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DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Yields and Nitrogen Dynamics in a Rice–Wheat System Using Green Manure and Inorganic Fertilizer

^a Dep. of Soils, Punjab Agricultural Univ., Ludhiana 141004, Punjab, India
^b USDA-ARS, East Campus, Univ. of Nebraska, Lincoln, NE 68583 USA
^c Farm Advisory Service, Punjab Agricultural University, Kapurthala, Punjab, India

jdoran1{at}unl.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions and recommendations REFERENCES

 
Integrating fertilizer N (FN) with legume green manures (GM) can foster sustainable and environmentally sound agricultural systems in subtropical, semiarid soils low in organic matter. A 4-yr replicated field experiment with irrigated rice (Oryza sativa L.) and wheat (Triticum aestivum L.) rotations was conducted on a sandy loam soil in the Punjab of India to evaluate the effects of incorporating cowpea (Vigna unguiculata L.) or sesbania (Sesbania aculeata L.) GM on crop productivity, N-use efficiency, and NO₃ leaching. Rice grain yields with pretransplant incorporation of 20 and 40 Mg GM ha^-1 (GM₂₀ and GM₄₀) ranged from 5.18 to 5.81 Mg ha^-1, which were equivalent to the 5.40 Mg ha^-1 obtained with 120 kg FN ha^-1 (FN₁₂₀). Rice yield with 60 kg FN ha^-1 (FN₆₀) and GM₂₀ was double the control yield and 6% greater than the FN₁₂₀ treatment. Nitrogen utilization by rice was greater for GM₂₀ than FN, as indicated by greater fertilizer N equivalents and an apparent N recovery by rice of 79 vs. 63%. While FN had no residual effects, residual GM produced greater wheat yields, indicating that FN applied to wheat could be reduced by 25%. Soil NO₃–N leaching to 60 cm in the rice crop was apparently used by the subsequent wheat crop, which has a deeper and more extensive rooting system. After 4 yr, use of FN₁₂₀ for both crops resulted in 35 kg of residual NO₃–N ha^-1 in the 150-cm soil profile (74% below 90 cm), whereas only 19 kg NO₃–N ha^-1 remained with GM₂₀ plus FN₆₀. Use of GM₂₀ with FN₆₀ for a rice–wheat rotation demonstrated greater yields of rice and wheat, 25 to 50% reduction in fertilizer N use, and decreased potential for groundwater NO₃ contamination.

Abbreviations: ANR, apparent N recovery • D_b, bulk density • FN, fertilizer N • FN₀, FN₆₀, FN₉₀, and FN₁₂₀ represent 0, 60, 90, and 120 kg fertilizer N ha^-1, respectively • FNE, fertilizer N equivalent • GM, legume green manure • GM₀, GM₂₀, and GM₄₀ represent 0, 20, and 40 Mg fresh weight green manure ha^-1, respectively • HI, harvest index • OC, organic C • RCBD, randomized complete block design

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions and recommendations REFERENCES

 
WHILE ONLY ONE-HALF of the 146 million ha under rice cultivation in the world is irrigated, it represents 75% of total rice production (IRRI, 1991). In Asia, irrigated rice is often a part of annual double cropping systems where upland crops such as wheat, maize (Zea mays L.), vegetables, or legumes follow rice, with rice–wheat as the major cropping system. Production of rice and wheat has responded to N rates up to 120 kg N ha^-1 on these subtropical, semiarid soils (Maskina et al., 1988), which are inherently low in organic matter and N levels.

In recent years, fertilizer cost and concern for sustainable soil productivity and ecological stability in relation to chemical fertilizer use have emerged as important issues (Aulakh and Pasricha, 1997a; Aulakh and Bijay-Singh, 1997). There is a renewed interest in organic manures, such as farmyard manures, composts, and green manures, as sources of plant nutrients (Yadvinder-Singh et al., 1988; Aulakh, 1994). Farmyard manures and composts are in limited supply and may have low and variable nutrient contents. The more readily available green manures constitute a valuable source of both N and organic matter (Buresh and De Datta, 1991; Ladha and Garrity, 1994; Ladha et al., 1996; Bijay-Singh and Yadvinder-Singh, 1997; Aulakh and Pasricha, 1997b).

