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Published online 1 January 2005
Published in Crop Sci 45:297-304 (2005)
© 2005 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Influence of Phosphorus and Potassium on Alfalfa Yield and Yield Components

W. K. Berg, S. M. Cunningham, S. M. Brouder, B. C. Joern, K. D. Johnson, J. Santini and J. J. Volenec*

Dep. of Agronomy, Purdue Univ., 915 West State St., West Lafayette, IN 47907-2054

* Corresponding author (jvolenec{at}purdue.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Phosphorus and K fertilization increases alfalfa (Medicago sativa L.) yield and stand persistence, but the changes in yield components as affected by P and K fertility level are not known. Our hypothesis is that P and (or) K fertilization will increase one or more alfalfa yield components, and those component responses may change with stand age. The objectives of this field study were to determine the impact of P and K fertilization on alfalfa forage yield and yield components during the initial 3 yr after establishment. Treatments were the factorial combinations of four P rates (0, 25, 50, and 75 kg P ha–1) and five K rates (0, 100, 200, 300, and 400 kg K ha–1) arranged in a randomized complete block design with four replications. Forage harvests occurred four times annually, and yield, mass shoot–1, and shoots area–1 were determined. Plant populations were determined in early December and late May each year. Incremental additions of P and K increased alfalfa yield in each year. Potassium fertilization did not influence plant population, while robust P-responsive alfalfa plants apparently crowded out smaller, less vigorous plants thus decreasing plants m–2. Stand assessments based on shoot counts, or aboveground plant counts may not accurately indicate alfalfa yield potential. Shoots plant–1 was not affected by application of either nutrient, while shoots m–2 generally declined with increased P and K fertilization. Improved forage yield of P- and K-fertilized plots was consistently associated with greater mass shoot–1. Because fertilizer-responsiveness is closely associated with greater mass shoot–1, cultivars possessing this trait may be relatively more productive under well-fertilized conditions.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE IMPACT OF MINERAL nutrition on alfalfa agronomic performance has been documented by several investigators (Nelson and MacGregor, 1957; Markus and Battle, 1965; Collins and Duke, 1981; Suzuki, 1991). Phosphorus and K fertilizer applications can significantly improve alfalfa yield; however, the morphological and physiological basis for increased yield with improved P and K nutrition is not completely understood. Understanding how alfalfa yield components (plants m–2, shoots plant–1, mass shoot–1) change with improved P and K nutrition may provide plant breeders with more specific morphological targets for enhancing yield potential in high fertility environments. Although alfalfa populations decrease following establishment, proper K nutrition is generally thought to promote plant persistence and stand longevity (Wang et al., 1953; Smith, 1975). Hanson and MacGregor (1966) reported that alfalfa stands receiving K after the first cutting, and K and P in autumn, maintained a denser stand after 10 yr when compared with plots provided P alone. Collins et al. (1986) correlated increased alfalfa stand persistence with increased K application. During 2 yr, alfalfa grown in a silt loam soil provided 224 kg ha–1 yr–1 of K fertilizer and had a 50% survival rate compared with only a 27% survival rate for plants not provided with K fertilizer.

Addition of P can increase alfalfa yield, but conflicting results have been published regarding the effect of P application on plant population across time. Jung and Smith (1959) stated that proper P nutrition was essential for plant survival. However, Sanderson and Jones (1993) reported that alfalfa stands declined from 495 to 98 plants m–2 during 2 yr where 59 kg P ha–1 was applied, whereas unfertilized stands contained 134 plants m–2 after 2 yr. Likewise, Collins et al. (1986) reported decreased plant populations in alfalfa stands provided P fertilizer when compared with unfertilized control plots.

Alfalfa yield is often positively associated with shoots area–1. Shoots area–1 and forage yield are maintained at higher levels across time if proper P and K fertility is provided (Suzuki, 1991). Using greenhouse-grown alfalfa, Li et al. (1997) reported that plants receiving adequate K averaged twice as many shoots plant–1 when compared with K-deficient plants. Sanderson and Jones (1993) found that P fertilization promoted increased shoots plant–1 of field-grown alfalfa when compared with alfalfa not receiving P. Current university recommendations in Wisconsin identify 430 shoots m–2 (40 shoots foot–2) as a minimum shoot density to sustain yield (Undersander et al., 1998). However, the interaction of P and K on alfalfa shoot production and how these nutrients impact forage yield, both as a stand ages and at harvests within years, have not been evaluated.

