Practical Uses of Crop Monitoring for Arizona Cotton

Practical Uses of Crop Monitoring for Arizona Cotton J. C. Silvertooth The use of crop monitoring and plant mapping has received a considerable amount of attention in the cotton production arena in recent years. Veteran cotton farmers and agronomists are well aware of the fact that any of them that have been successful have, by necessity, become expert in cotton crop monitoring. This type of crop monitoring is largely dependant upon experience and intuition, which can vary considerably among individuals. It is at least this authors opinion that there will never be a means by which the experience and skill of the farmer or crop manager is completely replaced by a new "systematic" approach. However, one of the important differences that is being brought about with this more recent interest and approach to crop monitoring is that cotton crop growth is being described in a more quantitative manner, which gives us a common set of standards to go by. Having a full set of standards to refer to, can then serve to assist even the most experienced of growers in reviewing crop conditions and making in -season management decisions. One of greatest incentives in developing a reliable and useful crop monitoring system is in the application of a feedback approach to crop management. In this case, a feedback approach simply refers to input management based on actual crop conditions, as opposed to a scheduled approach, which commonly directs inputs to the crop based upon stage of growth or even calendar date. In an effort to be more efficient and effective as managers, it is becoming increasingly important that inputs be directed to the crop based upon actual need rather than assumed needs. A good crop monitoring system becomes essential to the development of a feedback system to crop management. This is particularly true with a crop like cotton, which is very dynamic, highly variable, and very sensitive to management and environment. Some of the most common questions and concerns with cotton management center around the vegetative /reproductive balance of the crop, or it's vigor, which is related to the crop fruit load; and the stage of growth. In each case, whenever a cotton field is evaluated we have to address these questions and evaluate our information relative to where we think the crop should be. Good crop monitoring tools simply give us a set of references we can use to see where the crop should be regarding matters such as vigor or stage of growth. The next essential step in making a crop monitoring program useful is to apply it directly to management recommendations. There are many possible applications of a crop monitoring system to in- season management. The objective of this paper is to outline the use of crop monitoring, as we have developed it in Arizona, to the following areas of management: 1) early season fruiting, 2) nitrogen (N) fertility, 3) plant growth regulator (PGR) use, and 4) irrigation termination. Heat Units Quite often, the important stages of cotton crop development are described in terms of calendar dates or as the number of days after planting (DAP). Cotton (and plants in general) respond directly to the temperature and environmental conditions to which they are exposed. As a result, various types of heat unit (HU) systems have been developed to assist in predicting and /or projecting plant development. In Arizona we commonly use HUs with 86 and 55 F thresholds to describe the development of cotton and some insect pests. This information is collected routinely at numerous AZMET weather stations located throughout the state and operated through the expert and diligent efforts of Dr. Paul Brown with The University of Arizona. So fortunately, we have an easy access to crop /weather related information that can be useful in crop and pest management. Over the past seven seasons (1987-1993) we have collected a considerable amount of plant measurement data from 18

many field experiments that have been conducted across the state, involving numerous varieties of both Upland and Pima cotton. The intention was to not only provide a documentation of crop development in each of the experimental cases, but also to develop a collection of data from which general crop development patterns could possibly be described. Data on the number of HUs necessary to achieve specific points in crop development such as pinhead square (PHS), matchhead square (MHS), first bloom (FB), and cut -out have been collected in all cases. Measurements such as plant height, number of mainstem nodes, flowers per unit area, nodes above the top white bloom (NAWB), and plant mappings have also been routinely made over the growing season. All HU data was obtained from the AZMET system and the weather station closest to the field location under study. Review of this data has revealed some very consistent patterns in crop development and also a good description of normal variation that one may encounter, as well as what might represent abnormal development or a problem (or potential problem) field. It is our intention to develop from this data a set of guidelines that can be useful in crop management through the use of some simple measurements. The measurements need to be easy to make and there must be a set of guidelines or standards available for reference. We have found that the use of HUs provides a better way of describing cotton development than the conventional calendar and serves to standardize regional (location) or seasonal (year to year) variability. Common Terms Crop monitoring programs are currently being developed in many parts of the U.S. cotton belt, usually employing a common set of concepts. However, standards are being developed in each particular region that are specifically related to the growing conditions and varieties grown in that region. One of the things that can also happen in developing regionally based programs is that common terms can be slightly altered or new terms introduced. Therefore, for ease of reference, the following terms and abbreviations that are commonly used in the Arizona crop monitoring system are listed below: FFB first fruiting branch PHS pinhead square MHS matchhead square FB first bloom NAWB nodes above the top white bloom (first position fruit) HNR height(inches):node ratio FR fruit retention (% of fruiting positions one and two) Early Season Development The development of cotton plants in the early stages of the growing season can be predicted rather well by measuring HU accumulations since the date of planting (HUAP). As shown in Figure 1, PHS, MHS (first square size susceptible to pink bollworm), and FB can be expected to occur when approximately 700, 900, and 1200 HU have accumulated since planting, respectively. These HUAP accumulations are not offered as exact or absolute values, and in practice these values should be regarded as general signposts where the occurrence of such events may be expected to occur. For example, the presence of first PHS in a field should be expected at about 700 HUAP, but could be noticed as early as 600 HUAP. This should hold if plants are initiating first fruiting branches five to seven nodes above the cotyledonary nodes. If plants are initiating first fruiting branches substantially above node seven, then the occurrence of PHS, and all subsequent events will also be delayed accordingly. Scouting fields for PHS, MHS, and FB can help establish criteria for early season management for insects (pinhead square treatments for pink bollworms for example), timing of early irrigations, plant growth regulators, and N fertilizer applications. Identification of the occurrence of some of these events are important in that they are some of the first signs that we can use in determining how the young crop is beginning to develop. 19

