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. Basic phenological 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 (HUAP). Fruit retention guidelines and height: node ratios, which measure a crop's vegetative /reproductive balance, are developed as a function of HUAP. Similarly, the rate of canopy closure is described in terms 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 Upland and Pima cotton.

Several plant growth regulators were applied to DPL 5415 cotton to evaluate effectiveness on cotton lint yield. Growth regulators applied in small amounts over multiple applications early in the growing season reduced yields compared with the check while a single application applied at full-bloom slightly increased lint yields. High night time temperatures from late July into early September were detrimental for cotton production. Increased squaring of early season applications compared with the check may have resulted in reduced yields due to increased stress from temperatures and/or nutrient availability. Increased stress in the early treated (more fruit) plots may have had greater fruit abscission.

Cotton crop response was evaluated using the growth regulator FIX in a conventional liquid formulation compared to an equivalent rate of a newly developed dry flowable (DF) formulation. The DF formulation will be available in 1996 in water soluble packets with each 57 gram packet the equivalent of one pint liquid PIX. Both formulations regulated plant height and height:node ratios equally.

All plant growth regulators and nutritional products applied to Pima S-7 cotton during bloom economically increased lint yields, with increases ranging from 80 - 261 lbs. /acre. Greatest increase was noted with Boll-Set Plus, a product that contains raised levels of calcium and zinc. Cytokin and Boll -Set Plus significantly increased yields at one location. A lint yield increase of approximately 40 lbs/acre was noted when Tech-Flo Alpha was added to Cytokin. A single application of Foliar Triggrr resulted in equal yields as two applications of the Cytokin plus Tech-Flo Alpha.

Methanol, Cytokin, Cytoplex, PGR IV and Foli-Zyme plant growth regulators were tested on long and short staple cotton on the Safford Agricultural Center in 1995. It was a follow up study on Methanol, Cytokin and PGR IV and a first time look at Cytoplex and Foli-Zyme. Trends toward increased lint yield were seen with Cytokin and Cytoplex with the other treatments yielding near or below the untreated check Some increase in maturity was seen on the short staple plots by all of the treatments. Some differences in HVI data were also observed.

A series of boll measurements were taken at numerous locations across the state in 1995 in an attempt to continue to develop a yield prediction model that began in 1993. Results from 1995 showed the strongest relationship between final open boll counts and yield. Based on these results, data was collected in 1996 from several locations around the state. Boll counts were made just prior to harvest and then correlated to yield. Results showed that a good estimate for lint yield could be obtained using the factor of approximately 13 bolls/row-ft./bale of lint for Upland cotton on a 38 to 40 inch row spacing.

A single field experiment was conducted in 1996 at Maricopa, AZ to compare a scheduled approach (based on stage of growth) versus a feedback approach (based on vegetative status) to both nitrogen (1V) and mepiquat chloride (PIX™) applications on Upland cotton (Gossypium hirsutum L.). PIX feedback treatments were based upon fruit retention (FR) levels and height to node ratios (HNRs) with respect to established baselines. Scheduled and feedback PIX applications were made for a total of 0.75 and 1.25 pt./acre, respectively, with the scheduled treatments being initiated earlier in the fruiting cycle (early and peak bloom). Feedback PIK treatments consisted of a single 0.75 pt./acre application near peak bloom (approx. 2200 heat units after planting (HUAP), 86/55°F threshold). Scheduled applications of fertilizer N totaled 225 lbs. N/acre from three applications and feedback N treatments received a total of 135 lbs. N/acre from three applications. Treatments consisted of all combinations of scheduled or feedback applications of both N and PIX The highest lint yields were from a treatment receiving feedback N and PIX, but all treatment yields were not significantly different (P ≥ 0.05) from one another. From a practical (economic) standpoint, however, these treatments were different in terms of the differences of fertilizer N and the timing of the PIX applications required to produce comparable yields. Results from 1996 are consistent with 1993, 1994, and 1995 results from the same study.

Heat stress has been a subject of considerable concern among Arizona cotton growers due to a recent string of hot, humid summers. Research studies indicate heat stress develops when mean crop temperature exceeds 82.4F (28C). Serious heat stress develops when mean crop temperatures exceed 86E Several meteorological factors impact crop temperatures in Arizona; however, accurate estimates of crop temperature can be made using a model requiring air and dew point temperature. This model was used to evaluate heat stress conditions in Arizona over the past 10 years. Results from this evaluation show the past three years were difficult years for heat stress. Elevation and humidity levels are major factors impacting heat stress in any given year. Lower elevation areas are more prone to heat stress than high elevation areas such as Safford. Possible management options to minimize the impact of heat stress include early optimal planting dates, variety selection, field location and good water management.

The effects of late-season Pix™ on the growth characteristics and yield of Upland cotton was examined in this study. Three treatments were imposed late season (3447 HUAP), 1, a check plot, receiving no Pix™; 2 receiving 0.75 pt/acre, and 3; receiving 1 pt/acre. The imposed treatments did not have a statistically significant effect on plant growth characteristics or earliness, nor were there any significant overall yield differences detected among treatments.

Yield of upland cotton was related to heat stress in Yuma, LaPaz, Maricopa, and Pinal Counties for the period 1987-1999. Heat stress during the primary fruiting cycle was assessed using heat stress units (HSU) which were derived from mean daily canopy temperatures computed using a canopy temperature model and local AZMET weather data. Mean lint yields were computed for years with low, intermediate and high levels of HSU. Yields in years with low levels of heat stress were always significantly greater than yields in years with high levels of heat stress. Differences in yield between high and low heat stress years ranged from 100 lb/a in Maricopa County to 254 lb/a in Yuma County and averaged 166 lb/a across all counties. Differences in yield between the low and intermediate stress years, and intermediate and high stress years averaged 86 and 80 lb/a, respectively across all counties; however, these differences were not always significant in individual counties.