Breeding and evaluation programs develop improved turf varieties
Deciding factors during selection of Kentucky bluegrass cultivars for the new grass fields at Ohio State University included overall quality and compact growth habit. Winter dormancy was not such an important factor because the fields are typically not used until midspring.
Photos courtesy of Pamela Sherratt unless otherwise noted.
Selection of the proper turfgrass species is one of the most important decisions to be made when establishing a playing surface suitable for athletics. Since the field is meant to be permanent, it’s important to select a grass species adapted to the area and to the intended level of management. The most important criteria when selecting grass species for athletic fields can be summarized as:
- Ability to tolerate heavy traffic/wear and to rejuvenate or fill in any worn areas quickly.
- Quick seedling germination and establishment.
- Good annual color and ability to grow at low temperatures.
- Drought and heat stress tolerance.
- Pest, disease and weed resistance.
There are many cultivars/varieties of seed commercially available within the industry. The National Turfgrass Evaluation Program (http://www.ntep.org) is a nonprofit organization that has (together with the USDA) developed uniform evaluation trials of 17 turfgrass species in the U.S. and Canada. Trials are conducted at 46 locations, and data is collected and disseminated on an annual basis.
Trials include information such as turfgrass quality; color; density; resistance to diseases and insects; and tolerance to heat, cold, drought and traffic tolerance. This information is available through annual progress reports and online.
One of the most important considerations when selecting athletic field cultivars is its ability to withstand wear injury and compaction stress. As defined by the NTEP rating guidelines, wear injury occurs immediately upon trafficking a turf. Wear injury symptoms are often expressed within hours and definitely within days. Compaction stress injury is more chronic and expressed over time.
NTEP tall fescue trial.
Photo by Karl Danneberger.
Traffic tolerance ratings are conducted at several sites, and simulated traffic is applied using different types of equipment so there may be some variation among results, but the data does give a good indication of a particular cultivar’s tolerance level. Since not all states have a traffic trial, it’s important to use data from the closest regional trial. For example, there are traffic trials on seeded and vegetative bermudagrass cultivars in Florida, Arkansas and North Carolina.
Since so much perennial ryegrass is used on sports fields, another important consideration when selecting athletic field cultivars is disease resistance, particularly the level of tolerance to devastating diseases like gray leaf spot, Pythium and brown patch. It’s important to note, as reiterated by turfgrass pathologist Joe Rimelspach, that some cultivars are more resistant to disease than others, but none are immune.
Other criteria that are particularly desirable for turf on athletic fields that are evaluated by NTEP include spring green-up (crucial for early spring sports like lacrosse and baseball), seedling vigor and overall turf quality, which is a combination of color, texture and density. The ratings are subjective and done on a 0-9 rating, with 1 representing poorest and 9 representing best. A rating of less than 6 is considered “unacceptable.” People who evaluate the NTEP trials have guidelines on how to rate, and the results are collated each year and posted on the website. Over the years we have found the NTEP website to be an invaluable tool in selecting the best cultivars for athletic fields in a given location and level of management.
Turfgrass breeders may have a more difficult job than most other plant breeders. For the most part they deal with more than one species, usually at least four major species and many minor ones, and a vast geographic area for each species. What further complicates the process for the seed-propagated species is breeders must breed at the same time for turf quality, disease resistance and other characteristics for turf performance and also for seed yield.
Most turfgrass species are cross-pollinated, self-incompatible species, which means the same plant cannot be the mother and the father. This makes development of inbred lines for hybrids or seed-propagated varieties with one genotype unfeasible. Breeders must cross similar plants together, selected for the characteristics desired in the new cultivar, to start the breeding process. This means traditional breeding operates as a form of population improvement, with each individual seed in a variety genetically related to but distinct from others. By taking the portion of the population with the best of a certain characteristic – darkest green, highest stress tolerance, least disease – and crossing these together the breeder moves the mean of that characteristic up. The selected plants must still match for many other characteristics, such as color, date of seed head appearance and height, to make a uniform variety.
