Thanks for your comments!
@zionner: i've the free download version of TG2 (1.8.76.0). The quality settings of the render were 0.8 (Render time was 8h 40min) but i can't render larger than 800x600 pixels. Is it even possible with this free demo version to render images photo-realistic?
@ old_blaggard: i placed a pop of the grass right in front. i could see the rectangle and the instances of the pop in the preview but not the population in the rendered image
i changed the values (x,y,z) several times - but no chance. The other way would be to place 3 or 5 single objects of the grass in the foreground. The other thing was: TG2 crashes always (3x) during the rendering when it comes to the left bottom of the picture. So i have to crop the render that TG2 doesn't render the bottom of the picture.