How to take into account different treatments, doses and starting health

In addition to my previous post, is it possible to do post-hoc analysis with mixed models? Ideally I would have some kind of "Tukey" test to compare the effectiveness of the chemicals against each other. What would be perfect would be to be able to run a combined analysis of all three nematode species at once (with nematode, chemical and dose as factors, dose nested within chemical).