Hi Claire,
To include repeated measures in GLIMMIX, you need to add one more option to the random statement. Try:
random = round / type = AR(1) subject=subj
residual;
This will specify round as an R-side effect. Make sure that subject is included in the class statement, or that it is numeric and that the dataset is sorted in subject order (a good idea in any case).
Next question: are your timepoints equally spaced (or nearly equally spaced)? If not, AR(1) is probably not a good choice for the covariance error structure as it assumes a constant correlation between timepoints--look into SP(POW) as an alternative.
A note about autoregressive error structures: see Littell et al,
J. Anim. Sci. 1998. 76:1216–1231, especially the technical section at the end. The subject in the R-side specification should be included as a random effect, so the between subject error can be correctly modeled. So, I would try the following:
proc glimmix data=xxx;
class round treatment field transect subj;
model y = round treatment round*treatment / dist=poisson;
random = round / type = AR(1) subject=subj residual;
random subj transect(field);
run;
Good luck!