NAWB As the cotton crop goes through it's characteristic pattern of development, we commonly refer to the basic stages associated with the general flower curve shown in Figure 2. To follow a crop's development in relation to a flower curve requires the collection of flower (fresh blooms) counts within a given area on regular intervals. Another measure which has proven to be much easier to make and still very reliable is the NAWB (number of nodes above the top white bloom). By counting the NAWB for mainstem blooms only within the first two positions on the fruiting branches not vegetative branches) a good measure of a crop's development over the fruiting cycle can be made. The information shown in Table 1 outlines the NAWB values that relate to specific stages on the fruiting cycle. For example, at early bloom we can commonly expect about 10 NAWB. This value (NAWB) will decrease as the crop approaches cut -out, where it will move to five or less. The rate at which this occurs, or the length of the primary fruiting cycle may vary. The variation can be due in part to variety. Short season varieties will of course be expected to go through a fruiting cycle faster than medium or full season varieties. The general ranges in HUAP for a crop to move toward cut -out are shown in Table 2. These ranges can each be regarded as satisfactory or normal. Pre -mature cut -out could be an indication of crop stresses (i.e. water, salt, pests, etc.) that have occurred. Also delayed cut -out in this sense could be a strong indication of a light boll load (poor fruit retention possibly verified by high HNRs or plant mapping) and a good potential of having a late crop. The primary fruiting cycle is of critical importance to us in realizing our potential with developing good yields. By use of a simple measure such as NAWB, we can follow a crop over the first fruiting cycle and use it as a general signpost for development. This can be of particular value in projecting crop maturity and the occurrence of cut -out in relation to the timing of the final irrigation. HNR As a crop does progress through the critical stages of the first fruiting cycle, elements of major concern are often associated with the crop's vegetative /reproductive balance. We are working with plant mapping as a means of actually measuring a crop's fruit load and developing guidelines to determine what should be expected or acceptable. A measure that is easy to make and helps in this regard is the HNR. This is made by simply counting the number of mainstem nodes (above the cotyledonary nodes) and measuring the plant height in inches. The resultant height(inches):node (HNR) ratio can be used by reference to guidelines (Figure 3) that serve to describe an acceptable range in vegetative crop development. The HNR ratio is useful because we find that the mainstem nodes are developed rather regularly at a rate of about one node /100 HUs. Therefore, differences that exist between two similar fields, planted at a similar date, in terms of vegetative development can be taken into account by the length of the intemodes. We do not have to necessarily focus in on specific intemodes, but rather look for the expression of this condition through the overall HNR of the plant. A vegetative plant for example will have a larger HNR than a similar crop which is carrying a better fruit load. N Management The use of crop monitoring can be helpful in managing N fertilizer inputs. It is important to avoid pre -season applications of fertilizer N in irrigated cotton production systems, and to apply the N fertilizer to the crop in split applications over the course of the growing season to supply the crop with N at times of peak demand. Therefore, N applications should be made between the occurrence of PHS up to the period just before peak bloom. This generally corresponds to a period in crop development that ranges from 600 to about 2,000 HUAP. Growers can also split the N applications from PHS to the stage where NAWB = 7 to 8. Applications of fertilizer N past peak bloom can serve to delay crop maturity, promote rank growth, and hinder late season management, such as defoliation. 20

