Efficacy of sulfonylurea herbicides for improved transition on bermudagrass fairways

March 23, 2005

Summary: A replicated small plot study was conducted on ryegrass overseeded bermudagrass fairways to evaluate the ability of several herbicide products to remove overseeded perennial ryegrass and thus improve the quality of the underlying bermudagrass base.  Based on the results, the following conclusions were drawn:

• All treatments produced significantly accelerated increases in bermudagrass cover vs. the untreated check.  The greatest increases were seen with Revolver (0.2 oz/1000 sq ft), applied early in the season (5/7/03).
• All treatments produced relatively rapid (within 1-3 weeks) death of perennial ryegrass and Poa annua.
• All treatments produced some yellowing of turf or in some cases some bare spots, due to death of perennial ryegrass.  The period of significant yellowing and/or bare patches lasted from 5 weeks to greater than 9 weeks, depending on the treatment tested.  In this coastal environment with its very gradual spring/summer warming trend, significant yellowing of 5 weeks or more should be expected with any of the products tested.  If the spring is unseasonably cool (prolonged periods with average temperatures below 65F), this period can be significantly extended.
• No damage to hybrid or common bermudagrass was observed. However, due to its inherently slower growth rate, areas with common bermudagrass were slower to recover from herbicide applications than areas with hybrid bermudagrass.
• Treatments made later in the year, when average air temperatures were greater than 65F, produced less severe yellowing of turf than applications made earlier in the year.  Based on the 30-year normal data for Morgan Run Golf Club (Table 1), this threshold temperature would typically be reached by mid-June.
• The half rate (0.2 oz/1000 sq ft) of Revolver produced a less severe and a shorter duration of turf yellowing than the full rate, without any decrease in herbicidal activity.  While the lowered rate may result in decreased herbicidal activity for hard-to-kill weeds, the idea of reduced rates should be considered more seriously in geographic regions (such as coastal California) where the prolonged period of yellowing turf produced by sulfonylureas used at the full labeled rate may be unacceptable.  It may be that a slight loss in herbicidal activity would be a small price to pay if a significant reduction in turf yellowing accompanied it.
• Overall, the best combination of results (increased bermudagrass stand with the smallest duration and least severe yellowing of turf) was produced when applications were timed at bermudagrass cover greater than 50%, when average air temperatures were 65F or higher, and/or when average soil temperatures (6” depth) were 70F or higher.  The data suggests the possibility that lowered product rates may help to reduce the period of turf yellowing while still providing acceptable ryegrass control (see above). These conclusions apply to the Southern California coastal climate under which this trial was conducted, and may not apply in other locations with different weather patterns.
• Kerb is the current commercial standard for ryegrass removal.  It produced a more gradual transition to bermudagrass than either Revolver or Monument.  However, Kerb has the undesirable features of inconsistent performance (perhaps due to its inactivation in high organic matter [>4%] soils) and significant soil mobility.
• Variability on the golf course can produce varying results with sulfonylurea herbicides. In areas where bermudagrass cover is low and the percentage of cool season turf is high (greater than 50%), such as shaded, cool or wet areas, rye removal products are probably not a viable commercial option, unless long periods (8 weeks or more) of yellowing turf can be tolerated.  Likewise, during cool summers, bermudagrass cover may never reach the critical level to trigger herbicide applications.  Finally, the presence of common bermudagrass, which is an inherently slower growing turf than hybrid bermudagrass, will also result in a longer time period after application during which bare areas will persist.
• For all of the reasons above, the benefits of sulfonylurea herbicides for rye removal on overseeded fairways in coastal California remain mixed, since the risk of yellowing fairways for 5 weeks or more may outweigh the benefits of improved bermudagrass stands.  The decision to use these products will depend on the golf course and the situation:
• For golf courses that do not overseed, the benefits of sulfonylurea herbicides for control of rye, poa and other weeds are obvious and non-controversial.
• For golf courses that are committed to improved bermudagrass stands and are willing to accept yellowing turf during June and July, these products have a definite benefit, especially if a multi-year commitment (see Project II below) to this process is made.
• For golf courses that are looking for a quick and painless way to remove ryegrass from overseeded fairways, the benefits of sulfonylurea herbicides are less clear.  The rye can definitely be removed quickly by all of the products tested, but some pain will always be involved in the form of extended periods of yellowing turf.

Printable version of full report

Principal Investigators: Wendy Gelernter, Ph.D. and Larry J. Stowell, Ph.D., CPPP, CPAg

Cooperator:  Kevin Kienast, Morgan Run Golf Resort

Sponsors:  Chris Olsen, Bayer; Dean Mosdell, Syngenta