Increased mass shoot–1 has been positively associated with enhanced forage yield of alfalfa, whether differences were caused by improved genetics, soil fertility, or insect control (Volenec, 1985; Kitchen et al., 1990; Li et al., 1998). Cooper et al. (1967) attributed increased shoot mass with improved K fertility to increased leaves shoot–1. Using a controlled environment, he found that after 17 d of regrowth, plants provided 0 mg K L–1 of nutrient solution had 42 leaves plant–1, whereas plants receiving 195 mg K L–1 solution had 90 leaves plant–1.

Our hypothesis is that P and (or) K fertilization will increase one or more alfalfa yield components, and those component responses may change with stand age. The objectives of this study were to determine the impact of P and K fertilization on forage yield, plant persistence, and yield components of alfalfa.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Field Design and Sampling
In April 1997, a 1.4-ha site at the Throckmorton Purdue-Agricultural Center located 15 km south of West Lafayette, IN, was seeded to Pioneer Brand ‘5454’ alfalfa. This site was selected for study because soil tests indicated low concentrations of extractable P (9 to 15 mg kg–1 Bray P1) and low to moderate levels of exchangeable K (108–138 mg kg–1 exchangeable K) according to Vitosh et al. (1996) (Table 1). Five- by 10-m field plots were established, and treatments were the factorial combinations of four P rates (0, 25, 50, and 75 kg ha–1) and five K rates (0, 100, 200, 300, and 400 kg ha–1) arranged in four replicates of a randomized complete block design. Two replications were placed on a Drummer silty clay loam (fine-silty, mixed, superactive, mesic Typic Endoaquoll), while the other two replicates were placed on a Lauramie silt loam (fine-loamy, mixed, active, mesic, Mollic Hapludalf). Phosphorus and K were applied in split applications, with one-half the annual amount broadcast after the first forage harvest in late May, and the remainder broadcast after the last forage harvest in mid-September. Chemical control of pests occurred when threshold limits were surpassed.


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  		Table 1. Initial soil characteristics (15-cm depth) for the Drummer soil and Lauramie soil.

 
Soil test P and K levels were monitored following each growing season by sampling the soil in 5-cm depth increments to –20 cm. Twelve core composites per plot were taken for each depth increment. Each sample was air-dried and crushed to pass a 2-mm screen. Soil P and K availability was estimated using the Mehlich III extraction procedure with extract analysis by inductively coupled plasma emission spectroscopy (ICP). Mehlich III soil-P concentrations were interpolated to Bray P1 with a conversion provided by a commercial laboratory (Denning et al., 1998).

Four forage harvests occurred each year at 30-d intervals beginning in late May. At each harvest, herbage was removed to a height of 5 cm with a flail-type chopper of 0.91-m width, removing forage the length of the plot (9.1 m; 8.32-m2 area harvested). Fresh weights were determined, and then samples (400-g subsamples) were dried at 60°C until a constant mass was obtained. Forage yield was calculated on a dry-weight basis. A subsample of 50 shoots was hand-collected at several randomly selected sites within each plot before harvest, dried at 65°C, weighed, and mass shoot–1 was determined. Shoots m–2 was calculated by dividing yield m–2 by mass shoot–1. Roots in a 0.5-m2 area were sampled in May and December of each year to quantify changes in plants m–2 and to acquire root samples for laboratory analysis.

