CONTROL REQUIREMENTS FOR SPATIALLY SELECTIVE HERBICIDE SPRAYERS

Current agronomic practice is to estimate the mean weed density over t he whole field and use this information to decide whether or not to sp ray, and what herbicide dose to apply. It has been proposed that consi derable long-term economic and environmental benefits would accrue if the herbicide dose were varied according to spatial variations in weed density and species. The increased availability and reduced cost of i nformation technology and electronic control systems have made this co ncept more practical, but its implementation places a number of constr aints on the specification of the sprayer and these are outlined and d iscussed. Several spraying technologies and sprayer modifications are examined in relation to the concept of spatially selective herbicide a pplication and their ability to meet the required specifications. It i s concluded that the requirements for 'patch' spraying in terms of the ability to apply different herbicide mixtures can be met by a variabl e concentration injection metering system. Key performance parameters for spatially variable application include the accuracy, speed of resp onse and turn-down characteristics of the sprayer dose control system. Small bore pipes used with an injection metering system can give over all response times at the nozzle of less than 5.0 s. Variable concentr ation systems will however always require a larger turn-down ratio tha n systems operating with prediluted herbicide as they must account for the range of required recommended dose rates, in addition to the requ irement to apply reduced doses. The use of twin fluid nozzles, pulsed nozzle systems and rotary spray generators operating with prediluted h erbicides can achieve rapid response times (<1 s) and turn-down ratios greater than pressure control units. They may therefore have some com mercial role in the design of spatially selective application systems. Pressure control systems alone are unlikely to meet the requirements of such a system.