Herbicides application

Volatilization. The susceptibility of a herbicide to loss through volatilization has received much attention, due in part to the realization that herbicides in the vapor phase may be transported large distances from the point of application. Volatilization losses can be as high as 80—90% of the total applied herbicide within several days of application. The processes that control the amount of herbicide volatilized are the evaporation of the herbicide from the solution or soHd phase into the air, and dispersal and dilution of the resulting vapor into the atmosphere (250). These processes are influenced by many factors including herbicide application rate, wind velocity, temperature, soil moisture content, and the compound s sorption to soil organic and mineral surfaces. Properties of the herbicide that influence volatility include vapor pressure, water solubility, and chemical stmcture (251). 

Fig. 7. Historical trends in recommended herbicide application rates, by chemical class (77).

The following brief account identifies only major groups of herbicides not mentioned elsewhere in the text, and is far from comprehensive. Their mode of action is only dealt with in a superficial way. From an ecotoxicological point of view, there has not been as much concern about their sublethal effects upon plants as there has been in the case of mammals, and there has not been a strong interest in the development of biomarker assays to establish their effects. The major concern has been whether weeds, or nontarget plants, have been removed following herbicide application—a rather easy matter to establish as plants are fairly sedentary. For a more detailed account of herbicide chemistry and biochemistry, see Hassall (1990). 

Site-specific herbicide application, 13 328 Siting factors, in plant location, 19 523-527... 

The data provided clear evidence that anaerobic respiratory conditions must be taken into consideration when performing degradation feasibility studies and determining herbicide application practices in the future. 

These chemicals, as noted previously, are likely far more dangerous to children (especially neurotoxins such as chlorpyrifos) than has been generally suggested to date. Family pesticide use, including lawn herbicide application, has been shown to be related to childhood brain cancer. Malignancies for children linked to pesticides in case reports or case-control studies include leukemia. 

Vegetative coverage maps prepared In 1971 and 1973 (Figures 1 and 2 respectively) confirmed that rapid re-vegetation occurred Immediately after herbicide applications ceased. Table II... 

The broad spectrum of biological activities, including insecticidal and herbicidal applications, of diverse molecular species containing the oxime function is discussed in several reviews published between 2001 and 2005 by Abele and colleagues . Many of these oximes are potential candidates for drugs with a number of applications as outlined in the following. 

A number of issues influenced the selection of the dose-response model form and the treatment of the data prior to fitting the model. First, shoot weight and shoot length are continuous response measurements therefore, use of a standardized logistic model form is not appropriate. Second, the natural variation in plant growth often resulted in apparent increased shoot weight and shoot length measurements relative to the control at low herbicide application rates. A dose-response model needs to perform well even when some measurements in treatment levels exceed the controls. 

When these three cancers (soft-tissue sarcoma, non-Hodgkin s leukemia, and Hodgkin s disease) are considered as a whole, it is noteworthy that the strongest evidence for an association with exposure to phenoxy herbicides is the series of case-control studies conducted by Hardell [Lennart Harden, a Swedish physician] and colleagues and the cohort studies of herbicide applicators and agricultural workers.25 26... 

Noncheniical or traditional practices, such as weed seed removal, optimal crop seeding rates, crop selection, enhanced crop competitiveness, crop rotation, and mechanical weed control are all important components of an effective weed management program. In the context of modem intensive chemical herbicide application, noncheniical practices may represent an innovative approach to weed management and should receive careful consideration. 

The necessary distribution channels for reliable information and farmer education also were not well established. During the late 1950s, herbicide application recommendations were obtained through many of the same sources from which agricultural products were purchased, such as Farm Bureau stores, operators of local grain elevators, and feed... 

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. 

Bulcke, R.A. and E.M. Desmet (2005). Weed and crop responses to long-term repeated herbicide applications in continuous com. Weed Sci. Soc., Abstract 6, 45 2. 

Atrazine use in ecofallow usually is supplemented with other herbicides. For example, the first herbicide application to wheat stubble often uses glyphosate and 2,4-D or dicamba, with the atrazine application postponed until later in summer to coincide with the emergence of volunteer wheat, cheat, and downy brome. Atrazine can be applied with glyphosate, but antagonism with some atrazine formulations is associated with this tank mixture (Stahlman and Phillips, 1979 Wicks and Hanson, 1995) because of physical binding of inert components in the atrazine formulation with glyphosate (Ahmadi et al., 1980). Farmers know that if rainfall does not move atrazine off the wheat residue and into the soil, control of weeds, and volunteer wheat will be unsatisfactory. 

Historically, tillage and hand weeding were the main methods of weed control. Since the advent of herbicide use in 1950, essentially all sugarcane in Australia now receives one or more herbicide applications. Virtually all of the crop is produced on family-operated, commercial-scale farms and is processed at cooperative or privately owned mills. 

Miller, P.C.H. and J.V. Stafford (1991). Herbicide application to targeted patches. Proceedings of British Crop Protection Conference -Weeds, pp. 1249-1256. 

Chemicals such as simazine are used alone as herbicides, or more often in combination with other herbicides such as oxyfluorfen, oryzalin, or norflurazon, generally in four- to six-foot (1.2-2.0 meter (m)) strips down the tree row. The centers are either tilled or mowed parallel with the tree row. In order for strip chemical treatment to be successful, it is essential to eradicate perennial weeds by the use of repeated postemergence herbicide applications and tillage. 

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