The 15 farms have new treatments in common but the local variety which is considered to be a check variety varies from one farm to another. There was NO replication of each treatment within a farm. Therefore, each farm is taken as a block in order to estimate block effect and residual term for comparison. Does it make sense to make a pairwise comparison since the local check is unique from one farm to another? The SAS script for the analysis is as follows
proc mixed data=onfarm covtest;
class farm variety;
model fyld=variety nohav/ddfm=satterth; /* nohav is a covariate */
random farm;
lsmeans variety/diff adjust=tukey;
run;
quit;
Assuming that you randomized each block, it sounds like you have a slightly unusual augmented design (unusual because the checks are generally replicated, rather than the new treatments). I'd recommend that you read about augmented designs before continuing with your analysis (links below).
You should be able to make pairwise comparisons, but not with a thing called "check". Since your checks are not the same variety in all blocks, you should not estimate a combined mean for them. The augmented design analysis will allow you to compare each treatment variety with each control variety.
https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447275&topicorder=2&maxto=5&minto=1
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwigsNbq_vjLAhUGOJQKHRyNA1IQFggdMAA&url=http%3A%2F%2Fcropandsoil.oregonstate.edu%2Fsystem%2Ffiles%2Fclasses%2Fcss590%2Flectures%2FM19_augmented.ppt&usg=AFQjCNEFTGACBhc5DZe958HrSf5nemMYjw&sig2=9DOsuPxDnbWmRyuVn3I8jg