The experiment was designed to investigate whether defoliation and harvest timing differs for early, medium, and late maturing varieties and plant growth regulator strategies, and if an optimal timing may be defined for each to maximize profit. Field experiments were conducted at Upper Coastal Plains research Station near Rocky Mount, North Carolina. The test was planted with a four-row vacuum planter and was arranged in a split plot design with four replications. Plots were forty feet long by eight – 38-inch rows wide. Three varieties were used; an early maturing variety (DP 444), a medium maturing variety (DP 451), and a late maturing variety (DP 555). Three PGR strategies were also used; an early method (modified early bloom), a standard method (early bloom), and a late method (untreated control). Defoliation timings of 40%, 60%, and 80% open bolls, were implemented into each variety and PGR strategy. Two harvest timings; an early harvest (2 weeks after defoliation – H1), and a late harvest (4 weeks after defoliation – H2), were also implemented into each treatment. All other agronomic inputs were applied according to NC Cotton Extension recommendations.

When examining PGR strategy and defoliation/harvest timing interactions, there was an evident decrease in strength from H1 to H2 across defoliation timings, however there was a substantial increase in yield by delaying harvest. The MEB strategy yielded significantly higher than the other strategies across all defoliation and harvest timings.

Results from variety and defoliation/harvest timing interactions, include a yield advantage from delaying defoliation to a later than normal defoliation timing for DP 555, a late maturing variety. Also, yield was increased from H1 to H2 for DP 555, possibly because extra time was needed to fully mature the least mature bolls.