In a rice–wheat system, green manure crops such as cowpea and sesbania can be grown for 45 to 60 d in May and June when fields are fallow after the harvest of wheat. The green plants are incorporated into the soil in the third or fourth week of June prior to the transplanting of rice. Rice is grown during the monsoon season from July to October and wheat in winter from November to April. Information on benefits of integrating FN with legume GM or its use as an alternative source of nutrients will contribute greatly to development of sustainable agricultural management systems in semiarid, subtropical regions.

Nitrogen movement below the root zone and into the groundwater (Olson et al., 1970; Spalding and Kitchen, 1988) can cause human and animal health problems (USEPA, 1985). Leaching losses of N mainly occur as NO₃, while leaching of NH₄ is only a problem when applied in very large quantities on coarse-textured soils. Recent reviews (Aulakh, 1994; Bijay-Singh et al., 1995; Aulakh and Bijay-Singh, 1997) have revealed few studies on NO₃ leaching in intensively cultivated semiarid, subtropical or tropical soils and no reports have addressed rice – wheat cropping systems. In most of these studies, the distribution of NO₃ in the soil profile has been determined at the end of different field experiments. No studies are available that report changes in NO₃ and NH₄, during and after the harvest of each crop grown in a rotation, occurring under dry and flooded cycles, application of FN, and the incorporation of GM or crop residues.

Traditional flooded paddy soils are fine textured with very low percolation rates. Upon flooding, these soils become anaerobic and nitrification is severely reduced, resulting in the continuous accumulation of NH₄–N. However, with increasing food and fiber demands to support growing populations as well as the creation of additional canal and tubewell water irrigation facilities, coarse-textured highly porous soils of subtropical regions are now used for raising both upland crops as well as wetland rice (Aulakh and Bijay-Singh, 1997; Aulakh and Pasricha, 1997b). In porous soils under rice, continuous flooding cannot be maintained due to high water percolation rates. Thus the porous soils exhibit alternate aerobic and anaerobic conditions and can support nitrification (Aulakh et al., 1996). Nitrate enrichment of groundwater beneath several irrigated porous soils is evident from the NO₃ analysis of tubewell waters (Bajwa et al., 1993); however, no data have been reported on the effects of N fertilization and green manure crops on the dynamics of NO₃ and NH₄ in porous irrigated soils. Such studies would become more important as porous soils are increasingly used for irrigated rice–wheat cropping systems.

A 4-yr field study with an irrigated rice–wheat cropping system in a subtropical, semiarid porous soil was undertaken to investigate (i) the individual and combined effects of GM and FN on the yield response and N-use efficiency of irrigated rice, (ii) the capacity of GM to substitute for FN and its residual effect on the succeeding wheat crop, and (iii) N dynamics in soil during and after the harvest of rice and wheat crops, including the mineralization of GM-N and the amounts of NO₃–N and NH₄–N in the soil profile and the potential for their leaching below the rooting zone.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions and recommendations REFERENCES

 
A 4-yr field experiment was conducted during 1992 to 1996 on a semiarid irrigated Tolewal sandy loam (Typic Ustochrept) soil at Punjab Agricultural University Research Farm, Ludhiana, India. Ludhiana is located 30° 54' N and 75° 48' E and is 247 m above mean sea level. The mean monthly minimum air temperatures during the 4 yr of the study ranged from 5 to 8° in January to 26 to 27°C in June, while maximum temperatures ranged from 8 to 19°C in January to 35 to 40°C in June. The annual rainfall ranged from 593 to 1227 mm, with 74 to 85% occurring from July to September. Characteristics of the Tolewal sandy loam soil (Table 1) were determined by standard methods (Westerman, 1990). The plow layer (0–15 cm) had total N content of 0.4 g kg^-1. The infiltration rate of the unpuddled soil averaged 1.0 cm h^-1, and the puddled soil rate was 0.2 cm h^-1. The groundwater table was below 8 m.

