Plant Growth Regulator (PGR) Management Considerations for the 2013 Crop (Collins and Whitaker)

As is the case in many years, the 2013 Georgia cotton crop is widely variable in terms of growth stage depending upon planting date. Most of the early planted fields (late April to early May) are within the first couple of weeks of bloom, with several slightly later planted fields at various stages of squaring, and late planted cotton (early to mid-June) ranging from seedling to 4-5 true leaves. Most of the double-cropped cotton was planted noticeably later than normal this year. Soil moisture has recently been adequate to excessive in most of Georgia’s cotton belt due to frequent and hefty rains and effects have been variable with regard to growth and development. Many fields have been waterlogged which has noticeably slowed growth of younger cotton, while other fields of older cotton (near bloom or within the first couple of weeks of bloom) are currently showing signs of vigorous growth.

Due to wide variability in crop growth, we strongly recommend that growers make growth management decisions on a case-by-case and field-by-field basis, as opposed to a one-size-fits-all approach. When making plant growth regulator (PGR) decisions, it is important to recognize the actual effects that mepiquat-containing PGRs have on the plant, and how this may or may not benefit the grower. Mepiquat-containing PGRs reduce the production of plant hormones called gibberellins or gibberellic acid. Gibberellins are natural plant hormones that are involved in cell expansion. When mepiquat is applied, the internodes near the terminal of the main stalk or on lateral branches (regions where elongation occurs) may not elongate to the degree that non-treated plants would. This usually results in shorter plants, with more compact nodes. Additionally, most mepiquat-containing PGRs, with the exception of Stance, generally have similar effects on plant growth. Therefore, when applied at similar rates (except for Stance), similar results should be expected. Stance contains a higher concentration of mepiquat than other meqiquat products, and also includes cyclanilide. This product is used at much lower rates than standard mepiquat products.

It is also important to understand what mepiquat-containing PGRs do not do, as there are some misconceptions out there. Mepiquat does not stimulate flowering and does not create more bolls per plant. At best, mepiquat may improve retention of some bolls, but it does not cause the plant to produce more bolls. Lastly, and most importantly, yield responses to mepiquat are inconsistent at best….yields are improved in some situations, reduced in other situations, and in many cases, PGRs have no effect on yield at all.

So why use PGRs?? Some of the more beneficial effects, again which may or may not occur, are improved fruit retention on lower nodes and earlier maturity (generally more beneficial to later planted cotton, and especially later planted irrigated cotton), improved harvest efficiency, reduced impedance of insecticides/fungicides/harvest aids, reduced boll rot and reduced lodging of plants (all of which in the right circumstances could potentially increase yield). The likelihood of achieving one or more of these positive results greatly increases if the environment is likely to result in (or has historically and consistently resulted in) excessive vegetative growth, but even then, these results may or may not occur. Additionally, there are risks associated with mepiquat applications, especially when improper rates and/or premature application timings are implemented. Keep in mind that mepiquat should be used in a manner to prevent rank growth from occurring, but remember that plants still need to be tall enough to support an optimal boll load, thus an optimal yield. An optimal plant height generally varies depending on the situation, therefore this should be determined on a case-by-case basis and could be adjusted for other situations.

One other thing to consider in Georgia is the length of the growing season and our weather as it relates to cotton maturity and fruit retention. There are a couple of things to consider with PGR decisions which relate to Georgia’s conditions. Most importantly, one should consider that our relatively high temperatures and high humidity create a situation where fruit retention can be an issue to consider, such that fruit retention on lower nodes may explain why the crop is difficult to control with PGRs. It is also important to remember that conditions which occur during boll opening (especially in early planted fields) may impact the ability to harvest those bolls on lower positions, whereas boll rot can greatly impact the harvestability of fruit particularly those deeper in the canopy. These factors should be considered when attempting to decide the optimum plant height required to maximize yields. Another somewhat unique factor with Georgia’s environment is the length of the growing season. Our conditions often allow for cotton to be produced much later in the season than cotton grown in neighboring states (especially those north of us) and consideration should be made when comparing PGR ideas and recommendations from those regions. This is not intended to make Georgia growers weary of PGR recommendations from other states, but it can potentially explain differences in PGR philosophies in Georgia compared to other states.

