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a USDA-ARS, U.S. Dairy Forage Research Center, Univ. of Wisconsin Marshfield Agric. Exp. Stn., 8396 Yellowstone Dr., Marshfield, WI 54449
b Dep. of Animal Science, Univ. of Arkansas, Fayetteville, AR 72701
c Univ. of Arkansas Livestock and Forestry Branch Stn., 70 Exp. Stn. Drive, Batesville, AR 72501
d 126 Jessie Dunn, Northwestern Oklahoma State Univ., Alva, OK 73717
e Humphry Environmental, Inc., Fayetteville, AR 72702
f Stone County Extension Building, Mountain View, AR 72560
g North Carolina State Univ. Mountain Research Stn., Waynesville, NC 28786
h Animal Science Section, Arkansas Cooperative Extension Service, Little Rock, AR 72203
i Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, Fayetteville, AR 72701
j Dep. of Agric. Economics and Agribusiness, Univ. of Arkansas, Fayetteville, AR 72701
k Berea College, Berea, KY 40404. W.K. Coblentz, D.A. Scarbrough, J.B. Humphry, B.C. McGinley, J.E. Turner, and D.H. Hellwig all were associated formerly with the Dep. of Animal Science, Univ. of Arkansas, Fayetteville, AR 72701
* Corresponding author (coblentz{at}wisc.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSREFERENCES |
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Abbreviations: CP, crude protein • DM, dry matter • E+, endophyte-infected tall fescue • E–, endophyte-free tall fescue • IVDMD, in vitro dry matter disappearance • OG, orchardgrass • 2M, rotation to new paddocks twice monthly • 2W, rotation to new paddocks twice weekly
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSREFERENCES |
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Dilution of E+ pastures with other nontoxic grasses or legumes has improved both cow (Holloway and Butts, 1984; Gay et al., 1988; Waller et al., 1989) and stocker cattle performance (Coffey et al., 1990; McMurphy et al., 1990; Chestnut et al., 1991), but dilution of E+ pastures has rarely offset all of the performance reductions associated with tall fescue toxicosis. While competition with common bermudagrass creates an additional stressor for tall fescue, common bermudagrass and various other grasses and legumes also can provide a measure of natural dilution within many southern Ozark pastures. Throughout the southern Ozarks, the relative proportions of E+ and bermudagrass in mixed-species pastures that optimize cattle performance are unknown, and likely vary with precipitation, temperature, fertilization, and many other factors. To some extent, relative proportions of tall fescue and common bermudagrass can be managed to the advantage of one species or the other by the amount and timing of N fertilization, as well as by controlling closely the timing, frequency, and height of mowing or grazing (Wilkinson et al., 1968; Hoveland et al., 1978; Fribourg and Overton, 1979; Pitman, 1999). While it is unclear what relative proportions of E+ and common bermudagrass may optimize cattle performance, both species are likely important; E+ extends the grazing season by providing forage in the spring and fall when temperatures are cool, while bermudagrass grows actively throughout the summer and dilutes the concentrations of toxins produced within E+ plants.
Another option available to cow-calf producers in the southern Ozarks is to convert their acreage to fescue varieties containing novel, non-ergot-alkaloid-producing endophytes (Bouton et al., 2002; Nihsen et al., 2004). While promising, producers are not likely to convert all, or even a majority of their pastures to these novel endophyte–fescue associations because of expense and/or topography. However, other management options may be effective in reducing the effects of fescue toxicosis for cow-calf producers. One option would be to apply glyphosate [N-(phosphonomethyl)glycine] to mixed species pastures, thereby killing existing cool-season plants and releasing a base of common bermudagrass. Nontoxic forages, such as E– and OG could then be established by sod-seeding techniques, which producers in the region understand and have suitable equipment to carry out. This approach would not completely kill all existing (and presumably E+) tall fescue plants (Defelice and Henning, 1990), and reinfection of the pasture by surviving rogue E+ plants would likely occur. However, a spring or fall application of glyphosate would release the base of common bermudagrass within these pastures, thereby retaining considerable pasture productivity until OG or E– could be established. Furthermore, E– and OG have much lower seed costs than novel-endophyte tall fescues.