Rice (cv. PR 106) was seeded in a separate field adjoining the experimental plots in the second week of May and fertilized with 15 kg N ha^-1 and 17.5 kg P ha^-1 as diammonium phosphate, and 10 kg zinc sulfate ha^-1. Two weeks later, 25 kg N ha^-1 as urea was top-dressed prior to irrigation. Healthy 45-d-old plants were transplanted two plants per hill in the third or fourth week of June. Rows were 20 cm apart with an in-row spacing of 15 cm, and the individual plots measured 3 by 10 m. Fertilizer N as urea was applied in three equal amounts at 0, 3, and 6 wk after transplanting. The experimental area was bordered with 10 rows of nonexperimental rice plants on all sides of the field. The crop was irrigated on a daily basis during the first month and thereafter on an as-needed basis at intervals to prevent a dry spell (soil surface without overlying water) of more than 2 d. This is a recommended practice of intermittent submergence for rice as it saves water but does not reduce rice yields (Sandhu et al., 1980). Irrigation water was used either from canal or tubewell and contained nominal N (<0.5 mg L^-1). At maturity, the rice crop was harvested in the first or second week of October. Grain and straw yields were recorded and grain data are expressed on the basis of 140 g water kg^-1 dry matter (14%).

Wheat (cv. HD 2329) was drilled at 40 kg seed ha^-1 in early November in the same permanent plots as rice in rows 20 cm apart. A uniform application of 26.2 kg P ha^-1 as single superphosphate was drilled in all plots at the time of seeding. Four rates of FN (0, 60, 90, or 120 kg N ha^-1) as urea were broadcast in two equal splits, at seeding and 4 to 5 wk later (1 d after first irrigation). Thereafter irrigation was applied when required to maintain adequate moisture conditions (>50% of plant-available soil water). The middle 15 m² of each plot was harvested in the second or third week of April. Grain and straw yields were recorded and the data are expressed on a dry weight basis. Aboveground crop residues except stubbles of both crops were removed from the plots. Concentration of N in grain and straw samples of both rice and wheat was determined by the micro-Kjeldahl method (Westerman, 1990).

Of the 10 different treatment combinations (Table 2), five were selected for monitoring the dynamics of NO₃–N and NH₄–N. These were GM₀ FN₀ (Rice)–FN₀ (Wheat), GM₀ FN₁₂₀–FN₁₂₀, GM₂₀ FN₀–FN₀, GM₂₀ FN₆₀–FN₉₀, and GM₄₀ FN₁₂₀–FN₉₀. Four cores were collected with a 5-cm-i.d. tube auger and composited separately for six soil depths (0–15, 15–30, 30–60, 60–90, 90–120, and 120–150 cm) in 15 plots (5 treatments x 3 replications) at initiation and completion of the 4-yr study. Following the same protocol, soil samples were periodically taken from the upper three soil depths (0–15, 15–30, and 30–60 cm) before and/or after important field operations during three cycles of the rice–wheat rotation to monitor the dynamics of NH₄–N and NO₃–N during and after each crop.

Field moist soil samples were thoroughly mixed, and representative subsamples were extracted immediately by shaking with 2 M KCl solution (1:6 soil/solution ratio) for 1 h on a mechanical shaker followed by filtration. The extracts were analyzed for NH₄–N and (NO₃ + NO₂)-N by a micro-Kjeldahl procedure (Mulvaney, 1996).

Bulk density (D_b) of the soils was determined to a 150-cm depth with 5 cm i.d. by 15 cm long steel cores with open ends following the soil sampling protocol mentioned above for N dynamics. Bulk density measurements were made periodically during the 4 yr of the experiment, and mean D_b values for five selected treatments were used for converting NH₄–N and NO₃–N concentrations (mg N kg^-1 soil) to units of kg N ha^-1.

