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Forage Improvement Division, Samuel Roberts Noble Foundation, Ardmore, OK 73401
* Corresponding author (aahopkins{at}noble.org)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: E+, endophyte infected • E–, endophyte free • GxE, genotype by environment • KY-31, ‘Kentucky 31’ • rs, Spearman's rank correlation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Grasses may respond differently to grazing than clipping (Reardon et al., 1974; Wallace, 1990). Factors which may affect the plant under grazing, such as saliva (Reardon et al., 1972), trampling, fouling, and tiller pull-up, are not present, or may not be as prevalent, in a clipping situation. Bouton et al. (2001) found that, compared with indirect estimation of grazing tolerance by mechanical defoliation, grazing caused greater stand loss in tall fescue after 2 yr, particularly when in conjunction with bermudagrass competition. Similar results were reported for orchardgrass by Papadopoulos et al. (1995). Thus, evaluation of grazing tolerance of grasses is probably best accomplished by utilizing animals rather than machinery for defoliation.
Although continuous heavy grazing pressure is generally not a desirable pasture management practice, this scheme is thought to essentially eliminate selective grazing by animals, thus permitting the identification of grazing-tolerant germplasm while minimizing the possibility of selecting nonpalatable plants. Analogous procedures are used to evaluate insect (Berberet et al., 1991) and disease resistance (Berg et al., 1986; Zeiders et al., 1974) in forage crops. By exposing plants to substantial insect or pathogen pressure, sometimes repeatedly, escapes are minimized. Populations characterized as being poorly or nongrazing tolerant under continuous, heavy grazing pressure might be capable of performing well under moderate grazing stress. However, severe grazing pressure is useful when a breeder needs to identify only those accessions with outstanding levels of grazing tolerance and persistence. Heavy, continuous grazing has proven useful in developing alfalfa (Medicago sativa L.) cultivars able to withstand such extreme grazing pressure (Bouton et al., 1991, 1997).
Seed availability can be limiting for many applications in breeding programs. For example, the USDA National Plant Germplasm System typically distributes 500 seeds or less for a given grass accession. To evaluate such an accession in the field, the breeder is generally faced with the choice of either producing additional seed before evaluation in seeded sward plots (Casler and van Santen, 2001), or transplanting seedlings using a space-planted arrangement to conduct preliminary evaluations (Hopkins and West, 2002). The amount of effort and resources needed to evaluate grazing tolerance of a broad array of germplasm might be decreased by using space-planted trials to identify accessions warranting seed increase and further evaluation or selection.
Forage grasses do not always perform consistently across space-planted and seeded sward plot arrangements. In perennial ryegrass (Lolium perenne L.), data obtained for seed yield (Elgersma, 1990), forage yield (Hayward and Vivero, 1984), or crude protein (Humphreys, 1989) from spaced plants did not correlate well with those obtained from seeded sward plots. In contrast, water soluble carbohydrate concentration, dry matter digestibility, and forage yield in perennial ryegrass (Humphreys, 1989), as well as alkaloid concentration in reed canarygrass (Phalaris arundincea L.) (Hovin and Marten, 1975) have shown similar responses in seeded sward plots and spaced plants. Selection of spaced plants was effective in improving forage yield of Pensacola bahiagrass (Paspalum notatum var. saure Parodi) (Burton, 1974, 1982), perennial ryegrass (Hayward and Vivero, 1984), and digestible dry matter yield of timothy (Phleum pratense L.) (Surprenant et al., 1990), when measured in space-planted evaluations. However, selection was effective only in the case of Pensacola bahiagrass when these same populations were evaluated as seeded sward plots. It is not clear whether cool-season grasses display similar responses to heavy grazing pressure when evaluated in seeded sward and space-planted plots. Thus, the objective of this research was to evaluate grazing tolerance of several cool-season perennial grasses planted as seeded sward plots and spaced plants, to compare relative performance of entries across planting arrangements and time. Information regarding genotype x location responses of grazed cool-season grasses in the southern Great Plains was also of interest.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The initial number of live spaced plants for Trial 1 was counted on 15 March 2000, before any grazing, to take into account plant death due to transplanting. Winterkill was not observed in any of the plantings. Stands for seeded sward plots were also taken at this time using a grid method (Hopkins et al., 1993; Vogel and Masters, 2001). Briefly, a 1-m2 grid, divided into 25 equal subgrids, was placed randomly within a plot, and the number of grids not containing a live plant were counted. This was repeated in a second location within the same plot. The total number of grids not containing a live plant was multiplied by two, and the resulting number subtracted from 100 to give stand in percentage units.
