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Published in Agron. J. 95:1213-1219 (2003).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

PRODUCTION PAPERS

Corn Production on a Subsurface-Drained Mollisol as Affected by Time of Nitrogen Application and Nitrapyrin

Gyles W. Randall*,a, Jeffrey A. Vetscha and Jerald R. Huffmanb

a Univ. of Minnesota Southern Research and Outreach Center, 35838 120th Street, Waseca, MN 56093-4521
b Dow AgroSciences, 750 Double Springs Road, Almond, NC 28702

* Corresponding author (grandall{at}soils.umn.edu).

Received for publication May 13, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Time of N fertilizer application to corn (Zea mays L.) and use of a nitrification inhibitor are management strategies that can affect corn production and loss of NO3–N from the soil profile via subsurface, tile drainage. A field study was conducted from the fall of 1986 through 1994 on a tile-drained Canisteo clay loam soil [fine-loamy, mixed (calcareous), mesic Typic Endoaquolls] to determine the influence of time of N application and nitrapyrin [2-chloro-6-(trichloromethyl) pyridine] on yield of corn and soybean [Glycine max (L.) Merr.] in rotation and N uptake of corn. Four anhydrous ammonia (AA) treatments [(fall without nitrapyrin (NP), fall with NP, spring preplant, and split (40% preplant and 60% sidedress at V8 stage)] were replicated four times and applied at 150 kg N ha-1 (135 lb N acre-1) for corn each year. Fall applications occurred between 19 and 28 October when soil temperatures generally were <=10°C. Seven-year average corn grain yields were least for fall N without NP (8.27 Mg ha-1, 131 bu acre-1), intermediate for fall N with NP and spring N (8.72 Mg ha-1, 139 bu acre-1), and greatest for the split N treatment (9.11 Mg ha-1, 145 bu acre-1). Corn N uptake was not different among treatments in drier years but was generally greatest for the spring and split treatments in wet years. Apparent N recovery ranged from 31% for fall N without NP to 44% for the split treatment. Economic return to fertilizer was greatest for the split treatment ($239.40 ha-1 yr-1) and lowest for fall N without NP ($166.70 ha-1 yr-1). Application time strategies for AA considered to be best management practices for these poorly drained Mollisols include fall N with NP, spring preplant, and split application.

Abbreviations: AA, anhydrous ammonia • NP, nitrapyrin


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
DETERMINING THE OPTIMUM TIME to apply N for corn has been the subject of considerable research during the last 30 yr. The general conclusion among researchers has been that N should be applied nearest to the time it is needed by the crop, i.e., sidedressed several weeks after corn emergence (Aldrich, 1984; Fox et al., 1986; Olson and Kurtz, 1982; Russelle et al., 1981; Stanley and Rhoads, 1977; Welch et al., 1971). There is less time for N loss by leaching or denitrification when N is applied after plant emergence. Nitrate can easily be lost in leachate and subsurface drainage, particularly when precipitation exceeds evapotranspiration, as usually occurs in the spring in temperate climates. When N is sidedress applied in early summer, there is less likelihood of precipitation exceeding evapotranspiration and of nitrate leaching. Nitrate can also be lost by denitrification in wet soils, which is also most likely to occur in spring. Soils with subsurface drainage may be at higher risk for nitrate leaching, but at lower risk for N loss due to denitrification.

Application of N in the fall has advantages for both growers and the fertilizer industry. These economical and logistical advantages include better distribution of labor and equipment demands, time savings during the busy spring planting season, lower N costs in some years, and frequently more favorable soil conditions for field work (Bundy, 1986; Randall and Schmitt, 1998). Because of the risk of losing a portion of fall-applied N, recommendations for fall N usually specify use of ammonium forms and delaying time of application until soil temperatures are <10°C (Keeney, 1982). Comparisons of corn yield after fall or spring N application have been variable. Fall application (mid-November) produced lower corn grain yields than spring preplant or sidedress application regardless of N rate in Ontario (Stevenson and Baldwin, 1969). The yield reduction associated with fall application was greater on clay soils than on loam soils. Three-yr yield averages showed fall application on medium-to-fine textured soils in central and northern Illinois to be 90% as effective as spring application at 134 kg N ha-1 with equal yields for fall and spring applications at 201 kg N ha-1 (Welch et al., 1971). Considerable year-to-year variation among the 12 location-years was observed, suggesting the impact of weather variability on loss of N. In an extensive review of N application timing, Bundy (1986) concluded that fall N application is an acceptable option on medium-to-fine-textured soils where winter temperatures retard nitrification. However, under these conditions fall-applied N is usually 10 to 15% less effective than spring-applied N. The relative effectiveness is largely determined by soil characteristics and climatic conditions, and therefore, varies substantially among locations and years.

