The best grasses for an athletic field should ideally be able to tolerate heavy traffic, recover quickly from injury, plus germinate and establish rapidly. They should also have good annual color, meaning both the ability to grow and look good at cooler temperatures but have the ability to tolerate heat and drought as well. The perfect grass, based on these criteria, does not yet exist. However, in most (hopefully all) cases, the grasses you manage on your fields today are not your grandparents’ grasses, or maybe even your parents’ grasses. The industry of breeding new and improved turfgrasses for use in all facets of society is a big business and significant strides have been made in cultivar development for sports fields. This month’s article focuses on the changes in the agronomic characteristics that have occurred in each of the major cool-season turfgrasses that we manage as sports turf.
Poa – the bluegrasses
The most widely used species in the genus is Kentucky bluegrass (Poa pratensis). As a species, it has good heat tolerance and moderate to good drought tolerance. However, it has limited tolerance for shade or saline conditions. Kentucky bluegrass has finer texture, excellent mowing quality and, most importantly, the ability to spread by underground elongating stems known as rhizomes. As such, it has the ability to spread much more rapidly than bunch type grasses, such as the ryegrasses and fescues. Early Kentucky bluegrass varieties were actually selections out of pastures. With the advent of breeding and selection programs came the development of cultivated varieties or cultivars. Remember that a cultivar is an artificially created group of plants that has some distinguishing characteristic(s) that, when reproduced, retain those characteristics.
Kentucky bluegrass is the one species in which more cultivar development has occurred due to the fact that it is apomictic. Apomictic means that a mother plant will produce seed resulting in plants exactly like the original plant. Breeding efforts and the fact that Kentucky bluegrass is apomictic has resulted in the development of cultivars that are quite distinctly different in their agronomic characteristics and agronomic requirements. Because of this there are cultivars available that will perform very well under certain environments and/or management conditions but perhaps quite poorly in others.
Some of the common types of Kentucky bluegrass that dominated turf stands last century are still available for sale and, in some cases, have appropriate or ideal uses. They are very different compared to more elite cultivars. Common Kentucky bluegrasses have an upright growth habit, with leaves ascending at a 45-degree angle, a coarse texture and very thick vigorous rhizomes. These rhizomes give common Kentucky bluegrass the ability to spread aggressively. However, they are slow to breakdown, and as such contribute to what can become very thick thatch layers. These grasses are best suited for lower intensity management, with less fertility, less irrigation and higher mowing heights.
In 1947, the first cultivar of Kentucky bluegrass, Merion, was commercially released. A side-by-side inspection of the growth habit of this cultivar reveals some important differences between it and common Kentucky bluegrass (Figure 1). First, the leaf blades are more fine textured and grow at more of a prostrate or 90-degree angle. The importance of this is twofold: 1) the mowing height can be lowered without removing too much of the leaf blade, and 2) from eye level it gives the appearance of covering more of the turf surface (better density). The cultivar also had rhizomes that were less aggressive and fibrous. This gives the plant the ability to spread but reduces the potential for accumulating excess thatch. Merion also responded better to higher maintenance.
After Merion, more cultivars were developed but some of these early ones were limited because they had a limited range in which they performed well. Beginning in the 1960s and 1970s, a method to take advantage of the apomixis of Kentucky bluegrass was developed at Rutgers University, and since then literally hundreds of Kentucky bluegrass cultivars have been developed. Some of these cultivars are better for golf courses and others for sports turf. Based on growth and stress performance from field research trials, Kentucky bluegrass cultivars have been classified intro three general types: 1) elite, 2) BVMG or 3) common types. Within the elite category, the cultivars are further subdivided into about 10 different groups based on parentage and/or agronomic characteristics. Breeding and selection efforts continue. The focus now is to further improve color and lower the growth habit as well as enhance tolerance to heat, shade and various diseases. Another major effort is to improve seed production, which would lower production costs.
When selecting Kentucky bluegrass cultivars for an athletic field, you should choose a blend of elite types. It’s best that the cultivars have similar color, texture and growth rate. These improved cultivars will outperform common varieties in these shorter mowed and more intensively trafficked situations. However, in order to perform optimally, their cultural management requirements are significantly greater than the common varieties.
Within this genus are several other species that are managed as a turf, including annual bluegrass (Poa annua). Annual bluegrass appears on sports surfaces but it mainly is managed as a weedy infestation. But it’s important to understand that there are two naturally occurring varieties of annual bluegrass. Poa annua var. annua is a true annual. It is a bunch type grass with an upright growth habit and quickly produces a terminal inflorescence. This is one of our bigger weed issues. Poa annua var. reptans, however, is a perennial that produces stolons and has a more prostrate growth habit. These are not cultivated, but rather a naturally occurring variety of annual bluegrass. The reptans variety performs reasonably well in the northern tier of states. In fact, some efforts have been made to develop cultivars of var. reptans annual bluegrass. However, to date, no significant commercial releases have resulted. Some cultivars of rough bluegrass (Poa trivialis) have been developed. The species is stoloniferous but lacks the heat and wear tolerance necessary to be utilized on athletic surfaces. They are mainly utilized as apart of winter overseeding programs for the golf industry.
