The objectives of this study were to determine the effects of mower adjustment and Primo 1EC on the quality of creeping bentgrass (Agrostis palustris). Mowing quality and Primo response was assessed using a visual rating of the overall turf quality, a measurement of the leaf tip shredding, leaf tissue chlorophyll content, and evolved ethylene.

This study was conducted at the Iowa State University Horticulture Research Station using ‘Penneagle’ creeping bentgrass established on a ¹/₃ peat, ¹/₃ sand and ¹/₃ Nicollet soil growth medium.

Toro Greensmaster 1000 walk behind reel mowers were utilized in the study. The mowers had a cutting width of 21 inches and were bench set to a cutting height of ¹/₂ inch. The mowers were sharpened and backlapped to Toro specifications at the factory prior to the initiation of the study.

Four different mower adjustments were examined in this study (Table 1). Mowers A and C were maintained with slight reel-to-bedknife contact for the duration of the trial. Additionally, Mower C was operated for eight hours prior to the initiation of mowing treatments. This was to simulate a slightly dull mower that should be backlapped to perform optimally. Mower A represented a recently serviced mower that was adjusted and maintained with slight reel-to-bedknife contact, per manufacturer specifications.

Mowers B and D were adjusted so that there was no reel-to-bedknife contact. Mower D was also operated for eight hours to mimic a mower that needed backlapping. Mower B characterizes an alternative mower adjustment prevalent in the golf course industry. Many mechanics adjust mowers with no reel-to-bedknife contact in order to extend the time between scheduled mower maintenance. It is assumed that since the reel is not making contact with the bedknife, friction and heat buildup will be reduced, allowing the bedknife to remain sharp for an extended period of time. Mower D was a slightly dull mower that was adjusted according to this philosophy.

The reel-to-bedknife contact of mowers A and C was adjusted as necessary during the study so that the mowers were capable of cutting 20-lb. office paper the entire length of the bedknife. Similarly, Mowers B and D were adjusted over the duration of the trial so that they would cut cardstock along the entire length of the bedknife. Mowing treatments were performed three times a week beginning June 8, 2000.

Primo treatments were made at a four-week interval using a CO₂ - powered backpack sprayer calibrated to apply 3.0 gallons of material/1000 ft². Primo was applied at a rate of 0.25 fl. oz. of product/1000 ft² and compared to an untreated control. Primo applications were initiated on June 10, 2000 and a total of four applications were made over the duration of the study.

Turf assessments were made on a weekly interval. Data collected included an overall visual rating and a measurement of mowing injury. The overall visual rating was based on a comprehensive measure of the color, density and uniformity of the turfgrass. Ratings were assigned on a 9 to 1 scale, with 9=best, 6=lowest acceptable turf quality and 1=worst.

Mowing injury was determined by measuring the amount of shredding and browning on the tips of leaf blades that were randomly selected from each plot. This leaf damage evaluation was measured in millimeters under a microscope.

The chlorophyll content of grass samples from each treatment plot was determined every two-weeks. Chlorophyll content was determined as a means to quantify mowing injury. The necrotic and brown tissue damaged by mowing will not yield chlorophyll. Therefore, leaf tips that are torn and shredded rather than cut cleanly, will have larger brown and necrotic areas and will not contain as much chlorophyll.

To further quantify turfgrass mowing injury, ethylene production rates of grass samples from each treatment were determined. Ethylene production is induced when the turfgrass plant is stressed or damaged. Mechanical stress, such as mowing, will initiate ethylene production, and the amount of ethylene produced will increase as the severity of the injury increases.

The turfgrass quality of the Primo and untreated control PGR treatments can be found in Table 2. The untreated control plots rated higher in visual quality than the Primo treated plots. The reduction in visual quality associated with the Primo treated plots was primarily the result of minor discoloration the week after each PGR application. In the week following PGR application, Primo treated plots acquired a gray hue, causing a slight, but acceptable, reduction in visual quality. Primo treated plots achieved visual quality ratings similar to the untreated control the remaining three weeks between PGR treatments.

Mower A (reel-to-bedknife contact) and Mower C (reel-to-bedknife contact with 8 hours of use mower adjustments), generally resulted in the highest visual ratings of the mower adjustment treatments. Mowers B and D produced the lowest visual quality ratings. Mower B was adjusted with no reel-to-bedknife contact while Mower D was adjusted with no reel-to-bedknife contact with eight hours of use (Table 3). Necrotic, ragged, and frayed leaf tips caused by Mower D significantly lowered the visual quality of the affected turf.

No statistically significant differences in mowing injury were observed among the Primo and untreated control PGR treatments (Table 2).

Mower A consistently produced the smallest amount of mowing injury of the four mower adjustments, while Mower D, the no reel-to-bedknife contact with 8 hours of use adjustment, produced the largest (Table 3). Mowers B and C produced intermediate amounts of mowing injury, although at times Mower C resulted in mowing injury similar to Mower A.

Compared to the untreated controls, Primo treated plots had higher total chlorophyll concentrations (Table 2). Primo applications slightly reduced the visual quality of treated turf for one week after the treatment was applied. However, the Primo plots attained a darker color the remaining three weeks until the next application. This is reflected in the chlorophyll concentration data. Primo clearly increased the chlorophyll concentration of treated turfgrass.

Mowers A and C resulted in the highest chlorophyll content of the mower adjustment treatments, while Mowers B and D produced the lowest (Table 4). The numbers of frayed and shredded edges were limited in the Mower A and C plots. This resulted in higher chlorophyll content. Remember that brown or necrotic tissue damaged by mowing does not contain appreciable quantities of chlorophyll. The leaf tips of grass plants mown with the B and D mower adjustments were visibly damaged. Accordingly, their chlorophyll concentration was much lower.

Primo had no significant effect on the ethylene production rate as compared to the untreated control (Table 3). Mower A, however, consistently resulted in the smallest ethylene production rate of the four mower adjustments. Ethylene production increases as a result of stress, or wounding, and the amount of ethylene produced increases as the severity of the stress increases. Mower D had the highest ethylene production rate. Therefore, Mower A resulted in the least amount of stress, while Mower D resulted in the highest amount stress of the four mower adjustments (Table 3).

Primo had little affect on mowing quality, although it did cause a slight, but acceptable reduction in visual quality. This reduction was due primarily to the discoloration of the turfgrass the week immediately following Primo applications. In the three weeks before the next Primo application, however, the treated plots regained a healthy green color that at times surpassed the visual quality of the untreated control. This is reflected in the chlorophyll content data. Primo application increased the chlorophyll content of treated plots above that of the untreated control.

Mower adjustment had the greatest influence on mowing quality. Mower A, the reel-to-bedknife contact adjustment, consistently achieved superior mowing quality ratings as assessed by visual quality, mowing injury, chlorophyll content and ethylene production rate. Mower A most often resulted in the highest visual rating and chlorophyll content, while it produced the least amount of mowing injury and had the lowest ethylene production rates.

Mower D, the no reel-to-bedknife contact adjustment with eight hours of use, produced the worst mowing quality. This mower adjustment produced the lowest visual ratings, as well as the lowest chlorophyll content. Mower D also produced the largest amount of mowing injury and the highest ethylene production rate. These results indicate that the mowers utilized in this study should be maintained with reel-to-bedknife contact, per manufacturer’s specifications, to ensure the highest mowing quality.

Table 1. Mower Adjustment Treatments

Table 2. Average PGR Treatment Assessments

Table 3. Average Mower Adjustment Treatment Assessments