Managing bentgrass fairway growth with PGRs

The use of plant growth regulators (PGRs) in the management of fine turf such as golf-course fairways has changed greatly during the past 10 years. When researchers first evaluated PGRs (also called growth retardants or inhibitors) for their potential use on turf in the 1950s, they targeted them solely at reducing the mowing requirements of low-quality turf. Turf managers hoped that these early materials might reduce the cost and time involved in routine mowing. Most of the early materials met these objectives but with serious side effects. While these materials could reduce the number of mowings by up to 50 percent in many cases, they often discolored or injured the turf. The injury typically lasted only a few days, but sometimes several weeks. This loss of quality, even though it was usually only short term, prevented PGR use on fine turf. However, with newer materials, this has changed. Turf managers now can use PGRs on a wide variety of turf species.

One increasingly popular use of PGRs is in overseeding operations of both greens and fairways. Superintendents have found that PGRs can reduce competition from the existing stand so that the overseeded species can more easily become established. This aids winter overseeding as well as conversion to new varieties.

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Some of the new PGRs not only are safer to use on turf, they enhance the overall quality of the turf as well. This enhancement of quality has generated considerable interest in the routine use of PGRs in fairway maintenance, and many golf-course superintendents have been successful with this type program. However, questions remain, and several universities have active programs investigating the potential benefits and possible risks associated with PGRs.

Greens may be the most valuable real estate on any golf course, but fairways are catching up. As the desire for higher quality turf and more uniform playing surfaces increases, the level of maintenance superintendents must apply to golf-course fairways spirals upward. This is possible partially through lighter mowers that permit lower mowing heights. However, lower mowing heights in cool-season regions require the use of what was once thought of as an alternative fairway-turfgrass species-creeping bentgrass. Creeping bentgrass now is becoming the most popular species for championship-caliber fairway surfaces. Accordingly, interest in PGRs for bentgrass is increasing as well.

Using PGRs PGRs, applied at the appropriate time, can reduce the normal mowing frequency during periods of maximum growth. However, the rate and timing you use for a retardant depend on the turfgrass species and the intended use of the turf.

Turf's rapid growth helps it compete with pests and rapidly recover from the effects of stress. Thus, you should apply PGRs with caution during stressful growing conditions so the turf's recuperative potential in not impaired.

The successful application of a growth retardant depends on environmental conditions and the growth state of the turf, both at the time of application and during the period of growth suppression. In general, turf must be healthy, actively growing and, if applicable, free of signs of winter dormancy. Growth usually stops soon after application. If turf is still changing from winter dormancy to full green-up, this will leave the dead or dying older leaves visible, reducing visual quality.

Currently available PGRs Several chemical growth inhibitors of turfgrasses are available. The table on the bottom of the opposite page lists turfgrass growth retardants currently on the market. We group these products according to their mode of action. The first group, Type I, includes those retardants that suppress both the development and growth of a plant, preventing the normal transition of vegetative growth to a mature flowering plant. Mefluidide (Embark) and maleic hydrazide (Slo-Gro) are examples of this group. They reduce or eliminate the production of seedheads if you apply them before seedhead appearance. It is important to apply Type I retardants while the turf is still in a vegetative stage of growth. If any portion of a seedhead is visible from the tip of the sheath, the effectiveness of the growth retardant in reducing seedhead production will be minimal.

Mefluidide suppresses Poa annua seedheads, improving the appearance of infested fairways. Timing and accuracy of application are important. You must apply mefluidide during spring greenup but before seedhead formation. However, me-fluidide can discolor both the Poa and the desired turf species at growth-suppression rates (which are higher than seedhead-suppression rates). Maleic hydrazide also effectively suppresses Poa seed-heads, but it, too, can discolor bentgrass.

The second group of growth retardants, Type II, only suppress growth. They generally achieve this by inhibiting gibberellic acid, a plant hormone necessary for shoot elongation. The plant still usually matures and produces an inflorescence, even though the whole plant is much smaller in size. Flurprimidol (Cutless), paclobutrazol (TGR) and trinexapac-ethyl (Primo) exhibit these characteristics.

Type II PGRs have varying effects on different species. Superintendents take advantage of this by using them in mixed bentgrass + Poa annua turf to provide a competitive advantage for creeping bentgrass through greater growth suppression of Poa annua.

Superintendents commonly use PGRs to minimize mowing during peak growth periods in spring or summer, although sequential applications of growth retardants can be effective for long-term growth suppression. However, you should monitor the turf for signs of stress as environmental conditions change. Turf's response to an application of a growth retardant often depends on the local environment. Therefore, it's a good idea to consult your state turfgrass extension specialist for recommendations about the best use of growth retardants in your area.

