Cool-Season Turfgrass Reseeding Intervals for Sulfosulfuron

Darren W. Lycan and Stephen E. Hart, Program Associate and Assistant Professor, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901

Corresponding author: Darren W. Lycan. lycan@aesop.rutgers.edu

Abstract

Turf managers may wish to reseed turfgrass species into areas where weeds have been eliminated by sulfosulfuron treatments. Field studies were conducted in 2002 and 2003 in New Jersey to determine when creeping bentgrass (Agrostis stolonifera L.), Kentucky bluegrass (Poa pratensis L.), and perennial ryegrass (Lolium perenne L.) can be safely reseeded after a sulfosulfuron application. Sulfosulfuron at 34 or 67 g ai/ha was applied 6, 4, 2, or 1 week before seeding (WBS). Data pooled across sulfosulfuron rates revealed ground cover of creeping bentgrass, Kentucky bluegrass, and perennial ryegrass at 3 weeks after seeding (WAS) was reduced to 41, 30, and 70% of the untreated by applications 1 WBS, respectively. Our study suggests that perennial ryegrass can be safely reseeded two weeks after a sulfosulfuron application, whereas creeping bentgrass and Kentucky bluegrass may require two to four weeks before reseeding can occur.

Introduction

Sulfosulfuron belongs to the sulfonylurea herbicide family and inhibits acetolactate synthase (ALS, EC 2.2.1.6), a key enzyme in the branched-chain amino acid synthesis pathway (7). Sulfosulfuron has activity on various weeds of turfgrass including annual bluegrass (Poa annua L.) (9), roughstalk bluegrass (Poa trivialis L.) (10,11), and purple nutsedge (Cyperus rotundus L.) (6). Single, fall applications of sulfosulfuron at 34 g/ha reduced annual bluegrass cover in Kentucky bluegrass by 58% by the following spring (9). Other field research demonstrated that four spring applications of sulfosulfuron at 11 g/ha reduced roughstalk bluegrass cover by 74% nine weeks after initial application (10). Kentucky bluegrass and perennial ryegrass have demonstrated adequate tolerance to single applications of sulfosulfuron at 6 to 34 g/ha in field trials (8). Creeping bentgrass appears to be less tolerant to applications of sulfosulfuron than Kentucky bluegrass, and thus multiple applications of sulfosulfuron have the potential to selectively reduce populations of creeping bentgrass within Kentucky bluegrass stands (5).

Interseeding turf areas with a desired turf species is a common practice where new seed is introduced into a mature stand. Turf managers may interseed into mature turf stands attempting to introduce new cultivars with improved traits without completely renovating the area or to repair areas where turf is thinned due to abiotic or biotic stresses. Turf managers or sod growers may also wish to seed a desired species such as creeping bentgrass, Kentucky bluegrass, or perennial ryegrass to maintain adequate turfgrass density and uniformity in areas where weeds have been removed by sulfosulfuron applications. In addition, some golf course superintendents may choose to utilize perennial ryegrass in tee boxes to facilitate divot repair. Perennial ryegrass can rapidly fill in damage from golfers due to its rapid germination and emergence rate (3). Superintendents may have the option to utilize sulfosulfuron for weed control on tees, but it is not known if perennial ryegrass establishment is affected by previous sulfosulfuron applications.

Sulfosulfuron is currently labeled for weed control in perennial grasses native to the United States such as big bluestem (Andropogon gerardii Vitman), buffalograss (Buchloe dactyloides (Nutt.) Engelm.), and switch grass (Panicum virgatum L.) at rates ranging from 45 to 101 g/ha (2). The label stipulates that reseeding of approved grasses can occur no sooner than 14 days after the last sulfosulfuron application. Sulfosulfuron is also labeled for use in wheat (Triticum aestivum L.) (1). Establishment and development of grain sorghum (Sorghum bicolor (L.) Moench) and corn (Zea mays L.) are negatively impacted when seeded into areas previously treated with sulfosulfuron before adequate time has elapsed (4,12). Consequently, current herbicide labels for sulfosulfuron use in non-crop sites and wheat contain restrictions on reseeding various species into sulfosulfuron-treated areas (1,2). Therefore, it is important to understand the response of cool-season turfgrass species such as creeping bentgrass, perennial ryegrass, and Kentucky bluegrass when seeded into areas previously treated with sulfosulfuron. The objectives of this study were to determine the delay in reseeding of Kentucky bluegrass, perennial ryegrass, or creeping bentgrass after a broadcast sulfosulfuron application.

