Effet des passages de roues des engins de récolte sur la production de cultures de luzerne

National audience; Harvesting lucerne fields follows an industrial logic : the dates are chosen in order to optimize the forage production as well as the efficiency of the harvesting operations and of the factories. The impact of the traffic during harvesting is often visible, especially if the weather has been wet. The effect of the passage of the wheels on the forage production and on the perennity of the lucerne crop remains to be determined. This effect has been measured on 27 couples of lucerne fields ('Rolled' - 'Control') ; the fields were pure lucerne crops, 2 years old. The impacts of the wheels were quantified as regards the soil structure, the state of the pivot roots and the dry mater (DM) production of the current year. Four types of soil structure were defined, depending on the thickness of the compacted layer, a typology that explained well the increase in soil bulk density, which ranged between 0.25 and 0.45. Whatever the type of structure, and independently of the soil water status at the time of the harvest, the rolling led to a decrease of the DM yield of between 15 and 20% in the first re-growth after the harvest, and from 10 to 30% in the second. The number of lucerne plants and of stems per plant were significantly lower in the 'Rolled' zones than in the controls. Three types of effects of the wheels on the pivot roots were defined, of which one led to the death of the plant. The leaf/stem ratio was significantly reduced in the rolled zones. The nitrogen content appeared to conform on an average to Lemaire’s dilution curve, but in the more compacted zones, there seemed to appear nitrogen deficiencies. The quality of the harvested forage was identical between zones, notwithstanding the loss in DM yield. Lastly, the extrapolating of these data to the whole field gives a yield reduction of 1 to 3% in the next two cuts following the passage of the wheels.; L’impact des passages de roues sur l’état des pivots de luzerne et la production de l’année en cours est quantifié sur 27 couples de stations (“Roulé” - “Témoin”). Quel que soit l’état hydrique du sol lors des chantiers, la production des surfaces tassées est réduite de 15 à 20% lors de la première repousse qui suit la récolte, et de 10 à 30% lors de la seconde repousse. La qualité du fourrage récolté est inchangée. Les nombres de pieds de luzerne et de tiges par pied sur les zones “roulées” diminuent significativement par rapport au témoin. L’analyse de l’action des roues sur les pivots racinaires a permis d’identifier 3 types d’effets, dont l’un entraîne la mort du pied. Les passages de roues réduisent globalement de 1 à 3% la production des 2 repousses suivantes, selon les largeurs de travail des machines.