This management approach has several obvious uncertainties. Because of the potential for reinfection of pastures with E+ plants, this management option cannot be considered permanent. It is unclear how much livestock performance may be improved or how long this potentially higher level of performance may continue before it is limited by reinfection of pastures by E+ plants, poor persistence of E– or OG forages, increased frequency of bermudagrass, or various combinations of these potential occurrences. Persistence of E– overseeded into bermudagrass was improved in central Georgia with a 12-paddock rotation compared to continuous stocking (Hoveland et al., 1997). However, it is uncertain if rotational management will improve persistence of E– in the southern Ozark Highlands. Also, to the best of our knowledge, factors affecting the competitive balance between OG and common bermudagrass have not been evaluated, probably because OG is less tolerant of heat (Baker and Jung, 1968) and drought than tall fescue, and stands in the South generally do not persist more than 2 to 4 yr (Ball et al., 2002). Therefore, a 4-yr trial was initiated in January 2000 to evaluate the effectiveness of E– or OG overseeded into dormant common bermudagrass sods for spring-calving cows. Our objective was to evaluate forage characteristics for these nontoxic (E– or OG) forages at two rotational frequencies (2M or 2W), and to compare these forage systems with a typical pasture mixture of approximately 50% E+, plus common bermudagrass and other forages that is representative of those observed commonly throughout the southern Ozarks.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSREFERENCES |
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Establishment of Pastures
Before initiating this study, the experimental pastures had been used for a variety of grazing and/or supplementation studies; pastures generally contained 50% E+ or less and had a vigorous base sod of common bermudagrass, which is not unusual for mixed species pastures throughout the region. In the spring of 1997, nine of the 13 existing 4-ha pastures were sprayed with glyphosate at a rate of 1.68 kg a.i. ha–1 to eliminate annual and perennial cool-season grasses. The species composition of the remaining four pastures was mixed, but each contained approximately 50% E+ and a base sod of common bermudagrass; these pastures were retained as controls. Glyphosate was applied to each of the nine pastures only once because we felt this represented the likely limit of investment that many producers would be willing to make for eradication of E+ on marginally productive land. In addition, it was not a research goal to eliminate bermudagrass, which would have been damaged by multiple applications during the growing season. These nine pastures were then grazed as bermudagrass pastures during the summers of 1997 and 1998, and were used in another grazing study that required overseeding with cereal rye (Secale cereale L.) and/or annual ryegrass (Lolium multiflorum Lam.) in the fall of 1997.
All experimental pastures were grazed intermittently throughout the early summer of 1998. Beginning in August 1998, mob-grazing management was used to remove the existing summer forage growth from the nine pastures previously sprayed with glyphosate; at that time, the existing forage was primarily common bermudagrass. Mob-grazing techniques were used because several of the pastures contained rock outcroppings or other areas where the terrain was rough; therefore, they were unsuitable for haying. In late September and early October 1998, ‘Kentucky 31’ E– and ‘Benchmark’ OG were overseeded into their assigned pastures at respective rates of 26.5 and 24.7 kg pure live seed ha–1. All seeding was completed by 9 Oct. 1998. Before establishment, a randomly collected sample of E– seed was submitted for determination of endophyte-infection percentage (Fescue Diagnostic Center, Auburn University, Auburn, AL), and found to be 1% endophyte infected. Pastures were established with a 2.1-m-wide Tye Pasture Pleaser drill (The Tye Company, Lockney, TX) with rows spaced at 25 cm. Soil tests taken before establishment indicated that fertility levels for P, K, and lime varied only minimally from the soil test recommendations of the Arkansas Cooperative Extension Service (Chapman, 2001); therefore, any needed applications of P and K were deferred until 24 Feb. 1999. All pastures were fertilized with urea (46–0–0) at a rate of 50 kg N ha–1 on 18 Feb. 1999.