Nitrogen serves as one of the strongest growth stimulating factors, besides water, that we can provide to the crop. Accordingly, it is important to apply sufficient N to a vigorous, well fruiting crop to meet crop demand. Similarly, it is important to be able to hold back on the fertilizer N applications if a crop is developing vegetative tendencies, which can easily be determined with HNR measurements and referenced to established baselines, such as shown in Figure 3. If a crop were to be dropping in it's fruit load, for whatever reason, an excess of N can serve to further stimulate vegetative growth, and drive the plants beyond a reasonable point of control. This can also create a crop which is later in it's development, more vegetative late in the season, and ultimately more difficult to defoliate. So decisions concerning N inputs can be enhanced by the simple use of crop monitoring to determine stage of growth (HUAP and /or NAWB) and crop vigor (HNR). PGR Management The use of PGRs, particularly materials such as PIX,o serve as excellent applications of crop monitoring. The whole idea behind the use of a PGR such as PIX is to control any excessive vegetative growth, direct plant energy and resources into the fruit, improve earliness, and improve yield. Therefore, it is only reasonable to expect the best results from applications of PIX when it is applied to a crop that is expressing vegetative tendencies. In the past, many growers have applied materials such as PIX at specific stages of growth irrespective of actual crop conditions. Our work in Arizona has shown that the best responses of Upland and Pima cotton to PIX applications have come when the crop HNRs were trending above the baselines, towards the upper thresholds (Figure 3). If HNRs are at or below the baselines for a given variety type, we have found that yields can actually be reduced if PIX applications are made. It is under these types of conditions where a crop is in need of additional vigor to generate new fruiting sites, not a growth suppressant. The use of HNRs can be helpful in making decisions concerning the use of materials such as PIX at any stage of growth, provided that there are appropriate baselines (such as in Figure 3) to refer to. Irrigation Termination In irrigated cotton production, water serves as the most critical input we provide to the crop, and also it is often one of the most expensive. It is important to provide the crop with adequate water throughout the duration of it's fruiting cycle, but it is also important to know when to make the last or terminal irrigation. Irrigations beyond an optimal point in the growth of the crop do not benefit yields, but they can encourage late season rank growth, waste water, and cost money unnecessarily. The best approach to deciding on the timing of the last irrigation begins with an identification of the last set of bolls that one can realistically take to harvest. If a grower is using a full season (more indeterminate) variety, there is the option of terminating early (following the completion of the first fruiting cycle), or going for a top -crop development. Medium and short season varieties do not have any significant top -crop potentials. When these types of varieties cut -out they are basically done. They can develop additional topgrowth following cut -out, but this growth is not particularly productive in terms of lint yield. Therefore, the identification and anticipation of crop development into cut -out is very important. True cut -out occurs when the plant blooms all the way to the terminal, exhausting all of it's existing fruiting sites, which results in a cessation of flowering. For practical purposes a crop is progressing rapidly into cut -out when the NAWB <_ 5 (Table 1). When this stage is recognized, decisions need to be made concerning final fruit development and irrigation termination. When the last blooms are set which are intended for maturity and harvest one can project the staging of the last irrigation. We have found that approximately 600 HUs are required to develop a boll from the bloom to a full sized, hard boll. This is the stage in boll development where fiber elongation is taking place, and it is important to maintain adequate soil moisture to insure good plant -water relations over this period. In Arizona, this roughly equates to about three weeks in August and early September. The final irrigation for a given field can be made so that adequate soil moisture is provided throughout this final period of boll development. 21

Summary The purpose of the crop monitoring tools described in this paper are designed to assist a farmer or consultant in the process of making an assessment of a cotton crop's development and condition and using the information to manage inputs such as N, PIX, final irrigations, etc.. Measuring the NAWB, keeping track of HUs, or taking HNRs are only useful with a set of standard guidelines and tested recommendations concerning their use in managing important aspects of the crop. These types of tools also offer to us a means by which we can further refine our understanding of the cotton plant, it's growth characteristics and needs, and what inputs are needed to grow a good crop efficiently, which contributes to profits and sustainability for a cotton farm. 22

Table 1. Relationships between cotton growth stage and the number of nodes above the top white bloom (NAWB). Growth State NAWB * Early Bloom Peak Bloom Cut -out 9-11 7-8 55 *NAWB, first or second position fruiting sites on mainstem fruiting branches. Table 2. General cut -out occurrences for cotton variety types commonly grown in Arizona. Variety Type Short Season Mid Season Full Season HU at Cut -Out* 2000-2700 2300-3000 2500-3200 *Heat Units (86/55 F) accumulated since planting. 23