Analysis of variance P, K, and the P x K interaction effects were partitioned into orthogonal polynomial contrasts. Regression models incorporating terms for all significant polynomial contrasts were developed and evaluated on treatment means. Occasionally, a regression model exhibiting an anomalous relationship was rejected. Differences in equation slopes were determined using the standard error of the difference between the slopes adjusted for the experimental error. Statistical analyses were performed using SAS software (SAS Institute, 1989–1999).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Forage Yield
Application of K and P fertilizer increased total yield in 1998, 1999, and 2000 (Fig. 1 and Table 2). The ANOVA for yield contained significant or highly significant orthogonal polynomial contrasts for K, K2, P, P2, and (or) P x K for total yield and all harvests except Harvest 1 in all years and Harvest 2 in 1998 (Table 2). In every year, Harvest 1 had significant or highly significant orthogonal polynomial contrasts for P and (or) P2 but no response to K. The lack of K response suggests that soil K availability was not limiting forage yield in Harvest 1 of any year, even in control plots not receiving K for the previous 3 yr.


Figure 1
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  		Fig. 1. Influence of P and K fertilizer application on yield in 1998, 1999, and 2000. Contour plots are generated from the regression equations listed in Table 5 for total yield.

 

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  		Table 5. Regression equations for the influence of P and K on yield and yield components at each harvest and total forage harvest in 1998, 1999, and 2000.

 

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  		Table 2. Level of significance for the K and P treatment effects and orthogonal polynomial contrasts on individual harvest and total annual yield.

 
While the ANOVA showed a significant P linear x K linear interaction with forage yield only in Harvest 4 of each year, the additive effect of P and K on forage yield was apparent for total yield in 1998, 1999, and 2000 (Fig. 1). In each year, the highest yields were obtained by addition of both P and K fertilizers. Addition of 75 kg P ha–1 yr–1 without K, or 400 kg K ha–1 yr–1 without P reduced total yield each year when compared with plots provided both nutrients. Yields obtained from the control plots (0 kg P ha–1 yr–1, 0 kg K ha–1 yr–1) declined from 1998 to 2000 as nutrient removal in the forage depleted soil P and K levels (data not shown).

Yield Components
Alfalfa yield component analysis is needed to develop a more mechanistic understanding of yield increases due to P and K fertilization. Alfalfa forage yield can be described as the product of three components: plants area–1, shoots plant–1, and mass shoot–1 (Volenec et al., 1987). From December 1997 through May 1999, there were no P and K main effects or P x K interaction for plant population (data not shown). Plant population initially averaged 420 plants m–2 in September 1997, and declined to 280 plants m–2 by May 1998. Between May and December 1998, plant populations declined further to 200 plants m–2. Thereafter, a pattern emerged where plants m–2 declined during summer (May to December), but remained unchanged during winter (December–May), and this pattern was not affected by P and K fertilizer application (data not shown).

No significant orthogonal polynomial contrasts for K were evident for plant populations in 1998, 1999, and 2000. The ANOVA for plant populations contained significant quadratic and linear responses in May 2000 and December 1999, respectively, where addition of P led to decreased alfalfa populations. Using the regression equation for December 1999 [Y = 167 – 0.368(P); P < 0.0001], addition of 75 kg P ha–1 yr–1 reduced alfalfa population to 140 plants m–2 as compared with 167 plants m–2 in the control plots. The equation generated from the May 2000 plant population data, Y = 186 – 1.69 (P rate) + 0.015(P)2 (P < 0.0001), predicted similar P fertilizer-induced reductions in plant populations (186 and 144 plants m–2 for 0 and 75 kg P ha–1 yr–1, respectively).

Shoots plant–1 can only be calculated directly for Harvest 1 of each year when shoot counts and plant populations were simultaneously determined. Shoots plant–1 was not influenced by P and K fertilization at Harvest 1 of any year (data not shown). Averaged over fertility treatments, shoots plant–1 at Harvest 1 was similar in 1998 and 1999 (2.2 and 2.0 shoots plant–1, respectively), before increasing in 2000 to 2.7 shoots plant–1. Although shoots plant–1 increased at Harvest 1 of 2000, Harvest 1 forage yield was reduced in 2000 as compared with Harvest 1 forage yield in 1999. Although increased shoots plant–1 has been correlated with enhancing yield in previous studies, there was not a close relationship between shoots plant–1 and yield in this study. The discrepancies between this alfalfa trial and those performed previously (Suzuki, 1991; Sanderson and Jones, 1993) may reflect uncertainties in determining plant populations, such as quantifying plant populations using crown counts. In our study, plant populations were determined by digging and counting roots in a defined area; a method that avoids the ambiguities of associating crowns (and portions thereof) with specific plants. This discrepancy in methods may not only account for inconsistencies in plant populations but also for how plant populations ultimately influenced yield and shoots m–2.

