Kentucky Bluegrass Turf Research Articles

Seasonal Factors Affecting the Control of Annual Bluegrass (Poa annua L.) with Xanthomonas campestris pv. poae (JT-P482)

Abstract To ascertain the optimum timing for the application of Xanthomonas campestris pv. poae to control annual bluegrass ( Poa annua ), the differences in the efficacy of treatments initiated in consecutive months were investigated. October treatment and February through April treatments produced acceptable levels of fresh weight (FW) loss in annual bluegrass the following spring in pot experiments set in the field. More than 75% cumulative seedhead suppression was achieved by the October, November, and February treatments. Bacterial populations in annual bluegrass plants increased to 10 9 CFU/g of FW in the case of all application timings but the best control was obtained when maximum bacterial numbers were reached within 1 month of treatment. The differences in efficacy caused by different application timings (fall or spring) were compared in different turf fields. In two Kohrai ( Zoysia matrella ) turfs and in Kentucky bluegrass ( Poa pratensis ) turf, the fall and spring treatments were highly effective. However, in bentgrass ( Agrostis palustris ) turf, control of annual bluegrass was slower. October and/or March treatments effectively inhibited seedhead production, but April treatment was too late to prevent seed scattering. Taking into account late-emerging annual bluegrass, the optimum strategy for timing of treatments is application in early fall to obtain significant fresh weight loss and seedhead suppression or application in early spring, just before seed ripening, to obtain seedhead suppression and to control late-emerging plants.

Pendimethalin and Corn Gluten Meal Combinations to Control Turf Weeds

Concerns over ground water contamination and other environmental issues have prompted research to reduce synthetic herbicide use. Corn (Zea mays L.) gluten meal, a by‐product of wet milling, inhibits germination and the establishment of many annual and perennial weed species. Our objective in the field was to investigate crabgrass control (Digitaria spp.) in Kentucky Bluegrass (Poa pratensis L.) turf established on a Nicollet soil (fine‐loamy, mixed, mesic Aquic Hapludoll). In the greenhouse, we investigated the control of large crabgrass [Digitaria sanguinalis (L.) Scop.] on the same soil with no turfgrass cover. Experiments were conducted with corn gluten meal at 0, 49, 98, and 147 g m−2 combined in a factorial arrangement with pendimethalin [N‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitrobenzenamine] at 0, 29, 59, 88, and 117 mg ai m−2. The minimum label rate for pendimethalin is 176 mg ai m−2. Field crabgrass control was improved by application of a sublethal rate of pendimethalin in addition to corn gluten meal. As the application rate of corn gluten meal increased from 49 to 147 g m−2, the level of pendimethalin required to provide 75 to 85% control for 15 wk decreased from 88 to 29 mg ai m−2. There was no increase in crabgrass control in plots that received amounts of corn gluten meal and pendimethalin exceeding these combined rates.

Effects of slow-release fertilizers on growth and on uptake and leaching of nutrients in Kentucky bluegrass turfs established on sand-based root zones

Sports fields constructed on sand-based root zones are generally permeable to water and nutrients, and potential contaminants of the groundwater. This study was conducted to determine the effect of slow-release and water-soluble fertilizers on growth, nutrient uptake and leaching from a sand-based Kentucky bluegrass (Poa pratensis L.) turf. Two root zone mixtures and two N rates were investigated in two independent lysimeter experiments. In the first experiment (1) the 80:20 sand-peat mixture gave better growth and darker-colored turf than the 60:40 sand-peat mixture. Only RC-NPK and water-soluble NPK treatments in this experiment gave acceptable turf quality for the entire growing season. Frequent application with water-soluble fertilizer or spring application with slow-release carriers gave generally <5 mg L−1 NO3-N in the leachate. In the second experiment (2) application of 300 or 450 kg N ha−1 (either as spring or spring/summer application) as urea-ammonium-nitrate (UAN) (gel) did not give desirable turf growth during the entire period of investigation. Sulfur-coated urea gave generally better turf quality than UAN, but not as good as biweekly compound NPK fertilization. Irrespective of fertilizer N rate, the leaching losses of N varied from 1.1 to 2.9%, giving average NO3-N concentrations in the leachate for all treatments <2 mg L−1. Sandy soil used for sports turf, and fertilized with SCU and RC-NPK, or frequent and small amounts of water-soluble N carriers, appears not to create any serious environmental concern in southern part of Norway where the rainfall is moderate. Key words: Kentucky bluegrass, leaching, nitrogen, nutrient uptake, sand-based rootzone, slow-release fertilizers

