Agronomic Evaluations of Transgenic Cotton Varieties

Abstract

Several field experiments were conducted in many of the cotton growing areas of Arizona in 1997 for the purpose of evaluating agronomic characteristics of many new transgenic Upland cotton varietie. In many cases, the new transgenic lines were compared directly with their recurrent (non-transgenic) parents. Evaluations were carried out by collecting plant mapping data from each variety on a regular 14 day interval throughout the season and relating the resultant information to established baselines for Upland cotton in Arizona. Lint yield measurements were also taken on each variety at all locations. Results indicate that all transgenic lines tested are very similar to their recurrent parents in terms of growth, development, and yield. Some subtle differences were noted but they were very slight and should impact management of the varieties significantly in comparison to their recurrent parents.

Introduction

The cotton plant is the centerpiece to a cotton production system. Accordingly, the variety being grown is extremely important. Transgenic varieties can possess tremendous potential by providing unique tools to the farmer, being delivered directly in the plant system. However, it is important that the variety carrying the transgenic property is a strong variety for the situation in which it is being used. It is also important to note that although statements may be made relative to a new transgenic variety being "the same as" it's non-transgenic parent variety; the new transgenic variety is actually a separate and unique variety. Under these circumstances it becomes even more critical to have access to objective, well documented information regarding the agronomic nature (growth, development and yielding potentials in response to soil and environmental factors) of the variety in question. This point was illustrated quite dramatically in 1997 with concerns and reports of problems associated with several Roundup Ready (RR) cotton varieties in some of the mid-south states.

As an example, the most critical difference associated with a Bt variety is that it contains some genetic information that was extracted from a naturally-occurring soil bacteria called Bacillus Thuringiensis, or Bt, which has insecticidal properties. Essentially, this genetic information was spliced into the cells of cotton plants and crossed back into favorable varieties through conventional breeding techniques. Accordingly, it is important to note that the Bt varieties that we are dealing with in the field are very similar to their non-Bt counterparts, but they are unique varieties in themselves. With or without internally controlled insecticidal properties, the variety of cotton plants grown in a field has a tremendous impact on the yield potential of the crop. Therefore, monitoring the agronomic characteristics of a Bt variety, as with any variety, is an important part of the variety evaluation. This is true not only for Bt cotton, but also for any new transgenic variety that is developed (i.e. Roundup Ready or Buctril resistant varieties).

To be competitive and to maintain economic sustainability in either a short- or long-term sense, it is important that cotton growers in Arizona have access to the best and latest in technology. To use this technology effectively, it is equally important to have a complete understanding of what the technology offers and what it can and cannot do. To utilize transgenic varieties effectively and to provide appropriate management, it is very important to understand the agronomic characteristics of them.

The purpose of this study was to compare the crop growth, crop development, and yield of several new transgenic varieties against both their recurrent parents and also against established crop growth and development baselines for Arizona cotton.

Methods

A group of field experiments were conducted at several locations across Arizona in 1997 containing numerous transgenic cotton lines that are being evaluated for introduction into the commercial market in 1998 (Table 1). Lines include varieties with Bt genes and Roundup Ready (RR) genes. Several of these lines are being prepared for possible release in 1998 (i.e. DP 20b, 50b, 90b, and 32b). For each primary site (seven) in this project, a complete battery of crop growth and development parameters were conducted on all pertinent varieties throughout the season on approximately 14 day intervals (Table 2). The following measurements were made in each plot on all dates of sampling: plant height, number of mainstem nodes, node of the first fruiting branch, aborted sites at positions one and two, the number of nodes above the top white flower (NAWF), and petiole nitrate-N concentrations. From these measurements we calculate the height to node ratio (HNR) and percent fruit retention (% FR), which is plotted along with petiole NO3- -N and NAWF values for each treatment relative to established baselines for these parameters. Harvest and lint yield estimates were conducted at all locations (13 locations total). In each case, seedcotton yields, turnout, lint yields, and HVI measurements were conducted for each set of transgenic/non-transgenic.

Results

Results are shown in figures 1-34

Basic features evident from this data include the following points:

• Most transgenic lines are very similar to their recurrent parents.
• The problems associated with RR varieties reported from several locations in the mid-south in 1997 were not apparent in any of these studies.
• Misshapen bolls and lower rates of fruit retention were not detected for RR varieties.
• Slight differences were detected between some varieties in terms of vigor and in-season fruit retention.
• Most of the new varieties are sensitive to stress, which is commonly expressed by low vigor and reduced fruit retention.
• Differences in yield were not attributed to agronomic properties.
• DP 33b does not have greater growth/vegetative tendencies than 5415 (vigor / HNR).
• DP 33b was consistently a high yielding variety across locations. DP 32b also yielded very well and demonstrated good consistency.
• Agronomic evaluation of transgenic lines without a direct comparison to the recurrent parent is possible in Arizona due to the availability of regional baselines for vigor (HNR, FR, NAWF, etc.).
• Transgenic lines of this type are commonly not placed in advanced strain studies. Therefore, studies of this type are needed for the evaluation and comparison of transgenic lines.