Forage yield in this study was not closely associated with shoots m–2 (Tables 3 and 4). Although the ANOVA for shoots m–2 contained P x K interactions in 1998 and 2000, the regression models subsequently developed exhibited anomalous relationships that were ultimately rejected. Analysis of variance revealed highly significant orthogonal polynomial contrasts for P in Harvest 2 of 1998, and P and P2 in Harvest 1 of 2000. In 2000, significant orthogonal polynomial contrasts for K3 and K2 for shoots m–2 were determined in Harvests 4 and 3, respectively, while significant orthogonal polynomial contrasts for K linear were apparent in Harvest 4 of each year and Harvest 3 of 1999 and 2000. Although total yield was dramatically increased by addition of P and K, addition of these nutrients led to an overall decrease in shoot density, indicating that high forage yield is not dependent on high numbers of shoots m–2.


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  		Table 3. Level of significance for the K and P treatment effects and orthogonal polynomial contrasts on individual harvest for shoots m–2.

 

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  		Table 4. Regression equations, R2 values, and P values for the influence of shoots m–2 and mass shoot–1 on yield at each harvest in 1998, 1999, and 2000.

 
The ANOVA for mass shoot–1 contained significant or highly significant orthogonal polynomial contrasts for K, K2, K3, P, P2, and (or) K x P for mass shoot–1 in all harvests except Harvest 2 in 1998 (Table 4). Although the ANOVA indicated a significant orthogonal polynomial contrast for the K2 x P2 interaction in Harvest 2 of 1998, further examination revealed the interaction was irrelevant and the model was rejected. In each year, Harvests 3 and 4 contained significant polynomial contrasts for K and P. Addition of P induced a significant polynomial contrast in Harvest 1 of each year, while a significant response of mass shoot–1 to K was not observed.

Addition of P and K increased mass shoot–1 in nearly every harvest of each year of this study (Table 4). The influence of P and K on mass shoot–1 was similar to the effects of P and K on total forage yield, indicating a strong positive association between these traits (Tables 5 and 6). Although a quadratic response for P was observed in only 5 of 12 harvests, all harvests in 2000 contained a significant quadratic relationship between P fertilizer rate and mass shoot–1; a pattern that was similar to the quadratic relationships observed between P fertilizer rate and forage yield.


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  		Table 6. Level of significance for the K and P treatment effects and orthogonal polynomial contrasts on individual harvest for mass shoot–1.

 
The linear relationship between K fertilizer rate and mass shoot–1 was significant in each forage harvest where a linear relationship was observed between K fertilizer rate and forage yield, except Harvest 2 of 2000 (Table 2). The quadratic and/or cubic relationships between K fertilizer rate and mass shoot–1 were not common, but were observed in Harvest 3 of 2000 and Harvest 4 of 1998 and 2000. These higher-order polynomial responses in late season may indicate that other environmental factors, like water, limit forage yield and mass shoot–1 more than soil K availability. It is also noteworthy that, in a pattern similar to forage yield, mass shoot–1 was not influenced by K fertilizer rate in Harvest 1 of any year, indicating that soil K was not limiting spring shoot growth even though severe K deficiency occurred later in the growing season.

Regression identified a consistent positive linear relationship between forage yield and mass shoot–1 (Table 4 and Fig. 2) . Averaged across years and harvests, forage yield increased 2.6 Mg g–1 increase in mass shoot–1. While the positive relationship between mass shoot–1 and forage yield was fairly consistent, relatively low R2 values were obtained at Harvest 1 and 2 of 1998 when fertilizer application resulted in small or no difference in forage yield. Potassium application did not affect yield or mass shoot–1 at Harvests 1 or 2 of 1998, and P application did not impact yield and mass shoot–1 at Harvest 2 of 1998 (Table 2).