Leaching and Mass Balance of 15N‐Labeled Urea Applied to a Kentucky Bluegrass Turf

The fate of urea applied to Kentucky bluegrass (Poa pratensis L.) turf was studied over a 2‐yr period using a combination of intact monolith lysimeters and small plots. Soil type was a Marlette fine sandy loam (fine‐loamy, mixed mesic Glossoboric Hapludalfs). Urea was applied at a rate of 196 kg N ha−1 yr−1 in five equal applications of 39.2 kg N ha−1, using two application schedules. Treatments were fertilized at approximately 38‐d intervals with the “Spring” treatment fertilized from late April through late September and the “Fall” treatment from early June through early November. In 1991 only, the April and November applications used 15N‐labeled urea (LFN). For the Spring treatment, 31% of LFN was recovered from thatch at 18 DAT. This value remained constant for the next year, then gradually declined to 20% after 2 yr. Only 8% of the LFN was recovered from soil at 18 DAT and increased to only 20% 2 yr after application. Approximately 35% of the LFN was harvested in clippings over 2 yr. Through May 1993 (748 DAT), LFN in leachate totaled 0.18% of the amount applied. For the Fall treatment, 62% of the LFN was recovered from thatch at 18 DAT. This value declined to 35% by the following June. LFN in soil increased from 12% to25% over 2 yr. Approximately 38% of the LFN was harvested in clippings over 2 yr. Total leachate LFN recovery was 0.23% over the 2‐yr period. Total recovery of LFN was 64 and 81% for the Spring and Fall treatments, respectively, suggesting volatile losses of N. Whether the N was applied in the spring or late fall, rapid uptake and immobilization of the LFN resulted. A maximum of 25% of applied LFN was recovered in the soil from either application timing at any time over the 2 yr of the experiment. A well‐maintained turf intercepts and immobilizes LFN quickly making leaching an unlikely avenue of N loss from a turf system.

Ground Ivy (Glechoma hederacea L.) Control in a Kentucky Bluegrass Turfgrass with Borax

Abstract Field experiments were conducted to evaluate ground ivy (Glechoma hederacea L.) control and Kentucky bluegrass (Poa pratensis L.) tolerance to Twenty Mule Team Borax, Super Trimec, and Sharpshooter applications. Ground ivy control improved with time in 1991 whereas the inverse was true in 1992. Liquid borax at 610 g/100m2 and Super Trimec at 560 ml/100m2 controlled ground ivy consistently (≥85%) during 1991 and 1992. Ground ivy control with dry borax was not as good as liquid borax when each application was applied at 305 g/100m2. Unacceptable ground ivy control occurred with Sharpshooter. All treatments caused turfgrass injury in 1991. At 4 weeks after treatment (WAT), injury increased to 30 and 40% for the 305 and 610 g/100 m2 (10 and 20 oz/1000 ft2) liquid borax treatments, respectively. No turfgrass injury however was observed the following spring. In 1992, only Sharpshooter caused injury 2 WAT. Weather differences after the applications contributed to the variability in ground ivy control and turfgrass injury between years. These results indicate that boron can be used to selectively control ground ivy in a Kentucky bluegrass turf with a 610 g/100 m2 (20 oz/1000 ft2) liquid borax application with only temporary turfgrass injury.

