In the early 1990s, about the time that more than one plant growth regulating (PGR) compound became commercially available in turf, I remember being intrigued by these compounds and the management possibilities afforded by them.
Except, that is, in the context of managing an athletic field.
Around that same time, I remember attending a seminar by a sports turf colleague who explained simply that the goal of an athletic field manager is to produce green, so that the game is played on grass and not on bare soil. Given that mindset, while the idea of using a PGR on a golf course seemed good, the idea of using a PGR on a sports field seemed a bit counterintuitive—it would be sort of like putting a governor on your engine before entering your car in a race.
Today, however, we hear of many high-end athletic field managers incorporating PGRs into their maintenance programs, some with very good results. Also, the interest in using these products for sports turf seems to be on the rise. Considering this, all sports field managers need to be aware of the various types of PGRs, what we know about their benefits and side effects, how they’re being used and what we don’t yet know about them.
PGRs used in turfgrass
Simply put, a PGR is a chemical substance that is designed to alter either the growth rate or growth habit of a plant. There are several available, and a simple way to classify them is based on how they work. Some are called Type 1 regulators and some are called Type 2 regulators. But, a more recent classification system places them in five classes (Classes A-E).
- The Type 1 PGRs (also called Class C: Mitotic inhibitors) are older materials that act to limit plant growth by inhibiting cell division (fewer cells equals a smaller plant). Amidochlor (Limit), mefluidide (Embark) and maleic hydrazide are found in this class. These are good compounds for inhibiting seed head formation. But the phytotoxicity, which is primarily in the form of yellowing, can be an issue. A use of mefluidide in sports turf is meant to extend the life of lines painted on an athletic field. It can be mixed with the paint – per label directions – and the resulting growth reduction causes a delay in time before the painted leaves are removed by mowing.
- The Type 2 regulators act to inhibit the synthesis of gibberellin (GA). Gibberellins are plant hormones that cause cells to elongate. Thus, a Type 2 regulator doesn’t inhibit cell division, but acts to limit growth by preventing the cells from growing as large as they otherwise might (smaller cells equals a smaller plant). There are several Type 2 regulators on the market. These include flurprimidol and paclobutrazol (also called Class B: Early GA synthesis blockers) and trinexapac-ethyl and prohexadione calcium (also called a Class A: Late GA synthesis blockers). These products don’t suppress seed head development but, compared with Type 1 products, they do suppress vertical growth for longer periods of time. These regulators tend not to suppress lateral growth (tillers, rhizomes and other all-important structures for sports fields) as much as do Type 1 regulators. There are also some combination products available. Legacy, for example, is a combination of trinexapac-ethyl and flurprimidol.
- The most recent PGR to be registered for use in turfgrass is prohexadione calcium, marketed as Anuew. It’s also considered a Class A or Type 2 PGR but has a novel mode of action compared to the other PGRs. Among its potential advantages is that treated grass doesn’t display a “rebound effect” when the grass comes out of regulation. Because it’s new, we don’t yet know much about the side effects, good or bad, that use of this compound on an athletic field might have.
- Hormone or hormone producing PGRs (Class E: ethylene production enhancement PGRs) include the compound ethephon, which upon uptake by the plant produces ethylene, a naturally occurring hormone in the plant that functions to, among other things, act as an agonist to auxin. Auxin is yet another hormone that influences plant growth. Taking this into consideration, ethephon reduces the growth of the plant, and because of this, ethephon is also classified as a Type 2 PGRs. Ethephon was introduced to the turf market in the mid-1990s. It’s frequently used in conjunction with trinexapac-ethyl and research work in athletic turf is limited.
1. Many studies have been done that show trinexapac-ethyl, after a brief period of yellowing (phytotoxicity after application), can improve the visual quality of turfgrass. Since the cells don’t elongate as much, there ends up being more chlorophyll per unit area, causing the grass to appear darker green. This can be especially useful for sports such as baseball and soccer.
2. Studies also show that quality can be enhanced on turfgrass that’s being managed under artificial shade, as would be found in a stadium. Many of these studies, however, lack the confounding factor of assessing the effect the regulator had on wear tolerance.
3. Research has also shown that the benefit of improving shade tolerance can vary by turfgrass species and shade type.
Benefits of using a PGR
When the Type 2 PGRs came on to the market, the potential benefits of using them for golf course management were easy to recognize. But, for a very long time it was assumed that they had absolutely no place in athletic turf management. As our understanding of the side effects of these compounds on turfgrass improve, some argument can be made for using them on sports fields. Having said this, in some cases we have a good understanding of the potential benefits. But, there’s far less published literature on the management recommendations or effect that PGRs have on athletic turf compared to golf turf.
The obvious and easy-to-quantify benefit is the reduction in top growth, or clipping yield, by as much as 50 percent for a period of time. Related to this is the benefit of fewer required mowings, reduced mowing time and reduced equipment wear. In areas of heavy usage, this might not be a good thing. Thus, use in-season has for a long time been not recommended on fields that host a lot of sports. Many of the Type 2 regulators suppress shoot growth while not significantly impacting root or rhizome/stolon growth. There’s some evidence that the use of a Type 2 PGR, particularly during the off-season, might improve turf quality by increasing density and that any increase in root or rhizome growth might improve traffic tolerance.