In April 1999, three independent observers evaluated each pasture overseeded with OG or E– visually for continuous row coverage by cool-season seedlings. Observers walked each pasture in a zig-zag pattern, covering the entire pasture area before making their estimate. These estimates were averaged for each pasture before statistical analysis. During the 1999 growing season, overseeded pastures were grazed lightly to control forage growth and to allow new seedlings a chance to become well established.
Maintenance of Pastures
On 9–10 Sept. 1999, all pastures were fertilized with urea (46–0–0) at a rate of 67 kg N ha–1; similar applications were made each subsequent year in mid February, early June, and early September. Therefore, a total of 202 kg N ha–1 were applied annually after cattle were allocated and grazing was initiated. Soil tests were obtained each year in August and any needed P, K, or lime was applied each September based on soil test recommendations (Chapman, 2001). Broadleaf weeds were controlled as necessary throughout the trial with picloram (4-amino-3,5,6-trichloropicolinic acid, triisopropanolamine salt) and 2,4-D (2,4-dichlorophenoxyacetic acid, triisopropanolamine salt) applied in combination at respective rates of 0.3 and 1.1 kg a.i. ha–1.
Pasture Rotation Schedules
Experimental pastures were grazed with two rotational grazing schemes (Table 1) that included rotations to fresh paddocks either 2W or 2M. Pastures grazed with the 2W rotation frequency were subdivided into eight 0.5-ha paddocks that were grazed for 3 to 4 d during each rotation, and then rested for the balance of the month (26–28 d). Pastures grazed with the 2M rotation frequency were subdivided into two 2.0-ha paddocks. Cows assigned to these pastures were maintained on a specific paddock for 15 d before they were rotated to the other paddock for the remainder of the month. The OG and E– pastures were grazed with both 2W and 2M schedules, largely because it was hypothesized that rotation frequency may strongly affect the persistence of nontoxic E– and OG plants. The E+ control pastures were typical of mixed pastures of E+ and common bermudagrass throughout the region; these were grazed with the 2M rotation frequency only because that represented the likely upper limit of time and management that most part-time cow-calf producers would be willing to invest for pastures with E+.
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Whenever available forage was limiting, cows and calves were confined within a single 2W paddock (0.5 ha) or an area of comparable size constructed with temporary electric fencing within a 2M paddock and offered bermudagrass hay harvested from another location on the research station in round-bale feeders on an ad libitum basis. These 0.5-ha feeding areas were not relocated during the trial. These feeding techniques were used to prevent continued grazing over the entire pasture area when forage was limiting, and to confine both tractor and cattle traffic to a small, but convenient, area. The decision to feed hay was based on either of two criteria: (i) when forage mass dropped below 1000 kg ha–1 as measured by a calibrated disk meter (Bransby et al., 1977); or (ii) when the grazed stubble height of the appropriate cool-season grass (OG, E–, or E+) dropped below 7.5 cm. The second decision trigger was necessary, especially in the late fall, because cows grazed cool-season forages (particularly OG) preferentially over frosted bermudagrass.
In an effort to control the flush of forage growth that occurs in the spring, extra "thin" fall-calving cows were assigned to each pasture to improve their body condition. This technique was used because all pastures were not suitable for measuring any extra forage produced as hay. All details of these procedures and summaries of performance for these cows are described in detail within in a companion report (Coblentz et al., 2006).
Analytical Procedures and Measurements
Pasture Measurements
Except during the winter months when cows were offered hay, pastures were evaluated monthly for forage mass using a calibrated disk meter (Bransby et al., 1977). This was accomplished by walking each pasture in a zig-zag pattern and then estimating forage mass at 6 locations ha–1 (24 locations per pasture). Simultaneously, forage samples were collected at half (12 per pasture) of these locations by clipping forage to a 2.5-cm stubble height with hand shears. All forage samples were dried to a constant weight under forced air at 50°C, and then ground through a Wiley mill (Arthur H. Thomas, Philadelphia, PA) fitted with a 1-mm screen before determination of IVDMD and CP. On each sampling date, all disk meter readings and associated forage samples clipped for analysis of nutritive value were obtained from sites distributed uniformly over the entire 4-ha pasture.
