**5.3 Direct chemical injection with carrier control**

In this system, there is control of the herbicide injection rate and water flow rate to respond to changes in speed or application rate. A control circuit manages the injection pump, while a second controller operates a servovalve to provide a corresponding water flow (**Figure 24**). Such a system provides a mixture of constant concentration. The system can have many of the advantages of the previous two systems. There is no leftover mixing; the operator is not exposed to chemicals in the tank mixing process; the variation from one rate to another occurs quickly. The disadvantages include related to the complex system, higher initial costs, problem in delivering variable rates of liquid through in the nozzle spray, and modulated spraying nozzle control systems [10, 46].

### **6. Conclusions**

*Pests, Weeds and Diseases in Agricultural Crop and Animal Husbandry Production*

In this system, the mixture is prepared with direct injection of the chemical in a flow of water. This system (**Figure 23**) uses a controller and a pump to manage the chemical injection rate instead of the flow rate of a tank mix [46]. The water flow rate is constant and the herbicide injection rate is varied to accommodate changes in

*VRA spraying system that is a flow-based system of application rate. Source: Grisso et al. [7].*

With the chemical injection, there is no leftover mixture and the direct contact of the operator with toxic products is reduced [10]. The system allows you to control the desired size and spectrum of droplets, since the variation of the application rate does not depend on the flow and pressure on the spray nozzles. Its main disadvantage is the long transport delay between the chemical injection pump and the

*VRA spraying system that incorporates chemical injection technology. Source: Grisso et al. [7].*

**5.2 Chemical direct injection systems**

**Figure 22.**

soil speed or changes in the prescribed rate.

discharge nozzles at the ends of the boom.

**198**

**Figure 23.**

The variable rate application (VRA) of herbicides has great potential for use in agriculture because it allows better control of weeds at lower costs and reduction in the use of inputs and environmental contamination. The main techniques available are based on the generation of application maps and the use of sensors in real time to identify weed infestations, which can be used in the preemergence (PRE) and postemergence (POST) of weeds. Both modalities are equally important in integrated weed management. VRA systems still require relatively high investment, restricting their use. The constant improvement of the VRA should further increase its benefits and reduce the costs of adopting the system, allowing its use by more farmers. The use of precision agriculture in farming systems is a path of no return, in view of the conjuncture of food production needs and scarcity of natural resources. Thus, VRA tends to be used more and more frequently until possible complete replacement of the conventional way of using herbicides in agriculture.

*Pests, Weeds and Diseases in Agricultural Crop and Animal Husbandry Production*
