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Published online 19 October 2005
Published in Agron J 97:1473-1477 (2005)
DOI: 10.2134/agronj2004.0304
© 2005 American Society of Agronomy
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Production Papers

Reinfestation of Endophtye-Infected Tall Fescue in Renovated Endophyte-Free Pastures under Rotational Stocking

Benjamin F. Tracy* and Ian J. Renne

Department of Crop Sciences, Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801

* Corresponding author (bftracy{at}uiuc.edu)

Received for publication December 8, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Endophyte-infected (E+) tall fescue [Lolium arundinaceum (Schreb.) S.J. Darbyshire] is problematic in pastures because it produces alkaloids that can be toxic to cattle (Bos taurus). Replacement of E+ tall fescue pasture with endophyte-free (E–) fescue may effectively eliminate this problem. Endophyte-free cultivars, however, are less competitive than E+ fescue and are usually displaced over time. The main objective of this study was to determine whether E+ fescue would reinfest pastures planted with mixtures of E– fescue and other species and grazed under rotational stocking. In 2001, nine tall fescue pastures in western Illinois were renovated and planted with mixtures that contained E– fescue (Barcel) and two, four, or seven additional species. Pastures were grazed by beef cattle over 3 yr. Tall fescue tillers were collected each September and subjected to microscopic analysis for endophyte presence. After renovation, E+ still accounted for 18 to 38% of tall fescue. Relative to all other species, however, E+ fescue was <10% of pasture communities. The species mixtures sown with E– fescue had little influence on E+ fescue reinfestation (P = 0.70). Contrary to other studies, percentage of E+ fescue in E– pastures did not increase in the 3 yr of this study. Moderate grazing pressure under rotational stocking combined with relatively wet growing seasons likely favored E– fescue and suppressed E+ reinfestation. We also suggest that if pasture renovation can reduce E+ fescue percentage to <10%, it is unlikely that E+ fescue will reinfest pastures to an extent that may cause fescue toxicosis in cattle.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE ADAPTABILITY of tall fescue to diverse environmental conditions, and its resistance to grazing stress has made it one of the most valuable pasture grasses used in the USA (Buckner et al., 1979). Tall fescue usually harbors a systemic endophyte fungus [Neotyphodium coenophialum (Morgan-Jones and Gams) Glenn, Bacon and Hanlin], which confers stress tolerance characteristics to the plants (Clay, 1988; Bacon, 1993). Although endophyte-infected tall fescue (E+) is highly persistent, the wild-type endophyte fungus produces alkaloids that can cause toxicosis symptoms in cattle and other livestock (Read and Camp, 1986; Schmidt and Osborn, 1993; Roberts and Andrae, 2004). The best strategy to eliminate future toxicosis problems in cattle is to replace E+ fescue with nontoxic forages like endophyte-free tall fescue (E–) or cultivars that possess the endophyte but are nonergot alkaloid producing (Bouton et al., 2002). Although use of E– cultivars can improve animal performance (Hoveland et al., 1983), these cultivars are less persistent than E+ fescue (Latch, 1993; Shelby and Dalrymple, 1993). Compared with E– cultivars, E+ tall fescue is more drought tolerant, has greater herbivore resistance, and can be more competitive under environments that have significant levels of abiotic and/or biotic stress (Hoveland et al., 1983; Hill et al., 1998; Clay and Holah, 1999). The poor competitiveness of E– cultivars may explain why E+ fescue can rapidly infest swards established with E– fescue (Thompson et al., 1989; Shelby and Dalrymple, 1993; Gwinn et al., 1998).

In 2001, we initiated a field experiment to address relationships between forage species composition and pasture productivity under rotational stocking. The experiment involved renovation of nine pastures in western Illinois that contained tall fescue and sowing those pastures with seed mixtures that contained E– fescue and two, four, or seven additional species. As part of this study, we wanted to test the hypothesis that new E– pastures would be gradually infested by E+ fescue over 3 yr, but that the more complex mixtures (seven species) would show the least infestation. Recent evidence suggests that pasture communities with an even distribution of plant diversity are more effective at resisting weed invasion than simple communities (Wilsey and Polley, 2002; Tracy et al., 2004; Tracy and Sanderson, 2004). We also hypothesized that if E+ infestation was suppressed by resident forage species, then damage to the newly renovated pastures should promote spread of E+ plants. This spread may occur if E+ seed germinates in bare ground exposed from trampling or if E+ plants increase tillering in response to a more open canopy. Our specific study objectives addressed the following questions: Did the percentage E+ tall fescue in pastures increase after renovation? Did E+ fescue reinfestation differ among the three sown mixtures? Was the amount of E+ fescue reinfestation affected by past cattle trampling disturbances?