Pastures were evaluated for basal cover and species composition before initiating the trial (November 1999) by the modified step-point method (Owensby, 1973). Each pasture was walked in a zig-zag pattern placing the step-pointer on the ground at 40 locations ha–1 (160 per pasture). At each placement of the step-pointer, two observations were recorded: (i) whether the pointer directly touched the base of a plant; and (ii) the plant species either touching the pointer directly, or the nearest species within an 180° arc in front of the pointer. Total basal cover was calculated as the percentage of pointer placements that directly touched the base of any plant. Species frequency was calculated as the percentage of total placements that a particular species was either touched directly or was closest to the pointer. These procedures were repeated in June and November of each subsequent year to assess the cumulative effects of grazing on the species composition and basal cover within each pasture. The June evaluation date corresponded generally to the time period of peak forage mass for cool-season grasses. Conversely, the November evaluation date was scheduled immediately after the onset of fall dormancy to reflect bermudagrass that had accumulated throughout the summer months. By this time, below-freezing temperatures had caused the bermudagrass to turn brown, which made it easy to distinguish from cool-season grasses that were still growing actively. To simplify the presentation of results and subsequent discussion, June and November evaluations within each calendar year were averaged, thereby creating an annual mean for each pasture that was used for all statistical analyses of species composition data.
In 2000 and 2001, it was relatively easy to distinguish rogue (and presumably E+) tall fescue plants within E– pastures that were not killed by the application of glyphosate in 1997. This differentiation was based on large differences in crown size and spatial orientation of rogue plants relative to the drill rows. After the 2001 grazing season, these distinctions could not be made with any confidence, and separate identification of rogue fescue plants was discontinued. During all evaluations of species frequency, tall fescue plants were identified only visually; no attempt was made to establish the endophyte status of each plant by independent laboratory testing.
In June of each year, additional samples of tall fescue forage were obtained to quantify concentrations of total ergot alkaloids. These samples were collected by walking each pasture in a zig-zag pattern, and clipping tall fescue plants to a 2.5-cm stubble height with hand shears from approximately 4 random locations ha–1 (16 locations per pasture). Samples were limited to tall fescue forage only; all other species were removed from the sample. After collection, tall fescue samples were sealed immediately in plastic freezer bags, and then submerged in ice in an insulated cooler. Each hour during collection, samples were transported to a conventional freezer (–4°C) and stored for not more than 48 h before they were transported to Fayetteville on ice and then stored in an ultra-low temperature freezer (–80°C). Samples were then lyophilized, ground through a 1-mm screen as described previously, and then returned to the ultra-low freezer until they were analyzed for total ergot alkaloids. Only rogue fescue plants were sampled from E– pastures in 2000 and 2001. After that time, rogue plants could not be identified with confidence, and all fescue plants were sampled at random. Within some E– pastures during 2000 and 2001, species frequency estimates for rogue fescue plants were very low or 0%. If technicians sampling these pastures for total ergot alkaloids could not find rogue plants to sample, then all fescue plants were sampled at random to provide independent, corroborative evidence of a low rate of endophyte infection and concomitant low toxicity within these pastures.
Laboratory Analyses
Dried and ground forage samples were analyzed for total N by rapid combustion (AOAC, 1998; AOAC Official Method 990.93; Elementar Americas, Inc., Mt. Laurel, NJ), and for IVDMD by the batch techniques outlined by ANKOM Technology Corp. (Fairport, NY). Concentrations of CP within each forage were calculated by multiplying the percentage of total N in each forage sample by 6.25. Rumen fluid for IVOMD analysis was obtained from two ruminally cannulated crossbred steers that were offered a diet consisting of 850 g kg–1 alfalfa hay and 150 g kg–1 concentrate at a daily rate of 20 g kg–1 of BW (as-is basis). On an as-is basis, the concentrate mix contained 910 g kg–1 cracked corn, 40 g kg–1 liquid molasses, and 50 g kg–1 trace mineral salt. The steers were adapted to the diet for a minimum of 7 d before collecting the rumen fluid. Total ergot alkaloids in tall fescue forages were quantified using the procedures of Hill and Agee (1994; Agrinostics, LLC, Athens, GA).
