The necessity of decreasing the environmental impact of agricultural activities, while preserving in the same time the level of production to satisfy the growing population demand, requires to investigate new production tools. Mobile robotic can constitute a promising solution, since autonomous devices may permit to increase production level, while reducing pollution thanks to a high accuracy. In this paper, the use of several mobile robots for field treatment is investigated. It is here considered that they can exchange data through wireless communication, and a formation control law, accurate despite typical off-road conditions (low grip, terrain irregularities, etc), is designed relying on nonlinear observer-based adaptive control. The algorithm proposed in this paper is tested through advanced simulations in order to study separately its capabilities, as well as experimentally validated.