Movement of Pesticides under Two Irrigation Regimes Applied to Turfgrass

AbstractCurrent public concern has focused attention on the environmental effects of chemical applications to turfgrass areas. Our objective was to investigate the movement of isazofos ([O‐[5‐chloro‐1‐(1‐methylethyl)‐1H‐1,2,4‐triazol‐3‐yl]O,O‐diethyl phosphorothioate]), chlorpyrifos ([0,0‐diethyl‐0‐(3,5,6‐trichloro‐2‐pyridinyl) phosphorothioate]), metalaxyl [N‐(2,6‐dimethylphenyl)‐N‐(methoxyacetyl) alanine methyl ester], and pendimethalin [N‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitrobenzenamine] when applied to Kentucky bluegrass turf established on 50‐cm long and 20‐cm diameter undisturbed soil columns of Nicollet (fine‐loamy, mixed, mesic Aquic Hapludolls) in which the macropore system remained intact. This study was conducted for 28 d and used a heavy (four, 2.54‐cm applications) and a light (16, 0.64‐cm applications) irrigation regime. Average isazofos, chlorpyrifos, metalaxyl, and pendimethalin recovered (by extraction) from soil columns under the heavy irrigation regime, plus that collected in the leachate, were 8.8, 11.5, 23.8, and 7.9%, respectively. Isazofos, chlorpyrifos, metalaxyl, and pendimethalin recovery from soil columns under the light irrigation regime, plus that collected in the leachate, averaged 3.4, 6.6, 13.9, and 4.6%, respectively. On average, 6.3, 0.5, 7.7, and 0.2% of the applied isazofos, chlorpyrifos, metalaxyl, and pendimethalin, respectively, were measured in leachate from soil columns under heavy irrigation in contrast to 0.4, 0.0, 0.2, and 0.0%, respectively, from soil columns under light irrigation. Comparing the four applied pesticides, metalaxyl is most likely to be leached because of a low soil sorption, followed by isazofos, pendimethalin, and chlorpyrifos. Metalaxyl was found in the highest percentage in the leachate followed by isazofos, pendimethalin, and chlorpyrifos. From this research, we conclude that irrigation practices (frequency and depth of application) can have an impact on the movement of pesticides through soil profiles.

Pesticide Dissipation under Golf Course Fairway Conditions

The degradation and vertical movement of pendimethalin, chlorpyrifos, isazofos, and metalaxyl were monitored in Kentucky bluegrass (Poapratensis L.) turf managed as a golf course fairway. Intact turf‐soil cores from Mead, NE, on a Sharpsburg soil (fine montmorillonitic, mesic Typic Argiudoll) and Gilbert, IA, on a Nicollet loam (fine‐loamy, mixed, mesic Aqnic Hapludoll) planted to Kentucky bluegrass were periodically removed to 60‐cm depth through 113 d after pesticide application. Cores were sectioned into verdure, thatch, and multiple soil depths. While verdure contained high pesticide concentrations after application, precipitation, and clipping, and degradation reduced the amount in plant tissue with time. Thatch was highly retentive of the pesticides, generally containing the most residue throughout the monitoring period. Pesticide residues tended to be lower in soil at the Iowa site where more thatch was present. Little chlorpyrifos or pendimethalin moved through the thatch to the underlying soil. Isazofos was more mobile and metalaxyl moved through the entire soil column. Soil contained an average of 58% less pesticide than thatch over all sampling times, and concentrations in soil were highest at the 0‐ to 5‐ and 5‐ to 10‐cm depths. Average time for 50% dissipation in the turf‐soil profile (DT50) was 16, 12, 10, and 7 d for metalaxyl, pendimethalin, chlorpyrifos, and isazofos, respectively. The pesticides appeared to degrade more rapidly in the turfgrass environment than typically reported for other agronomic cropping systems. Variability in pesticide residue concentrations for each soil depth among the turf‐soil cores indicated non‐uniform dissipation in the field.

