Do PIX® Application Guidelines Change for Bt Cotton?

Stephen. H. Husman, Pinal County Cooperative Extension


Two PIX experiments were conducted on commercial cooperator sites in Waddell and Buckeye, Az in 1997 to evaluate the validity of the published University of Arizona (UA) PIX application guidelines for Bt cotton. Experimental treatments consisted of an untreated check, a calendar based application schedule (early bloom, peak bloom, cutout), and a feedback approach using plant growth measurements based on the UA PIX guidelines (height :node ratio, fruit retention). There were no significant yield differences at the Waddell site where height:node ratios and fruit retention values were above the optimum baseline season long, conditions not supportive of PIX applications. There was a significant yield decline at the Buckeye site between the untreated check and the calendar based treatment. Due to low plant vigor season long, there were no feedback based PIX applications. PIX applications under low vigor conditions can further compromise plant vigor and ultimately yield. The UA PIX application use guidelines are valid and should be used for both Bt and non-transgenic Upland cotton varieties.


Upland cotton varieties containing a Bt gene which produce toxins providing effective lepidopteran pest control were produced on approximately 70% of the Arizona cotton acreage in 1997. DP 33B is a popular variety which was produced on the majority of the Bt acres in 1997. DP 33B possesses similar genetic and resultant agronomic characteristics of its recurrent parent, DP 5415. After production experience and observation in 1996, many producers and pest control advisors felt that DP 33B had a tendency to become rather vegetative in the absence of routinely scheduled PIX applications. As a result, common sentiment was that DP 33B required PIX applications to control height, maximize fruit retention, and maximize yield.

The University of Arizona (UA) has developed and offers PIX application usage guidelines based on in field measurements of plant growth and development parameters as a function of heat unit accumulation and growth stage. The guidelines have been developed from a multiple year and site basis where optimum growth and yield has been attained. The parameters used for PIX application guidelines include height:node ratio and fruit retention (Figure 1). The guidelines are based on an optimum, lower, and upper baselines indicative of low, optimum, and high vigor and fruit retention. It is suggested to measure the height:node ratio and fruit retention on regular in season intervals which will result in trend observations over time. Relative to PIX application, an example of a trend measurement that would be worthy of PIX application consideration would be when vigor is increasing towards or in excess of the upper limit with a corresponding decline in fruit retention indicative of a plant with potential to become more vegetative.

Due to the observations and experiences in 1996, the major objective of these field studies were to evaluate the validity of the UA guidelines for PIX application and usage in order to accurately determine whether a need existed for modification relative to Bt cotton. The experiments were designed to include an untreated check, a calendar based approach, and a feedback approach based on in field plant growth and development measurements.

Materials and Methods

Two experiments were conducted in 1997 in Buckeye and Waddell, Az on commercial cooperator sites. Each experiment consisted of three PIX treatments including an untreated check, pre-determined calendar approach, and plant based feedback approach using the UA PIX guidelines. The calendar based treatment was designed to receive PIX applications at early bloom, peak bloom, and cutout. The rate used would be a judgement call from 0.5-1.0 pt./acre dependent on growth stage and degree of increasing vigor. The feedback treatment used height:node ratio and fruit retention measurements for PIX application decisions based on the UA guidelines.

Plots were 18 and 12 rows (38 inch) wide running the entire field length of 800 and 1200 feet in Waddell and Buckeye respectively. Each treatment was replicated four times and arranged in a randomized complete block design. Plant growth and development measurements (height:node ratio, fruit retention) were made every two weeks beginning at early bloom continuing until harvest. PIX applications were made by ground rig with a carrier volume of 18 gpa. Harvest was accomplished by machine picking the center four rows into trailers with weight measurements recorded with portable electronic field scales. Results were analyzed statistically using SAS ANOVA.

Results and Discussion

Tables 1 and 2 include treatment protocol, application dates and rates, heat unit accumulation at time of application, and yields for both experiments. The Waddell experiment resulted in no statistically significant yield differences between treatments. Figures 1 and 2 illustrate the height:node ratio and fruit retention trends throughout the season. The Waddell site received the first calendar treatment application on June 13 with the measurements indicating that application was not warranted on the feedback treatment.

Height:node ratio measurements on July 3 (peak bloom) indicated that an increasing trend towards the upper limit was occurring which justified a feedback treatment application as well as a pre-scheduled calendar treatment. In addition, the commonplace occurrence of ensuing monsoon flows and resultant fruit retention decline potential contributed to the decision to make a feedback application. The final application on the calendar treatment occurred on July 23 with both the feedback and calendar plant based measurements indicating a decline in height:node ratio and fruit retention, conditions worthy of treatment omission on the feedback treatment.

The Buckeye site did not receive feedback based treatments due to height:node ratio measurements below the optimum baseline the duration of the season and fruit retention levels towards the high optimum baseline. As expected, there were no significant yield differences between the untreated check and the feedback treatment. However, there was a significant yield decline with the calendar approach contrasted with the untreated check. This is indicative of applying PIX under low vigor conditions further compromising potential vigor season long.


The UA PIX application guideline recommendations are valid and should be used for sound decision making purposes for both Bt and non-Bt cotton. PIX should be applied according to plant based growth and development measurements including height:node ratio and fruit retention, not a routine calendar based approach. Generally, PIX applications do not effect yields negatively and do offer management based height control. However, plants which have low vigor as indicated by height:node ratios can experience yield depression as a result of a slight and subtle decrease in node generation and resultant fruiting site potentials. PIX is an excellent management tool but should be used in accordance with measured plant growth and development parameters and the PIX application use guidelines.


Sincere appreciation is extended to Sycamore Ranches and H-4 Farms for their interest and cooperation necessary to complete these field studies.

This is a part of publication AZ1006: "Cotton: A College of Agriculture Report," 1998, College of Agriculture, The University of Arizona, Tucson, Arizona, 85721. Any products, services, or organizations that are mentioned, shown, or indirectly implied in this publication do not imply endorsement by The University of Arizona. The University is an Equal Opportunity/Affirmative Action Employer.
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