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a Dep. of Agronomy, Univ of Wisconsin-Madison, Madison, WI 53706-1597
b Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108 (P.R. Peterson formerly, Dep. of Crop, Soil, and Envt. Sci., Virginia Poly. Inst., Blacksburg, VA) 24061
c Dep. of Agronomy, The Pennsylvania State Univ., University Park, PA 16802
d Dep. of Agronomy, Iowa State Univ., Ames, IA 50010
e Dep. of Plant Breeding and Biometry, Cornell Univ., Ithaca, NY 14853-1902
f Agric. Res. Stn., Univ. of Wisconsin-Madison, Ashland, WI 54806
g Dep. of Horticulture and Crop Sci., Ohio State Univ., Columbus, OH 43210
h Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40506
i ITB Tech. Inc., Spring Green, WI 53588
j AgResearch Intl. Inc., Madison, WI 53719
k Pure Seed Testing Inc., Hubbard, OR 97032
* Corresponding author (mdcasler{at}facstaff.wisc.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Meadow fescue contributes midsummer growth, winter hardiness, and drought tolerance (Humphreys and Pasakinskiene, 1996; Thomas and Humphreys, 1991) while the ryegrass parent contributes rapid establishment and forage quality (Casler, 1990; Thomas and Humphreys, 1991) to the hybrids. Forage quality of festulolium cultivars is similar to that of ryegrass cultivars (Casler, 1990), which is higher than for any other agronomically important cool-season forage grass (Jung et al., 1996).
On the negative side, the ryegrass parent generally reduces winter survival of festulolium hybrids and their derivatives. The lack of adaptation of festulolium and other ryegrasses (Lolium spp.) to the northcentral and northeastern USA and southern Canada severely limits their use in pasture systems. Severe cold, dessicating winds, and lack of snow cover cause severe winter injury and mortality, limiting stand life under these extreme conditions (Casler and Walgenbach, 1990). Festulolium cultivars tend to have better winter survival than ryegrass in the northern USA, but not equal to that of meadow fescue. Three festulolium cultivars ranked 1, 2, and 3 out of 50 ryegrass and festulolium cultivars in a four-location evaluation of survival following two winters in Wisconsin (Casler and Walgenbach, 1990). The three festulolium cultivars averaged 56% ground cover, compared with 50% for the highest-ranked ryegrass cultivar and 38% for the mean of all 47 ryegrass cultivars.
Natural selection for survival was practiced in festulolium by digging and intercrossing survivors from swards that had been exposed to three winters (Novy et al., 1995). Selection increased progeny survival as measured by ground cover and botanical composition of mixed swards. Genetic variation for winterhardiness may be caused by genetic differences in fall dormancy, winter dormancy, soluble carbohydrate concentration, freezing tolerance, and possibly other factors. The objectives of this study were to (i) quantify freezing tolerance of four festulolium populations created by natural selection for field survival and their parental populations and (ii) determine if natural selection for survival can increase forage yield or persistence at multiple locations across the northern USA.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Kemal festulolium was seeded to a 2-ha pasture near Spring Green, WI, in 1985. The pasture was set stocked for 5 yr, with 2 cows plus calves (Bos taurus) per ha. Following five winters, festulolium plant density in the Kemal pasture was less than 1 plant m-2. A random sample of 160 plants were dug from the pasture, 80 representing heavily grazed plants in open areas and 80 representing plants ungrazed and relatively hidden from cattle by tall perennial broadleaf weeds. These plants were transplanted near Hubbard, OR, in August 1991. In 1992, 11 clones were selected for resistance to stem rust and intercrossed in two separate groups. The progeny populations were named W4KG (five clones from the grazed group) and W4KU (six clones from the ungrazed group).
Freezing Tolerance Test
Seed from the populations W4KG, W4KU, WFL-a89C, and WFLr and the cultivars Kemal, Elmet, Prior, and Tandem were planted in a glasshouse in June 1995. Each population was planted in 20 replicates of a completely randomized design. Each experimental unit was a rack of 67 30- by 130-mm cones, each with one seedling.
When seedlings reached a 56-d age, racks were placed in a controlled-environment chamber for 35 d of cold hardening at 2°C with an 8-h photoperiod. After hardening, four replicates of each population were placed in one of five controlled-environment chambers, maintained at one of the following five constant temperatures: -5, -7, -11, -14, or -17°C. Plants were maintained at that temperature for 3 d in the dark, after which they were removed and returned to the glasshouse, which was maintained at 18/13°C day/night.
After 30 d of recovery in the glasshouse, the number of living and dead tillers were counted on each plant. Tiller survival was computed on a plant basis and plant survival was computed on an experimental unit basis. Plants were harvested at a 0.5-cm cutting height, weighed, dried for 5 d at 60°C, and reweighed to determine dry matter concentration. Data for each freezing temperature were analyzed by analysis of variance. Means of each selected population were compared with means of their parent population(s) by contrasts.