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This study was part of a rotational grazing experiment conducted at the University of Illinois Orr Center Beef Unit located in Baylis, IL (39.7° N lat; 90.9° W long). The center has 24 pastures ranging from 3 to 6 ha located on rolling deep-loess soils (primarily Hapludalfs, Ochraqualfs, and Albaqualfs). The pastures have been used to supply forage for beef cattle herds housed at the research station since 1990. Pasture management previous to our experiment could best be described as low input. In August 2001, nine of these pastures were renovated by first treating existing vegetation with one application of glyphosate isopropylamine salt herbicide (1.12 kg a.i. ha–1). Existing sods then were turned with a moldboard plow, disked, and finally harrowed to prepare a clean seedbed. Previous to renovation, pastures contained mostly tall fescue, Kentucky bluegrass (Poa pratensis L.), and red clover (Trifolium pratense L.). Tall fescue in these pastures accounted for approximately 50 to 80% of ground cover, but we did not determine the proportion of E+ in pastures before renovation. Aside from occasional red clover frost seeding, the nine pastures had not been planted with any new species since at least 1979. Soil tests previous to renovation showed that pastures had a mean pH of 6.8, whereas P and K averaged 59 and 333 kg ha–1, respectively. Before planting, all pastures received 44 kg N ha–1 as urea and individual pastures were fertilized with DAP or potash as recommended by soil test.

Three mixtures—which consisted of E– tall fescue (Barcel) and two, four, and seven additional species (Table 1)—were drilled into a clean seedbed in early September 2001. The mixtures were randomly assigned across the nine pastures so that each mixture was replicated three times. One pasture sown with five species was excluded from the experiment due to heavy weed invasion. Each pasture was sown to a target seeding rate of 22 kg ha–1 based on total seed weight (Table 1). In May 2002, each renovated pasture was divided into six paddocks (~0.8 ha each) for rotational stocking. Cow–calf pairs (black purebred Simmental cattle) were assigned to pastures to achieve an initial stocking rate of ~2.5 cow–calf pairs ha–1. For most of each grazing season, each paddock was grazed for 6 d with 30 d of rest. A faster rotation was used in spring to keep up with forage growth. Put-and-take cattle were added or removed based on subjective evaluation of forage availability. Pregraze canopy height in paddocks usually averaged between 16 and 25 cm and post-graze height 6 to 9 cm. Pastures were grazed from 17 May 2002 to 9 Sept. 2002, 23 Apr. 2003 to 28 Oct. 2003, and 26 Apr. 2004 to 28 Oct. 2004. Forage yields will be reported in a future paper, but yields among the three mixtures were generally similar during most of the growing season (B. Tracy, unpublished data, 2005).


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  		Table 1. Plant species and seeding rates for the seed mixtures used in renovated pastures.

 
Tall fescue tillers were collected in late September each year when fescue plants were still actively growing. We collected a total of 72 tall fescue tillers from each pasture. Thompson et al. (1989) suggested that 6 to 8 samples ha–1 were sufficient to characterize E+ status in pastures, and our sampling intensity was approximately 10 to 12 samples ha–1. Two of the six paddocks within each pasture were sampled for the presence of E+ fescue. One paddock in each pasture received significant trampling disturbance by cattle during a heavy rain in May 2002. This event increased the amount of bare ground relative to other paddocks in the pastures. We wanted to compare this ‘disturbed’ paddock with an adjacent, "undisturbed" paddock to determine if the extensive trampling affected the extent of E+ fescue infestation into the E– pastures. In each disturbed and undisturbed paddock we collected tillers from 12 randomly located stations stratified by topography—six stations were in top slope positions and six from bottom slopes. This stratification was originally done to test whether topography affected potential differences in E+ infestation. Preliminary analysis indicated that topography was unimportant in explaining E+ infestation so it was not included in the eventual statistical analysis. At each station, one tiller was clipped from three randomly selected tall fescue plants within a ~2 m2 area (n = 18 tillers per slope position). For comparison, we also sampled three old field pastures at the Orr Center in the same manner. The old field pastures were similar to the pastures used in the experiment except they had not been renovated in at least 15 yr.

