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Published in Agron J 98:1221-1226 (2006)
DOI: 10.2134/agronj2006.0007
© 2006 American Society of Agronomy
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Production Papers

Stand Persistence and Animal Performance for Tall Fescue Endophyte Combinations in the South Central USA

A. A. Hopkinsa,* and M. W. Alisonb

a Forage Improvement Division, Samuel Roberts Noble Foundation, Ardmore, OK 73401
b LSU Agricultural Center, Winnsboro, LA 71295

* Corresponding author (aahopkins{at}noble.org)

Received for publication January 6, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Infection by an endophyte [Neotyphodium coenophialum (Morgan-Jones & Gams.) Glenn, Bacon, & Hanlin comb. nov.] can improve persistence of tall fescue (Lolium arundinaceum (Schreb.) S.J. Darbyshire = Festuca arundinacea Schreb.). Novel endophytes may minimize toxicity problems associated with wild-type endophytes, thus encouraging tall fescue use in the south central USA. The objectives of these studies were to compare stand persistence and grazing animal performance among tall fescue cultivars with different fungal endophyte combinations in the south central USA. Trials were planted in 1999 in Oklahoma and Louisiana. Tall fescue entries contained no endophyte (‘GA-5’ E–, ‘Dovey’), a novel endophyte (‘Jesup’ MaxQ, ‘GA-5’ MaxQ), or a wild-type endophyte (‘KY-31’ E+, ‘GA-5’ E+). Weight gain data for beef cattle (Bos taurus) were collected during fall and/or spring. Estimates of stand persistence were collected periodically. In Oklahoma, average daily gain (ADG), at 0.71 kg d–1, did not differ between entries in fall. During spring, ADGs (kg d–1) were greater from Dovey (0.64), GA-5 MaxQ (0.67), and GA-5 E– (0.56) than from KY-31E+ (0.29) and were intermediate for GA-5E+ (0.47). In Louisiana, ADGs were greater from GA-5 E– (1.20), GA-5 MaxQ (1.06), and Jesup MaxQ (1.12) than from GA-5E+ (0.76). In Oklahoma, stands were greater than 80% 4 yr after establishment. In Louisiana, stands declined in some E– paddocks 3 yr after planting. Thus, E– and novel endophyte–infected tall fescue resulted in greatest animal performance, whereas wild-type and novel endophyte infection led to enhanced stand longevity. However, improved persistence is needed for broader adaptation of tall fescue to the south central USA.

Abbreviations: ADG, average daily gain • E+, endophyte infected • E–, endophyte free • GA-5, Georgia 5 • GPH, gain per hectare • KY-31, Kentucky 31 • RCBD, randomized complete block design


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE south central USA is a major production region for grazing livestock, particularly beef cattle (Bos spp.). As of 2004, beef cow numbers totaled greater than 8.9 million head in Arkansas, Texas, Oklahoma, and Louisiana, with a large number of young, growing animals (i.e., stocker animals) in this region as well (www.nass.usda.gov/Statistics_by_State; verified 26 June 2006). During the fall to spring months (approximately October to May), producers primarily maintain breeding herds by feeding hay or other conserved forage. Where availability of high-quality forage is an important consideration, as in the case of stocker animals, cool-season annual grasses are widely used during the fall to spring period (Redmon et al., 1995; Rouquette et al., 1997; Ball et al., 2002).

