Because they never touch theirs really (theirs have been in the shed for the last 4 years and only have rounded edges), and as they are milking fine I felt that I do not want to overdose on Copper for them. The BT's go through at least a lick per year roughly, and as they are milking twice the amount of the GG's I felt that more Copper wouldn't be amiss....
All of my adults (milkers and goatlings) also get a 6-weekly mineral/multi-vit drench with added Copper, GG's and goatlings get half the dose of the large BT's. I have some really fussy eaters this year (same goats as last year, just getting fussy), so have stopped givin Caprivite, as they were refusing to eat their food once Caprivite was in it... (same bucket as before kidding, so no idea what's going on).