Obtaining Least Square Means with lme (mixed models)

Hello,

I was wondering how one could obtain LSM for fixed factors in a 
mixed-effect model using lme. In other words, if I have model such as:

model<-lme(yield~period+treatment+period*treatment, 
data=data,random=~1|cow)

how can I get LSM for period, treatment and their interaction?

Thanks a lot!

Alex