Factors to Consider When Making PGR Decisions (Collins & Edmisten)

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The early part of the 2016 has been challenging to say the least, with cool/wet weather and resulting slow early season growth that magnified some challenges with regard to thrips, seedling disease, stand establishment, etc. The resulting slow growth and need for replanting in many areas, as well as the general lack of suitable planting days, have collectively caused the 2016 crop to be behind schedule, as a general consensus. However, the past two to three weeks have brought warmer temperatures and many fields are showing signs of healthier plants and renewed vigor. Some cotton has reached the squaring stage (or will very soon), and top-dress fertilizers applications are beginning to be made as well. If rains continue, this will likely spur growth of cotton in many areas, which will trigger some of the early PGR applications. Not all fields will need pre-bloom PGR applications, but we will need to manage the 2016 crop for earliness if rains persist through first bloom and thereafter. Let’s be clear…..this does not necessarily mean that growers need to automatically be aggressive with PGRs, however we will need to be timely with scouting for both growth parameters and insects (mainly plant bugs) and timely with any necessary action. In years such as this, when the crop development is delayed, we can not afford to be late on these applications, especially for later planted cotton.

There are several tools and strategies for PGR management (rates and application timing recommendations) available to NC cotton producers. These tools are discussed in detail in the “Suggestions for Growth Regulator Use” chapter in the 2016 Cotton Information book. This chapter also discusses various scenarios in which a PGR application may result in favorable responses and how to go about assessing the need for PGRs. Many growers are very familiar with the various plant measurements described in this chapter (internode length near the terminal, fruit retention, plant height, nodes above white bloom, height to node ratio, etc.) that assist in making the best PGR management decisions possible, however, below are a few items that many understand but do not always consider when making PGR decisions. These considerations could help you make the best decisions with regard to growth management

1. The underlying premise when deciding to apply a PGR is that growth is currently vigorous and will continue to be vigorous for the foreseeable future. In most cases, this requires continuous rainfall for quite a bit of time following a PGR application, especially during the pre-bloom period. Therefore, if you are considering making a PGR application, it never hurts to look at weather forecast. If rain is unlikely and hot temperatures are forecasted, you may want to consider delaying any PGR application until you know how the weather unfolds. Hot dry conditions often suppress growth more so than any PGR, therefore you don’t want to waste money on an unnecessary application nor penalize yield from such application if rains don’t continue. Remember, in our sandier soils, we are never more than a few days away from a drought at any given time. Unpredictability of summer weather really complicates these decisions. The downside or risk of delaying or eliminating pre-bloom PGR applications is that conditions ultimately continue to promote rank growth and rains prohibit you from making timely applications later. However, if pre-bloom PGRs are applied and rains subside soon thereafter and hot/dry conditions prevail, there is a risk of unnecessarily suppressing plant height and perhaps a penalty in yield, especially if dry weather persists through the early weeks of the bloom period.

2. Yield responses to PGR applications are variable, inconsistent, and many time unpredictable. There is a school of thought among some growers that PGRs always improve yields or at least never reduce yields. However, multi-year research has shown that there are many situations in which PGRs have no impact on yield at all, a few cases in which yields are slightly increased, and a few cases in which yields are reduced. The reason we mention this, is that aggressive PGR rates applied early can (not always) reduce yield if drought follows that application and persists for a while thereafter. In these cases, you run the risk of sending your crop into a premature cutout, potentially reducing yields. If you decide that a PGR application is necessary prior to bloom to control vegetative growth, consider using appropriate rates and only when rainfall or soil moisture is very likely to remain adequate.

3. The ultimate goal of any PGR management program should be artificially slow terminal growth to a point where the developing boll load can take over restraining terminal growth. Therefore you want to manipulate growth just enough so that you can guide the plant to an optimal end-season plant height. Final plant height should be sufficiently tall enough to support an adequate boll load for high yields, but short enough to be harvest efficient and to avoid any penalties associated with rank growth. There is no optimal plant height that applies to every situation, however general observations suggest that plant should loosely be somewhere between 35 and 50 inches tall at the end of the season, depending on several other factors, such as planting date etc. Generally speaking, the taller end of this spectrum applies to earlier planted cotton, and the shorter end of the spectrum applies more-so to later planted cotton, depending on fruit retention and other factors.

4. Timeliness of PGR applications often results in more effective growth suppression than do higher rates. There have been several reports in years past of excessive rates being used to control growth. This may be necessary if soil moisture approaches the point of being excessive for prolonged periods of time, however, in many situations where growth is aggressive, research has shown that moderate rates can usually suppress growth adequately if applied in a timely manner, and that excessively high rates don’t really suppress growth much more than moderate rates if applied in a timely manner. Naturally, there are equipment or labor limitations, along with weather, that can affect our ability to be timely, so these factors also play a role in our PGR decisions. Thorough scouting on a field by field basis is necessary to accurately assess plant growth, so avoid making any windshield assessments of growth. For example, cotton could appear to be at first bloom from your truck when in fact it is actually closer to the second week of bloom and growing more aggressively than you thought.