Livestock producers in the south central USA are interested in adopting cool-season perennial grasses that are persistent and productive. Except for much of Arkansas and eastern Oklahoma, currently available cultivars of cool season perennial grasses, including tall fescue, generally do not persist for more than 2 to 4 yr in the south central USA (Pitman, 1999; Malinowski et al., 2003; Hopkins, 2005), although summer-dormant cultivars may extend the use of tall fescue into drier areas (Malinowski et al., 2005b). Infection of tall fescue with a Neotyphodium endophyte often results in improved tolerance to abiotic (Malinowski et al., 2005a) and biotic (Popay and Bonos, 2005) stresses and thus improved persistence (Read and Camp, 1986). The deleterious effects on health and performance of animals grazing tall fescue infected by toxic or ‘wild-type’ endophytes have also been well documented and reviewed extensively (Ball, 1997; Thompson et al., 2001; Stuedemann and Seman, 2005). Two alternative approaches for dealing with this situation involve the development of E– tall fescue cultivars with improved persistence, which in at least one instance has been shown to be ineffective (Bouton et al., 2001), or infection of tall fescue cultivars with endophytes that do not produce ergot alkaloids (Bouton et al., 2002; Nihsen et al., 2004), referred to as novel endophytes in this report. The objective of this research was to determine grass persistence, under moderate seasonal grazing, and performance of animals grazing various tall fescue/endophyte combinations in the south central USA.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Burneyville, OK
Research was conducted at the Noble Foundation Red River Demonstration and Research Farm (33°53' N, 97°15' W; elevation approximately 200 m) near Burneyville in south central Oklahoma. Soil type was a Norwood clay loam (fine-silty, mixed, superactive, hyperthermic Fluventic Eutrudept). The site had previously been planted to cereal rye (Secale cereale L.) for winter pasture, followed by crabgrass [Digitaria sanguinalis (L.) Scop.] for summer pasture. Glyphosate [N-(phosphonomethyl)-glycine] was applied to kill existing vegetation before planting tall fescue. Paddocks, approximately 0.47 ha in size, were planted no-till using a drill on 14 and 15 Oct. 1999 at a rate of 22.4 kg of bulk seed ha–1. The experiment was designed as a randomized complete block design (RCBD) with three replicates. Entries contained no endophyte (Dovey, GA-5E–), a novel endophyte (GA-5 infected with MaxQ endophyte), or a wild-type endophyte (KY-31E+, GA-5E+). Limited grazing was used in the spring of 2000 to control annual ryegrass (Lolium multiflorum Lam.). Because of poor establishment, all or part of each paddock in the third replication was replanted in the fall of 2000. Nitrogen was applied in early October (84 kg ha–1) and mid-March (28 kg ha–1) as ammonium nitrate each grazing season (i.e., fall 2000 to spring 2004). Paddocks were mowed in September 2000 and 2001 before fall grazing, and seed heads were clipped during the spring 2002 grazing period.

Beginning in the fall of 2001, growing beef steers consisting of Angus and Angus x Tarentaise cross animals were purchased from eastern Oklahoma, where it was likely that animals had been previously exposed to tall fescue infected with a toxic endophyte. Steers were kept on a bermudagrass [Cynodon dactylon (L). Pers.]–annual ryegrass pasture and fed bermudagrass hay plus protein supplement for at least 30 d before being placed on the tall fescue treatments. Animals received recommended vaccinations and anti-parasite treatment during this period. Fall grazing generally commenced in late November and ended in January to March, depending on forage availability (Table 1). Fall stocking rates, at two animals per paddock, averaged roughly 1050 kg body weight ha–1. The height of residual forage after fall grazing in 2001 to 2003 was approximately 10 cm. Spring stocking rates averaged roughly 1500 kg body weight ha–1. Water and salt were offered free choice, as was Stillwater Sweet Mag mineral supplement (Stillwater Milling Co., Claremore, OK), with a guaranteed minimum of the following (g kg–1): Ca, 150; P, 30; NaCl, 155; Mg, 100; K, 4; Cu, 0.83; Se, 0.03; and Zn, 2. Data from fall 2000 and spring 2001 were excluded from analyses because of problems associated with animals jumping fences. After the 2000–2001 grazing season, half-sibs from a given sire were assigned to each of the five paddocks of a replicate, and half-sibs from a different sire were assigned likewise to those same five paddocks. In total, half-sibs from six different sires were used for a given grazing season. Animals assigned to a given paddock for the fall grazing period returned to that same paddock for the spring grazing period. Animals were maintained on rye hay and protein supplement between fall and spring grazing periods.


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  		Table 1. Grazing dates for tall fescue paddocks at Burneyville, OK, and Winnsboro, LA.

