Late-Season Crop Evaluation

by Jeffrey C. Silvertooth,
Extension Agronomist - Cotton

Conditions among cotton fields across Arizona are highly variable at this stage in the season. In general, the crop is still significantly later than usual as a result of a number of factors that I have enumerated in earlier bulletins in this series (e.g. 8 August 1998). In many areas we are approximately one month behind normal stages of growth and management operations. Due to delayed crop maturity and low fruit retention (FR) levels that are common to many fields, yield potentials have been reduced. Reliable and accurate predictions are difficult to develop at this stage in the season. However, based on field reviews in many areas of the state, I believe statewide average yields for 1998 are going to be notably lower than those recorded in 1997.

The difficulties in FR that many fields have experienced commonly stem from three basic factors: 1) abortion of many early squares, 2) lygus bug damage, and 3) heat-related stress. The monsoon weather patterns began the first of July for most cotton producing areas of the state and many cotton fields responded to the heat and humidity with characteristic drops in FR (primarily in lower elevations). Dr. Paul Brown recently wrote an article describing the weather patterns across the state this season in relation to the Level 1 and Level 2 categories of heat stress that he proposes in relation to cotton growth and development. Upon review of many cotton fields across the state we certainly find a good relationship between these categories of weather-related stress and FR levels. We also commonly find a higher proportion of malformed bolls (parrot-beaked bolls) resulting from poor pollination in blooms that were apparently damaged during the square stage of development as a result of Level 2 stress conditions. The monsoon conditions have persisted now for over eight weeks and it has certainly taken some toll on the fruit load we are carrying in many fields. Heat-related damage is also greater in fields that were subjected to water stress during this period of the season.

However, in my view, the negative impacts on the crop due to heat are relatively minor in relation to the damage that we have suffered from lygus bug infestations in many areas of the state. Lygus infestations seemed to start early for many areas of the state this season, apparently in relation to enhanced desert vegetation due to the winter and spring rains. When the problems associated with a delayed crop are compounded by the effects of lygus bug damage that were encountered in many areas this season (in addition to the heat), we have many fields that have a very light fruit load at this time.

The problems that we have encountered this season in relation to lygus bug populations and their management in cotton, brings to our attention the need to consider the overall crop ecology of a system that we are dealing with. With the current economic conditions in the agricultural community, we are seeing growers responding to markets with a broader diversification of crops in many areas of Arizona. Crop diversification brings with it many positive factors (agronomically and economically). However, there are other changes in the cropping system that can present special challenges to crop managers. For example, the changes in insect pest complexes that may occur with a change in the proportions of various crops that are grown in an area may require special attention. This may be a mute point for some parts of Arizona this season. However, I would venture to guess that many farmers and crop managers (e.g. PCAs) that were trying to grow cotton in close proximity to safflower or seed-alfalfa this season would agree that this aspect of crop ecology is an important issue. Crop ecology can be complex but it represents an area where farmers, PCAs, entomologists, weed scientists, plant pathologists, economists, agronomists, etc. need to plan ahead and try to anticipate the various problems that a given cropping system or combination of crops may pose. We need to address this in the development of cropping systems in Arizona.

Crop evaluation is important at any stage of the season, but as we approach the prospect of harvest many fields often receive a few extra inspections. Important factors to consider in field evaluation late in the season include:

• Total number of mainstem nodes on the plant and the total number of fruiting branches;
• Plants should commonly show signs of cut-out at about 25-28 mainstem nodes;
• This should translate to about 20 to 23 fruiting branches;
• If the first fruiting branch was late, this commonly results in less fruiting branches and sites;
• The cut-out zone on the mainstem can often be found by an area of about 4 to 5 compressed internodes;
• Generally, fruit retention (FR) should be greater than about 45% (considering first two fruiting sites on each fruiting branch) at cut-out for the plant to have reasonable yield potential;
• Yield potentials will be lower with a given FR estimate if fruiting sites are reduced or boll size is abnormally small;
• Boll counts per unit area (Upland:15-20 bolls / foot of row (38-40 in. spacings) ~ 1 bale / acre; and 30 bolls / ft. for Pima);
• Cotton yields arise from the number of individual fibers developed in a given area, and therefore basic yield components are: (fibers/seed) X (seeds/boll) X (bolls/acre);
• Yield estimates from boll counts can also be affected by the number of seeds/boll (which can relate to effective pollination), overall boll size, and fiber development (weight);
• Each of these components can also be affected by variety, environment, and management.

In evaluating fields at this stage in the season one should try to identify any patterns associated with gaps in the fruit load (high rates of fruit abortion) or areas where FR was high. Often these patterns in FR can be traced back in relation to either some management-related event or weather. For example, it is important to identify crop development in response to the occurrence of lygus infestations (and their management), mepiquat chloride (e.g. PIX™) applications, water stress, periods of heat stress, etc. In back-tracking on crop development, one can use an estimate that approximately 2.0 to 2.5 nodes are produced by cotton plants each week when good plant-water relations are maintained (no water stress). Many fields are in need of later irrigations (than normal) just to complete the first or primary fruiting cycle (mature the bolls that are set up to cut-out). Fields that are carried later in the fall for the development of a top-crop (second fruiting cycle) should be monitored in terms of FR levels and boll maturity rates that are realized in September and October (in the nodes above the cut-out zone).

For fields that are carrying light fruit loads and/or have significant gaps in fruiting patterns, the potential impact of this condition on fiber quality should be recognized. In recent years we have experienced a number of cases of high micronaire. Fields that experienced low FR levels in the mid to later stages of the season, may be subject to high mike fiber. This can be due to the tendency of the plant to provide abundant carbohydrate supplies to the existing bolls (and developing fibers) during the cellulose deposition phase of development (following fiber elongation).

Late season management in relation to defoliation should also be taken into account. This is particularly true for fields that are going to be maintained for production through October and defoliated under cooler conditions, which are typically more difficult to defoliate. Also, fields that have a low boll load but have lush vegetative growth, may offer some challenges in terms of defoliation. Important factors to consider for defoliation include: plant-water status, N fertility levels, weather conditions, the rate of the defoliant of choice, and the time of application. We have a number of bulletins addressing defoliation and management that are available through the University of Arizona Cooperative Extension county offices.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

Any products, services, or organizations that are mentioned, shown, or indirectly implied in this web document do not imply endorsement by The University of Arizona.