I have struggled with the lighting of the grass. The problem is threefold. First: The grass has several shaders. Because I thought the colors were too dark, instead of adjusting each color individually to make them lighter, I used a color adjustment shader to correct them all at once. This probably gave some colors a value higher than '1', so light was emitted.
Second: The grass patches do not have much volume, so there is not much area to receive shadow.
Third: The sand color is very light and therefore reflects a lot of light, exactly to the spot on the grass where you would expect shadow.
I've now set the reflection of the grass to 0, didn't use a color adjust, and increased the population by 50%. It looks better, but not much.