The thornless blackberries are really producing this year.
After doing some research, I implemented a new plan. The berries can produce to maximum potential when the shoots out of the ground are in their second year. So last year I pulled all the canes in this row to this side of the fence. Keeping them pruned to under 3 feet, I let them grow but they did not produce berries. I didn't let anything grow on the other side of the fence.
This year, they filled out and then started producing berries ~ lots of them! All the new canes coming out of the ground are being pulled to the other side of the fence to be next years canes.
And so the cycle goes ~ every year 1st year canes are on one side not producing and 2nd years canes are on the other side producing blackberries like crazy. After these canes finish producing they will be cut to the ground making way for the new canes.