Plots were grazed so that usually within 5 d of initial grazing, stubble height was reduced to, and subsequently maintained at, approximately 5 cm or less. Very little or no summer regrowth occurred because of onset of summer dormancy. Grazing was continuous within a given period, whereas stocking rates for beef cattle varied from roughly 1800 to 11000 kg live weight ha–1, depending on visual estimates of forage availability. It should also be noted that heavy infestations of grasshoppers occurred at the Burneyville location in summer 2000 and 2001, with densities of 32 grasshoppers m–2 being observed in nearby fields (Jeff Ball, Noble Foundation, personal communication, 2001).
Following grazing (Table 2), spaced plants were assigned a vigor rating, with 0 being dead, 9 being maximum vigor. Vigor ratings were based on visual estimates of the amount of fall growth, density of tillering, and plant diameter, or for rhizomatous species, plant spread. Fall re-growth had essentially ceased when vigor ratings were taken. Recovery score for a given space-planted plot was calculated based on the vigor ratings and initial number of live plants within that plot as follows.
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Data Analyses
For Trials 2 and 3, data were analyzed across locations with replications nested within a location. Within a trial, data were analyzed across age of stand as a split plot in time. Because of the prevalence of entry x location and entry x age of stand interactions, data were also analyzed separately for each trial x age of stand combination (i.e., environment) as a randomized complete block design. Mixed models were used for all analyses with entry and age of stand considered fixed effects, and replication and location as random effects. Analyses were performed using PROC MIXED of SAS (SAS Institute, 1999), with error terms and denominator degrees of freedom being specified using the DDFM = SATTERTH option. Rank correlations between stand, survival, and recovery scores of entries were calculated based on data averaged across all replications for a given environment, using the PROC CORR procedure with the Spearman option of SAS (SAS Institute, 1999). Means were calculated as generalized least squares means (LSMEANS) and compared using the PDIFF option of SAS. Except where noted, the 0.05 level of probability is used to declare significance.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Seeded sward plots established well, with average stands for all entries exceeding 65% (Trial 1) and 80% (Trials 2 and 3) before the start of grazing. Similar establishment was obtained for spaced plants. Exceptions to this in Trial 1 included the wheatgrass ‘Jose’, with an average stand of 48%, as well as ‘Luna’ and ‘Oahe’, both of which had few transplants; all data for these entries in Trial 1 were subsequently dropped from the analyses.
Strong GxE interactions occurred throughout the experiment. Entries responded differently to contrasting levels of stress at Ardmore and Burneyville, resulting in a significant entry x location interaction for stand in Trials 2 and 3. For example, at Burneyville stands of all entries except Barton western wheatgrass were nearly or completely eliminated within 26 mo of planting (Tables 3 and 4). In contrast, more than 48 mo after planting at Ardmore, stands of Barton were 31%, compared with more than 70% for ‘Maru’ hardinggrass and more than 10% for the PDF entries. Likewise, survival of spaced plants after 2 yr in Trial 2 was nil except for a trivial number of Barton plants, whereas roughly 25 to 50% of PDF E+ and E–, Barton, and Maru spaced plants survived for 4 yr at Ardmore (Table 4). Considering that the Ardmore and Burneyville sites are located within 65 km of each other, I speculate that differences in grazing tolerance at the two locations can be attributed mostly to differences in soil type. The Konsil soil at Ardmore, characterized by loamy fine sand up to 35 cm deep over sandy clay loam subsoil (Moebius and Maxwell, 1979), would be expected to retain moisture better than the Minco soil at Burneyville, which is described as having a coarse-textured surface layer and subsoil (Maxwell and Reasoner, 1966). These results help illustrate the need for farmers and ranchers to establish current cultivars of cool-season perennial grasses on more moisture-retentive soils in the Southern Plains. From a breeding standpoint, the GxE situation can be addressed by developing cultivars of a number species, each tailored to a narrow range of soil and climate conditions, and/or cultivars that are more broadly adapted to the region. Both approaches will require evaluating populations in a broad array of environments to characterize their adaptation.
