soil pH values of no-till systems were not significantly modified by the presence of glyphosate-desiccated rye, wheat, crimson clover and subterranean clover cover crops. In this field study a substantial portion (estimated to be roughly 50%) of the cover crop residues was located on the soil surface and thus only had an indirect impact on soil pH by way of leachates generated... 

Fig. 3.16 The effects of soil moisture, and wheat and crimson clover cover crop residues on percent pigweed seedling emergence in Cecil A soil, where C equals crimson. Wheat inhibitory, C-clover inhibitory, and C-clover non-inhibitory were collected 2,1, and 4 months after glyphosate desiccation, respectively. The absence of standard error bars indicates that the error bars are too small to be visible. Figures adapted from Lehman and Blum (1997). Figures used with permission of International Allelopathy Foundation...

If available phenolic acids in soil come from root tissues/residues, then the distribution of available phenolic acids will be consistent with root tis-sue/residue distribution in the soil, movement of gravitational and capillary water, mass flow of soil solutions driven by transpirational pull , and the action of soil processes. Concentrations released will be highest shortly after glyphosate desiccation. Note This is also true for all other organic and inorganic compounds, and... 

Con 2 Usually within hours to days after glyphosate desiccation cell membranes are disrupted and all the soluble and readily available organic and inorganic compounds (including phenolic acids) in the dead cells are released into the soil by leaching during the first adequate rainfall event. The most dramatic effects on weed... 

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