Differences in ball-roll distance among various ultradwarf cultivars may occasionally be statistically significant, but rarely are they practically significant when cultivars are under similar management programs. Many factors create the friction that slows ball roll on a green. A uniform stand of dead grass may provide the fastest possible "greens."

At least one ultradwarf bermudagrass is marketed for "faster greens." Wouldn't it be nice if a grass could actually provide longer Stimpmeter ratings? This might reduce the need for rolling, topdressing and close mowing.

To evaluate the idea of developing cultivars for speedy greens, I compared the ball-roll distances of 12 bermudagrass cultivars maintained at greens height. With few exceptions, their results were disappointingly, but predictably, similar.

The experiment involved small, 8-by-10-foot plots of greens bermudagrasses in four replications at the University of Florida-Fort Lauderdale. I used a 19-centimeter modified Stimpmeter (4). The data were converted to values expected for a standard 76-centimeter Stimpmeter.

The Stimpmeter is no more than an inclined plane that imparts a constant initial energy to the golf ball as it rolls away from the end of the plane. The distance traveled from the Stimpmeter is inversely proportional to the rolling friction of the greens surface. If we could reduce friction to zero, ball-roll distance would be infinite, and golfers would presumably have no complaint about slow greens.

Slight variations in ball weight are not important, because both friction and the kinetic energy of the ball are directly proportional to the weight of the ball. Of little consequence is the fact that the ball's kinetic energy has two components: translational and angular, or spin. These components have also been described as "whole ball speed" and "peripheral speed" (6). The fact that the ball's kinetic energy is divided into two components does not affect how far it will roll, because the components are interchangeable.

For more than a year, a student and I made more than 1,500 measurements of ball roll under varying conditions. Mowing height varied between .125 and .155 inch. Even though the research greens appeared to be flat, we made up-slope and downslope measurements and used a slope correction formula (1,2). This took care of the problem of the slight, but steady, prevailing trade winds that cut across southern Florida. Because we were extremely careful in our Stimpmeter procedures, the degree of error in our measurements was minuscule. For the pooled plot-to-plot comparisons, the coefficient of variation (a measure of the relative error) was only 1.7 percent.

Student Susan Boyer compares ball-roll distances on a contaminated bermudagrass green.

There were two groups of grasses in the experiment: seven hybrid bermudagrasses such as Tifdwarf and TifEagle, and five Cynodon transvaalensis genotypes, also called African bermudagrasses. Hybrid bermudagrasses, first represented in the United States by the Tifton series, are the interspecific hybrids between African bermudagrass and common bermudagrass (Cynodon dactylon).

The African bermudagrasses were very slow, with Stimpmeter ratings 7-9 inches shorter than the slowest hybrid bermudagrass. These results were obtained during cooler months when the African bermudagrasses were lush. By the summer of 1995, most of the African bermudagrasses had died out, and their plots were pocked with bare soil, so it is only meaningful to talk about the hybrids.

The speediest hybrid bermudagrass, Quality Dwarf, had a Stimpmeter rating of 104 inches; the slowest hybrid, TifEagle, had a rating of 100 inches. Although differences in ball-roll distance were statistically significant, they appeared to be too small to be of any practical significance (3).

Another example of unremarkable Stimpmeter differences among bermudagrass greens grasses appears in the 1999 data from the National Turfgrass Evaluation Program's "On-site evaluation of bermudagrass for putting greens" (5). In addition to the commercial standards Tifgreen and Tifdwarf, there were several newer ultradwarf varieties. The ultradwarf bermudagrasses are generally those that have shorter internodes and are promoted as being more tolerant of close mowing (.125 inch and lower).

Averaging 12 sets of evaluations over six locations, the speediest hybrid bermudagrass, Champion, had a Stimpmeter rating of 110 inches; the slowest, MS-Supreme, had a rating of 106 inches. When the data were statistically analyzed using locations as replicates, there was no difference. The probability, or P value, for cultivar differences was 44 percent -- so close to 50-50 that it's of no value in deciding whether to plant one grass or another.

Species vs. species
If there are no differences among greens bermudagrass cultivars, are there observable differences among species? Many Florida golf course superintendents suspect that creeping bentgrass (Agrostis palustris) greens are faster. We tested this on a green with patches of creeping bentgrass and bermudagrass growing side-by-side.

A shortened Stimpmeter allows ball-roll readings on plots too small for a full-length roll.

