Herbicides Soil-applied

Volatilization of NDPA, NDEA, NDMA and Ji-nitrosopendime-thalin were examined in a model system. The nitrosamines were either mixed into predetermined depths of the soil or applied to the soil surface (the conditions were chosen to represent those that would be encountered by nitrosamines coapplied with dini-troaniline herbicides). Volatilization of nitrosopendimethalin was extremely slow regardless of application. The volatile nitrosamines, NDPA, NDEA, and NDMA, in contrast, volatilized so rapidly after application to the surface of moist soil that we predicted that a substantial proportion of the nitrosamine thus applied would enter the atmosphere within a few hours. Incorporation of the nitrosamine in the top 7.5 cm of soil (as might be the case when the herbicide was applied and incorporated in a single operation) decreased total volatilization by at least an order of magnitude. 

Chloroacetanilides are soil-applied herbicides used for pre- and early post-emergence control of annual grasses and broadleaf weeds in crops. Representative chloroacetanilide compounds, alachlor, acetochlor, and metolachlor, are extensively used worldwide. Other chloroacetanilides with limited usages include propachlor, bu-tachlor, metazachlor, pretilachlor, and thenylchlor. Public environmental concerns and government regulatory requirements continue to prompt the need for reliable methods to determine residues of these herbicides. There now exist a variety of analytical methods to determine residues of these compounds in crops, animal products, soil, and water. The chemical structures and major crops in which these compounds are used are summarized in Table 1. 

The degradation rate of paraquat in certain soils can be slow, and the compound can persist for years — reportedly in a form that is biologically unavailable. But data are missing or incomplete on flux rates of paraquat from soil into food webs and on interaction dynamics of paraquat with other herbicides frequently applied at the same time. It seems prudent at this time to keep under close surveillance the residues of paraquat in soils in situations where repeated applications have been made over long periods of time (Summers 1980). 

Much of the early work in characterizing the environmental behavior of chemicals was accomplished in the area of agricultural chemistry. Work surrounding the behavior of plant nutrients in the soil has provided a large base of information about the processes of environmental chemistry. Workers investigating the effectiveness of soil-applied herbicides determined that the herbicidal activity of organic chemicals varied with soil properties. It was determined that clay and OM contents of the soil were related to the ability of a soil to diminish the effectiveness of an organic herbicide applied [17,21]. 

Weber, J.B. and Best, J.A. Activity and movement of 13 soil-applied herbicides as influenced by soil reaction, Proc. Soil Weed Sci Soc., 25 403-413, 1972. 

In the biodegradation plots and In the Herbicide Storage Areas, high concentrations of herbicides were applied In a short time period and Incorporated Immediately Into the soil profile, and hence, the long persistence time. Nevertheless, these studies do show that the soil chemistry and the soil microbial populations can effectively combine to degrade massive concentrations of the phenoxy herbicides and that recovery of the sites occur as documented by the re-establlshment of the vegetative community. 

Pre-emergence herbicides are applied pre-weed emergence and this will usually mean pre-crop emergence as well. Such compounds are taken up underground by the roots or hypocotyls of the weed. It is important for such compounds to possess some water solubility, in order that they become available to the germinating weed, but not so much that they are leached away from the weed germination zone. They must also be relatively persistent in the soil so that weeds that germinate over a period of time are all controlled. 

Pre-plant incorporated herbicides are applied before the crop is sown and are incorporated into the soil. Hence, they are also applied before weeds emerge. The reason for incorporation is usually because the herbicides are volatile and would be lost if they were not incorporated, or light unstable and they would be degraded if they remained on the soil surface. Volatility is a useful characteristic as it allows the redistribution of the compound throughout the soil following incorporation. 

During the late 1950s and early 1960s, most soil-applied herbicides typically did not receive mechanical incorporation, and when rain was insufficient, lacked reliability in controlling weeds. The reason for the slow adoption of mechanical incorporation techniques was probably three-fold (a) banded applications were preferred from a cost standpoint (b) the advantages of soil incorporation were not universally recognized and (c) the equipment and techniques for adequate incorporation were not available. Poor soil incorporation not only resulted in poor weed control, but in some cases resulted in crop injury. Simazine, in spite of its good weed control spectrum in corn, was never widely used in the Central Plains. Its lack of acceptance was attributed to its soil residual with the potential to injure... 