Seeding Cool-Season Turfgrass into Sulfosulfuron-Treated Plots

Field studies were conducted in 2002 and 2003 at the Rutgers Plant Science Research Center in Adelphia, New Jersey. Soil type was a Holmdel sandy-loam (fine-loamy, mixed, active, mesic Aquic Hapludult) with a pH of 6.5 and organic matter content of 2%. Sulfosulfuron (MON 44951, 75% active ingredient, water dispersible granule; Monsanto Co., St. Louis, MO) was applied at 34 or 67 g ai/ha one, two, four, or six weeks before seeding (WBS) (Table 1) to a sward of ‘Midnight’ Kentucky bluegrass with >90% ground cover maintained at a 4-cm mowing height. Treatments were applied to the interior 2 by 4 m of 3-by-4-m plots with a four-nozzle CO₂ backpack sprayer system utilizing 11004 DG nozzle tips (TeeJet, Spraying Systems Co., Wheaton, IL) which delivered 374 liters/ha of spray solution at 276 kPa. Non-ionic surfactant (MON 0818, Monsanto Co., St. Louis, MO) was added to all treatments at 0.25% vol/vol. Within one week after each application the test site was irrigated with approximately 2.5 cm of water. Regular maintenance mowing continued as needed with a reel mower and clippings were returned. The entire test area was treated 1 WBS with 2.2 kg ai/ha of glyphosate (Roundup Pro, Monsanto Co., St. Louis, MO) to kill all existing vegetation to facilitate evaluation of stand establishment of seeded turf species. Plots were seeded with ‘L-93’ creeping bentgrass at 24 kg/ha, ‘Kenblue’ Kentucky bluegrass at 98 kg/ha, and ‘Pizzazz’ perennial ryegrass at 244 kg/ha on September 6, 2002 and September 12, 2003. A commercially available slicer/seeder (Lesco Renovator, Lesco, Inc., Cleveland, OH) was used for the seeding operation. Seeding depth was set at 0.4 cm and seeding swath was 0.5 m wide. Two seeding swaths of each species were made across each plot. The entire test site received 163 kg/ha of 15:15:15 (N:P₂O₅:K₂O) fertilizer after seeding and adequate irrigation was applied to ensure germination. At 3 and 7 weeks after seeding (WAS) and 7 months after seeding (MAS) percent ground cover of each seeded turf species was evaluated on a scale of 0 to 100 where 0 equals no emergence of seeded species and 100 equals complete cover by seeded species. Values were converted to percent of the untreated.

Table 1. Sulfosulfuron application and turfgrass seeding dates in 2002
and 2003.

Operation		2002	2003
Sulfosulfuron applications	6 WBS^x	July 26	August 1
	4 WBS	August 7	August 14
	2 WBS	August 23	August 28
	1 WBS	August 30	September 5
Seeding		September 6	September 12

^x WBS = weeks before seeding

Experiments were conducted as randomized complete block designs with four replications. Each species was analyzed separately to adequately determine an appropriate reseeding delay interval for each species. Data were subjected to analysis of variance (ANOVA) procedures and pooled across years when year by treatment interactions were not significant at the 5% probability level. Arcsine of square root transformation did not change the analysis; therefore, actual means are presented. No significant year by treatment interactions were detected by ANOVA procedures, and thus data for each species were pooled across years.

Creeping Bentgrass

Creeping bentgrass ground cover in untreated plots averaged 61 (± 5.8 standard error, SE) and 31% (± 8.2 SE) 3 WAS in 2002 and 2003, respectively (data not shown). The main effect of sulfosulfuron application timing was significant at each evaluation date, while sulfosulfuron rate and the interaction of timing and rate were not significant at any evaluation (Table 2). Data pooled across rates revealed sulfosulfuron applied 1 or 2 WBS substantially reduced creeping bentgrass ground cover 3 WAS as compared to applications 6 WBS. Ground cover in plots treated with sulfosulfuron 6 WBS was 82% of untreated; while applications 1 or 2 WBS reduced ground cover to 41 and 70% of untreated, respectively (Table 2). By 7 WAS, ground cover increased to 78 to 89% of untreated in plots treated 2 to 6 WBS. Density of creeping bentgrass continued to increase throughout fall and early spring and by 7 MAS ground cover in plots treated 2 to 6 WBS was 91 to 96% of untreated, while cover was only 78% of untreated when sulfosulfuron was applied 1 WBS.

Table 2. Creeping bentgrass ground cover 3 and 7 weeks and 7 months after seeding into sulfosulfuron-treated plots in 2002 and 2003^wx.