5. Three factors that are often overlooked or underutilized when making PGR decisions are variety maturity or growth potential, planting date, and field history. Modern varieties vastly differ in regard to growth potential and maturity so understanding these characteristics can provide some insight on how a variety is likely to behave. Keep in mind that some early maturing larger-seeded varieties can often exhibit more vigorous growth during the early season (from planting to the 6-8 leaf stage when squares become visible) than later varieties, however, once these varieties enter the bloom period, they can often rapidly develop a large boll load towards the bottom of the plant which can drastically reduce the rate of growth, resulting in shorter plants by the end of the season. Therefore, failing to consider the variety’s ultimate growth potential could lead growers to utilize PGR approaches that are too aggressive on early maturing, short-statured varieties during the pre-bloom period. Likewise, underestimating the ultimate growth potential of later maturing varieties could lead to rank cotton, although many of these varieties may be less vigorous than earlier varieties during the pre-bloom period. The tables below illustrate data from 2015 on-farm trials regarding how some early maturing varieties may in fact be more vigorous during the pre-bloom period but result in shorter plants once a rapid boll load accumulates. Likewise, some of our latest maturing varieties with the greatest growth potential were less vigorous than other during the early part of the season, but once blooming began, these later varieties were able to continue vegetative growth for a longer period of time, resulting in larger plants.

Plant Height Versus Vigor 2015

Additionally, the next table below illustrates the percent of plant height reduced by an aggressive PGR strategy for all varieties that appeared in the on-farm trials in 2015. As you can see, plant height of varieties with the greatest growth potential was reduced 18 to 22 percent whereas plant height of the earlier maturing varieties with less growth potential was reduced 24-28 percent. This can be used as a general guide to help determine how to categorize varieties according to growth potential which can provide insight on how they may need to be managed.

2015 Variety Responses to PGRs

Keeping this in mind can influence the PGR management approach (especially pre-bloom applications) that you decide to take for each variety on your farm. Let’s be clear though….although variety growth characteristics play a role in PGR decision, the environmental factors, primarily rainfall and soil moisture, have a far greater influence on plant growth than variety characteristics!!!

Secondly, planting date can influence the outcome of a PGR strategy. Generally speaking as mentioned earlier, cotton planted in late April or early May can tolerate slightly taller plants (but not excessively tall), as it has a little more time to mature upper bolls that may develop. Later planted cotton (late May or later) with similar plant height may encounter cooler weather when upper bolls are developing, which increases the risks that they may not fully develop and open, therefore later planted cotton generally requires slightly more aggressive PGR management (if growth is likely to be excessive). There are situations in which early planted cotton could benefit from PGR applications, such as excessive growth or if some early planted fields are to be harvested first, which would allow harvest to begin slightly earlier than in other fields. Lastly, field history is often underutilized but can be a good indicator of growth potential in a particular field. Usually field history can be tied to soil moisture retention, and knowing the likelihood of excessive growth when normal rainfall occurs can influence the PGR strategy (rates and timing) you use.

6. If you are on the fence as to whether or not to make a PGR application, consider the aforementioned items and all the tools available to you to make the best decision. Considering the factors listed above and evaluating plant growth using all the various plant measurements (internode length near the terminal, fruit retention, plant height, nodes above white bloom, height to node ratio, etc.) should lead you to the best approach. In addition to these measurements, also look for larger terminal squares and a toughened or reddened terminal main-stem that are difficult to detach from the plant. This could indicate that the plant is undergoing some degree of stress and that terminal growth is slowing, but this should be accompanied by other measurements that indicate the same. There is no one-size-fits-all approach and no approach will be effective 100% of the time until we can predict the weather with long-term accuracy.

7. There are always questions about rainfastness of various mepiquat products. Usually this is a sign that things are going good and folks are getting adequate rainfall. Labels for many of the commonly used mepiquat-based PGRs state that if rain is expected within 8 hours, applicators should use a high-quality EPA-exempt surfactant or adjuvant to make the product rain-safe after 4 hours. However, this is not a rule of thumb, so read the label for your specific product to know with certainty. There are variations from label to label with regard to rain-free periods: for example, Pentia has a 2-hour rain-free period when applied alone, or 1 hour when tank-mixed with an EPA-exempt adjuvant. The label for Stance suggests that it is generally rain-safe after 4-8 hours when applied alone, or 2 hours when applied with an EPA-exempt surfactant. Other deviations may exist, so read the label for the product you purchase.