 
Cattle were weighed twice within 72 h at the beginning of each grazing period; the average of these two weights was used in calculating average daily gain (ADG) and gain per hectare GPH. The same procedure was used to determine cattle weights at the end of each grazing period, except in spring 2003, when cattle were weighed once at the conclusion of the grazing season. Rectal body temperature was taken at this time. Blood samples were collected from the upper tail, kept on ice, and centrifuged to separate plasma samples for prolactin analysis. Serum prolactin levels were analyzed by Dr. Duane Keisler at the University of Missouri using a radioimmunoassay procedure (Henson et al., 1987). Prolactin analysis was performed in triplicate on each sample and averaged for statistical analysis.

Available forage was estimated before and after fall grazing. Forage was harvested at a cutting height of 7.6 cm using a sickle bar forage harvester from three 2.3 m2 areas in each paddock or by hand clipping three 0.19-m2 grids from each paddock. Forage was dried in a forced draft oven set at 50°C for 3 to 4 d and weighed. Average herbage mass per unit area from each paddock was used for statistical analyses of available forage.

A grid method (Hopkins, 2005) was used to collect stand percentage data in March 2001 and 2002, June 2003, and July 2004. Stands were determined from five random locations per paddock and averaged for statistical analysis. Botanical composition of paddocks was estimated after grazing each spring from 2002 to 2004. Data were taken from 15 total points, spaced approximately 12 m apart, along two perpendicular transects in each paddock. Data were taken from different points each year. The dominant feature (e.g., tall fescue, crabgrass, bare soil, manure, etc.) in a 1 cm2 area at each point was determined. Each component was calculated as a proportion of paddock composition by dividing the number of points containing that component by 15.

Presence or absence of endophyte and presence or absence of endophytes producing ergot alkaloids were determined using commercial immunoblot and ELISA test kits (Agrinostics, Watkinsville, GA), respectively, following the manufacturer's instructions. Identical tillers were used for both tests. Forty-five tillers per paddock from replications 1 and 2 were collected in mid-May 2001, and 18 to 20 tillers per paddock were collected from all three replications during late May to early June in 2003 and 2004.

Winnsboro, LA
Paddocks 1.09 ha in size were planted using a drill on 2 Nov. 1999 at a rate of 28 kg of bulk seed ha–1 at the Louisiana State University Macon Ridge Research Station (32°8' N, 91°42' W; elevation approximately 22 m) near Winnsboro in northeast Louisiana. Soil type was a Gigger silt loam (fine-silty, mixed, thermic Typic Fragiudalf). The experiment was planted as a RCBD with two replicates. Entries consisted of GA-5E–, GA-5 MaxQ, GA-5E+, and Jesup MaxQ. The test area was limed to raise soil pH to approximately 6.0 before planting. Annual grasses had been planted in the area the previous 2 yr, and tall fescue was planted in a prepared seedbed during fall 1999. Mature cattle were allowed to graze the tall fescue paddocks during April and May 2000.

Nitrogen was applied on 20 Oct. 2000 (67 kg ha–1), 9 Feb. 2001 (76 kg ha–1), 11 Dec. 2001 (53 kg ha–1), and 14 Feb. 2002 (68 kg ha–1). All applications were as ammonium nitrate except for February 2002, which was applied as urea. Phosphorus and K were applied according to soil test recommendations on 2 Oct. 2000. Paddocks were grazed by steers during a single season, from early January to early May in 2001 and 2002 (Table 1), with three tester animals per paddock, at initial stocking rates of approximately 650 and 600 kg ha–1, respectively. In 2001, the GA-5E– paddock in the second replication was stocked with only two tester animals because of limited forage availability. A put-and-take stocking system was used, with stocking rates adjusted as needed every 28 d, so that an approximately equal amount of forage was available per animal in each paddock. As a result, stocking rates, in animals ha–1, ranged from 1.7 to 5.4 in 2001 and 2.7 to 4.9 in 2002. Calves were weaned on location 45 to 60 d before being stocked on the test and maintained during this period on bermudagrass hay with a protein and energy supplement or limit grazed on annual ryegrass. Animals were de-wormed and given recommended vaccinations during this post-weaning period.

Tall fescue stands were evaluated from a random location in each quarter section of each paddock. Percent stand was estimated by counting the number of 3-cm segments containing a live tall fescue plant from a 3-m row. Stand data were taken 8 Jan. 2001, 17 Aug. 2001, 20 Mar. 2002, and 11 Dec. 2002.

