Going wild with bentgrass

I recently drove by a billboard promoting nutrition education and it asked if I could name seven kinds of berries. I failed miserably, even being married to a dietitian. I started thinking of other categories, including different turfgrass species. Bermudagrass, buffalograss, creeping bentgrass, Kentucky bluegrass, perennial ryegrass, tall fescue and zoysiagrass quickly came to mind, but I wanted more of a challenge.

I envisioned a billboard showing a picturesque golf course putting green, with the billboard copy asking passersby to name seven different bentgrass species. The bentgrasses represent the Agrostis genus, and creeping bentgrass (Agrostis stolonifera) often is regarded as the gold standard for a high-quality putting green surface and is the one that most readily comes to mind.

Considering water movement through plants and photosynthetic capacity, it has always amazed me that a plant can tolerate and even thrive when mowed at the exceptionally low heights of cut used on golf course putting greens. Considering the type of stress creeping bentgrass is subjected to as part of its daily use and management on courses, it is among the hardiest cool-season grass species grown for turf. The other commonly known bentgrass species, but to a lesser degree than creeping bentgrass, are Colonial bentgrass (A. capillaris), Highland bentgrass (A. castellana), redtop bentgrass (A. gigantea), and Velvet bentgrass (A. canina).

Even if you could name those five species, it likely would be difficult to name just two more, even though there are more than 150 different bentgrass species. I was able to rattle off California bent (A. densiflora), Northern bent (A. mertensii), Dune bent (A. pallens), Cloud grass (A. nebulosa), Idaho redtop (A. idahoensis), Clavate bent (A. clavata), and Brown bent (A. vinealis) along with 10 others, but I am a self-proclaimed bentgrass enthusiast who studied the genus for my Ph.D.

Early bent

Early bentgrass cultivars have roots in South German mixed bentgrasses that were introduced to the United States in the early 1900s. These mixed bentgrasses often consisted of several different species and biotypes. The mixed bentgrass stands would segregate depending on management and environment, and from within stands a few patches were recognized for their characteristics important for golf course putting greens. The best performing patches were selected and ultimately became the first vegetative bentgrass cultivars developed in the United States.

Since these early selections, bent-grass breeders have spent considerable time and effort developing uniform, stable, high-quality bentgrass cultivars. Turf uniformity describes the consistent appearance of the turf, and a stable bentgrass cultivar is one that doesn’t change over time.

Development of stable and uniform populations through plant breeding can be challenging in highly outcrossing species such as the bentgrasses. Highly outcrossing species tend to pollinate unrelated plants of the same species, often regulated by physical or molecular barriers that prevent or reduce the likelihood of self-pollination. Outcrossing increases diversity in bentgrass populations. From a plant-breeding perspective, these heterogeneous populations often are desirable in early breeding steps to increase the chance of combining desirable traits and for finding unique off-type plants. Diverse populations are less desirable at the later breeding stages, when uniformity and stability are more important.

The promiscuous nature of bent-grasses also can be useful for moving traits from one genetic background into another and in bridging species boundaries. Plants of the same genus often are labelled as separate species if they are not sexually compatible. Many grasses don’t adhere to these species boundaries and are able to pollinate across the species divide. A great example lies with creeping and Colonial bentgrasses, which have innate differences in disease resistance.

Creeping bentgrass generally is susceptible to dollar spot disease whereas Colonial bentgrass is resistant, and dollar spot is the most economically important disease in golf course management. Hybrids naturally occur between creeping and Colonial bentgrass, and researchers have transferred dollar spot resistance from Colonial bentgrass into experimental creeping bentgrass lines using traditional plant-breeding methods. With more than 150 different bentgrass species adapted to different growing environments and tolerant of a variety of stresses, there are opportunities to identify important traits from wild or non-domesticated bentgrasses and move those traits into elite creeping bentgrass cultivars through conventional plant breeding.

Wild challenges

Bentgrass breeders have more challenges when working with wild plant material than just overcoming hybridization problems. Because humans have not selected wild bentgrasses, they often carry poor turfgrass-quality traits. For example, when introducing stress tolerance from a wild species, the process also will introduce a sparse canopy, coarse leaf texture, poor traffic tolerance or other traits that would take several generations to remove through traditional plant breeding. As such, many bentgrass breeders who have worked tirelessly to genetically stabilize uniformity and turf quality traits are reluctant to introduce wild material into their breeding programs.

Creeping bentgrass cultivar development already is a long process, typically taking as long as 12 years from the early crosses to a line ready for commercial release, and the introduction of wild material would slow the process and set back breeding efforts. To overcome the challenge of working with wild bentgrasses, a two-year study sponsored by the United States Golf Association was initiated in 2014. The project, titled Low input performance of Highland, heat, and drought tolerant bentgrasses, focused on the evaluation of plant introductions from hot and dry climates. The project evaluated the performance of 72 bentgrass accessions (entries) representing nine different species in three separate studies. The purpose of the project was to initially evaluate diverse bentgrass plant material and then establish breeding efforts to improve the genetic background of that material so that bentgrass breeders could take advantage of those unique traits without jeopardizing turf quality in their elite breeding material.