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With few exceptions, stand and survival declined in time. Stand increased for Barton at Burneyville from Year 1 to Year 2 in both Trial 1 (P = 0.0551) and 2 (Tables 3 and 4) because of substantial rhizome formation. Stand as well as survival of Barton at Ardmore remained stable for the first two years but declined by Year 4. It should be noted that estimates of survival for Barton may be inflated, as it was exceptionally difficult to determine identity of individual plants because of the highly rhizomatous nature of this entry. Despite a numerical trend, at Ardmore stands of PDF E+ and PDF E– did not significantly increase with time. Survival of these two entries as spaced plants was equivalent to Barton (P > 0.70) and had declined to around 50% by Year 4 (Table 4). Although entry ranks for stand were correlated between Years 1 and 2 in Trials 1 (Spearman's rank correlation, rS = 0.75), 2 (rS = 0.63), and 3 (rS = 0.80), several of the wheatgrass and smooth bromegrass cultivars that showed promise after 1 yr failed to persist for an extended period (Tables 3 and 4). The data suggests that grazing tolerance of cool–season grasses can be characterized after 1 yr in some cases, but it is generally best to conduct such evaluations across several years, as has been found in numerous environments (Casler et al., 1998; Brummer and Moore, 2000; Bouton et al., 2001; Culvenor et al., 2002).
Stubble height after grazing appeared to vary with species based on visual estimates. For example, at Ardmore, both seeded and space-planted plots of Maru were grazed to approximately 1 to 2 cm, whereas tall fescue plots were grazed to about 5 cm of stubble height. It is not known if these slight differences in stubble height after grazing affected grazing tolerance of the entries examined in this research. Differences in stubble height after grazing were not apparent in the trials at Burneyville. Maturity differences and endophyte infection status has been reported to affect grazing preference in tall fescue (van Santen, 1992). Slight differences in grazing preference by beef cattle could probably be managed by segregating grazing tolerance trials by species, maturity, and endophyte infection status.
As the plantings became older, populations in a given environment responded more similarly to heavy grazing pressure across the planting arrangements. Rankings of entries for stand were correlated with survival following 2 yr at Burneyville for Trial 2 (rS = 0.74). Entry ranks for stand and survival in Trial 1 were not correlated after 2 yr (rS = 0.56; P = 0.12) most likely because of minor differences among entries with very limited grazing tolerance. Barton was the only entry with any appreciable level of grazing tolerance in both trials at Burneyville. In contrast, Paiute orchardgrass consistently ranked at or near the bottom of all entries in terms of stand and survival (Tables 3 and 4). At Ardmore, rankings for stand and survival were strongly correlated but only after 4 yr (rS = 0.73). Maru had the greatest grazing tolerance in seeded sward plots, but did not survive as well as Barton and the PDF entries when space planted. Maru perhaps would have been classified as potentially grazing tolerant in a space-planted germplasm evaluation, however, given that it ranked among the top third of entries for survival. Survival and recovery scores were very closely correlated within environments, ranging from rS = 0.81 to 1.00. In this research, rank correlations between stand and recovery scores were very similar to those between stand and survival. Thus, estimates of grazing tolerance for populations can be determined by counting surviving plants, without having to consider plant vigor.
On the basis of these results, it appears likely that populations with promising levels of grazing tolerance can be identified, in some cases within 2 yr, using space-planted arrangements. Where limited amounts of seed are available, the Noble Foundation grass breeding program has used this approach to screen large numbers of accessions at a single location. Subsequently, seed is produced from the most promising accessions and used to establish seeded sward trials, typically at multiple locations, for further evaluation. These small plots can also be used directly as selection nurseries by recovering the most vigorous plants following multiple years of grazing pressure. Deposits of animal waste can cause plants to appear to be vigorous due to high fertility patches or lack of uniformity in grazing, so care is needed to avoid selecting such plants from grazing tolerance trials or pastures.
Various techniques may be useful in evaluating grazing tolerance in cool-season perennial grasses. Individual seeded rows, often referred to as headrows, may be an alternative to spaced plantings for evaluating accessions with limited seed supply. The effect of factors such as continuous vs. rotational stocking, grazing with cattle versus sheep (Ovis aries L.), grazing deferment during the establishment year, and plot border effects may need to be considered in grazing tolerance evaluations. Additional research would be helpful in refining protocols for determining grazing tolerance of perennial grasses.
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ACKNOWLEDGMENTS |
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Received for publication June 10, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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This article has been cited by other articles:
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  A. A. Hopkins and M. W. Alison Stand Persistence and Animal Performance for Tall Fescue Endophyte Combinations in the South Central USA Agron. J., August 3, 2006; 98(5): 1221 - 1226. [Abstract] [Full Text] [PDF]
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