Field Test
Three festulolium cultivars were seeded into 14 field trials in 1997. The cultivars were Spring Green (Casler et al., 2001), Kemal, and Tandem. The latter two cultivars comprise 67% of the parentage of Spring Green. Elmet and Prior, comprising the other 33% of Spring Green's pedigree, were unavailable. Previous field tests at four locations indicated that Elmet, Prior, and Tandem had similar survival over a 3-yr period (Casler and Walgenbach, 1990).
Each experiment was designed as a randomized complete block with three to 10 replicates (Table 1) . In some cases, the festulolium cultivars were included in larger cultivar trials, but generally as a split-plot design in which species were whole plots. Plots were seeded with drill-type seeders, in rows spaced 15 cm apart, to simulate solid swards. Plot size varied among locations (Table 1). Plots were seeded at a rate of 18 kg ha-1 on a pure-live-seed basis. Germination was tested in January 1997 according to AOSA standards (AOSA, 1998). Plots were clipped during the establishment year to control annual weeds.
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Plots at Lancaster were grazed with beef steers (Bos taurus) with a stocking rate sufficient to remove most standing forage within a 4- to 6-h period. Forage yield was determined with a capacitance meter calibrated to predict herbage mass from clipped samples (Murphy et al., 1995). Forage yield at all locations was computed as the sum over all cuttings or grazings.
Forage yield data and ground cover data were analyzed by mixed models analysis. For forage yield, cultivars, locations, and years were assumed to be fixed effects. Locations were assumed fixed because they belonged to different hardiness zones and we expected this to be an important factor influencing cultivar x location interaction. Cultivar x location interactions were partitioned into cultivar x region and cultivar x location(region), where two regions were defined based on locations in USDA hardiness zones 2 through 4 (Iowa, Minnesota, and Wisconsin) and zones 5 through 6 (all other locations).
Years were assumed to be fixed because of the potential importance of stand age for a species with marginal winter hardiness. All error terms involving replicates (R/L, Y x R, and Y x R/L, where R = replicates, Y = year, and L = location) were assumed to be random. Years were treated as repeated measures for which the optimal model was a heterogeneous-variance compound-symmetric covariance structure (Littel et al., 1996). Locations also had highly heterogeneous errors, so each location was modeled as a separate variance group within a single mixed model analysis, resulting in one residual variance estimate for each location–year combination (Littel et al., 1996). The resulting model was chosen as superior to simpler (homogeneous variance) models on the basis of both Akaike's Information Criterion and Schwartz'Bayesian Criterion (Littel et al., 1996). Ground cover was analyzed similarly, but without a source of variation for years. Means for Spring Green were compared with means of Kemal and Tandem by contrasts.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Selection for survival in the replicated row plots of Elmet, Prior, and Tandem was also successful at increasing plant survival, but only when measured at -11°C (Table 2). The progress made in this germplasm appeared to be considerably less than that observed in Kemal. This appeared to be due to the large differences in plant survival among the parental cultivars at -11°C (Elmet = 6%, Prior = 83%, and Tandem = 11%). The selected populations represented dramatic improvements in plant survival at -11°C over Elmet and Tandem, but not Prior. Differences in freezing tolerance among Elmet, Prior, and Tandem were unexpected, because field survival of these three cultivars, following two winters at four locations ranged from 52 to 60% and were not significantly different (Casler and Walgenbach, 1990). Field survivors from Elmet and Tandem likely represented significant improvements in freezing tolerance, while survivors from Prior may have been more or less random plants with respect to freezing tolerance. Stand losses for Elmet and Tandem may have been due to mortality of freezing-intolerant plants, but stand losses for Prior were likely due to other factors. This is supported by results of Hides (1979), who found that selection for freezing tolerance of ryegrass seedlings did not confer increased adult-plant survival, suggesting the existence of alternative mechanisms of winter survival.
The difference between WFLr and WFL-a89C at -11°C was significant (P < 0.05) and suggested that selection for rust resistance in Oregon resulted in some loss of freezing tolerance that was accumulated among the initial selections. This may not signify a genetic correlation between rust resistance and freezing tolerance, but is more likely due to relaxation of selection pressure for survival during the cycle of selection for rust resistance in Oregon.
Differences in tiller survival were similar to those for plant survival (Table 2). Selection progress was evident in tiller survival at all three of the warmest freezing temperatures for the Kemal selections and at -11 and -7°C for the Elmet, Prior, and Tandem selections. These changes indicate that improved freezing tolerance can be manifested in two ways—reduced plant mortality and reduced injury (tiller mortality) of surviving plants. These two responses are positively correlated with each other, but not identical.