Cultural Practices Affect Root-Feeding White Grubs (Coleoptera: Scarabaeidae) in Turfgrass

Cultural practices were manipulated before or during seasonal flights of Japanese beetle, Popillia japonica Newman, and masked chafers, Cyclocephala spp., to study effects on grub densities in tall fescue or Kentucky bluegrass turf. Masked chafer grubs were consistently smaller and less abundant in turf that had been treated with aluminum sulfate to reduce soil pH and in high-mown turf. High mowing or application of aluminum sulfate before beetle flights reduced total biomass of white grubs in tall fescue by as much as 55 and 77%, respectively. However, where spatial gradients in soil moisture occur, the positive response of grub populations to moisture may override effects of those treatments. Grub densities were not affected by spring applications of lime or urea or by aerification of plots before beetle flights. Plots that were irrigated during beetle flights incurred significantly higher densities of both P. japonica and Cyclocephala spp. grubs than did nonirrigated turf. The use of a 2,247- kg roller to compact the soil before beetle flights did not affect subsequent grub populations, and the roller was not effective for remedial control of 3rd instars in the fall. Application of organic fertilizers (composted cow manure or activated sewage sludge) resulted in significant increases in grubs of green June beetle, Cotinis nitida L., in 1 of 2 yr. Cultural practices may have general or species-specific effects on densities of white grubs. This study suggests that witllolding irrigation during peak flight of beetles, raising cutting height, and light application of aluminum sulfate in spring may help to reduce the severity of subsequent grub infestations.

Fate of Nitrogen Applied to Turfgrass-Covered Soil Columns

Current public concern for the environment has focused on the environmental effects of chemical applications to turfgrass areas. There is little research on the environmental effects of nitrogen (N) applied to turfgrasses. Our objectives were to investigate the hydrology of 50 cm of undisturbed soil columns with a Kentucky bluegrass turf and intact macropores under a heavy (four 2.54 cm applications) and a light irrigation regime (sixteen 0.64 cm applications), and to measure the fate of N, using 15 N as a tracer, when applied to an undisturbed soil column. Mean leachate values for columns under the heavy irrigation regime totaled about six times the amount collected from columns under the light irrigation regime. We found that the heavy irrigation increased N, which leached through the 50 cm of undisturbed soil columns by 30 times and decreased the volatilization of liquid urea compared with columns under the light irrigation.

Fate of amended urea in turfgrass biosystems

Abstract The fate of nitrogen (N) has been studied under several agronomic crops and agricultural profiles, but relatively little information has been collected from areas managed as turfgrass. The turfgrass industry has become the focus of environmental concerns in recent years and is often identified as a source of ground water contaminate. The objectives of this study were to: i) investigate the hydrology of 20‐cm diameter by 50‐cm deep undisturbed soil columns covered with a Kentucky bluegrass (Poa pratensis L.) turf under a heavy (one 2.54‐cm application) and a light (four 0.64‐cm applications) irrigation regime, and to ii) quantify the fate of 15N‐labeled urea when it is applied to an undisturbed soil columns having intact macropores. Clipping, verdure, and thatch/mat samples were taken from each column, and the soil was excavated in 10‐cm layers at the end of the 7‐day test period. A glass collection chamber was used to collect volatilized N and a plastic bag for leachate collection. All samples we...

Application of Prodiamine to Immature Turfs

Turf managers use prodiamine to control annual grasses in mature turfs. Earlier, evaluations of prodiamine have indicated it has an elevated soil persistence and high activity on susceptible species. This persistence raises concern for prodiamine's effect on juvenile turf. An experiment was established to evaluate the effect of prodiamine on juvenile Kentucky bluegrass turf. Significant loss of quality occurred on turf less than one year old receiving a single application of prodiamine at ≥ 1.4 kg/ha. Split applications of prodiamine did not significantly decrease turf quality. Prodiamine at 0.8 and 2.2 kg ai/ha and DCPA at 23.5 kg ai/ha, applied in the fall the previous three years, significantly reduced the number of overseeded Kentucky bluegrass seedlings two years after planting. In a subsequent study, prodiamine at ≥ 0.8 kg/ha and DCPA at 23.5 kg/ha applied once the previous fall significantly reduced the number of overseeded Kentucky bluegrass dlings 56 d after planting. An analysis of prodiamine rate versus seedling density was found to be significantly correlated (r2= 0.79).