Statistical Analyses
Individual pastures served as the experimental unit for all statistical analyses. Forage mass and concentrations of IVDMD and CP were analyzed as a split-plot design with forage system as the whole-plot term, and the 41 sampling dates as the subplot (repeated measures) term. The whole-plot term (forage systems) was arranged in a completely randomized design and tested for significance using the pasture within forage system mean square as the error term. Sampling date, and the forage system x sampling date interaction were tested for significance with the residual error mean square.
Species composition, basal cover, and concentrations of total ergot alkaloids were all analyzed as a split-plot (repeated measures) design with forage systems as the whole-plot term and year as the subplot or repeated measures term. In each of these cases, forage system was tested for significance as described previously; the repeated measures and interaction terms were tested with the residual error mean square. All statistical analyses were conducted with PROC GLM of SAS (SAS Institute, 1989). Generally, least square means were separated with the PDIFF option; however, main-effect means of sampling date were balanced, thereby permitting calculation of LSD. Statistical significance was declared at P < 0.10, unless otherwise noted.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSREFERENCES |
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0.615) in establishment across OG2W, OG2M, E-2W, and E-2M pastures. The overall mean of 68% (data not shown) suggests that initial establishment was relatively good considering the rough and rolling terrain, shallow soils, and competition from vigorous stands of common bermudagrass that comprised the vast majority of the existing sod. Generally, two specific conditions were responsible for poor establishment; either the terrain was too rough for proper drill function, or there was incomplete removal of the existing canopy of bermudagrass by mob grazing techniques before seeding. Of these, the latter could be better managed by close mowing before establishment as described by Hoveland et al. (1997); however, the rough terrain (rock outcroppings) on several of these experimental pastures prohibited this approach.
Forage Mass and Nutritive Value
Forage Mass
A critical management goal during the trial was to prevent overgrazing of the perennial cool-season forages. Bermudagrass is a C4 forage that has a higher photosynthetic rate and efficiency at high radiation than C3 forages (Nelson, 1995), but also is less tolerant of shading in mixed pastures than upright C3 forages (Hoveland et al., 1997). Under the stressful climatic conditions of the southern Ozarks, avoiding overgrazing is critical for the persistence of E– and OG because it preserves the stem and tiller bases, which are the primary storage organs for carbohydrate reserves (Nelson, 1995). In addition, overgrazing allows more light to penetrate to the soil surface, thereby increasing the competitiveness of bermudagrass. Over the entire 4-yr trial, the mean forage mass was 3809 kg ha–1; this was more than twice that reported by Hoveland et al. (1997) for pastures of E– overseeded into common bermudagrass and grazed with a rotational management strategy. We postulated that this conservative management approach was essential for persistence of both E– and OG, but especially for OG, which is known to be less tolerant of heat (Baker and Jung, 1968) and drought (Ball et al., 2002) than tall fescue.
Overall, forage mass was affected by the interaction of forage system x sampling date (P = 0.001); however, a closer inspection of these data indicated that forage mass changed over sampling dates in similar patterns for all forage systems. The interaction of forage system and sampling date was probably created by superior forage production for OG and E+ pastures relative to E– pastures during September 2002, and superior production from E+ and E– pastures compared to OG during June, July, and August 2003 (data not shown). This conclusion was supported by an additional statistical analysis (ANOVA) by year, in which no interaction of main effects was detected (P 0.727) for either 2000 or 2001. Because the overall interaction of main effects was not likely related to divergent patterns of forage production for individual forage systems across sampling dates, the interaction will be ignored to permit a concise presentation of results; therefore, only main effect means of sampling date (P < 0.0001) are presented (Table 3) and discussed.
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1362 kg ha–1; forage mass declined (P < 0.10) erratically thereafter, but remained at 3695 kg ha–1 as late as January 2003. Forage mass during 2003 was 4215 kg ha–1 from June through the conclusion of the trial; from May through December of that year, precipitation exceeded the 30-yr normal by 37%, and in all months except October.