Statistical analysis of experimental data was accomplished by standard analysis of variance (ANOVA) in RCBD (Gomez and Gomez, 1976) using IRRISTAT software (Bartolome et al., 1999). As three of the 10 treatments were not included in Year 1, only data from Years 2, 3, and 4 for rice and wheat yields, N uptake and recovery, and harvest index (HI) were used for repeated-measure pooled analysis. The data for the GM₀ FN₁₂₀–FN₉₀ treatment were not included for 3-yr pooled analysis as this treatment was missing in Year 2 (Table 2). Using a paired t test, no-N control (GM₀ FN₀–FN₀) and FN (GM₀ FN₁₂₀–FN₁₂₀) or control (GM₀ FN₀–FN₀) and GM (GM₂₀ FN₀–FN₀) treatments were compared for the data on NH₄–N and NO₃–N concentrations in the soil profile during rice and wheat crops. Harvest index of rice and wheat was calculated from the ratio grain yield/total yield (grain + straw). The fertilizer N equivalent (FNE) of GM in wetland rice is the quantity of FN that must be applied to obtain grain yield equal to that obtained with GM alone and was calculated following the procedure of Bouldin et al. (1984) and Yadvinder-Singh et al. (1995). First, from rice yield data obtained with different FN rates, the best-fit regression was searched using the scatter diagram technique and greatest R² values (Gomez and Gomez, 1976). Then quadratic regression equation y = ax² + bx + c was computed separately for each year using the regression module of IRRISTAT software (Bartolome et al., 1999). Where y is rice grain yield (Mg ha^-1) obtained with FN, x is applied FN (kg ha^-1), a and b are constants, and c is an intercept. Thereafter, FNE of GM was calculated by solving the x as follows

(1)

Where y is the yield data obtained with the GM₂₀ or GM₄₀ rate. The same procedure was followed for computing FNE of GM using N uptake data.

Apparent N recovery (ANR) was calculated as follows

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions and recommendations REFERENCES

 
Rice Yield
Grain yield of rice was significantly greater in the plots treated with 120 kg FN ha^-1 (GM₀ FN₁₂₀) than in the no-N control (GM₀ FN₀) plots (5.40 vs. 2.71 Mg ha^-1). Three-year pooled analysis of rice yields for Years 2 to 4 revealed that rice yields and N responses did not differ significantly between years (Table 3) .

View this table: [in this window] [in a new window]   Table 4 Fresh and dry matter, and N additions from green manure applied to rice

The narrow C/N ratio of GM (Table 4) resulted in rapid mineralization and allowed the GM to serve as an efficient source of N. This was reported earlier (Yadvinder-Singh et al., 1988; Aulakh et al., 1991) and was confirmed in a recent laboratory incubation study, where Khera et al. (1999) observed 36% mineralization of sesbania GM-N within 15 d. Efficient use of GM-N may be due to the synchrony between N release from incorporated GMs and crop plant N demand. Also, unlike FN, GM-N is less prone to losses through leaching of NO₃ beyond the rooting zone (see below).

Residual Effects of Green Manure on Wheat
Fertilizer N applied to wetland rice in subtropical, semiarid soils often has no residual effect on succeeding crops (Maskina et al., 1988), and a fresh application of 120 kg FN ha^-1 is recommended for wheat that follows wetland rice (Gill, 1995). However, information on the residual effects of green manures in rice–wheat systems is not available. In our study, the 3-yr mean wheat grain yield of 2.24 Mg ha^-1 without FN and GM (GM₀ FN₀–FN₀) increased to 4.53 Mg ha^-1 with the GM₀ FN₁₂₀–FN₁₂₀ treatment (Table 5) . Green manuring of rice exhibited a significant residual effect on the grain yield of wheat. Mean 3-yr yields of wheat increased 15% due to the residual effect of 20 Mg GM ha^-1 (GM₂₀ FN₀–FN₀) applied to the preceding rice crop and 22% following 40 Mg GM ha^-1 (GM₄₀ FN₀–FN₀). However, application of 120 kg FN ha^-1 to wheat in plots not amended with GM (GM₀ FN₁₂₀–FN₁₂₀) and 90 kg FN ha^-1 to wheat in GM₂₀-amended plots (GM₂₀ FN₁₂₀–FN₉₀) resulted in statistically equivalent wheat grain yields and N uptake (Table 5). These results suggest that the use of green manuring in a rice–wheat system may provide a similar effect as the application of 30 kg FN ha^-1 to the following wheat crop and thus reduce its fertilizer N requirement by 25%.

Dynamics of Ammonium and Nitrate in Soil
In general, the amount of NH₄–N and NO₃–N in the soil showed significant increases after application of urea to rice or wheat during different years. For example, Year 2 data for one rice–wheat cycle presented in Fig. 1 illustrate that early-season rises in the amount of NH₄–N, and NO₃–N were generally greatest in the surface layer (0–15 cm) and less prominent in the 15- to 30- and 30- to 60-cm layers. Nevertheless, rapid distribution of applied FN to lower soil depths in this irrigated porous soil was evident. In all layers, the amount of NH₄–N or NO₃–N differed significantly between no-N (GM₀ FN₀–FN₀) and fertilizer N (GM₀ FN₁₂₀–FN₁₂₀) treatments according to paired t tests.