Herbicide Programs for Large Crabgrass and Goosegrass Control in Kentucky Bluegrass Turf

Low rates of preemergence herbicides were applied with postemergence herbicides as sequential applications or as tank-mixed applications for large crabgrass [Digitaria sanguinalis (L.) Scop.] and goosegrass [Eleusine indica (L.) Gaertn.] control in Kentucky bluegrass (Poa pratensis L.). In 1991, a higher percentage of large crabgrass control (83%) was obtained in late August from sequential applications of oxadiazon at 2.2 kg·ha-1 applied in early March and followed by fenoxaprop at 0.20 kg·ha-1 in June than when oxadiazon (62%) or fenoxaprop (11%) was applied alone. During this period, sequential applications of oxadiazon at 2.2 kg·ha-1 and monosodium salt of MAA (MSMA) at 2.2 kg·ha-1 achieved 79% control of large crabgrass, while the control was at ≤46% when MSMA or fenoxaprop was applied to plots previously treated with pendimethalin at 1.7 kg·ha-1 or dithiopyr at 0.28 kg·ha-1. There was no advantage from sequential applications of preemergence and postemergence herbicides for large crabgrass control in 1992, when compared with postemergence herbicides applied alone. Large crabgrass control in late August was higher over 2 years from tank-mixes of pendimethalin at 1.7 kg·ha-1 with fenoxaprop at 0.14 kg·ha-1 (80%) than when pendimethalin (28%) or fenoxaprop (55%) was applied alone at the same rates. The control was also higher from tank-mixes of pendimethalin at 1.7 kg·ha-1 with MSMA at 2.2 kg·ha-1 (92%), when compared with MSMA (77%) alone. The response was similar from tank-mixes of dithiopyr at 0.28 kg·ha-1 with fenoxaprop at 0.14 kg·ha-1 (98%) or with MSMA at 2.2 kg·ha-1 (98%), when compared with dithiopyr applied alone (81%) at the same rate. Quinclorac applied alone at 0.84 kg·ha-1 controlled large crabgrass as effectively as when applied to plots previously treated with preemergence herbicides or when applied tank-mixed with preemergence herbicides. None of the preemergence herbicides applied at low rates with postemergence herbicides (fenoxaprop, MSMA, or quinclorac) controlled goosegrass. Chemical names used: S,S-dimethyl 2-(difluoromethyl)-4-(2-methylpropyl)-6-(trifluoromethyl)-3,5-pyridinedicarbothioate (dithiopyr); (±)-2-[4-[(6-chloro-2-benzoxazolyl)oxy]-phenoxy]propanoic acid (fenoxaprop); 3-[2,4-dichloro-5-(1-methylethoxy)-phenyl]-5-(1,1-dimethylethyl)-1,3,4-oxadiazol-2-(3H)-one (oxadiazon); N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzen-amine (pendimethalin); and 3,7-dichloro-8-quinoline-carboxylic acid (quinclorac).

Toxicity of pesticides to earthworms in Kentucky bluegrass turf.

Toxicity of pesticides to earthworms in Kentucky bluegrass turf.

High Phosphorus Applications Influence Soil-available Potassium and Kentucky Bluegrass Copper Content

The objectives of this study were to determine the effects of high P applications on `Baron', Kentucky bluegrass (Poa pratensis L.) turf quality, chlorophyll content, soil test levels of P and K, and foliar nutrient concentration. In this 5-year field study, P was applied at 0, 22, 43, 86, 172, or 258 kg·ha-1·year-1 using triple superphosphate (210 g P/kg) in single-applications in May. Phosphorus applications did not affect overall yearly quality, chlorophyll content, or soil pH, but increased available soil P and reduced available soil K and Cu concentration in clippings.

