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a Dep. of Crop and Soil Sci., Univ. of Georgia, Athens, GA 30602
b AgRes. Grassl., Tennent Dr., Palmerston North, New Zealand
c Dep. of Anim. Sci., Univ. of Georgia, Athens, GA 30602
d College of Veterinary Medicine, Univ. of Georgia, Athens, GA 30602
* Corresponding author (jbouton{at}arches.uga.edu)
Received for publication March 1, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: AR, AgResearch • E+, endophyte infected • E-, endophyte-free
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Summer drought results in the greatest loss of tall fescue stands in the Southeast, with cultivars infected with their endemic N. coenophialum endophyte demonstrating much better survival in very hot, dry summers than the same cultivars with their endophyte removed (Bouton et al., 1993a). Therefore, the toxicity of E+ tall fescue presents livestock producers with a dilemma of whether to grow current E+ cultivars for stand persistence and risk reduced animal performance due to the inherent toxins.
Animal toxicity can be reduced in current E+ pastures with pasture management such as interplanting with clovers to dilute the toxins directly in the forage before consumption (Ball, 1997) or controlling of toxicosis directly in the animals with drugs, vaccines, feed additives, or detoxification agents (Oliver, 1997; Stuedemann and Thompson, 1993). Cultivar improvement to develop either more persistent endophyte-free (E-) cultivars or E+ cultivars with reduced or nil production of toxic alkaloids is also being pursued (Bouton, 1996, 1998). For the E+ cultivar development, the main objectives would be to reduce the levels of alkaloid production through plant genetic selection of the plant genotype–endemic strain association (Adcock et al., 1997); isolate and genetically manipulate the E+ endophytes themselves to reduce their toxicity for later reinfection (Wilkinson and Schardl, 1997); and select naturally occurring, nontoxic endophyte strains for later reinfection into elite cultivars (Latch, 1997; West et al., 1998). The approach of isolating naturally occurring, nontoxic endophyte strains for reinfection into elite cultivars was found to provide better animal performance and stand survival in the perennial ryegrass (Lolium perenne L.)–N. lolii complex (Fletcher and Easton, 1997; Tapper and Latch, 1999). Because nontoxic strains of the tall fescue endophyte were also available, this strategy could be attempted for tall fescue (Latch, 1993).
Two tall fescue cultivars that had previously been released for the southeastern USA were selected for use in this study. Georgia 5 was developed as an E+ cultivar for the southern Gulf Coast region (Bouton et al., 1993b), and Jesup (Bouton et al., 1997) was developed as an E- cultivar for use in the main fescue-growing regions, primarily because of its excellent animal performance results (Hoveland et al., 1997). However, as is the case with most tall fescue cultivars, these two exhibit better summer survival when grown E+ (Bouton et al., 1993a). Therefore, the adaptation and excellent performance of Georgia 5 and Jesup, especially when infected with endophytes, made them good candidates for reinfection with non-ergot alkaloid–producing N. coenophialum endophytes.
Our objective was to assess the strategy of killing the toxic resident endophyte in seed of tall fescue cultivars and reinfecting seedlings with non-ergot alkaloid–producing N. coenophialum endophytes. This paper presents testing results for animal performance, stand survival, and yield trials of Georgia 5 and Jesup after their reinfection with non-ergot-producing endophyte strains.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Statistical Analyses
All data were statistically analyzed by PROC ANOVA or PROC GLM and means separated via LSD (SAS Inst., 1982). However, the means for total ergot alkaloid concentration and serum prolactin showed nonhomogenity among their variances due to some treatments having near-zero values and others possessing values in the hundreds or even thousands. For this reason, the ergot and prolactin data, and only these data, were subjected to a square root transformation, and statistical analyses were then performed on these transformed values.
Stand Survival Trials
Stand survival was assessed in separate trials at two Georgia locations, Eatonton and Tifton, using proven procedures to separate E+ and E- tall fescue with bermudagrass [Cynodon dactylon (L.) Pers.] competition and grazing (Bouton et al., 2001). The Tifton trial was established in a Tifton sandy loam soil (fine-loamy, siliceous, thermic, Plinthic Kandiudult) at the University of Georgia Coastal Plain Experiment Station, Tifton, GA. The Eatonton trial was established in a Mecklenburg sandy loam (fine, mixed, thermic, Ultic Hapludalfs) at the University of Georgia Central Georgia Branch Station, near Eatonton, GA.