Tall fescue tillers were clipped near ground level, bagged, and returned to the University of Illinois. Samples were kept moist at 3°C until examination—usually within 3 to 10 d. Presence of the endophyte infection was evaluated by microscopic examination of tiller leaf sheath (Shelby and Dalrymple, 1987). Leaf sheaths were dissected with a scalpel and stained with aniline blue dye. During microscopic examination we classified any tillers with detectable hyphae as infected (E+) plants. To assess soil seed bank for tall fescue seed, we collected 30 soil cores (2.54 cm diam.) in each pasture before renovation to a 10-cm depth. Soils were frozen for ~3 mo and then germinated in a greenhouse over 8 wk. Fescue seedlings were counted after positive identification, but not tested for endophyte infection. Aboveground species composition of pastures was estimated in early June 2004 by sampling vegetation in three of six paddocks. In each of the three paddocks, we visually estimated percentage ground cover of plant species in eight randomly located quadrats (0.5 by 2 m). Percentage cover of species in the three paddocks was averaged because the paddocks were at different stages of defoliation and regrowth. In the oldfield pastures we sampled vegetation in the same manner except we sampled 20 (0.5- by 2-m quadrats) in randomly located locations across each pasture. Visual estimation of the old pastures showed that tall fescue accounted for 58 to 79% of ground cover.

We expressed the amount of E+ fescue in pastures in the following manner. If we sampled 50 tillers in a particular experimental unit and 25 were classified as E+, we classified 50% of the fescue plants in that sampling unit as endophyte infected (E+). Tall fescue also occurred in mixture with other species, and we expressed the amount of tall fescue in pasture by calculating the mean ground cover that fescue occupied relative to other species in the pasture. So if tall fescue accounted for 50% of the relative ground cover in a pasture, then all other species accounted for the remaining cover. If we classified 10% of the fescue plants as E+ in this pasture, then the amount of E+ in the pasture actually would be 5% expressed on a relative cover basis. In other words, relative to all other species in the pasture, we assumed that 5% of the cover was E+ fescue. The percentage of E+ tall fescue in pastures was analyzed using a split-plot arrangement in a completely randomized design using the GLM procedure of SAS (SAS Inst., 2000). Seed mix (three, five, or eight species) was used as a main plot effect with disturbance history (disturbed, undisturbed) as the split plot. Year (2002, 2003, and 2004) was analyzed as subplot effect. The percentage of tall fescue was compared between renovated pastures and old field pastures using a one-way ANOVA. For this analysis, we averaged all E+ data from each pasture and compared renovated (n = 8) and oldfield pastures (n = 3).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Climatic conditions during the study were generally average for this site (Table 2). Although the site experienced occasional dry months, no extended drought periods (e.g., >1 mo) occurred in any of the years. Tall fescue was a significant component in renovated pastures. Tall fescue cover was highest in the three species mixtures averaging approximately 40% in 2002 and 2003, then declining to 27% in 2004. In contrast, tall fescue cover generally increased after 2003 in the other two mixtures (Table 3). We found germinable tall fescue seed in five of eight pastures, with the amount of seed varying from 65 to 526 seeds m–2.


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  		Table 2. Growing season (April–October) mean temperate (°C) and rainfall (mm) during the study.

 

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  		Table 3. Percentage ground cover of tall fescue relative to all plant species in pastures. Values are means (±1 SE) sampled in June of respective years.

 
Endophyte-infested tall fescue accounted for 18, 28, and 38% of the fescue in mixtures sown with two, four, and seven additional species, respectively. When expressed on a whole pasture basis, however, the amount of E+ fescue accounted for <10% of the forage cover (Fig. 1) . The amount of E+ fescue did not differ among the three mixtures (P = 0.70), nor did it increase over time. Paddocks disturbed by hoof trampling in May 2002 averaged 7.1% E+ compared with 4.6% in the undisturbed paddocks, but this difference was not statistically significant (P = 0.25). None of the interactions among main effects were significant (P > 0.05). In the three old field pastures, E+ tall fescue averaged 38% and was significantly higher (P = 0.002) than E+ levels in the renovated pastures (Fig. 2) . Endophyte-infested levels in the oldfields did not change appreciably from 2002 to 2004.