Control of Annual Bluegrass (Poa annua) in Kentucky Bluegrass (Poa pratensis) Turf with Linuron

Effects of linuron on annual bluegrass control and Kentucky bluegrass cultivar tolerance were studied in field and growth chamber experiments. In controlled environment experiments, linuron at 0.06, 0.12, 0.25, 0.50, and 0.75 kg ai ha-1 was applied to pure stands of annual bluegrass and eight Kentucky bluegrass cultivars. Linuron at the two highest rates controlled annual bluegrass, reducing the clipping dry weight by more than 85% 4 wk after treatment, and by 65 to 92% 6 wk after treatment. Growth of Kentucky bluegrass was reduced with the most severe reduction occurring 2 wk after linuron application. All cultivars exhibited normal growth 8 wk after treatment. In field experiments, linuron at rates from 1.5 to 2.0 kg ai ha-1 controlled annual bluegrass in old (> 5 yr) Kentucky bluegrass stands, and in 16 cultivars of 1-yr and 2-yr-old Kentucky bluegrass stands, with little or no damage. At rates of 1.5, 2.0, and 2.5 kg ai ha-1 linuron, damage to newly seeded cultivars was moderate to severe. However, 6 to 7 wk after linuron application to newly seeded cultivars, stand density and turf quality were equivalent to untreated checks.

Evaluation of Urease Inhibitors as Urea Amendments for Use on Kentucky Bluegrass Turf

Urea applied to turf areas can result in loss of urea N through NH3 volatilization. One approach to decrease N loss is the use of urease inhibitors. The objectives of this study were to evaluate the urease inhibitors N‐(n‐butyl) thiophosphoric triamide (NBPT) and phenylphosphorodiamidate (PPD) as fertilizer amendments for increasing N‐use efficiency of urea applied to Kentucky bluegrass (Poa pratensis L.) turf in both the laboratory and the field. A forced‐draft system used to measure NH3 volatilization in the laboratory incorporated four levels of NBPT and three levels of PPD combined with 49 kg urea N ha−1. Cumulative loss of urea N as NH3 without inhibitors was 49.9% over 7d. Percentage reduction of NH3 volatilization in 7 d was 34.4, 46.5, and 50.1% for 1, 2, and 3% (w/w) PPD treatments, respectively; and 41.8, 50.7, 54.2, and 59.2% for 0.125, 0.25, 0.5, and 1.0% (w/w) NBPT treatments, respectively. A 2‐yr field experiment was also conducted that measured turf response to urea N at 0 to 73.5 kg ha−1 with NBPT at 0, 0.25, and 0.5% of the weight of N. There was little quality or clipping yield response to NBPT in the 1988 and the 1989 field trials. When responses were measured, they ranged from negative, due to tip burn, to a positive clipping response at the high rate of NBPT late in the 1989 season. More work will be necessary to further assess the usefulness of urease inhibitors for reduction of NH3‐N loss under field conditions.

Impact Absorption Characteristics on Turf and Soil Surfaces

AbstractPlaying quality of athletic fields is critical to an athlete from the safety and performance standpoints. Use intensity and maintenance practices affect playing surface quality and impact characteristics within and among fields. The objective of this study was to quantitatively assess the effects of cutting height, compaction, soil water content (date), and turf cover on the impact absorption characteristics of Kentucky bluegrass (Poa pratensis L.) turf. Treatments were four cutting heights (0 [bare soil], 19, 38, and 57 mm), and two compaction levels (with and without) for Exp. I, and three cutting heights (19, 38, and 57 mm), two compaction levels, and three surface types (full turf, no verdure, and no thatch) in Exp. II. Surface impact characteristics measured were peak deceleration, time to peak deceleration, and impact duration. Two hammer weights, 2.25 kg and 0.5 kg, were used to assess impact characteristics on four dates and to obtain data for soil water conditions of 0.263, 0.197, 0.163, and 0.064 kg kg−1. The highest peak deceleration values and shortest time periods were associated with the periods of low soil water and compacted conditions. Only the 0.5‐kg hammer detected differences in impact absorption characteristics between cutting heights. Significant cutting height‐by‐compaction‐ by‐date interactions occurred for each impact characteristic, indicating interdependence of one of the factors on another. It appears that field management practices that influence soil water content, soil compaction, and turf cover are more important than cutting height in altering the impact absorption capability of Kentucky bluegrass turf.