In both trials, the experimental design was a randomized complete block with either four (Tifton) or six (Eatonton) blocks and tall fescue entries as treatments. The fescue entries were Jesup (E+), Jesup (E-), Jesup (AR502), Jesup (AR510), Jesup (AR542), Georgia 5 (E+), Georgia 5 (E-), and Georgia 5 (AR542). The experimental area was established uniformly with bermudagrass, which was clipped to a 7.5-cm height with a flail mower just before seeding fescue entries. Each entry was sown with a Hege precision drill at a rate of 22.5 kg ha-1 seed on 11 Nov. 1996 at Tifton and 20 Oct. 1997 at Eatonton. Plot size was 1.5 by 3.5 m. The experimental areas were fertilized uniformly with 67, 15, and 28 kg ha-1 N, P, and K, respectively, as a complete fertilizer at establishment and in September or October in each subsequent year. Only broadleaf weeds were controlled on an as-needed basis in the experimental areas with 2,4-D herbicide (2,4-dichlorophenoxyacetic acid) at a rate of 0.45 kg a.i. ha-1. The experimental areas were continuously grazed with beef cattle from April until November of each year. Animals were placed into or removed from the paddock area to maintain a grazing height of approximately 7.5 cm.
Stand survival was quantified using a 1.5-m rod graduated into 10-cm sections, recording the number of sections in three of the seven drill rows comprising the plot that contained a living tall fescue tiller, and dividing by the total number of sections. At Tifton, stand assessments were made on 4 Mar. 1997 (initial), 11 Dec. 1997, and 22 Jan. 1999. At Eatonton, stand assessments were made in a similar manner on 7 May 1998 (initial), 18 May 1999, and 7 Dec. 1999.
Yield Trials
Dry matter yield was assessed using replicated small plots at two locations in Georgia; these were the Plant Sciences Farm near Athens and the Mountain Experiment Station at Blairsville, GA. The soil at the Athens site was a Cecil sandy loam soil (clayey, kaolinitic, thermic, Typic Hapludult), and at Blairsville, it was a Congree clay loam (fine-loamy, mixed, non-acid, thermic Fluventic Dystrochrepts). The experimental design was a randomized complete block with six replicates (blocks) and tall fescue entries as treatments. Entries were Jesup (E+), Jesup (E-), Jesup (AR502), Jesup (AR542), Georgia 5 (E+), Georgia 5 (E-), and Georgia 5 (AR542). However, at the Athens location, Jesup (AR510) was an additional entry. Plots of each entry (1.5 by 3.5 m) were established using a Hege precision drill in plowed and disked soil during October 1997. Plots were mowed and fertilized uniformly with 67, 15, and 28 kg ha-1 N, P, and K, respectively, as a complete fertilizer at establishment and in early September in each year. An additional 56 kg N ha-1 was applied as liquid N in the late winter of each year. All plots were harvested with a flail mower three times during 1998 (May, June, and November) and four times in 1999 (May, June, August, and November) at both locations. At each harvest, subsamples were taken to determine dry matter percentage, and all plot yields were calculated on a dry matter basis.