Available forage was determined monthly during the 2001 and 2002 grazing seasons, beginning just before grazing. Forage was clipped to 5-cm stubble from two 1.4-m2 areas in each paddock, dried, and weighed. Data from the two locations within a paddock were averaged for statistical analysis. Re-growth was determined in the same manner by clipping forage monthly from 1.4-m2 areas within two cages that had been constructed in each paddock before grazing. Caged areas differed each year but remained in the same location throughout a grazing season. Re-growth data were not collected in February 2001 because of limited growth.

Tall fescue was not grazed in fall 2000 because drought limited growth before late fall. Climatic conditions in late summer and early fall 2001 were adequate for tall fescue growth, and cows grazed in the paddocks during October and early November. Cows were removed, and tall fescue growth was allowed to accumulate for 47 d before stocking the paddocks with test animals in January 2002.

Cattle were weighed before grazing and every 28 d thereafter during the 112-d grazing period. Initial and final weights were determined after a shrink period during which cattle were kept in a lot without food for 20 h and water for 12 h.

Endophyte testing was conducted at the conclusion of the study in spring 2002 using the same procedures as for the Burneyville trial, with 22 to 25 tillers being collected per paddock.

Data were averaged over animals within a paddock for ADG, temperature, and prolactin level; stand data were averaged within paddocks as well. Gain per hectare was calculated using total weight gain of tester animals from a paddock and adjusting for paddock area and, for the Louisiana trial, stocking rate. Within a trial, data were analyzed across age of stand as a split plot in time. A mixed model was used with entry considered a fixed effect and replication and age of stand random effects. Stand data were analyzed using this same approach, except that age of stand was considered a fixed effect because changes in stand composition over time were anticipated. Age of stand was considered random in relation to animal performance results because even with such anticipated changes in stand composition, forage availability was assumed to be adequate, and any weeds that encroached were summer annuals or perennials, which were senesced during most of the grazing period, so the animals consumed essentially only tall fescue. Analyses were performed using PROC MIXED of SAS (SAS Institute, 2002), with error terms and denominator degrees of freedom being specified using the DDFM = SATTERTH option. Spring and fall data from Oklahoma were analyzed separately. The LSMEANS PDIFF option was used to compare treatment and age of stand differences. Significance was declared at P ≤ 0.10 throughout this research.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Burneyville, OK
Rainfall and temperatures during this research were within a typical range for Oklahoma and generally allowed production of sufficient tall fescue forage for fall and spring grazing. Periodic drought occurred, such as June 2003 through January 2004 when precipitation totaled less than half of normal. Conversely, after grazing in spring 2004, heavy rains resulted in standing water in several paddocks. Summer precipitation data were considered for the fall grazing period because of possible effects on summer survival of tall fescue and available soil moisture in fall, both of which could influence the amount of pasture growth in fall. Although seasonal precipitation totals were consistently less than average (Table 2), this was evidently counter balanced by timely rains within a season and by the high fertility and moisture holding capacity of the Norwood soil.


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  		Table 2. Precipitation totals (Precip), average temperatures (Temp), and departures from long-term averages (Departure) for grazing periods at Burneyville, OK and Winnsboro, LA.

 
Age of stand-by-entry interaction was significant for initial prolactin level during the spring grazing season; otherwise, animal responses to entries (i.e., ADG, GPH, and body temperature) were stable across years, as indicated by a lack of age of stand-by-entry interactions, for fall and spring grazing seasons at Oklahoma (data not shown). In fall, ADG and GPH did not differ among cattle grazing the various tall fescue/endophyte combinations in Oklahoma (Table 3). Similarly, at the conclusion of fall grazing, the body temperature (Table 3) and serum prolactin level (Table 4) of the animals did not differ.


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  		Table 3. Average daily gain (ADG), gain per hectare (GPH), and body temperature (Temp) for steers grazing various tall fescue/endophyte combinations at Burneyville, OK and Winnsboro, LA.