View larger version (29K): [in this window] [in a new window]   Fig. 1 Dynamics of (a) NH4–N and (b) NO3–N in three soil layers of no-N control (GM0 FN0–FN0) and fertilizer N (GM0 FN120–FN120) treatment in a rice–wheat cropping system during Year 2 (1993–1994). GM is green manure and FN is fertilizer N; subscripts denote application rates (Mg ha-1 and kg ha-1, respectively). Arrows denote fertilizer N application. * Indicates significant difference (P 0.05) between no-N control and fertilizer N treatment. The periods under fallowing (F), rice, and wheat are marked on the x axis

After each FN application to the rice crop (June–August), NH₄–N usually exceeded NO₃–N (Fig. 1), presumably due to reduced nitrification in the partially anaerobic soil systems that resulted from frequent irrigations. However, increased levels of NO₃–N in soil were frequently observed 1 to 5 d after each FN application. During the period of upland wheat growth (November–early April), the amount of NO₃–N increased rapidly after FN application (Fig. 1b). These results confirmed findings of earlier controlled-environment studies that nitrification of applied ammoniacal N in subtropical near neutral soils is very rapid under aerobic conditions (Kuldip-Singh et al., 1996) and that such porous soils under partially anaerobic condition can support substantial nitrification of applied ammoniacal N (Aulakh et al., 1996).

Mineralization of Green Manure Nitrogen and Soil Organic Nitrogen during Fallow
Incorporation of fresh GM 2 d prior to transplanting rice was associated with rapid accumulation of mineral N in soil, mostly as NH₄–N, for 1 to 2 wk, followed by a slow decline during a 2- to 3-wk period (Fig. 2) . Rapid mineralization of GM-N was supported by the accumulation of 103 kg mineral N (NH₄ + NO₃) ha^-1 on 23 June 1993 (5 d after GM incorporation) in the GM₂₀ treatment, as compared with 56 kg N ha^-1 for the GM₀ soil in Year 2 (Fig. 2). Similar results occurred in Year 3 with accumulation of 106 kg mineral N ha^-1 on 28 June 1994 (14 d after GM incorporation), as compared with 76 kg N ha^-1 in GM₀ treatments (data not shown). As mentioned above, Khera et al. (1999) measured 36% mineralization of sesbania GM-N within 15 d in growth chamber studies conducted at 35°C. The rapid mineralization of GM-N explains the enhanced efficiency of GM, compared with FN, in supplying N to growing plants and increasing rice yields, N uptake, and N recovery. After a 3-wk post-rice fallow period, the top 60 cm of soil in the GM₂₀ FN₀ treatment contained 17 kg more mineral N ha^-1 than the control (GM₀ FN₀) soil on 11 November (Fig. 2), presumably due to the mineralization of residual GM. These results suggest that mineralization of residual GM-N to the succeeding wheat crop reduced the fertilizer input by 25% for optimum wheat yield.

View larger version (46K): [in this window] [in a new window]   Fig. 2 Total amount of (a) NH4–N and (b) NO3–N in the 60-cm soil profile of no-N control (GM0 FN0–FN0) and green manure (GM20 FN0–FN0) treatment in a rice–wheat cropping system during Year 2 (1993–1994). GM is green manure and FN is fertilizer N; subscripts denote application rates (Mg ha-1 and kg ha-1, respectively). *Indicates significant difference (P 0.05) between no-N control and green manure treatment. The periods under fallowing (F), rice, and wheat are marked on the x axis

