# TukeyHSD with factorial ANCOVA

I'm trying to run post-hoc comparisons on a factorial ANCOVA model for a project of mine and I'm having some difficulty. First off, lets start with the experimental design:

1. Factors include: Focal plants infection status (2 levels), Competition (2 levels) and Damage (2 levels) crossed in a fully factorial design (i.e. 8 treatments)
2. Response variable is simply total biomass of focal plants.
3. Competitor plant biomass and Initial focal plant biomass are being included as continuous covariates.

The model that I ran was as follows:

model1<-aov(TotalFB~Focal*Competition*Grazing+TotalIB+TotalBB, data=dat2)


I realize I cannot use the TukeyHSD function as it does not handle continuous covariates. I have played around with the ghlt from the multcomp package and cannot seem to get it to work correctly.

I am specifically interested in the significant Focal*Grazing interaction and was wondering how I could see if there are significant differences within the levels of focal plant infection status but across the levels of grazing. Any help would be greatly appreciated. Feel free to ask me if you require additional information. Thanks

What you're describing is an interaction. If Focal:Grazing is significant that is the effect you want. It's in your model output and easily found using summary(model1).