In Vitro Dry Matter Disappearance
Concentrations of IVDMD varied with effects of sampling date (P < 0.0001), but not with any other main effect or interaction (P 0.105). Averaged over all forage systems and sampling dates, the mean concentration of IVDMD was 586 g kg–1. Concentrations of IVDMD reached a numerical maximum between 700 and 800 g kg–1 in April or May of 2000, 2001, and 2003 (Table 4), and then declined (P < 0.10) throughout the summer as cool-season plants aged and bermudagrass comprised a greater percentage of the forage mass. The trend was similar in 2002 except that the numerical maximum was only 685 g kg–1.
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213 g kg–1 in March or April of each year (Table 5); in contrast, minimum concentrations of 93 and 99 g kg–1 were observed in September of 2000 and 2002, respectively. Overall, the mean concentration of CP was 157 g kg–1, which is more than adequate to support the needs of either lactating or nonlactating beef cows (NRC, 1996), and likely reflects the aggressive annual N fertilization management (202 kg N ha–1) utilized during the trial. Generally, concentrations of CP followed closely the trends described for IVDMD, exhibiting effects of only sampling date (P < 0.0001), but not forage system (P = 0.933) or the interaction of main effects (P = 0.790).
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Bermudagrass
For frequencies of bermudagrass (Table 6), there was a tendency (P = 0.073) for an interaction of forage system with year. Within OG2M pastures, bermudagrass frequencies ranged from 41 to 58%, and generally increased following 2000, averaging 52% from 2001 through 2003. Frequencies of bermudagrass in OG2W pastures increased sharply (P < 0.10) by 11 percentage units in 2003 from numerical minima (35 and 36%) observed in 2001 and 2002, respectively. Over the entire trial, the mean frequency of bermudagrass in OG2M was 9 percentage units less than observed during 5 yr for OG2W. For both OG systems, there was an inverse relationship between frequencies of OG and bermudagrass, and these data suggest that bermudagrass was opportunistic, readily filling in areas vacated by declining frequencies of OG (Table 6).
Within E+2M and E–2W pastures, frequencies of bermudagrass varied (P < 0.10) over dates, but these differences did not represent a clearly defined pattern, and were confined to relatively narrow ranges (28–37 and 27–37%, respectively; Table 6). For E–2M pastures, bermudagrass did not differ (P > 0.10) over years, averaging 30% for the entire trial. Generally, the relatively static responses of bermudagrass in all tall fescue pastures indicates that bermudagrass was effectively controlled by management practices that promoted shading of the soil surface.
Rogue Fescue Plants
Neither rotation frequency (P = 0.522) or the interaction of rotation frequency and year (P = 0.160) affected percentages of rogue tall fescue plants within OG pastures. However, frequencies of rogue fescue plants (Table 7) increased (P < 0.10) over years from a numerical minimum of 4% in November 1999 to a maximum of 10% during 2003, which was greater (P < 0.10) than observed for all other years except 2002 (8%; P > 0.10). Within E– pastures, no treatment effect was significant (P 0.209), and the mean frequency from 1999 through 2001 was 2%. Generally, these findings are consistent with a recent report (Tracy and Renne, 2005) that found frequencies of E+ did not increase in pastures renovated with E– and other species over a 3-yr period in which they were grazed under rotational stocking.
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While there were rogue fescue plants that escaped herbicide kill scattered throughout most of the OG and E– pastures, their frequency was neither uniform across nor within individual pastures. Overall, frequencies of rogue fescue plants within individual OG and E– pastures ranged from 0 to 19%; however, greater frequencies were found most commonly in low-lying areas that tended to remain hydrated during periods of heat and drought stress. These observations were especially obvious for OG; three pastures exhibited a mean frequency across 5 yr of 2%, which agrees closely with that observed for E– pastures, and suggests that there was little reinfection by rogue E+ plants during the entire trial. The other two OG pastures contained low-lying areas (
15% of the total pasture area), and frequencies within these pastures averaged about 13% over the entire trial, ranging from about 10% in 1999 to 17% in 2003. While this clearly indicates that rogue E+ plants spread within OG pastures, it should be noted that frequencies were calculated on the basis of the entire pasture area; high frequencies of rogue fescue plants were extremely localized within low-lying areas, and reestablishment of presumed E+ plants appeared to occur at a much faster rate within these areas than across the pasture generally.
