There are a number of ways of burning in at different contrasts Marcus and you are getting a variety of answers already. I understood from your question that you wanted to burn in and at the same time raise contrast, right? A more detailed description of the print and problem might help us here.
Originally Posted by brummelisa
You mention grass. This is a little different to burning in a highlight area like, say, sky. If you want to darken the grass overall, and yet make the light grasses stand out whilst darkening the darker areas, you need to raise contrast in that area.
This will be hard to do by just burning in, even on Gr 5, because you have tones already laid down by the first exposure.
If I have sussed your problem correctly you would be better off holding back the grass during all of the (say) Gr 2 1st exposure and then burning it back on Gr 5.
Alternatively, using a bleach (ferricynaide - it's not as dangerous as it sounds!) which acts on the light areas preferentially. For this application (if I guess your needs correctly) use strongish, swab on and hose off swiftly and repeat as required. this will slowly lighten the lighter grasses without affecting the darker ones, i.e. expanding local contrast. You can't due this by dodging grass stems, they are too fine. It does take a little practice though.