A real-time variable rate Pix application system that integrated an optical plant height sensor, the MEPRT (MEPiquat Chloride Rate and Timing) relationships, and a chemical rate controller was developed. The height sensor used an array of light beams that were either blocked or unblocked by the plants. The height estimate was calculated from the histogram of the blocked beams. The height estimate along with the operator supplied average plant density and number of nodes was used in the MEPRT relationships to predict the optimal Pix application rate. The desired rate for an area of the field was transmitted to the chemical rate controller for application to the measured plants.

Pix was applied using a paired observation design of alternating variable and uniform application rates in two 20-acre fields. The variable application rate was based on the developed system and the uniform application rate was calculated using the operator observed average plant height. The average application rate from the variable blocks were greater than the uniform blocks in Field A, but lower in Field B. At the time of variable application, the plant height throughout the field was systematically overestimated by 7 cm, resulting in over-application of Pix.

Four rows received no Pix application the entire season. These rows were used as a control to compare the treatments. Yield differences between the treatment and control were calculated for seed cotton yields in Field B and lint and seed in Field A. No statistical difference was observed between the treatment blocks.