Grass render only on some part (squares) of landscape

[VJokl]

Hi,
I’m struggling with rendering using the movie render queue - I need the highest possible resolution to render the image (using 14000x7875px. However, when rendering, grass only renders on certain parts of the landscape. I use Brushify for landscape. Grass is set to:
end cull distance 8000

any suggestions why is this happening?
Thank you very much!