Turf Response to Urease Inhibitors and Cationic Materials Applied in Combination with Urea

We evaluated the response of Kentucky bluegrass (Pea pratensis L.) turf to urea amended with the urease inhibitors PPD, NBPT, and ATS and with the cations K+ (KCl) and Mg+2 (MgCl2). Treatments for the 2-year field experiment included liquid urea applied monthly in June to Sept. 1985 and 1986 at 49 kg N/ha with PPD (1%, 2%, 3% by weight of applied N), NBPT (0.5%, 1%, 2%), ATS (5%, 15%, 25%), K+ (5%, 15%, 25%), and Mg+2 (5%, 15%, 25%). The NBPT was included only in the 1986 field study. The Mg+2 and K+ reduced foliar burn and increased turf quality during mid- and late Summer 1985 at the 5% rate, but clipping yield was not affected by any treatment. In 1986, under milder climatic conditions, PPD and NBPT increased clipping yield by 13.2% and 15.2%, respectively. At the 15% rate, ATS increased clipping yield by 15.1%, but, on average, PPD and NBPT were much more effective. Chemical names used: phenylphosphorodiamidate (PPD), N-(n -butyl) thiophosphoric triamide (NBPT), and ammonium thiosulfate (ATS).

Pendimethalin Dissipation in Kentucky Bluegrass Turf

Pendimethalin dissipation was studied following annual 1.7 kg ai ha–1applications to 3-yr-old Kentucky bluegrass turf growing on a Sharpsburg silty clay loam soil and an 85/15 by volume sand/Sharpsburg soil mixture in 150-cm-deep rhizotron containers. Plant tissue, thatch, and soil were sampled periodically between application and 168 days after treatment (DAT). Soil and leachate were collected to monitor pendimethalin movement. Most of the herbicide appeared to remain within the turfgrass system. Pendimethalin concentration was highest in plant tissue and thatch. The 4-hydroxymethyl pendimethalin metabolite was detected in turfgrass tissue up to 42 DAT. No pendimethalin was detected at the 30-, 60-, or 120-cm depths in the rhizotron containers. Traces (≤0.003 mg kg–1) of pendimethalin detected in rhizotron leachate collected between 6 and 14 days after heavy rainfall (88 and 95 DAT, respectively) were attributed to gravitational displacement of soil colloids containing adsorbed herbicide. Pendimethalin application to established turfgrass would not appear to pose a high risk of groundwater contamination.

Kentucky Bluegrass Recovery of Urea-Derived Nitrogen-15 Amended with Urease Inhibitor

Urea is a commonly used N fertilizer on turfgrass areas. Urease inhibitors represent a possible method of reducing volatile loss of NH3 from treated turf. In this study, the recovery of urea-N by a Kentucky bluegrass (Poa pratensis L.) turf was determined in the field by using a 15N tracer. The urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) also was evaluated as a fertilizer amendment to increase urea-N efficiency. The study was conducted on a Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll) soil. Treatments included urea labeled with 5% 15N applied at the rate of 49 kg ha−1 with NBPT at 0 (urea alone), 0.25, and 0.5% of the weight of N. Grass clippings were collected weekly for 5 wk. Shoots, thatch, and rootzone samples were collected at termination. Total urea-derived N recovered from 5 wk of clippings ranged from 7.5% from the areas treated with urea alone to 8.1% in the urea + 0.5% NBPT treated areas. The 0.25% NBPT treatment increased N recovery in the 0 to 7.5 cm depth of soil from 13.6 (urea without NBPT) to 22.4%. This NBPT rate also increased N recovery from the whole soil-plant system from 28.8 (urea without NBPT) to 45.0%, whereas the 0.5% NBPT rate either had no effect or numerically decreased N recovery. The increased N recovery at the low rate of NBPT was probably due to a reduction in NH3 volatilization. The reducing effect of the higher NBPT treatment may have been due to delayed urea hydrolysis and to subsequent increased free-urea loss below the rooting zone after an unusual 130 mm rain during the fifth and sixth days after treatment.