Animal Toxicity Trials
Jesup (AR502), Jesup (AR542), and Georgia 5 (AR542) were selected for animal trials and were initially tested against E+ and E- Jesup for a 10-wk spring grazing period (March to June, 1998–2000) and a 8-wk fall grazing period (October to December, 1998–1999) at the University of Georgia Central Georgia Branch Station, Eatonton, GA. Paddock size was 30 by 30 m. There were two paddocks (reps) of each tall fescue entry sown at the rate of 25 kg ha-1 on 4 Sept. 1997. Paddocks were fertilized uniformly with 67, 15, and 28 kg ha-1 N, P, and K, respectively, as a complete fertilizer at establishment. In March and September of each of subsequent year of the trial, 100 kg N ha-1 was applied to each paddock as liquid N. The initial stocking rate was four to five 25-kg lambs per paddock. Two animals were initially designated as testers and the others as grazers. Based on forage availability, a put-and-take system was then employed to adjust stocking rate during the 10-wk grazing period by removal of one or more tester animals. The testers stayed on the paddock the entire time. Initially, and every 2 wk during each grazing period, lamb weight gain, rectal temperature, and serum prolactin were recorded. Forage-available yield and ergot alkaloid concentration were also determined initially and every 2 wk by sampling ten 0.09-m2 quadrat samples throughout the paddock area. These grazing studies were conducted under University of Georgia animal use regulations in compliance with the Animal Welfare Act of 1966 & 9 CFR Subchapter A.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Stand Survival
By the second evaluation date (11 Dec. 1997) at the Tifton location, Jesup (E+) was found to have better (P < 0.05) stand survival compared with Jesup (E-) and Jesup (AR510) (Table 2). Jesup (AR502) and Jesup (AR542) showed stand survival that was no different (P < 0.05) than the E+ or E- Jesup checks at this sampling date. This was also the case for Georgia 5 (AR542) when compared with its E+ and E- checks; however, it is noted that the Georgia 5 (E+) check exhibited poor survival (Table 2). By the third sampling date (22 Jan. 1999; after two summer seasons), Jesup (AR542) exhibited stand survival that was better (P < 0.05) than Jesup (E-) and no different (P < 0.05) than Jesup (E+) (Table 2). Jesup (AR502) and Jesup (AR510) showed stand survival similar (P < 0.05) to Jesup (E-) but inferior (P < 0.05) to Jesup (E+). Similarly, Georgia 5 (AR542) was no different (P < 0.05) in survival than either Georgia 5 (E+) or Georgia 5 (E-) at the 22 January sampling date (Table 2).
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Animal responses over the 10-wk spring grazing period showed some immediate and consistent results for the E+, E-, and AR542 versions of Jesup. First, only the E+ forage possessed high total ergot alkaloid levels which were found to increase at each 2-wk interval until the last sampling date when the levels dropped slightly (Fig. 1) . Both the E- and AR542 forage showed similar and baseline levels of ergot alkaloids at all sampling dates. Secondly, average daily weight gain and blood prolactin levels showed significant differences (P < 0.05) among treatments by the 2-wk sampling period, and these differences remained the same for the rest of the grazing season (Fig. 2 and 3) . Animals grazing E+ forage showed less average daily gain (P < 0.05) at each sampling date than animals consuming E- or AR542 forage (Fig. 2). The serum prolactin levels of animals on E+ forage dropped to near zero (within the detection limits of the assay) by the 2-wk sampling date and remained very low for the duration of the trial (Fig. 3). Animals on E- and AR542 forage did not exhibit the depressed blood prolactin responses of E+ animals (Fig. 3). Finally, during the first 2 yr, rectal temperatures were sampled during the morning hours, with no differences seen among entries. However, in 2000, rectal temperatures were recorded in the afternoon, and differences were observed (Fig. 4) . Although body temperatures were initially variable, animals on E+ forage showed consistently elevated body temperatures (P < 0.05) over the following 6 wk compared with contemporaries on E- or AR542 forage.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Based on this background information on perennial ryegrass, it was speculated that the same approach could be attempted for the tall fescue–N. coenophialum association (Latch, 1997; West et al., 1998). For tall fescue, nontoxic endophyte strains (e.g., nil production of total ergot alkaloids, yet capable of producing pest-deterring peramine and loline alkaloids) were isolated from field-grown plants collected throughout the world (Latch, 1993; G.C.M. Latch, personal communication, 1994). The main initial criteria when infecting cultivars, which are populations of different plant genotypes, would be high infection and transmission rates of the reinfected strains during the seed increase phase. In these current studies, initial seed production on the 200 inoculated genotypes of each cultivar–strain combination was conducted in isolation glasshouses. This seed was then planted into a field isolation block. Tillers from these field isolation blocks showed differences among various cultivar–strain combinations for infection rate (Table 1). For example, the AR572 and AR577 strains did not hold a high infection frequency in either cultivar, AR502 and AR510 transmitted or maintained at a high frequency in Jesup but not in Georgia 5, and AR542 transmitted equally well in both cultivars. All combinations, however, produced nil or near-nil levels of total ergot alkaloids (Table 1) and nil levels of ergovaline (data not shown). They also maintained their high initial level of infection during all agronomic and animal toxicity trials (Tables 2–7). These results support the plan that any reinfection of an isolated strain into a new cultivar will need to be intensely screened for compatibility and infection transmission and maintenance (West et al., 1998).