Comparisons of preplant and sidedress N applications on medium- and fine-textured soils indicate that sidedress applications are not likely to increase corn yields relative to preplant N treatments in most growing seasons (Bundy, 1986). In a 12-yr comparison of fall, spring preplant, and sidedress N applications for continuous corn on a tile drained Webster clay loam (fine-loamy, mixed, superactive, mesic Typic Endoaquolls), Nelson and MacGregor (1973) found no difference in corn yield between preplant and sidedress N treatments. Corn yields obtained with spring preplant and sidedress N were also similar in eight experiments conducted on clay and loam soils in Ontario (Stevenson and Baldwin, 1969). Welch et al. (1971) found no differences in corn yield between preplant and sidedress N applications in 3 of 4 yr on poorly drained Illinois soils. However, in a year of above-normal June rainfall, corn yields with sidedress N were greater than with preplant N, indicating greater losses of preplant N when climatic conditions conducive for N loss occur.

Applications of N split between preplant and in-season have given mixed, site-specific results. Split application (40% at planting, 60% at V8) of N at four sites in Pennsylvania eliminated the potential for early season N deficiency that was associated with sidedressing all N at V8 and minimized the potential for early season loss that occurred when all of the N was applied at planting (Smith et al., 1996). They concluded that split application of N would be the best recommendation on fields with low residual N. Conversely, splitting N application between preplant and sidedressing did not increase corn grain yields compared with preplant application at eight field sites in a 3-yr Iowa study (Polito and Voss, 1991) or at three ridge-till sites in Minnesota (Randall et al., 1997).

Nitrification inhibitors such as nitrapyrin (N-Serve) have been evaluated as a method to improve the efficiency of ammonium forms of N applied in the fall (Meisinger et al., 1980; Hoeft, 1984). Delaying nitrification should reduce the potential for loss of fall-applied N. Addition of a nitrification inhibitor to fall-applied N has been suggested as a strategy to allow N application before soil temperatures decline to 10°C, thereby extending the fall N application season (Nelson and Huber, 1980).

Many studies have shown that nitrification inhibitors are effective in delaying conversion of ammonium to nitrate when N is fall-applied (Hoeft, 1984), but use of nitrification inhibitors with fall-applied N has not resulted in consistent crop yield responses. Corn grain yields were increased significantly in 15 of 18 comparisons when nitrification inhibitors were added to fall-applied AA in Indiana when soil temperature was <12°C (Nelson and Huber, 1980). Yield increases averaged 24% over the 4-yr period with the greatest increase being 104% on a silty clay soil. Studies conducted from 1975 to 1979 on fine-textured soils in Illinois showed that continuous corn yield was not affected by time of application or by use of NP in the first 4 yr (Hoeft et al., 1981). However, in 1979, addition of NP to early and late fall N applications increased yield, but yields were still highest with spring application. Yields with early fall applications were lower than yields with late fall application, both with and without NP. In an 8-yr continuous corn study in Ohio, grain yields were increased by adding NP to fall-applied AA but not to spring-applied AA (Stehouwer and Johnson, 1990). Hendrickson et al. (1978) found that NP added to fall- and spring-applied N did not significantly increase corn yields on medium-textured Wisconsin soils. Bundy (1986) concluded that nitrification inhibitors can improve the effectiveness of fall-applied N, but spring N is more effective than fall N applied with an inhibitor when conditions favoring N loss from fall application develop. To maximize the effectiveness of fall N applied with an inhibitor, he suggested applications should be delayed until soil temperatures are below 10°C.