Recent research The quest for high quality, low mowing height and uniform playability in bentgrass fairways has served to promote the use of PGR compounds such as flurprimidol, paclobutrazol and trinexapac-ethyl. These materials provide a more uniform surface by reducing bentgrass' vertical growth. All three products inhibit the production of one or more gibberellic-acid compounds in the bentgrass plant, altering its hormone balance. Giberellic acid causes cell elongation, and by inhibiting its production, you end up with miniature or dwarf plants.

This physiological phenomenon prompts the question of whether hormonal changes affect the plant's metabolism. My initial curiosity was whether changes in metabolism are greater when you apply regulators more than once during the season. Do single high-rate applications of a PGR have similar or greater impact than multiple low-rate applications? From a visual standpoint, lighter, multiple applications have less overall visual impact. Is the same true for plant sugar reserves and other components of metabolism?

With these questions in mind and the support of the Illinois Turfgrass Foundation, we embarked on a 4-year study to determine how Type II PGRs impact the level of total non-structural carbohydrates (TNC). These sugar and sugar-related compounds are the food reserves plants need for growth and to recover from stress or injury. It is important, therefore, to maintain relatively high levels of TNC to ensure rapid recovery from any summer maladies.

In our greenhouse experiments, Primo, Cutless and TGR significantly increased the TNC content (see graph at the top of page G 4) of creeping-bentgrass verdure 2 weeks after the initial treatment (WAT). This accumulation was probably due to growth suppression, which reduced the demand for carbohydrates. However, TNC levels began to decrease 4 WAT compared to the control, probably due to a post-inhibition growth response. (This response is common following PGR use but not substantial enough to create a maintenance problem with heavy clippings.) TNC levels remained lower than the untreated control for the remainder of the sampling dates. Even plants that we continually suppressed with frequent applications (which, therefore, might not undergo a post-inhibition growth spurt) failed to reach a normal carbohydrate level.

It is possible that the turfgrass plants used the available carbohydrates for something other than vertical growth. In preliminary work, we did not detect changes in photosynthetic rates. In other words, the plants were still producing as much food. However, we observed that plants treated with growth retardants had a darker green color and thicker stems and roots compared to the untreated plants (see photo, at left). Thus, the plants may have converted the carbohydrates to structural ("non-food") forms. This is an area that we hope to study further. Nevertheless, these results suggest that high PGR application rates may have limited use in fine turf.

However, based on previous observations, we suspected that repeated low-rate applications of PGRs-as many superintendents already use-might have a relatively small effect on TNC. If true, this would reduce the potential for negative effects on stress tolerance or recuperative potential.

To verify this, we conducted field experiments using trinexapac-ethyl with a range of application rates and application intervals in 1995 and 1996. However, we kept the total amount of material applied to the treated turf the same over the course of the experiments. We collected samples of the bentgrass verdure for TNC analysis each week of this study and found a trend similar to what we observed in the greenhouse experiments. Generally, either no difference or a slight elevation in TNC levels occurred in the first 4 weeks after the initial application. After this, a significant depression in TNC levels occurred for up to 12 weeks. However, the magnitude of the TNC reduction in the treated turf was comparatively small when we used lower rates and more frequent applications. Though we only used trinexepac-ethyl in the field portion of our study, the same may be true of flurprimidol and paclobutrazol as well.

I should note that I do not consider these drops in TNC to be agronomically significant. After all, we did not undertake this study because of problems superintendents were experiencing with PGR use. Actually, superintendents generally have little trouble using PGRs, including in this manner, on healthy turf.

Mowing reduction The effect that a PGR has on clipping production is always of interest to golf-course superintendents. We collected and weighed clippings from Primo-treated turf weekly during the experiment. See the graph at the bottom of page G 4 for the 1996 clipping data.

A measurable reduction in clipping production occurred on many of the Primo treatments from the 2nd week after initial application through the 11th week of the study. For the remainder of the experiment, clipping production on treated turf was similar to the untreated control plots.

Knowledge in the area of turfgrass growth retardants is still rapidly expanding. It is important to remember that PGRs will not totally substitute for mowing. Superintendents should integrate them with normal maintenance practices. However, they can be an important tool for improving quality as well as efficiency.

Dr. Tom Fermanian is associate professor of turfgrass science at the University of Illinois, Champaign-Urbana.

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