 

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  		Table 4. Serum prolactin levels, averaged across 2 yr, for steers grazing various tall fescue/endophyte combinations at Burneyville, OK.

 
Toxicity responses were very apparent during the spring grazing period. Weight gains from KY-31 E+ were about one half as much as from Dovey, GA-5E–, and GA-5 MaxQ, whereas gains from GA-5E+ were intermediate (Table 3). Likewise, at the conclusion of spring grazing, KY-31E+ and GA-5E+ led to the lowest and intermediate serum prolactin levels, respectively, whereas serum prolactin levels were greatest for animals grazing Dovey, GA-5E–, and GA-5 MaxQ (Table 4). Serum samples were not available at the beginning of spring grazing in 2003, but within 28 d animals grazing KY-31E+ had prolactin levels less than half that of other animals in the trial (data not shown). Body temperatures were also elevated in spring for animals grazing KY-31 E+ compared with the other treatments (Table 3).

All entries persisted well in Oklahoma, with stands exceeding 80% after 4 yr (Table 5). Although differences existed between entries in 2001 and 2003, healthy stands existed in Oklahoma throughout this research. Botanical composition estimates at the end of the grazing season indicated a greater amount of tall fescue, and concomitantly less bare soil, in KY-31E+ vs. all other paddocks in 2002 (Table 6). This trend continued in 2003, except that differences in bare soil among entries were not significant, and by 2004 botanical composition of paddocks did not differ, with minimal bermudagrass encroachment occurring. Endophyte status and infection level remained largely stable during this research (Table 7).


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  		Table 5. Stands for various tall fescue endophyte combinations grown at Burneyville, OK, and Winnsboro, LA.

 

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  		Table 6. Botanical composition of various tall fescue paddocks grown at Burneyville, OK.

 

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  		Table 7. Endophyte infection levels and type for paddocks of tall fescue grown at Burneyville, OK, and Winnsboro, LA.

 
Available forage was equal among entries at the outset of grazing each fall, averaging around 950, 2950, and 1550 kg ha–1 in 2001, 2002, and 2003, respectively. Available forage did not differ among entries after the fall 2001 grazing period. After fall grazing in 2002, available forage was greatest in KY-31E+, Dovey, and GA-5 MaxQ paddocks and least in GA5E– and GA5E+ paddocks. The same result occurred after the 2003 fall grazing period, except that ranking switched between GA-5 MaxQ and GA5E+.

Winnsboro, LA
The only extensive departure from 30-yr temperature and precipitation norms occurred in 2002, when precipitation amount was much lower than normal during the grazing period. Although the precipitation reduction appears drastic, the amount and distribution of rainfall during the period was probably adequate to minimize the occurrence of drought symptoms, considering the moderate temperatures that occur during most of this time of year. Although climatic conditions were relatively conducive to tall fescue growth, there were 10 to 14 d during the last month of grazing in both years when maximum temperatures reached at least 29°C.

Age of stand-by-entry interaction was not significant for any variable in Louisiana (data not shown). Animals grazing E– or novel endophyte combinations in Louisiana gained at least 0.3 kg d–1 more than animals on GA-5E+ (Table 3). Because of variable stocking rates, the improved individual animal performance did not result in significant differences in GPH, although numerical differences were large and followed similar patterns.