Typically, animal responses to the effects of tall fescue endophytic toxins can be grouped into four categories (Stuedemann and Thompson, 1993): (i) decreased weight gain and pregnancy rate, (ii) behavioral criteria demonstrated with decreased feed but increased water intake, (iii) physiological responses such as increased respiration and elevated rectal and core body temperatures, and (iv) changes in sera or plasma levels of constituents such as decreased serum prolactin and cholesterol. One or more of these responses are usually measured when assessing fescue toxicity (Stuedemann and Thompson, 1993). In our lamb toxicity trials, only the E+ treatment was found to produce toxic levels of ergot alkaloids (Tables 6 and 7), which continued over the course of the grazing season (Fig. 1). Removal of ergot alkaloids eliminated the main animal responses associated with fescue toxicity, regardless of the AR strain or cultivar host (Tables 6 and 7). This response was also very stable over years and grazing seasons. Another interesting trend was how quickly the toxins affected animal weight gain, serum prolactin, and rectal temperature; these responses were significantly affected by 2 wk of exposure for animals on the toxic E+ forage compared with their contemporaries on E- or AR542 forage (Fig. 2–4).
The main dilemma for livestock producers using tall fescue in the USA is whether to grow current E+ cultivars for stand persistence and risk reduced animal performance due to the inherent toxins. Because E- cultivars eliminate fescue toxicity and are an alternative for producers willing to risk stand loss, any cultivar reinfected with a nontoxic endophyte strain must, at a minimum, provide better agronomic performance than E- cultivars to be a viable option for producers. The most successful combinations should provide the agronomic advantage of E+ cultivars.
For agronomic yield and stand survival, the substitution effect of a non-ergot alkaloid–producing strain was more complex and variable than for the animal responses. For example, of the three strains intensively tested in Jesup, only AR542 gave a response approaching that of the E+ version for dry matter yield and stand survival across all testing environments (Tables 2– 5). In Georgia 5, this response was not as apparent as stand survival during the second year, and dry matter yields in some instances were less for the AR542 version compared with its E+ version (Tables 2–5). However, although the difference between Jesup (E+) and Jesup (AR542) means for the final stands in the survival trials (Tables 2 and 3) is within the LSD value, the coefficient-of-variation percentages are high, and Jesup (AR542) was never numerically as good as the Jesup (E+). These responses are interpreted as meaning it may be difficult to achieve the exact duplication of E+ agronomic performance with a reinfected strain, but the positive effect of AR542 in Jesup is encouraging and should be important for stressful pasture situations. Finally, any new cultivar reinfected with even a previously successful strain such as AR542 will need to be intensely tested for agronomic performance before release into the commercial seed trade.
Overall, these studies demonstrated that reinfection of naturally occurring, non-ergot–producing endophyte strains into elite tall fescue cultivars after removal of their toxic, endemic strain(s) is a good strategy for eliminating fescue toxicosis and providing better animal performance while keeping the agronomic performance close to that desired by producers. In this regard, the AR542 strain was the most successful substitute for the endemic strain in Jesup in these current studies. Finally, the AR542 strain is being marketed by Pennington Seed, Madison, GA, under the trademark name of MaxQ.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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J. A. Parish, M. A. McCann, R. H. Watson, C. S. Hoveland, L. L. Hawkins, N. S. Hill, and J. H. Bouton Use of nonergot alkaloid-producing endophytes for alleviating tall fescue toxicosis in sheep J Anim Sci, May 1, 2003; 81(5): 1316 - 1322. [Abstract] [Full Text] [PDF]
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Crop Science | Vadose Zone Journal | |||
| Journal of Natural Resources and Life Sciences Education |
Soil Science Society of America Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
