Yellow Starthistle Information

 Mech control
   -hand
   -tillage
   -mowing

 Cultural control
   -grazing
   -burning
   -re-vegetation

 Biocontrol
   -insects
   -biocontrol table
   -plant pathogens

 Chem control
  - risks
    --spray
    --water
    --toxicology
    --herbicide resist
    --effects
   -herbicides
    --preemergence
    --postemergence
    --late season
    --pre- & post-
    --imazapic
    --clopyralid
    --picloram

 Integrated app

 Summary

Management

Chemical control

Herbicides are the most widely used method for controlling weeds, both in agricultural and non-crop environments, and are generally considered the most economic and effective. Of the 400 million ha of rangeland in the United States, about 25% were treated with herbicides in 1997 (Bussan and Dyer 1999). Herbicides can be applied to rangeland and grasslands by a number of methods, including fixed wing aircraft, helicopter, ground applicators, backpack sprayers, and rope wick applicators (photo of rope wick applicator).

Herbicide applications can be conducted by individual landowners or managers or through companies specializing in non-crop areas. Most landowners or managers that treat large areas themselves use off-road four wheelers, such as ATVs, that are attached to a spray boom. For specific instructions to build this equipment click on this sentence (Henry 1998).

The potential risks associated with herbicide use have been widely publicized both in the scientific literature and the public press. Although these risks are often greatly exaggerated, improper use of herbicides can lead to several potential problems, including spray or vapor drift, water contamination, animal or human toxicity, selection for herbicide resistance in weeds, and reduction in plant diversity.

Risks

Spray and Vapor Drift
Herbicide drift may injure susceptible crops, ornamentals, or non-target native species. Drift can also cause non-uniform application in a field and/or reduce herbicide efficacy in controlling weeds (DiTomaso 1997).

Several factors influence drift, including spray droplet size, wind and air stability, humidity and temperature, physical properties of herbicides and their formulations, and method of application. For example, the amount of herbicide lost from the target area and the distance it moves both increase as wind velocity increases. Under inversion conditions, when cool air is near the surface under a layer of warm air, little vertical mixing of air occurs. When applications are made above the inversion layer drift is most severe since small spray droplets will fall slowly and move to adjoining areas even with very little wind. Low relative humidity and high temperature cause more rapid evaporation of spray droplets between sprayer and target. This reduces droplet size, resulting in increased potential for spray drift.

Vapor drift can occur when a herbicide volatilizes. The formulation and volatility of the compound will determine its vapor drift potential. Potential of vapor drift is greatest under high temperatures and with ester formulations. Ester formulations of 2,4-D, dicamba and triclopyr are very susceptible to vapor drift and should not be applied at temperatures above 80^oF (26.5^oC).

Nozzle height will depend on the type of application. It controls the distance a droplet must fall before reaching the weeds or soil. Less distance means less travel time and less drift. Wind velocity is often greater as height above ground increases, so droplets from nozzles close to the ground would be exposed to lower wind speed. Applications are more likely to be above the inversion layer when herbicides are aerially applied. This will not allow herbicides to mix with lower air layers and increase long distance drift.

A number of measures can be taken to minimize the potential for herbicide drift. Chemical treatments should be made under calm conditions, preferably when humidity is high and temperature is relatively low. Most applications, particularly aerial are made during the morning hours. Ground equipment reduces the risk of drift, and rope wick or carpet applicators nearly eliminate it. Use of the correct formulation under a particular set of conditions is important. For example, applying an ester formulation of postemergence herbicides during the hotter periods of the summer is not recommended.

Groundwater and Surface Water Contamination
Most herbicide groundwater contamination results from "point sources." Point source contaminations include spills or leaks at storage and handling facilities, improperly discarding containers, and rinsing equipment in loading and handling areas, often times into adjacent drainage ditches. Point sources are characterized by discrete, identifiable locations discharging relatively high local concentrations. These contaminations can be avoided through proper calibration, mixing, and cleaning of equipment.

Non-point source groundwater contaminations of herbicides are relatively uncommon. They can occur, however, when a mobile herbicide is applied in areas with a shallow water table. In this situation, the choice of an appropriate herbicide or alternative control strategy can prevent contamination of the water source.

Surface water contamination with chemicals can occur when herbicides are applied intentionally or accidentally into ditches, irrigation channels or other bodies of water, or when soil-applied herbicides are carried away in runoff to surface waters. Direct application into water sources is generally used for control of aquatic species. In these cases, there is a restriction period prior to the use of this water for human activities. Accidental contamination of surface waters can occur when irrigation ditches are sprayed with herbicides or when buffer zones around water sources are not wide enough. In many situations, alternative methods of herbicide treatment, including rope wick application, will greatly reduce the risk of surface water contamination.

Loss of a preemergence herbicide through erosion may occur when a heavy rain follows a chemical treatment. It is possible to minimize herbicide runoff to surface waters by carefully monitoring weather forecasts before applying herbicides. Applications of preemergence herbicides should be avoided when forecasts call for heavy rainfall. Predictions of precipitation between 0.5 and 1 inch should allow a preemergence herbicide to percolate into the soil profile, thus minimizing the subsequent risk of surface runoff and maximizing the effectiveness of the compound.

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