Figure 2
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  		Fig. 2. Relationship between forage yield and mass shoot–1 at the first (May 1998) and last (Sept. 2000) harvests of the study.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Fertilization with P and K has long been known to improve persistence and yield of alfalfa (Markus and Battle, 1965; Powell, 1972; Smith, 1975), but the physiological and morphological mechanism responsible for yield increases remains unclear. Large reductions in alfalfa population following emergence have been reported (Suzuki, 1991), but the extensive plant losses we observed each summer, with virtually no plant losses during winter, were not expected given the importance of K nutrition in alfalfa winter hardiness. During the winters of 1998–1999 and 1999–2000, plant losses were two and zero plants m–2, respectively, but in summer 1999, between the aforementioned winters, plant losses totaled 47 plants m–2. Stout et al. (1992) also found that alfalfa populations can decline extensively during the summer and speculated that root and crown diseases, particularly verticillium wilt (Verticillium albo-atrum Reinke & Berthier), were the cause of stand thinning during the growing season. Stresses associated with defoliation and diseases, as well as competition for water, light, and nutrients during summer, may contribute to plant death in summer. In addition, winter-injured plants may survive to be counted in May, but ultimately succumb during the summer, resulting in a misperception that plant loss reflects summer-related stresses. Additional work is underway to understand causes of alfalfa plant death in summer, knowledge that is essential for improving stand persistence.

Improved plant nutrition (especially K) is thought to increase plant persistence, but through the 3 yr of this study, K fertilizer applications have not influenced plant populations. Unexpectedly, addition of P fertilizer reduced alfalfa populations. The decrease in alfalfa populations observed in response to enhanced P fertilization agrees with trends reported by Sanderson and Jones (1993) where alfalfa populations declined from 109 to 98 plants m–2 when P fertilizer application increased from 29 to 59 kg P ha–1 yr–1. The decline in alfalfa populations with P fertilization may result from enhanced interplant competition for light, water, and nutrients that eliminate smaller, less vigorous plants from the genetically heterogeneous population of plants that comprise an alfalfa stand. Robust, P-responsive plants acquire greater mass and have more rapid regrowth after harvest, eventually crowding out smaller, less vigorous, slower-growing alfalfa plants. Plant population is inversely related to plant size in natural communities; as plants increase in size, the space requirement of the larger plant increases, and as a result, plant populations decrease. Yoda et al. (1963) characterized the relationship between plant size and population by plotting the log of plant mass as a function of the log of plant density. Linear regression indicated that in natural ecosystems, this relationship had a slope of –3/2. Although debate has arisen on which plant tissue to use as the mean weight (especially in forest species where total mass is difficult to quantify), the total aboveground mass vs. plant population has garnered results well correlated with the –3/2 slope (Mohler et al., 1978). White and Harper (1970) reported similar self-thinning regression slopes (–3/2) using rape (Brassica napus var. napus) and radish (Raphanus sativus L.). They suggested that plants established a hierarchy of resource exploration that ultimately resulted in differential growth rates among competitors. This variation in growth rate creates populations of suppressed and dominant plants where competition eventually eliminates members of the suppressed plant population.

Using data from Harvest 1 of 2000 where shoot mass plant–1 and plant population data were measured simultaneously, we examined whether P and K fertility influenced this size-density relationship in alfalfa (Fig. 3) . Regression analysis revealed that plots not fertilized with P had a slope near the expected –3/2 value (–1.7), whereas the slope ranged from –1 to –1.2 for plots where P fertilizer was applied (Fig. 3A). Plots not fertilized with P represent natural ecosystems where this relationship has been most frequently tested, whereas P fertilization increased the size of individual alfalfa plants, resulting in greater interplant competition and ultimately enhanced mortality. Nevertheless, P application permitted larger plants to exist at a higher population density than would be predicted from the –3/2 law (Fig. 3A, 102.2 to 102.4; 150 to 250 plants m–2); a factor that contributes to the higher forage yields obtained with P fertilization.