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  		Fig. 1. Percentage tall fescue cover, relative to all species in pastures, that was classified as E+. Bars are means of three pastures in each seeding category. Error bars represent 1 SE. Solid line represents threshold infection (~20%) when potential symptoms of cattle toxicosis may be observed (Fribourg et al., 1991).

 


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  		Fig. 2. Percentage tall fescue cover, relative to all species in pastures, that was classified as E+ in renovated pastures (n = 8) and old field pastures (n = 3) at the Orr Research Center. Error bars represent 1 SE.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Despite herbicide application and tillage, E+ tall fescue was not completely eliminated from pastures. Pastures sown with three species consistently had the lowest average E+ percentage, averaging 18% of fescue plants. Tall fescue in pastures sown with five and eight species averaged 28 and 38% E+ by comparison. This result should be expected if the amount of volunteer E+ fescue and establishment rate of E– fescue was similar. For example, in the three species mixture, E+ fescue was probably more diluted by E– simply because those mixtures were sown with more E– seed. When expressed relative to all plant species, however, the percentage of E+ fescue accounted for <10% of pasture composition, and it did not differ among the mixtures. Although we did not test for E+ levels before pastures were renovated, infection levels were probably similar to the old field pastures surveyed in our study (mean of 38% E+). If this were true, it suggests that renovation successfully reduced E+ below levels thought to be safe for cattle consumption, ~20% E+ (Fribourg et al., 1991).

Pastures sown with E– tall fescue can become infested with E+ fescue by several mechanisms (Barker et al., 2005). These mechanisms include (i) contamination from sowing E+ seed or transport of E+ seed from animals and machinery, (ii) establishment of fescue plants that emerge from buried seed, or (iii) tillers that survive herbicide treatment and cultivation. In this study, it was unlikely that pastures were planted with contaminated E+ seed. In a different experiment that explored potential allelopathic effects of tall fescue, Renne et al. (2004) tested E– seed (Barcel) from a similar batch that was sown in our study. Percentage E+ infection was 0% for E– seed, whereas 58% of Kentucky-31 E+ seed was infected. Some E+ seed could have entered the renovated pastures from mowing equipment or wildlife, but probably not in significant amounts. Pastures also can be contaminated from E+ seed that establishes from the soil seedbank. We assessed the soil seed bank for tall fescue seed before pastures were renovated in 2001. We found germinable tall fescue seed in five of the eight pastures, and the amount was highly variable, ranging from 65 to 526 seeds m–2. The pasture with most fescue in the seedbank (526 seeds m–2) had intermediate E+ infestation (29% of fescue plants). In pastures with highest E+ infection (46 and 52% of fescue plants), we found little or no germinable tall fescue seed in the soil. So although some E+ fescue probably came from soil seedbanks, we suspect it made a minimal contribution. We should mention, however, that we did not sample soils for fescue seeds after they were tilled and prepared for planting. Tillage could have facilitated germination by bringing fescue seeds to the soil surface. Sampling the soils post-tillage may have given a more accurate assessment of fescue seed that contributed to the future aboveground vegetation. Most E+ fescue likely came from tillers that survived pasture renovation. Defelice and Henning (1990) used higher glyphosate applications than our study (1.68–2.52 kg a.i. ha–1) and found that they were ineffective at reducing E+ fescue from pastures. The herbicide applications appeared to kill fescue plants initially, but many tillers survived. Regardless of the mechanism, pasture renovation in our study appeared to reduce E+ levels enough so that symptoms of fescue toxicosis (Roberts and Andrae, 2004) should not occur in cattle grazing the pastures. We should note that during 3 yr of grazing during this study, we observed no distinct symptoms that could be ascribed to fescue toxicosis in our cattle.

Most studies that have evaluated populations of E+ and E– fescue have reported rapid increases of E+ fescue into E– swards. Shelby and Dalrymple (1993) found that E+ fescue increased from 27 to 84% during a 4-yr period in E– plots sown with varying rates of E+ fescue. Barker et al. (2005) showed that E+ fescue rapidly invaded E– swards, but swards of E– fescue that contained a beneficial endophyte were more resistant to invasion. Other studies have found that E– swards can become highly infested with E+ fescue in just 2 yr, but the amount of infestation is dependent on initial amounts of E+ (Thompson et al., 1989; Gwinn et al., 1998). Unlike these studies, the percentage of E+ fescue measured in our study remained low (<10%) and stable after pasture renovation. Several factors may explain why E+ fescue did not increase in the E– mixtures.

