Thanks for comments,
@Pibuz: Indeed I'm not very happy with that dark foreground plant but too lazy to recal
@StompinTom: I'm not very satisfied with the displacement for grass and the particules hair or strand produce huge scene files so I finally opted for bilboards with a grass and alpha mask (I found that somewhere on that site) and a particle emitter using this bilboard as an instanciated object facing the camera, and I mixed several particle emitters, some with grass and other with flowers, really efficient and very CPU cost less.