Kentucky bluegrass is the exception due to its apomictic reproduction. It’s hard to get hybrids, with often only 10 percent of plants in a cross being the hybrids, the rest being genetically identical to the mother plant. These hybrids usually have all the chromosomes of the mother plant and about half of the father. Each plant is a shot in the dark, but if you do get a good plant that is apomictic the progeny will all be the same and it can be a new variety.
The general outline for breeding is:
Improvement of turfgrass varieties is dependent on being able to efficiently screen large numbers of plants for the desired characteristic(s). The selected plants need to be crossed together and the progeny (offspring) evaluated again for the characteristic(s). If the characteristic is highly heritable, the majority of the population may then have the characteristic, or additional cycles of selection must be performed. Due to the complex inheritance of many desired characteristics, being able to concentrate many of them in one population or variety is often difficult. It’s often necessary to evaluate the selected plants and progeny over a number of years and environments to reliably screen for some characteristics.
Screening for wear tolerance was often difficult since the size and speed of the machines made it difficult to screen large numbers of turf plots. Rutgers University recently developed the Rutgers Wear Simulator (Turf Slapper), which can apply wear over a large number of plots and has enabled wear tolerance and recovery to be more easily integrated into varietal development.
Tall fescues with improved wear tolerance and the ability to demonstrate wear tolerance as younger plants has been a recent emphasis. Current information by Dr. David Minner of Iowa State University suggests that the addition of large seed quantities early in the season, even with wear applied, increases the percentage of tall fescue, perennial ryegrass or Kentucky bluegrass in the stand. Increased establishment occurred at rates up to 200 kilograms per hectare for perennial ryegrass and 40 kilograms per hectare for Kentucky bluegrass. Tall fescues were only slightly less effective than perennial ryegrass in establishing during play. Additional breeding work in tall fescues has emphasized drought tolerance as well as brown patch resistance in multiple locations for durable resistance.
Cultivars of Kentucky bluegrass vary greatly in color, growth habit and stress resistance. Rutgers University developed a classification system to group cultivars together with similar traits. For example, the Midnight types are very dark green with a compact growth habit.
Photo by Karl Danneberger.
For many years, perennial ryegrass breeding in the U.S. has emphasized darker green dwarf varieties with high turf quality. An important new characteristic has been the discovery and incorporation of genes for resistance to gray leaf spot disease, which is the same pathogen as rice blast disease, and has decimated stands of perennial ryegrass in the U.S. transition zone. A balance must be achieved between resistance to this disease and resistance to other important pathogens, such as brown patch and red thread.
Wear tolerance is a major component in selecting new ryegrasses. Breeders have been crossing more winter-active wear-tolerant ryegrasses with American germplasm to increase the wear tolerance during the colder months. An additional high priority in perennial ryegrass breeding has been salt tolerance. This is due to the increasing use of effluent water at many golf courses in the U.S. and elsewhere.
Hybrids of Texas bluegrass and Kentucky bluegrass have the potential to expand the range of Kentucky bluegrass. Many of these are more heat and drought tolerant than traditional Kentucky bluegrasses, although some new cultivars of Kentucky bluegrass have also been selected for more heat and drought tolerance. The other major advantage of these hybrids is their extensive rhizome systems. In wear trials they have shown excellent wear tolerance, rapid recovery and more winter-active growth, making them better suited for many applications.
In breeding of all species seed yield is just as important as disease resistance or turf quality. If you find cultivars or experimentals that are only on the market a short time, or are never marketed, it’s often due to inadequate seed yield. We must often cycle one generation for a turf characteristic and then another for seed yield. Turf breeders have been successful in improving turf quality and seed yield at the same time, but we may not find that to be true in the future.
Development of new turf cultivars takes many steps, and the diverse needs make it different from many other crops. Turf breeders must have patience and understand the many needs of customers and seed growers.
Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011. Dr. Leah Brilman is director of product management and technical services, DLF-Pickseed.