Mepiquat chloride (1,1-dimethylpiperidinium chloride) are used to control excessive vegetative growth in cotton (Gossypium hirsutum L.) broadcast sprayed by ground or air. As proven by previous researches the variability of the cotton plants height in the field is large enough to justify the application of Mepiquat at variable rate. The major advantages of it are: (i) yield increase; (ii) economy of the applied input; (iii) reducing the potential of environmental pollution. The main objective of this project was to develop a prototype device capable to accomplish the application of Mepiquat at variable rate in real time. This project used an equipment previously developed for automatic mapping of the of cotton plants height based on ultrasound transducers. An electronic control device for ground sprayers was designed and constructed, based on microcontroller PIC, using Java for Linux operational system and IDE Netbeans. The device controls the pressure and flow in the spray line and uses Varitarget® nozzles due to its characteristics of linearity of flow and pressure. The calculation of the Mepiquat dosage to be applied is based on the obtained information of the plants heights and the map of previous application.  The laboratory results demonstrate the acuracity of the equipment, with low response time considering the  commercial sprayers velocity (4 m.s^-1 to 5 m.s^-1).