Mature 'Chester Thornless' blackberry plants were trained to the rotatable cross-arm (RCA) trellis to determine the effect of retaining two, four, or six primocanes on plant productivity. Retention of only the two oldest primocanes and generally the most vigorous primocanes per plant yielded 14.1 kg of fruit compared to 17.1 kg per plant in which as many as six primocanes were retained. Increasing the number of canes did not result in significant yield increase (P = 0.09) because the primocanes trained in late-June and July produced only a few, and, in some cases, no lateral branches. Thus, retaining only those canes that become trainable early in the season decreased labor inputs and allowed primocane training to be completed prior to the onset of harvest. As a result, the effort to train and retain only those primocanes that reach the trainable height before mid-June may be advantageous to minimize labor costs, but will not effect plant productivity.