Other Species
A wide variety of other species were found throughout the study. Among these, the most common were little barley (Hordeum pusillum Nutt.), Kentucky bluegrass (Poa pratensis L.), southern crabgrass [Digitaria ciliaris (Retz.) Koel.], knotroot foxtail [Setaria geniculata (Lam.) Beauv.], johnsongrass [Sorghum halepense (L.) Pers.], dallisgrass (Paspalum dilatatum Poir.), chickweed [Stellaria media (L.) Vill.], henbit (Lamium amplexicaule L.), and annual ryegrass. Frequency of other species was affected only by year (P = 0.001). Estimates declined (P < 0.10) over years (Table 8), ranging from 18% in November 1999 down to 9% during 2003; however, the greatest decline (5 percentage units; P < 0.10) occurred during 2000, with only slow changes thereafter.
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Total Ergot Alkaloids and Percent Endophyte Infection
Total Ergot Alkaloids
Within OG and E+ pastures, concentrations of ergot alkaloids were affected by the interaction of main effects (P = 0.001). Within OG2M and E+2M pastures, concentrations of ergot alkaloids were greater (P < 0.10) in both 2002 and 2003 than in either 2000 or 2001 (Table 9). For both forage systems, there was an approximate threefold increase in ergot alkaloids during the last 2 yr of the trial, relative to the first 2 yr. In contrast, concentrations of ergot alkaloids within OG2W pastures varied (P < 0.10) erratically over years, which likely contributed to the interaction.
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0.527) of rotation frequency during this time period, and the overall mean concentration of total ergot alkaloids in rogue fescue plants was 472 ng g–1 (SEM = 50.9 ng g–1). During 2002 and 2003, all fescue plants in E– pastures were sampled because rogue plants could no longer be identified visually. There was a tendency for an effect of year (P = 0.055) during this time period, but a single extreme value may have contributed heavily to this observation. Over the final 2 yr of the trial, the mean concentration of total ergot alkaloids in E– pastures was 523 ng g–1 (SEM = 68.8 ng g–1), which was approximately 25% of that observed for E+ and OG pastures. For purposes of interpretation, it should be noted that concentrations of total ergot alkaloids were determined for tall fescue plants only; dilution effects by other forage species are not reflected in these results.
Since glyphosate was applied only once before establishment of E– and OG pastures, complete control of preexisting and presumably E+ plants was not expected (Defelice and Henning, 1990), and did not occur (Table 7). In part, reinfection of recently renovated E– pastures with E+ may occur because mature, rhizomatous plants escape herbicide kill (Defelice and Henning, 1990), and because survival rates for E+ and E– plants may differ (Shelby and Dalrymple, 1993). Clearly, reinfection occurred by these or other mechanisms because measurable concentrations of ergot alkaloids were found in E– pastures in both 2002 and 2003. However, we are not aware of other grazing studies with other treatments (OG) in which rogue fescue plants could be counted visually. Theoretically, reinfection should occur by identical mechanisms within both E– and OG pastures. Considering the widely held view throughout the southern Ozarks that OG is less persistent than E–, it is reasonable to expect that E+ plants would increase even more rapidly in OG than in E– pastures. These data suggest that reinfection of OG occurred at a relatively slow rate during the trial, reaching a maximum frequency of 10% during the final year of the trial, but averaging only 6% over the previous 4 yr (Table 7). Furthermore, livestock performance did not suggest that there were large increases in pasture toxicity. Mean annual weaning weights for calves raised across all pastures ranged only slightly from 246 to 250 kg during the trial, and effects of year and forage system x year were not significant (Coblentz et al., 2006). Depressions in livestock performance (Paterson et al., 1995) and significant effects of year and/or interactions with year would be expected with increases in pasture toxicity.
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IMPLICATIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSREFERENCES |
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Received for publication March 23, 2006.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSREFERENCES |
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