Two field studies were conducted in 1991 to further evaluate Upland cotton yield and development responses to PIX application timing as a function cotton growth stage. Treatments imposed in 1991 were intended to further clarify some response trends observed in previous field studies. Treatments in 1991 were all at the maximum label rate of 1.5 pt./acre with application timing the main variable. Timing was based on growth stage and heat unit accumulation since date of planting. The study in Waddell, Az. resulted in no significant yield differences across all treatments. In contrast, the study at the Maricopa Agricultural Center resulted in a statistically significant yield response by approximately 100 lbs. lint /acre for all FIX treatments compared to the untreated check plots. Plant growth and development measurements indicated the height: node ratio counts to be a good reflection of vegetative tendencies under field conditions in the two studies.

Plant Growth Regulators (PGR's) are used in cotton production to reduce excessive vegetative growth. This study was conducted to determine the effect of mepiquat chloride (PIX) on growth, and yield of Pima S7 and Deltapine 90 cotton. Single and multiple applications of PIX were conducted over the growing season. Plant mapping measurements were performed 13 times. Seed cotton yield estimates were obtained with a mechanical picker. Analysis of the mapping data showed that vigor index for control and PIX treated Pima and Deltapine plants was linear for the first 9 weeks, then leveled off as flowering and boll set occurred. PIX treated Pima plants were approximately 5cm shorter than the control after the 9th week. Heights of PIX treated Deltapine were similar to the controls. The number of nodes above white bloom in the FIX treated plants was not modified. PIX treatment of Pima cotton reduced the seed cotton yield compared to controls but yield of PIX treated Deltapine was similar to the controls.

A field study was conducted at the Maricopa Agricultural Center to evaluate the effect of a late - season application of PIX on Upland cotton termination. A 28 August application of PIX or FIX + Prep had only a small effect on late-season growth and no effect on defoliation or yield.

Summaries of cotton crop phenology, as a function of heat units (HU, 86/55 °F limits) have been developed across a wide range of production conditions in Arizona. Optimum ranges of HU accumulations since 1 January are used to describe optimal planting dates for full season varieties. Basic events such as the occurrence of pinhead squares, squares susceptible to pink bollworm, and first bloom are described in terms of HU accumulations since planting. Fruit retention guidelines and height: node ratios measures a crop's vegetative/reproductive balance, are developed as a function of HUAP. The use of the number of nodes above the top white bloom to the terminal (NAWB) is developed as a measure of a crops progression towards cut-out. Also, the expected ranges of HU 's accumulated since planting that are required to accomplish crop cut-out are shown for three general maturity types of Upland cotton.