In our study, the use of rotational stocking methods and a moderate climate may have favored persistence of E– fescue. Rotational stocking may reduce selective grazing since it forces cattle to graze small areas for short durations. If cattle graze E– and E+ plants equally, it may reduce the chance that E+ fescue will displace E– plants. If cattle selectively graze E– fescue, it could give a competitive advantage to E+ fescue and allow it to increase in abundance. van Santen (1992) found that steers preferentially grazed E– fescue rather than E+ fescue if given the choice. Also in our study, paddocks were typically rested for 30 d between grazing bouts. This rest period may have allowed E– plants to recover from grazing and compete more effectively against E+ plants. We also used relatively moderate stocking rates in this study (~2 cow–calf pairs ha–1). Gwinn et al. (1998) found that E+ infestation did not increase under similar stocking rates (2–3 steers 1.2 ha–1), but did increase at a stocking rate of 5 to 7 steers 1.2 ha–1. They hypothesized that low to moderate stocking rates may alleviate physiological stress that plants experience and this may favor E– fescue. Lastly, growing season weather conditions were also moderate for this site during the experiment. From 2002 to 2004, the site experienced no extended dry periods (e.g., >1 mo) and temperatures were generally cool or around the long-term average. The relatively moist and cool climatic conditions may have benefited E– fescue since it is considered less stress tolerant than E+ fescue (Read and Camp, 1986; Bacon, 1993; Malinowski and Belesky, 2000).

Another factor to explain why E+ fescue did not increase in pastures may be that the renovation reduced E+ tall fescue below a threshold where it is unlikely to increase. Several studies have shown that when E+ fescue is initially 10% or less, it does not increase substantially over time (Thompson et al., 1989; Shelby and Dalrymple, 1993; Gwinn et al., 1998). Thompson et al. (1989) showed that E– swards established with no E+ fescue had only 5 to 9% E+ infestation after 27 mo. During the same time frame, E– swards sown with 15% E+ nearly tripled that amount, whereas swards sown with 30% E+ doubled. Using three levels of grazing pressure during 3 yr, Gwinn et al. (1998) found that the greatest increases in E+ (26–38%) occurred in swards initially infected with 25 or 60% E+. Swards with no initial E+ increased to a maximum of 9% E+ under the same conditions. Renovation in our study likely reduced E+ fescue to less than 10% in pastures. Perhaps at this low abundance, the competitive environment created by other plant species prevents E+ fescue from increasing significantly over time. Manipulative experiments to test this hypothesis could shed light on these observations.

Another objective of this study was to determine whether E+ infestation was affected by disturbance history. We hypothesized that E+ fescue would be more abundant in paddocks that were disturbed by cattle trampling in May 2002. Bare ground created by the hoof disturbances may have given greater opportunity for E+ seed to germinate or E+ plants to produce more tillers. We found no conclusive evidence that cattle trampling disturbance caused an increase in E+ fescue. Although mean E+ cover was higher in disturbed paddocks compared with undisturbed paddocks (7 vs. 5%), this difference was not statistically significant. The resident forage species appeared to recover rapidly after trampling and filled open spaces created by the disturbance. White clover was especially effective at filling in disturbed areas by spreading of its stolons. Even if E+ seed was abundant in the soil seedbank, it is unlikely that fescue seedlings could have established after the rapid vegetation recovery. Moreover, inherent variation among the different pastures used in this study simply made it difficult to distinguish disturbance effects.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Renovation successfully reduced the percentage of E+ fescue below 10%, which is below the level thought to cause fescue toxicosis in cattle (~20% E+ in pasture). Contrary to other studies, we found no evidence that E+ fescue increased over time in E– pastures. Even a cattle trampling disturbance in May 2002, which opened up bare ground in paddocks, did not promote E+ infestation. The lack of E+ fescue increase likely resulted from a combination of factors that included relatively moist climatic conditions, grazing management that used rotational stocking, and a strong competitive environment created by plants sown in the different mixtures. We also suggest that there may be a threshold where E+ infestation into E– swards will not occur if E+ fescue is reduced to 10% or less. At this low abundance, the competitive environment created by E– fescue or other plant species may prevent E+ fescue from increasing over time. Manipulative experiments to test this threshold hypothesis may provide insights into mechanisms that allow E+ fescue to infest E– swards.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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