Also keep in mind that we now are dealing with some newer varieties that tend to be earlier maturing than DP 555 BR with generally less growth capacity than that of DP 555 BR. The range of maturity and growth capacity is very wide among these newer varieties, with some varieties showing somewhat similar characteristics to that of DP 555 BR, so it is important to familiarize yourself with these characteristics of the varieties you choose to plant. When DP 555 BR was still widely planted, most growers began their PGR applications at the 8- to 10-leaf stage, which was generally followed by applications at or near first bloom and again 2 to 3 weeks later. This program was a more preventative-type prophylactic program that generally worked well for DP 555 BR, especially in irrigated fields, as this variety could consistently result in (and was likely to result in) excessive growth and extremely tall plants. Most (but not all) of the newer varieties tend to develop a larger boll load slightly quicker than DP 555 BR, which can restrain terminal growth to some degree, therefore growers can be more reactive than proactive/preventative with PGR management in some situations, especially with the earlier maturing varieties.

In other words, some of the earlier maturing varieties with less growth capacity may not need a pre-bloom PGR application in order to prevent excessive growth, and delaying these decisions until first bloom may allow for better growth management decisions. Keep in mind that some of the earlier maturing varieties may exhibit vigorous or aggressive growth prior to first-bloom, however the growth rate may rapidly decrease once these varieties enter the bloom period when the rapidly developing boll load begins to retrain terminal growth. Aggressive, preventative approaches for early maturing varieties may in fact prevent plants from reaching an optimal plant height in some environments, thereby risking yield loss associated with inadequate numbers of fruiting sites. For later maturing varieties with greater growth potential, especially in timely irrigated fields, a more aggressive preventative approach (which may include pre-bloom applications) may be necessary to prevent excessive growth, but a one-size-fits-all approach is no longer suitable. Below are several modern cotton varieties categorized by growth potential and necessity of PGR applications (if signs of vigorous growth are evident, and conditions are currently and will likely remain favorable for excessive growth).

When making PGR applications factors other than variety play an important role in making proper PGR decisions. This table is intended to provide information pertaining to the varietal impact on PGR decisions, and recs. are not precise due to other factors involved. Varieties are grouped based on similar vegetative growth and response to PGR applications. The recommendations for each group of varieties are intended to adequately control vegetative growth while allowing varieties to reach their full potential with regards to growth and yield. 

Regardless of the variety planted, it is important to remember that the environment (primarily soil moisture status) has much greater influence on the need for PGRs than any other factor. If conditions favor extremely excessive growth, then even early maturing varieties can result in excessively tall plants. If conditions become hot and dry (especially if irrigation is not an option), even vigorous later maturing varieties may be forced into a premature cutout and yield may be limited. Due to the recent and excessive rains, many growers will be tempted to apply PGRs as soon as fields dry out enough to allow passage of equipment. In many cases, these applications may be warranted, especially in blooming cotton or near-bloom cotton that is already taller than normal and showing the signs of vigorous growth. However, if the recent rains have been excessive and potentially slowed growth of younger cotton, PGR applications may not be necessary, therefore growers should evaluate their crop on a field-by-field basis to determine if PGRs are necessary.

Impact of sequential applications – It is also important to remember that the timing of sequential (or follow-up) applications likely has a large impact on final plant height and overall effectiveness of PGR applications. In most cases, aggressive vegetative growth can be regulated adequately with two or three appropriately timed PGR applications. When the number of days between applications is much longer than two weeks, then suppression of aggressive vegetative growth can become more difficult. In situations where a grower has delayed initial applications and vegetative growth has the potential to get out of hand, be sure to closely monitor growth soon after the initial application and make any necessary sequential applications within 10 to 14 days after the initial application, if expected results from the first application were not observed or terminal growth hasn’t adequately slowed. Using this approach may allow us to use lower initial rates, especially in dryland fields, in order to not “overdo it” while properly maintaining vegetative growth management.

Once consideration has been given to the variety’s growth potential, the environment (irrigated versus dryland environments), field history, planting date, etc., growers should then make PGR decisions based on other factors that may indicate whether or not excessive growth is likely. Such factors include current soil moisture status, weather forecast for the near future, and signs of current vigorous growth when examining plants. PGRs should not be applied to drought stressed cotton, or cotton that is rapidly approaching cutout, especially if an optimal plant height has not yet been reached. Below are a few plant parameters to consider when making PGR decisions.