The USGA project obtained seeds from the National Plant Germplasm System (NPGS) for bentgrass accessions collected from Afghanistan, Ethiopia, Iran, Greece, Kenya, Mongolia, Portugal, South Africa, Spain, the United States and Turkey (Figure 1). The NPGS is part of the USDA-ARS Germplasm Resources Information Network and is a germplasm repository, maintaining the largest collection of publicly available bentgrass accessions. The project evaluated the performance of the bentgrasses when mowed at a fairway (0.5 inch) or lawn (2 inches) height of cut. A third study was established and not mowed to evaluate canopy architecture and timing of seed set. The research was conducted at the University of Nebraska turfgrass research farm near Mead, Neb. Minimal supplemental irrigation was applied to keep plants alive, and 2 pounds of N per 1,000 sq. ft. was applied annually. During the two-year study, plants were evaluated for several important turfgrass characteristics, and Penncross, T-1 and Alpha creeping bentgrass cultivars were included for comparisons with the NPGS bentgrasses.

Evaluated criteria

During the establishment year of the project, color, canopy density, leaf texture and establishment rate were evaluated and statistical differences were observed among the bentgrass entries in each rating category (Figure 2). Thirteen plant introductions established as quickly as the named cultivars. Establishment rate is an important characteristic for turfgrass species because it reduces the length of time required to grow in a new turf stand and it can be an indicator of the line’s ability to recover from stress or damage. Four plant introductions were as dense as the named cultivars. Increased canopy density helps the turf naturally outcompete weeds and is important for golf because it can impact ball roll and lie. Not many differences were observed for color, and 45 entries were as dark green as the named cultivars. There has been a push in the turfgrass industry for darker green cultivars, using Kentucky bluegrass as the model because most turf-type cultivars have dark green color. More recently, the emphasis has been on uniformity of color because a light green turf is recognized for its light color only when dark green plants disrupt its uniformity.

At the end of the establishment year, several entries stayed green late into the fall, while most of the entries turned straw color as they transitioned to winter dormancy (Figure 3). Some of the entries that maintained late-season color did not appear to have the winter dormancy response and suffered from winter injury, evident as reduced-percent plot coverage in the spring of the following year. One entry maintained both late-fall color and good plot coverage the following season, and it may be better equipped to tolerate winter stress while extending the growing season of bentgrasses used on putting greens.

Leaf texture, or the width of the leaf blade, also was visually estimated. Twelve entries had finer leaf texture than the named cultivars and eight had coarser leaf texture. Finer-textured grasses contribute to increased canopy density and improved mowing response, and generally are preferred for golf course putting greens.

We generally don’t consider bent-grasses for lawn applications because we think of creeping bentgrass as the only bentgrass species and it does not do well when managed with a rotary mower. However, entries that have more upright growth and coarse texture may tolerate a 3-inch mowing height. One entry, a Clavate bentgrass, had a good establishment rate, coarser leaf texture and good quality at the higher mowing height. This entry has potential for use in low-input golf course roughs or lawn areas.

The final measure of importance in this study was stand persistence, measured at the end of the project as a visual estimate of percent-plot coverage. Many bentgrasses are annual types and would not persist in a turf stand. In the fairway study, 16 entries had as much plot coverage as the named entries, and in the lawn-height study nine entries had as much plot coverage as the named cultivars. At the lawn-mowing height, 20 entries had at least 50-percent plot coverage and 26 had at least 50-percent plot coverage at the fairway height of cut. This measure of plot coverage is important, but no one reading this article would be satisfied with 50 percent of his or her lawn being covered by turf. These data, however, give an indication as to which lines under study are likely perennials and which are annuals, which is important information when making crosses to improve the bentgrasses.

Going forward

Previously, importance was claimed for novel traits from wild bentgrasses and how they could be used to improve bentgrass cultivars, but this study just describes evaluation of traits important for any turf. So, where is the novelty? For wild germplasm to be useful it must be aesthetically and functionally compatible with cultivated bentgrasses. This project provides the first data of important turf traits from this bentgrass collection, and it was successful because wild extant bentgrass germplasm was shown to be diverse, with potential as a source of stress resistance and other traits of interest. Research still needs to be done to move desirable traits from wild germplasm into stable genetic backgrounds with higher quality that could be used by bentgrass breeders without diminishing quality of the elite plant material.

Co-principal investigators on the USGA project were Bill Kreuser, Ph.D., and Leah Brilman, Ph.D. Kreuser is a turfgrass extension specialist at the University of Nebraska and has considerable experience evaluating turfgrasses in different environments and in response to various treatments. Brilman is a turfgrass breeder working with Seed Research of Oregon. Like me, Brilman is a bentgrass enthusiast and evaluates the commercial appeal of traits and plant material identified in this study. If the next phase of the project is successful and high-quality germplasm is developed, Brilman or another bentgrass breeder could use that material directly to further improve their elite bentgrasses. Our turfgrass breeding program at the University of Nebraska is focused on developing turfgrasses suitable for reduced-management environments. As such, the bentgrass entries that persist with reduced fertility and irrigation or those that perform well at the higher mowing height are of interest to our program. It will be several years before any of this material will be useful to breeding programs, but the project is an important first step for identifying the turf potential of NPGS wild bentgrass accessions.

Acknowledgements
I would like to thank the United States Golf Association for financial support of this project.

Keenan Amundsen is an assistant professor of turfgrass genetics at the University of Nebraska-Lincoln. You may contact him at kamundsen2@unl.edu.