Since 1980, the discovery of many herbicides with very low application rates has shifted the application timing and chemistry of herbicide applications. The majority of the newly discovered low-rate herbicides were best suited to postemergence applications, and their adoption has resulted in a gradual shift away from preplant or preemergence soil-applied treatments. 

Atrazine is the key herbicide facilitating ecofallow corn and sorghum production in the semi-arid Great Plains, where crop production is often uncertain and profits to farmers are often marginal. The success of atrazine in ecofallow is attributable to its duration of weed control as a soil-applied herbicide, the broad spectrum of weeds controlled, the low cost per area treated, and its safety to crops. In this semi-arid environment, maintaining weed-free fallow with repeated applications of nonresidual herbicides is not an economically viable alternative to atrazine. 

Atrazine remains the standard herbicide for making the transition from wheat to sorghum or corn in Great Plains cropping systems. Even where more intensified crop rotations have been developed, they are built around winter wheat followed by ecofallow sorghum or com. The success of atrazine is due to its persistence as a soil-applied herbicide, to the broad spectrum of weeds controlled, to its low cost per acre, and to its safety on sorghum and com. In the Great Plains, repeated burndown of weeds in fallow with nonresidual herbicides is not a viable alternative to the role that atrazine plays. 

Chemical weed control practices in citrus have been thoroughly reviewed by various researchers (Ryan, 1969 Jordan et al, 1977 Jordan, 1978 Tucker and Singh, 1983 Mersie and Singh, 1989 Singh el al., 1990 Sharma and Singh, 1999). Herbicides used in citrus can be divided into two groups soil-applied (preemergence) and... 

Gaillardon, P., J.C. Gaudry, and R. Calvet (1981). Adsorption des herbicides par des acides humiques Remarques sur les sites d adsorption et le role cations fixes sur les acides humiques. In Proceedings of Symposium on Theory and Practice of the Use of Soil Applied Herbicides. Paris European Weed Research Society, pp. 3-9. 

Koren, E. and E. Shlevin (1977). Effect of trickle irrigation on soil-applied herbicides. Weed Res., 17 133-138. 

Walker, A. and S.J. Welch (1992). Further studies of the enhanced biodegradation of some soil-applied herbicides. Weed Res., 32 19-27. 

Uses soil applied herbicide to control many annual and perennial broadleaf weeds. 

The herbicides listed above represent both soil-applied herbicides (Glean and Treflan) and foliar-applied herbicides (Round-up, MCPA and Dicamba) and research with these herbicides has recently demonstrated an increase in root disease of several crops. Tottman and Thompson (57) showed that a mixture of dicamba 2,3,6, TBA, mecoprop and MCPA, and Mecoprop, loxynil and dicaoba alone increased the severity of take-all infection of roots. 

Unsubstituted uracils have no herbicidal activity but certain derivatives substituted in the 3,5,6-positions are active. The most important examples are bromouracil, terbacil, and lenacil. These compounds were introduced by DuPont in 1963. These herbicides are applied to the soil and are used for selective weed control. Bromacil and terbacil are pre- and postemergence herbicides they control a wide spectrum of grass and broadleaf weeds when applied early in the growing season. These compounds must be carried to the roots by soil moisture, where they are absorbed and translocated to the rest of the plant. This family of herbicides is used for asparagus, sugar cane, pineapple, apple, and citrus crops. 

Baer U. and Calvet R. (1999) Fate of soil applied herbicides experimental data and prediction of dissipation kinetics. J. Environ. Qual. 28, 1765-1777. 

Thiolocarbamates, e.g. butylate (70) (R = R = Me2CHCH2, R"= Et), are valuable soil-applied herbicides for pre-emergence weed control in a wide range of crops. They probably kill weeds by interference with lipid biosynthesis. 

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