You’ll have to use the :label_method option for this.
<%= f.association :owner_type, :include_blank => false, :label_method => lambda { |owner| "#{owner.name} | #{owner.subtype_name}" } %>
or, if you define a select_label method on the owner’s class, you can do
<%= f.association :owner_type, :include_blank => false, :label_method => :select_label %>