Main effect		3 WAS	7 WAS	7 MAS
Main effect		Creeping bentgrass ground cover^y (% of untreated)
Application timing	6 WBS	82 a^z	89 a	91 a
	4 WBS	79 ab	78 a	96 a
	2 WBS	70 b	80 a	92 a
	1 WBS	41 c	53 b	78 b
Sulfosulfuron rate	34 g ai/ha	70 a	79 a	88 a
Sulfosulfuron rate	67 g ai/ha	66 a	71 a	89 a
Timing		****	****	*
Rate		NS	NS	NS
Timing by rate		NS	NS	NS

^w Abbreviations: WAS = weeks after seeding; MAS = months after seeding; WBS = weeks before seeding; NS = not significant.

^x Seeding dates were September 6, 2002 and September 12, 2003.

^y Creeping bentgrass ground cover in untreated plots averaged 61, 73, and 87% in 2002 and 31, 53, and 75% in 2003 at 3 WAS, 7 WAS, and 7 MAS, respectively.

^z Means within a column and main effect followed by the same letter are not statistically different according to Fisher’s protected LSD test (0.05). * and **** signify P-values of <0.05 or <0.0001, respectively.

Kentucky Bluegrass

Kentucky bluegrass is slow to establish from seed (3). Therefore, Kentucky bluegrass ground cover averaged only 10 (± 1.4 SE) and 15% (± 2.0) in untreated plots 3 WAS in 2002 and 2003, respectively (data not shown). The main effects of sulfosulfuron application timing and rate were significant at each evaluation date, but the interaction of timing and rate were not significant at any evaluation (Table 3). Data pooled across rates revealed Kentucky bluegrass ground cover 3 WAS was 97% of untreated when sulfosulfuron was applied 6 WBS (Table 3). Ground cover was reduced to 74 and 79% of untreated by applications 2 and 4 WBS, respectively. Sulfosulfuron applied 1 WBS resulted in substantial reductions in Kentucky bluegrass cover 3 WAS (30% of untreated). Kentucky bluegrass ground cover in plots treated 2 to 6 WBS increased to 87 to 97% of untreated and 101 to 107% of untreated by 7 WAS and 7 MAS, respectively. Significant reductions in Kentucky bluegrass cover were still evident 7 WAS (56% of untreated) and 7 MAS (88% of untreated). Data pooled across application timings revealed sulfosulfuron at 67 g/ha reduced Kentucky bluegrass ground cover more substantially than 34 g/ha at each evaluation date (Table 3).

Table 3. Kentucky bluegrass ground cover 3 and 7 weeks and 7 months after seeding into sulfosulfuron-treated plots in 2002 and 2003^wx

Main effect		3 WAS	7 WAS	7 MAS
Main effect		Kentucky bluegrass ground cover^y (% of untreated)
Application timing	6 WBS	97 a^z	97 a	101 a
	4 WBS	79 b	90 a	107 a
	2 WBS	74 b	87 a	103 a
	1 WBS	30 c	56 b	88 b
Sulfosulfuron rate	34 g ai/ha	77 a	91 a	107 a
Sulfosulfuron rate	67 g ai/ha	63 b	74 b	92 b
Timing		****	***	*
Rate		**	**	**
Timing by rate		NS	NS	NS

^w Abbreviations: WAS = weeks after seeding; MAS = months after seeding; WBS = weeks before seeding; NS = not significant.

^x Seeding dates were September 6, 2002 and September 12, 2003.

^y Kentucky bluegrass ground cover in untreated plots averaged 10, 24, and 50% in 2002 and 15, 50, and 80% in 2003 at 3 WAS, 7 WAS, and 7 MAS, respectively.

^z Means within a column and main effect followed by the same letter are not statistically different according to Fisher’s protected LSD test (0.05). *, **, ***, **** signify P-values of <0.05, <0.01, <0.001, or <0.0001, respectively.

Perennial Ryegrass

Perennial ryegrass ground cover in the untreated averaged 53 (± 3.2 SE) and 34% (± 3.1 SE) 3 WAS in 2002 and 2003, respectively (data not shown). The main effect of sulfosulfuron application timing was significant at 3 and 7 WAS, whereas sulfosulfuron rate and interaction of timing and rate were not significant at any evaluation (Table 4). Perennial ryegrass ground cover 3 WAS was reduced to 70% of untreated by sulfosulfuron applied 1 WBS. All other application timings resulted in ground cover ranging from 90 to 93% of untreated 3 WAS (Table 4). By 7 WAS, perennial ryegrass ground cover in plots treated 1 WBS increased to 91% of untreated; however, this level was still lower than those exhibited by applications 4 or 6 WBS. Perennial ryegrass density continued to increase in plots treated 1 WBS; by 7 MAS there was no significant effect of application timing on perennial ryegrass ground cover (> 98% of untreated for all timings).