Effects of Rice and Wheat on Ammonium and Nitrate Distribution in the Soil Profile
The distributions of NO₃–N and NH₄–N in different soil layers changed frequently; however, changes in NH₄–N in subsurface layers (15–30 and 30–60 cm) were relatively small compared with those of NO₃–N (Fig. 1 and 2). For example, data after rice harvest of Year 2 presented in Table 7 illustrate that NO₃–N present in the 60-cm soil profile was significantly greater in GM₀ FN₁₂₀–FN₁₂₀ (39%), GM₂₀ FN₀–FN₀ (58%), GM₂₀ FN₉₀–FN₉₀ (67%), and GM₄₀ FN₁₂₀–FN₉₀ treatments (120%) than in GM₀ FN₀–FN₀. Further, the amount of NO₃–N remaining in different soil layers after rice harvest was 37 to 109% greater than after wheat harvest (Table 7). Rice, a shallow-rooted crop, primarily uses nutrients in the surface soil layer and allows NO₃–N to move deeper in the soil, where it is prone to loss via leaching to groundwater (Bajwa et al., 1993; Aulakh, 1994). However, significant amounts of the NO₃–N from all soil layers up to 60-cm depth were used by wheat following rice because of its deeper and more extensive rooting system that enables it to take up N from lower soil layers (Bijay-Singh and Sekhon, 1977). From these data, however, it is not possible to verify such effects of wheat beneath the 60-cm soil depth.

Leaching of Ammonium and Nitrate
The amount of NH₄–N and NO₃–N present in different layers of soil at the end of four cycles of a rice–wheat rotation revealed that the different amounts of applied FN and incorporated GM did not affect NH₄–N content of the soil profile to 150 cm (data not shown) but resulted in significant differences in NO₃–N ranging from 59 to 101 kg N ha^-1 (Table 8) . Application of the recommended rate of 120 kg FN ha^-1 to both rice and wheat (GM₀ FN₁₂₀–FN₁₂₀) in split applications for 4 yr resulted in a significant increase of 35 kg NO₃–N ha^-1 greater than the no-N control, 74% of this increase occurred in the 90- to 150-cm soil depth. The increase of 16 kg NO₃–N ha^-1 with the GM₂₀ FN₀–FN₀ and 19 kg NO₃–N ha^-1 with the GM₂₀ FN₆₀–FN₉₀ treatment was not significant. However, incorporation of 40 Mg GM ha^-1 in conjunction with 120 kg N ha^-1 in rice (GM₄₀ FN₁₂₀–FN₉₀) resulted in a residual of 42 kg NO₃–N ha^-1 greater than the control; 70% of this NO₃–N was located in the 90- to 150-cm soil layer, subject to leaching to deeper soil layers and subsequent NO₃ contamination of groundwater. The wheat crop that followed rice in this treatment only received 90 kg FN ha^-1, 75% of the recommended N rate, but produced dry matter, grain, and N yields that were similar to a wheat crop supplied with 120 kg FN ha^-1 (Table 5). However, all NO₃–N reserves in the soil profile, especially in the 90- to 150-cm layer, were not removed by the wheat crop. Perhaps the amount of NO₃–N present in the soil profile was in excess of wheat crop needs or was moved beyond the active zone for nutrient absorption by roots. Because farmers tend to apply N in excess of crop requirements (Aulakh and Bijay-Singh, 1997), this could cause alarming groundwater contamination in some cases, as indicated by earlier reports (Singh et al., 1987; Singh, 1992). The results of this study clearly demonstrate that excess N applied through FN and GM reduces crop yields and results in leaching of NO₃ to deeper soil layers. A major portion of enhanced soil NO₃ (70–74%) occurring in the 90- to 150-cm depth indicates the possibility of NO₃ leaching below 150 cm and into groundwater. However, NO₃–N enrichment of the groundwater lifted by the tubewell located in the vicinity of the present experimental site, which was used for irrigation, remained uncontaminated. Groundwater NO₃ levels should be closely watched in the future; if NO₃ leaching occurs below 150 cm, groundwater might become a significant contributor to N fertilization of crops but be unfit for human and animal consumption.

	Conclusions and recommendations

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions and recommendations REFERENCES

	ACKNOWLEDGMENTS

 
We gratefully acknowledge the financial assistance received from the Office of International Cooperation and Development, USDA, and the suggestions and critical review of an earlier version of this manuscript by Dr. Wally Wilhelm, USDA-ARS, University of Nebraska, Lincoln, NE; Dr. Changsheng. Li, University of New Hampshire, Durham, NH; and Dr. Kevin Bronson, Texas A&M University, Lubbock, TX.

Received for publication April 29, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions and recommendations REFERENCES

 

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