Can you show us some examples?
Ideally, you would want to meter off of something that is mid toned and is in the same light as your subject. The green field should be pretty close. So first, decide what your creative goal for the shot is. It's probably getting a nice sharp shot, so you probably want to choose a faster shutter speed. Then fill the frame with green grass, then adjust your aperture and ISO (in manual mode) until the needle gets to the --0--.
You should now have pretty good exposure of anything that is in the same light as that grass. And because you set the exposure in manual mode, you shouldn't have to change your settings, until the light changes.
Of course, if you see that this exposure isn't what you want (if it's too bright) then just subtract exposure by adjusting either the aperture, the shutter speed or the ISO. Once you get to where you want it, leave it alone.