Dry matter concentration was determined as a measure of living tissue, on the assumption that the amount of living tissue was highly correlated with the concentration of moisture in the plants. Results for dry matter confirmed the combined results for plant and tiller survival, that selection for field survival increased freezing tolerance in both selection experiments, and that progress was greater for the grazed Kemal vs. the ungrazed Kemal (Table 2).
Genetic variation for freezing tolerance appears to be a mechanism by which adaptation of festulolium can be improved. Freezing tolerance can be increased in festulolium by field selection under either grazing or hay management. Although many interacting environmental factors combine to determine field survival of festulolium plants in harsh environments, selection of field survivors can be a useful means of increasing tolerance to freezing temperatures.
Field Test
Cultivar x location and cultivar x location x year interactions were significant (P < 0.01). Although there were some changes in ranking of the three cultivars across years and locations, these interactions were largely due to lack of significant differences at some locations (Table 3)
. On the basis of trials at Ames, IA, Ashland, WI, Lancaster, WI, Marshfield, WI, Lexington, KY, Blacksburg, VA, and Blackstone, VA, it was not possible to distinguish among festulolium cultivars over a 2-harvest-year period. Trials at Rosemount, MN, Arlington, WI, Wooster, OH, Rock Springs, PA, and Ithaca, NY, all showed some evidence that Spring Green had superior forage yield to either Kemal or Tandem. The cultivar x location x year interaction was manifested as inconsistency in these results across years; Rosemount, Arlington, and Wooster showed the largest differences among cultivars in 1998, while Rock Springs and Ithaca showed the largest differences in 1999. One location was an anomaly compared with the others; the trial at Orange, VA, showed superior forage yield for Tandem compared with Spring Green (P < 0.05). There was no pattern to soil types, N fertilization rate, latitude, longitude, or USDA hardiness zone of the locations that successfully discriminated among cultivars vs. those that did not discriminate.
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Spring Green averaged 4.0 and 5.3% higher forage yield than Kemal in the first and second production years, respectively (Table 3). Spring Green ranked higher than Kemal for forage yield at 11 of 14 first-year locations and nine of 13 second-year locations. Averaged over years, Spring Green was 5.0% higher in forage yield than Kemal and ranked higher than Kemal at 11 of 13 locations. Differences in forage yield between Spring Green and Tandem were inconsistent and only occasionally significant. Spring Green ranked higher than Tandem at five of 14 first-year locations and six of 13 second-year locations. Averaged over locations within each year and over years, Tandem and Spring Green were not significantly different in forage yield.
Spring Green had significantly lower yield than Tandem at Orange, VA. Orange, VA, was one of only two locations in USDA hardiness zone 7 (along with Blackstone, VA); all other locations were in zones 2 through 6. The consistent superiority of Tandem to Spring Green at Orange may indicate that Spring Green is less adapted to warmer climates.
Mean ground cover in spring 2000 ranged from 27 to 91% at the 10 locations that experienced some loss of stand. The three Virginia locations were excluded from all ground cover analyses, because they suffered no stand losses. With the obvious exception of Rosemount, MN, mean ground cover was highly correlated with USDA hardiness zone classification (Table 4) . Longer and more reliable snow cover may explain the higher ground cover at Rosemount.
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Differences in ground cover among cultivars were greatest for the six locations with the lowest USDA hardiness zone ratings (Table 4). For these six locations, Spring Green averaged 42% higher ground cover than Tandem (52 vs. 37%; P < 0.01) and 22% higher ground cover than Kemal (52 vs. 43%; P < 0.01). The cultivar x region interaction (2 df) accounted for 46% of the cultivar x location interaction (18 df) for ground cover. Both the cultivar x region and the cultivar x location(region) interactions were significant (P < 0.01). While there may be other climatic, soil, and/or management factors that might account for differences in cultivar performance across locations, USDA hardiness zone classification clearly accounts for approximately half of the cultivar x location interaction. It is unlikely that any single environmental factor could be found to account for a significant portion of the remainder of this interaction.
Because the five locations used to select field survivors (Arlington, Ashland, Prairie du Sac, Spring Green, and Spooner, WI) all fell within USDA hardiness zones 2 through 4, it is not surprising that persistence was improved in this region, but not outside this region. Freezing tolerance is important for survival in these hardiness zones, while it may be unimportant or less important at locations in hardiness zones 5 through 7. Thus, Spring Green festulolium appears to represent an adaptive improvement to a specific group of environments, defined by relatively severe winter conditions. Our ability to measure improvements in Spring Green across the range of soil types, N fertilization rates, and locations within hardiness zones 2 through 4 suggests that this adaptive improvement is quite broad within these hardiness zones.
Received for publication August 27, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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