Figure 3
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  		Fig. 3. Regression of log of herbage mass plant–1 vs. log of plants m–2 as influenced by P (A) and K (B) fertilization in Harvest 1 (May) of 2000. Slopes of regression equations designated with the same letter are not different (P < 0.05). Data presented in Fig. 3A are averaged across the K fertilizer treatments, while data presented in Fig. 3B are averaged across the P fertilizer treatments.

 
Addition of K fertilizer did not influence plant size or plant population when compared with plots not provided K. Averaged across P fertilizer treatments, the slopes of the plant density–plant size regressions were similar irrespective of K fertilizer application and averaged –1.6 (Fig. 3B), a value close to the –3/2 slope reported for self-thinning in natural ecosystems by Yoda et al. (1963) (Fig. 3B). This indicates that increases in plant mass are independent of K fertility, and are more related to plant density and its impact on interplant competition.

Shoots plant–1 is the yield component that is thought to increase as stand density declines to maintain high yield (Suzuki, 1991). However, in this study, shoots plant–1 did not increase significantly in response to P or K, nor was this yield component positively associated with forage yield in any harvest of this study. Li et al. (1997) found that hydroponically grown alfalfa provided 6 mM K and produced an additional shoot per plant when compared with plants receiving no K. However, the wide spacing of potted plants in the greenhouse may have permitted greater shoot production in response to enhanced fertility than was possible in our field study, where we had approximately 220 plants m–2. Although a decline in plants m–2 was observed between May 1999 and May 2000, plant populations still may not have reached a threshold where an increase in shoots m–2 was possible.

Undersander et al. (1998) suggested that alfalfa forage yield could be predicted using shoot number estimates taken the previous fall, and described this relationship using the equation: yield (tons acre–1) = (0.10 x shoots foot–2) + 0.38. We tested the accuracy of this equation in predicting forage yield for each of the 80 plots in the study in 1999 and 2000 using shoots m–2 obtained for each plot in September 1998 and 1999, respectively (after converting shoots m–2 to shoots foot–2). Regression of predicted forage yield vs. actual May forage yield resulted in a low R2 values and negative slopes in both years (1999, y = 8.93 – 0.35x, R2 = 0.07; 2000, y = 9.3 – 0.45x; R2 = 0.10). To better understand the discrepancies between the predicted and actual yield, a detailed seasonal analysis of shoots m–2 was performed. No relationship was found between shoot numbers at Harvest 1 and shoot numbers present the previous fall (shoots m–2 fall 1998 vs. shoots m–2 Harvest 1 1999, y = 333 + 0.095x, R2 = 0.02, P = 0.21; shoots m–2 fall 1999 vs. shoots m–2 Harvest 1 2000, y = 365 + 0.039x, R2 = 0.003, P = 0.63). The large seasonal variation in shoot numbers indicates that using this yield component to estimate future yield potential of alfalfa may not be appropriate.

Mass shoot–1 was the yield component most closely associated with yield increases in response to P and K fertilization. In 11 of 12 harvests during 3 yr, mass shoot–1 was positively associated with yield, whereas other yield components were either negatively (shoots m–2) or not (plants m–2) associated with forage yield.

Causes of increased mass shoot–1 have been examined previously (Volenec, 1985). Rapid shoot initiation rates after defoliation permit shoot regrowth to resume quickly after harvest and result in high mass shoot–1 (Singh and Winch, 1974). Plants receiving P and K initiated new shoot growth quicker than unfertilized plants because increased nutrient mobilization occurs between roots and actively growing shoots (Li et al., 1997). Second, after new shoot initiation, shoot growth rates (mg d–1) are enhanced with increased K fertility, leading to greater mass shoot–1 (Li et al., 1997). Future work defining how P and K nutrition alter shoot initiation and regrowth rates after defoliation will enhance our understanding of mechanisms responsible for increasing mass shoot–1 and ultimately the responses of alfalfa yield to enhanced P and K nutrition.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Contribution from the Purdue Univ. Agric. Exp. Stn., Journal Series No. 17282.

Received for publication December 2, 2003.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


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