Penneagle bentgrass had been overseeded once six years before, and it was persisting by growing vegetatively from stolons, forming permanent patches in the bermudagrass green. Because the patches were only 6-10 feet across, we again used the shortened Stimpmeter (4); because there was slope, we again used a slope correction (1,2).

Stimpmeter ratings were bermudagrass, 108 inches; bentgrass, 103 inches. Those results were statistically significant at a P value of 1 percent, which means that I am about 99 percent sure that bentgrass was slower than bermudagrass on this green. This conclusion would be true only for the one experiment, done on one green in February 1998, at one mowing height. Yet the idea that bentgrass is faster than bermudagrass may be mere legend spread by northern golfers returning to Florida in December.

The physics
To give the idea of speedy greens cultivars another chance, let's examine the physics in more detail. Golf ball roll distance, also called "greens speed," is a physical characteristic of a putting green interacting with a rolling sphere. The golf ball stops rolling because its energy of movement (kinetic energy) has changed to imperceptible heat energy because of friction.

Rolling friction occurs when a golf ball rolls across a green. The more friction, the shorter the roll. By standardizing the initial speed of a golf ball using a Stimpmeter or other constant initial velocity, one can measure rolling friction. The longer the golf ball roll distance, the smaller the friction. To overstate the obvious, smooth greens are faster.

Normally we think of friction as occurring when objects slide over one another. A common frictional force is traction, which allows us to walk without slipping. But in the case of a rolling object such as a golf ball, sliding friction is minimal.

Small plots of multiple cultivars provide a laboratory for studying greens speeds of various bermudagrasses.

Probably the biggest component of rolling resistance is the deformation of the ball and the surface. When a golf ball sinks into a green, ever so slightly, the greens surface bows downward, but because the surface is not perfectly elastic, it doesn't spring back up instantly. One could visualize the relative deformational loss by dropping a golf ball onto a greens surface and noticing that it doesn't bounce back to the same elevation from which it was dropped. In comparison, if one were to drop a ball onto a steel plate, it would bounce back to nearly the same elevation as the drop elevation, because the steel surface is elastic and will return energy to the ball. This also explains why the ball will roll farther on the steel plate than across the green.

There is another component of rolling resistance. As the ball sinks into the greens surface, it contacts the surface at many points. Edges of the ball's center of contact will turn at a slightly slower linear speed than the center of contact. This will result in a slight amount of slippage resistance.

Brown or dormant grass, or no grass, would offer the least resistance to ball roll, whereas luxuriant grass (such as the African bermudagrasses and the bentgrass grown in the cool months) will have the shortest ball roll, by simple physics.As an example, a plot of ultradwarf bermudagrass was sprayed with Roundup (glyphosate), a nonselective herbicide. The plot area was still mowed and maintained the same as the adjacent plots with the 100- to 104-inch ratings. After the bermudagrass plot was dead, the average Stimpmeter rating was 137 inches. This should be a good lesson for anyone who is attempting to establish speedy greens. It can be done, but you will no longer have greens.

Acknowledgments

The experimental plots were made available by Monica Elliott, Ph.D., and the Florida GCSA. Thanks to turfgrass student Susan Boyer, who took hundreds of Stimpmeter readings.

Literature cited

1. Brede, A.D. 1990. Measuring green speed on sloped putting greens. USGA Green Section Record 28(6):10-12.
2. Brede, A.D. 1991. Correction for slope in green speed measurement of golf course putting greens. Agronomy Journal 83:425-426.
3. Busey, P., and S.E. Boyer. 1997. Golf ball roll friction of Cynodon genotypes. International Turfgrass Society Research Journal 8:59-63.
4. Gaussoin, R., J.L. Nus and L. Leuthold. 1995. A modified stimpmeter for small-plot turfgrass research. HortScience 30(3):547-548.
5. National Turfgrass Evaluation Program. 1999. 1998 USGA, GCSAA, NTEP On-Site Bermudagrass Test -- 1999 Data. Progress Report NTEP No. 00-7. NTEP, Beltsville, Md. Available at www.ntep.org/reports/bg98o_00-7/bg98o_00-7.htm (verified Jan. 2, 2001).
6. Stobbs, A., and A. Cochran. 1968. The search for the perfect swing. Golf Society of Great Britain. Dover, Kent, United Kingdom.

Philip Busey, Ph.D., is associate professor of environmental horticulture in turfgrass science at the University of Florida in Fort Lauderdale.