The overwhelming majority of published work on PGR use on athletic fields is with trinexapac-ethyl (Primo). Trinexapac-ethyl applied two weeks prior to sod harvest has been shown to increase root tensile strength eight weeks after the sod was transplanted. But the results were not consistent. Tiller density and root mass can be increased by 10 to 15 percent with May, June and July applications of trinexapac-ethyl. However, studies to examine the impact that trinexapac-ethyl has on either wear tolerance or lateral growth for turf recovery have been mixed. This probably has a lot to do with the timing of application – for heavy traffic sports, better results have been seen if trinexapac-ethyl is used in the off-season. Spring and summer applications improve traffic tolerance for fall sports due to the increased root mass and tillering.
Faster establishment when overseeding is also a potential benefit. This is because the energy that would go into leaf production may be redirected into root and/or rhizome production. But studies specifically examining the effect of trinexapac-ethyl on seedling quality and seedling establishment in perennial ryegrass showed no effect. Furthermore, others have reported that using a reduced rate of PGR applied more frequently in conjunction with nitrogen can reduce establishment time and provide denser cover. We do know that different species react differently to trinexapac-ethyl and, in some cases, there are differences in how different cultivars react (particularly those of Kentucky bluegrass).
PGRs have also been researched for use as pre-stress conditioners. The idea is that application(s) prior to heavy wear or stress periods may enhance tolerance and/or enhance recovery of the turf after the stress period. Trinexapac-ethyl, because of the way that it works, causes four weeks of growth suppression followed by two weeks of faster growth, compared to untreated turf. The reason for this is that trinexapac-ethyl prevents synthesis of active GA, but the intermediate continues to pool in the cell. After the regulator wears off, all of the intermediate converts to active GA, thus causing a flush of growth. If timed correctly (four to six weeks prior to start of play), the use of this PGR can cause the growth of your grass to be “pushed” into the season of play.
Positive results have been seen when using trinexapac-ethyl with winter turf covers. Specifically, it allows the tarp to be pulled off later in the spring without a flush of growth. At Ohio State University, studies showed that an August or September application of trinexapac-ethyl resulted in significantly faster spring green-up without excessive growth, which can be very useful for early spring sports such as soccer, baseball and lacrosse.
PGR Use and POA
- Paclobutrazol (Trimmit) is labelled for use in sports fields. But the label cautions not to use the product on athletic fields under heavy traffic where maximum growth potential of turf is desired.
- Stressed Kentucky bluegrass is particularly sensitive to paclobutrazol. this regulator, along with flurprimidol (Cutless), are labelled for the suppression of annual bluegrass by weakening it, thus making it less competitive with the other grasses.
- These PGRs are typically used on golf courses as part of a program to reduce annual bluegrass, or Poa annua. Since Poa is a big problem on athletic fields, this seems like a potential use. but trinexapac-ethyl doesn’t appear to affect Poa populations and the PGRs that do (mefluidide, fluprimidol and paclobutrazol) don’t produce the same desirable effects on the athletic turf that trinexapac-ethyl does.
- Also, on golf courses, these PGRs change the competition dynamics so that creeping bentgrass 9which spreads by stolons) becomes more competitive with Poa. But they don’t necessarily make the grasses used on athletic turf in northern parts of the country more competitive against Poa.
- Perhaps the best strategy when trying these PGRs is to be aware of how much Poa you have so that you can quantify the effect (good or bad) and use these PGRs during the off-season in conjunction with a ryegrass overseeding program.
Is a PGR right for your field?
Some work has been done to examine how to use PGRs on athletic fields. But we don’t yet have consistent or reliable management recommendations for every sport/grass/management level combination.
When deciding if the use of a PGR may be appropriate for your fields and management situation, you should proceed with caution. First and foremost is to read and understand the label. Some have specific turf species for which they’re registered and species for which negative results have been observed are left off the label. Some of the products can be used on shorter heights of cut turfgrass, while others can’t. Certain PGRs are labelled for use as a part of a seeding or overseeding program, and there are differences among the products in the lag time prior to regulation of the grass, the duration of regulation and after effects. In addition, certain products can cause yellowing of certain turf species.
For all of the work that’s been done with PGRs in turfgrass, we still have many questions about how to use them on athletic fields. For example, are the current label rates and application schedules adequate, or would sports field managers be better served by applying more often with lower rates of product?
There are also questions about appropriate application timing. If you’re applying in the fall, should applications cease prior to frost? If a field is regulated, at what point prior to the season of play should you stop applications? While four to six weeks prior to the season of play is a generic guideline, we don’t know the impact of different timings. For that matter, depending on the sport, maybe the PGR can and should be used in-season.
We have also seen undesirable effects when a PGR is applied to stressed turf. When in doubt, or if using for the first time, it’s always best to test a small area or an area adjacent to the field, or at least leave an untreated area so you can quantify the differences the PGR causes. While some evidence exists that they can increase the stress tolerance of certain turfgrass, they shouldn’t be applied to already stressed turfgrass, as would occur in the heat of the summer or if battling a disease or insect infestation.
Note that there are differences in application rates for different turf species and sites. Generally speaking, most athletic field managers are using the lower rates and applying more frequently as this provides more consistent regulation over time. You should also pay attention to whether the label recommends irrigation following application (as with Class B regulators that are root absorbed) or to allow the foliage to dry prior to irrigating if the PGR is absorbed through the leaves (Class A regulators such as trinexapac-ethyl).