Two other species, Supina bluegrass (Poa supina) and Texas bluegrass (Poa arachnifera) have been either investigated for use in certain parts of the country or for use in breeding programs. Supina bluegrass can be used in cooler parts of the country. It has average wear resistance but excellent recuperative potential because it is aggressively stoloniferous. It’s more widespread use is limited by its lack of heat tolerance. Hybrid bluegrasses, such as Thermal Blue, which is a cross between Kentucky bluegrass and Texas bluegrass, are becoming more popular due to improvements in rhizome growth (thus recovery potential) and also drought tolerance.
Lolium – the ryegrasses
Two species of ryegrass are used in turfgrass, perennial ryegrass (Lolium perenne) and annual ryegrass (Lolium multiflorum). Perennial ryegrass is often mixed with Kentucky bluegrass because modern cultivars are similar in appearance but ryegrass has the advantage of much more rapid germination (as few as three to four days) and establishment rate compared to Kentucky bluegrass.
The first turf-type perennial ryegrasses became available during the 1960s. The older perennial ryegrass cultivars had a lighter green color, wider leaf blade, lacked density and had tough fibers in the leaf blade that reduced the mowing quality (Figure 2).
Perennial ryegrass breeding efforts and cultivars differ from that of bluegrass. Ryegrass cultivars are referred to as synthetic, which is defined as an advanced generation of an open-pollinated population composed of selected inbreds, hybrids or clones. Because of this, a different system had to be developed to produce improved ryegrass cultivars compared to what is used to produce bluegrass cultivars. Early breeding efforts with perennial ryegrass focused on improving the color and producing finer textured leaf blades, better density and the ability to be mown at lower heights of cut. Some improvements in mowing quality have also been produced, as well as a reduction in springtime stemmy growth following flowering.
Ryegrasses (as well as the fescues) may also contain endophytes. These symbiotic fungi can improve ryegrass and fescue resistance to certain surface feeding insects, such as billbugs. Breeding efforts to develop high-quality endophytic ryegrasses began in the 1980s. The level of endophyte infection varies between cultivars so the challenge has been to produce cultivars that are both highly resistant to insects but also have good agronomic qualities and many of the newer perennial ryegrass cultivars achieved both objectives.
Another more recent and major accomplishment of ryegrass breeding efforts has been the development of cultivars that are more resistant to grey leaf spot, which can be very damaging on sports fields in the Midwest and transition zone.
There are also intermediate or transitional ryegrasses that are cultivars resulting from an artificial cross of perennial ryegrass and annual ryegrass. These grasses have characteristics that are intermediate (as the name implies) of the two parent species. They have a moderate green color, finer texture and slower growth of perennial ryegrass. They will germinate and establish in cooler temperatures but only last one or two seasons. Because of this, they will not over- dominate a turf stand the way that perennial ryegrass can when used for overseeding.
Schedonorus (festuca) – tall fescue
Within the fescues, the one species that is by far most frequently used on athletic surfaces is tall fescue. In class, we teach our students that learning Latin binomials is useful because a plant species may have many common names, which can cause confusion. However, with tall fescue there are also many Latin binomials that, since the turn of the century, have been used including Festuca arundinacea, Schedonorus phoenix, Schedonorus arundinaceus (the currently accepted binomial) and Lolium arundinaceum. There is much confusion about the classification of this species. Just think, its either closely related to perennial ryegrass, or to the fine fescues or to neither (which is the current thinking).
Regardless of its botanical classification, tall fescue has many agronomic characteristics that potentially make it quite desirable for use as an athletic turf. Most importantly though is that it is very tolerant of wear. Its tolerance of heat and drought are also significant factors when choosing it for lower maintenance athletic fields.
The tall fescue of old, including the first commercially available varieties Kentucky-31 and Alta, were very different compared to the modern cultivars. Tall fescue has a very coarse texture and naturally performs better at higher heights of cut and with lower cultural intensity compared to Kentucky bluegrass or perennial ryegrass. Through breeding efforts, the first generation of improved tall fescue types was developed, including cultivars such as Rebel and Falcon during the 1980s. These cultivars have a finer texture and higher density (Figure 3). Further improvements resulted in release of cultivars such as Bonzai and Trailblazer, which had yet finer texture and the ability to be mowed at lower heights of cut.
However, they also are more susceptible to the disease brown patch and have reduced heat and drought tolerance compared to forage-type tall fescue. Further breeding work has resulted in another category of tall fescues, including cultivars such as Millennium, Plantation and Rembrandt. These cultivars are intermediate, or semi dwarf, in their growth habit but have fine texture, high density and are more disease resistant. They are also less restricted geographically compared to the dwarf tall fescues. Breeding efforts with tall fescue continue. Many of the newer cultivars have a fine leaf blade that allows them to be mixed with Kentucky bluegrass. They also respond better to lower mowing heights.
Through breeding and selection, the grasses we have available today are very different than the grasses 50 or even 20 years ago. Efforts to breed better grasses continue, including for grasses that are more tolerant of drought, more resistant to disease, and less reliant on fertilizer applications in order to perform at optimal levels for use as athletic turf.