Stands did not significantly differ at any date in Louisiana (Table 5). However, stands in one of the GA5E– paddocks averaged 6% on 11 Dec. 2002, leading to a large numerical divergence between GA5E– (23.5%) and GA-5E+ (64%). No differences among entries occurred for residual forage or re-growth at any sampling date. Residual forage averaged approximately 2275 and 1500 kg ha–1 in 2001 and 2002, respectively. Re-growth began in late January to February and generally averaged about 1750 and 850 kg ha–1 every 28 d in 2001 and 2002, respectively.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Several factors may have contributed to the minimal toxicity response (e.g., lack of elevated body temperatures or depressed ADG) among animals grazing KY-31E+ and GA-5E+ during the fall in Oklahoma. Research in eastern Oklahoma (McMurphy et al., 1990) and other locations (Crawford et al., 1989; Hoveland et al., 1997) suggests that toxicity response tends to be greater and more consistent in spring vs. fall for animals grazing tall fescue infected with a wild-type endophyte. Warm to hot air temperatures in spring exacerbate the elevated body temperature of animals grazing wild-type endophyte. Animals consume more water and reduce their grazing time in an attempt to cool off, resulting in decreased forage intake, which contributes greatly to reduced weight gain (Parish et al., 2003). In the present research, the lack of a clear toxicity response in fall was probably due largely to mild fall temperatures. The fact that animal gains in Louisiana tended to be similar among treatments early in the grazing season (data not shown) but much lower from cattle grazing GA-5E+ later in the season is another indication that milder temperatures may moderate the toxicity effect. Reduced levels of ergot alkaloids have been reported in E+ tall fescue for leaves older than 6 wk of age (Belesky and Hill, 1997), for fall vs. early summer growth (Hill et al., 2000), and for stockpiled growth sampled in winter (Kallenbach et al., 2003). In addition, reduced levels of P nutrition can lead to decreased production of ergot alkaloids by endophyte-infected plants (Malinowski et al., 1998). Available P was at moderate levels of 40 and 65 mg kg–1 in Oklahoma and Louisiana, respectively, but may have been further decreased by soil alkalinity (pH 7.9) in Oklahoma. As such, reduced levels of toxins in grazed forage may have occurred in Oklahoma during fall grazing and, if so, could have helped mitigate any toxicity response in the present research. For farmers and ranchers with existing stands of tall fescue infected with wild-type endophyte in the south central USA, animal health and performance problems might be minimized by stockpiling and grazing such pastures in late fall to early winter.

The intermediate response in Oklahoma of animals grazing GA-5E+ in spring may be attributable to slightly lower levels of endophyte infection compared with KY-31E+ and/or other factors, such as host–endophyte combinations and the environment. Animal gains have been estimated to decrease by 45 to 68 g d–1 for each 10% increase in rate of wild-type endophyte infection in tall fescue (Hume and Barker, 2005; Crawford et al., 1989). Using the latter estimate, and an average difference of 16% in infection level that occurred during this research, animals would be expected to gain about 0.11 kg d–1 more on GA-5E+ vs. KY-31E+ paddocks, which is comparable to the observed difference in spring of 0.18 kg d–1. However, others have reported a diminishing effect on animal performance as infection rate of wild-type endophyte increases, with no differences observed in ADG between cattle grazing pastures infected at 60 vs. 80% (Fribourg et al., 1991). An additional or alternative possibility is that differences in alkaloid production contributed to observed differences between animals grazing GA-5E+ and KY-31E+ paddocks. The levels of alkaloids produced by host–endophyte combinations can vary depending on plant genotype (Hill et al., 2002), with differences in mycelial mass perhaps explaining much of this variation (Easton et al., 2002). Possible genotype or genotype by environment effects of the plant and endophyte may have resulted in GA-5E+ plants producing toxic ergot alkaloids in lower concentrations than KY-31E+ plants. Finally, GA-5 may have inherently greater nutritive value than KY-31, which could lead to improved animal gains, although previous research has found no difference between these two cultivars for neutral detergent fiber concentration (Vaylay and van Santen, 1999).

Our results provide further evidence of the value of nontoxic endophytes in eliminating animal health and performance problems in a variety of grazing situations, including stockpiled forage in late fall and winter and actively growing paddocks in early to late spring. On soils with favorable moisture holding capacity, stands of tall fescue were maintained regardless of endophyte infection status and type. The seasonal, moderate level of grazing used in this research probably also contributed greatly to stand longevity. Heavy grazing pressure during spring and summer can lead to almost complete loss of tall fescue stands in southern Oklahoma on soils with a range in moisture-holding capacity (Hopkins, 2005). This observation, coupled with the partial loss of stands in Louisiana, particularly for GA-5E–, indicates that cultivars with improved persistence are needed for broader use of tall fescue in the south central USA and that infection with a novel endophyte is important for the success of such cultivars.


   ACKNOWLEDGMENTS
 
The authors thank Mack Armstrong, Brian Motes, Jim Johnson, Dennis Walker, and Johnny Ashley for their assistance in this research.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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