Table 4. Perennial ryegrass ground cover 3 and 7 weeks and 7 months after seeding into sulfosulfuron-treated plots in 2002 and 2003^wx.

Main effect		3 WAS	7 WAS	7 MAS
Main effect		Perennial ryegrass ground cover^y (% of untreated)
Application timing	6 WBS	90 a^z	97 a	98 a
	4 WBS	93 a	98 a	99 a
	2 WBS	92 a	95 ab	100 a
	1 WBS	70 b	91 b	98 a
Sulfosulfuron rate	34 g ai/ha	89 a	96 a	99 a
Sulfosulfuron rate	67 g ai/ha	84 a	95 a	99 a
Timing		****	**	NS
Rate		NS	NS	NS
Timing by rate		NS	NS	NS

^w Abbreviations: WAS, weeks after seeding; MAS, months after seeding; WBS, weeks before seeding; NS, not significant.

^x Seeding dates were September 6, 2002 and September 12, 2003.

^y Perennial ryegrass ground cover in untreated plots averaged 53, 84, and 80% in 2002 and 34, 88, and 89% in 2003 at 3 WAS, 7 WAS, and 7 MAS, respectively.

^z Means within a column and main effect followed by the same letter are not statistically different according to Fisher’s protected LSD test (0.05). ** and **** signify P-values of <0.01 or <0.0001, respectively.

Recommended Reseeding Intervals

These field studies demonstrate that sulfosulfuron applied 1 WBS can reduce ground cover of creeping bentgrass, Kentucky bluegrass, or perennial ryegrass by 30 to 70% compared to the untreated 3 WAS. Significant reductions in creeping bentgrass and Kentucky bluegrass cover were still evident 7 MAS; however, perennial ryegrass completely recovered from sulfosulfuron applied 1 WBS. The initial reductions from sulfosulfuron applied 1 WBS may prolong the time until full turf canopy and this may allow weedy species to establish or encroach due to decreased competition from the turfgrass. Sulfosulfuron applied 2 WBS reduced creeping bentgrass and Kentucky bluegrass cover 3 WAS compared to applications made 6 WBS; however, by 7 WAS cover was similar in plots treated 2 to 6 WBS. A significant rate effect was observed for Kentucky bluegrass data at each evaluation. Since ANOVA procedures did not reveal a significant interaction between application timing and rate, statistical separation of means of rates within application timing are not warranted in a factorial analysis. However, for discussion purposes it is interesting to note that Kentucky bluegrass cover 3 and 7 WAS was 87 and 93% of untreated, respectively, when sulfosulfuron was applied 2 WBS at 34 g/ha; however, the higher rate (67 g/ha) applied at the same timing resulted in cover that was only 61 and 80% of untreated 3 and 7 WAS, respectively (data not shown). This significant rate effect demonstrates that the higher rate of sulfosulfuron reduced Kentucky bluegrass ground cover more substantially than the lower rate. Due to the level of annual bluegrass activity displayed by sulfosulfuron at 34 g/ha in previous research (9), 34 and 67 g/ha were chosen as proposed 1× and 2× rates for the purposes of this study. However, recent product development has focused on controlling roughstalk bluegrass using single or sequential applications of sulfosulfuron at 11 g/ha (11), which will likely be the maximum labeled rate in cool-season turfgrass (Domingo C. Riego, personal communication). Therefore, due to the relatively rapid recovery of creeping bentgrass and Kentucky bluegrass from sulfosulfuron applied 2 WBS at 34 and 67 g/ha and the significant rate effect observed with Kentucky bluegrass, it is feasible that these two species could be reseeded earlier than 2 to 4 weeks after an application of sulfosulfuron at 11 g/ha with very little negative impact on establishment. Other research has demonstrated that reductions in the establishment of creeping bentgrass, Kentucky bluegrass, and perennial ryegrass due to preemergence herbicides were minimized when lower rates of dithiopyr, oxadiazon, and prodiamine were applied (13).

Our study suggests that turf managers could safely reseed perennial ryegrass two weeks after a sulfosulfuron application, while creeping bentgrass and Kentucky bluegrass may require a longer delay, 2 to 4 weeks, before seeding into areas treated with sulfosulfuron at 34 or 67 g/ha.

Acknowledgments

The authors thank Nola Bobsin and the farm staff at the Adelphia Research Center for their technical assistance and Monsanto Co. and the Rutgers Center for Turfgrass Science for partial funding of this research.

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