Utilizing plant height prior to, or at, first bloom: When we make PGR decisions, remember that we are trying to achieve a final plant height that can support adequate fruiting sites in order to maximize yield potential, without excessive height which could result in several adversities including: poor retention of earlier-set fruit or delayed maturity, dense canopies that impede other over-the-top applications, increased risk for boll rot diseases, reduced harvest efficiency, lodging of plants, and potentially yield loss. PGRs should be applied in a manner to slow terminal growth, and guide the plant into the bloom period to a point where the developing boll load can further restrain terminal growth and hopefully terminal growth will cease within the optimal plant height range. PGR applications could be justified when plant height reaches 25 to 30 inches during the first week of bloom (the onset of the bloom period occurs when one bloom every 5 to 6 feet of row is visible) and when plants exhibit signs of vigorous growth and weather is likely to continue to promote vigorous growth. However, growers should consider the other signs of potentially excessive growth, before treating cotton just based on plant height. Also consider variety growth potential when deciding upon prebloom or at bloom PGR initiation (see chart above).

Considering the number of nodes above white flower (NAWF) and fruit load throughout the bloom period: The number of nodes above a first position white flower (NAWF) can be as high as 9 to 10 soon after first bloom (14 to 16 total plant nodes) in healthy, vigorously growing cotton and 7 to 8 at peak bloom (2 to 3 weeks after first bloom). This number should gradually decrease throughout the bloom period until the cessation of fruiting. Usually, terminal growth slows as this number decreases and a boll load accumulates. If NAWF is significantly less than 9 or 10 soon after first bloom (7 or less), then this could be an indicator of some sort of stress or slowed growth which occurred prior to bloom. This could also be a result of prior PGR treatments, thus a PGR application may not be necessary, especially in dryland fields. If NAWF is 9 to 10 or greater at first bloom, a PGR application may be justified if soil moisture is sufficient and there are no signs of current drought stress. Remember that the application of PGRs can slow terminal growth to the point in which cutout is reached more quickly than desired. When physiological cutout is reached the plant will generally not continue to produce additional main-stem nodes. Think of NAWF as a measure of the time left before cutout, and that PGR applications can speed this process along. Therefore, the higher the NAWF, the more time the plant has to grow prior to cutout, and the more potential the PGR application can have on overall plant growth and development. Monitoring retention of bolls and squares throughout the bloom period may also provide some indication of growth potential. If retention is relatively low at or near first bloom, and soil moisture is high, vigorous growth could result, potentially necessitating a PGR application.

Measuring internode distances: A very strong, and one of the best indicators of vigorous growth is the distance between adjacent plant nodes, between the 4th and 5th leaf from the terminal, which is generally the uppermost internode that has fully expanded. Longer distances between these nodes is an indicator of greater terminal “horsepower” or growth potential. This is often a much better indicator of growth potential than height-to-node ratio, because height-to-node ratio may sometimes fail to account for the current growth rate. If internode distances between the 4th and 5th leaf from the terminal are around 3 to 5 inches, then the plants are growing vigorously, possibly requiring a PGR application. If internode distances are much less than 3 inches, then the plants may be encountering some sort of stress or may be a result of the developing boll load, and is a sign that terminal growth is slow, therefore a PGR application may not be necessary.

These are just a few of several general guidelines that are useful in determining the need for PGRs for cotton that is approaching the bloom period. The most accurate assessments are made when evaluating the “big picture” or utilizing several of these guidelines in concert. No single approach can determine the optimal PGR rate or timing without predicting the future, nor is a single approach accurate enough to account for all the variation in crop development and environments. Again, it is critical to make these decisions on a case by case basis, and to monitor growth frequently to determine the necessity of additional mid-bloom PGR applications. Remember that environment tends to play a very strong role in the necessity of PGR applications…..just because you grow cotton in a dryland field, does not mean that excessive growth can’t occur, and vice versa for irrigated fields. The same idea applies to variety maturity. PGRs should only be applied on an as-needed basis, when there are signs of current and expected vigorous growth. PGRs should not be applied when there is insufficient soil moisture, especially in dryland fields.