Herbicides the chemical The combination of acute toxicity plus exposure to a pesticide during its expected use determines the hazard it poses to humans The EPA uses these data to set a tolerance level for each agricultural pesticide, which is the maximum amount of the chemical allowed on a particular crop These and many other data, including chronic toxicity, reproductive effects, teratogenicity, and carcinogenicity, are used by the EPA in setting limits for pesticide use to ensure that hazard to humans from the use of pesticides will be at acceptably low levels Toxicology and exposure studies are also required on certain species of wildlife, including birds, fish, and invertebrates, before pesticides can be registered These tests utilize studies of pesticide residues in foods that these species may consume, as well as potential concentrations of pesticides in water or air to determine hazard to nontarget species from pesticide use Pesticides vary widely in their toxicological properties In relation to all pesticides, which include insecticides, fungicides, and rodenticides, most herbicides are relatively nontoxic to mammals, since many of the processes or pathways they inhibit in plants are not present in mammalian systems For the purposes of regulation, pesticides are classified according to their toxicity, which is a relative term used to describe the amount of a chemical that causes harm to a particular species The most common unit of measurement for toxicity is the lethal dose, LD50, or lethal concentration, LC50, which is the dose or concentration that kills 50% of the test population, respectively High values for LD50 or LC50 indicate lower toxicity, as higher doses are required to produce lethal effects The categories of acute toxicity that must be shown on herbicide labels are listed in Table Because all herbicides are toxic to some degree, container labels are required to give specific directions for-use as well as ingredients, properties, hazards, exposure limits, first aid procedures, and other information When handled according to the label directions, hazard to humans and wildlife from exposure to herbicides can be avoided Effects of Herbicides on Biodiversity Weeds All weed control practices exert selection pressure on weeds and thus can have short- and long-term effects on the composition, structure, and dynamics of weed communities (Radosevich et al., 2007) Agricultural weed communities tend to have lower species diversity than natural plant communities and are often dominated by a few key species The primary short-term effect of weed control is a reduction in weed Table Toxicity, signal words, and lethal dosages (LD) used in herbicide labeling Toxicity Signal word Oral LD50 (mg/kg) Dermal LD50 (mg/kg) Very high High Moderate to low Danger Warning Caution r50 51–500 4500 r200 201–2000 42000 91 density, particularly of the dominant species, which is desirable in order to improve crop yields or facilitate land use Over a longer time frame, weed control practices rarely eliminate weeds altogether; rather they generally result in changes in species composition and structure of weed communities In the case of herbicides that act on specific plant processes, selection pressure over time can eliminate susceptible genotypes and thus cause evolutionary changes in weed populations Most research in weed control to date has emphasized reducing weed density and improving crop yields Only recently the attention has been focused on changes in weed community dynamics as a result of weed control practices Thus, only a few generalities can be made about specific effects or directions of change in weed diversity caused by biological, cultural, mechanical, and even chemical methods of control The most information available on the role of weed control in shaping weed communities comes from documented cases of herbicide-resistant weeds, which have been selected by repeated use of the same herbicide or herbicide group (Powles and Holtum, 1994) It is clear that herbicides are a powerful evolutionary force acting on weed community composition However, herbicide effects on weed diversity are generally transient and occur immediately following application Over longer time periods, herbicide effects on weed diversity are either inconsistent or not apparent, and in many cases are overshadowed by more pronounced effects of other weed control and cropping practices Effects of Weed Control Practices Changes in weed community composition and structure due to agricultural practices have been documented; however, assessments of the effects of these practices on weed species diversity are relatively limited Clements et al (1994) calculated diversity indices from an array of published data to compare the impacts of conventional and alternative weed management practices on weed species diversity When compared to weed control by mechanical cultivation only, broadcast applications of herbicides resulted in lower weed species diversity over time When herbicide use was reduced by placement of applications only in bands over the crop row, higher diversity of weeds resulted than when broadcast herbicide applications were made Although these findings suggest that use of herbicides reduces weed species diversity, case studies show that this generalization is too simplistic Impacts on diversity depend on several factors, including the periodicity of tillage practices, weed species susceptibility to the herbicide product and rate (dosage), and variability in weather and its interaction with herbicide efficacy (Grundy et al., 2010) When different herbicides were evaluated, in some cases applications of pre-emergence herbicides (those having residual soil activity) reduced weed species diversity more than the applications of postemergence herbicides (those with no residual activity) By exerting continuous selection pressure on susceptible species from the time of emergence, pre-emergence herbicides can reduce the richness and diversity of weed communities In contrast, postemergence herbicides are present in the environment only after emergence and for a shorter time period, which may permit a more diverse weed flora to establish both before and after the disturbance of herbicide application is imposed