Because of agronomic, economic, and environmental concerns with fall application of N, we established a study to determine the influence of time of AA application and the use of a nitrification inhibitor (NP) on (i) yield, N uptake, and profitability of corn grown in rotation with soybean and (ii) NO3 losses via subsurface, tile drainage from a fine-textured, poorly drained soil. Results addressing objective (ii) are reported in a companion paper (Randall et al., 2003). Most previous studies reported in the literature either focused on continuous corn or did not include a specific objective linked to water quality.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
A field experiment was conducted on a poorly drained Canisteo clay loam soil from the fall of 1986 through 1994 at the University's Southern Research and Outreach Center, Waseca, MN. The site consisted of 32 individual subsurface tile drainage plots installed in 1976. After completing a research project using this drainage facility in 1983, the experimental area was cropped to corn with a blanket N rate in 1984 and 1985 to establish uniformity.

Beginning in 1986, corn was planted on one-half of the experimental area, including 16 drainage plots. Soybean was grown on the other half, which included another 16 drainage plots. Each year the corn and soybean areas were rotated. Four N treatments [fall without NP, fall with NP, spring preplant, and split (40% preplant and 60% sidedress at V8 stage) (Richie and Hanway, 1984)] were replicated four times and were applied for corn on 16 drainage plots. The N treatments were not rerandomized each year; thus, each treatment was applied to the same plots in 1987, 1989, 1991, and 1993 for one set of drainage plots and in 1988, 1990, and 1992 for the other set of drainage plots. A randomized, complete-block design was used with restricted randomization based on previous annual tile drainage discharge from 1977 to 1983. A 2-way ANOVA was used to statistically compare plant data from the four N treatments (SAS Inst., 1988). Four check plots (no fertilizer N) were located adjacent to each set of drainage plots. These plots did not contain an individual tile drain, and thus were not included in the statistical analysis.

Anhydrous ammonia was applied at a rate of 150 kg N ha-1 (135 lb N acre-1) for all N treatments. This was 12% higher than the rate recommended for a yield goal of 9.5 Mg ha-1 (150 bu acre-1) (Rehm and Schmitt, 1990). Nitrapyrin (N-Serve) was applied at 0.56 kg a.i. ha-1. Dates of application for all treatments are shown in Table 1. Soil temperatures at the 15-cm depth on the date of fall application and in the following 10-d period are found in Table 2. Initial soil test P (27 mg kg-1 Bray P1) and K (135 mg kg-1) were high. Broadcast P and K were applied uniformly to all plots in 1990 and 1992 at rates of 35 and 115 kg ha-1, respectively, to maintain high soil test levels.


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  		Table 1. Nitrogen application dates, corn hybrid, and planting dates used each year.

 

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  		Table 2. Average soil temperature at the 15-cm depth on the date of fall N application and in the following 10-d period and cumulative soil heat units (SHU) between fall N application and 1 December each fall.

 
Corn was no-till planted in 75-cm wide rows at 75900 seeds ha-1 with a John Deere Max-Emerge planter equipped with waffle coulters. At the V5 stage, the plots were thinned to a uniform population of 71200 plants ha-1 (28800 plants acre-1). The hybrid and planting date are shown in Table 1. The same planter was used to plant soybean following corn ground that had been chisel plowed in the fall and field cultivated in the spring. The seeding rate was 378000 seeds ha-1 (153000 seeds acre-1). Soybean varieties were Hardin in 1988–1991 and Sturdy in 1992–1994. Weeds were controlled successfully each year with preemergence and post-emergence application of herbicides plus one cultivation in June.

Surface residue cover measured just before planting by the line-transect method (Sloneker and Moldenhauer, 1977) averaged 32% (soybean residue) for the plots planted to corn and 59% (corn residue) for the plots planted to soybean. Corn grain yields were taken at physiological maturity by hand-harvesting four 4.6-m long rows, shelling the grain, and determining grain moisture content. Corn stover yields were measured on two 4.6-m long rows. Soybean yields were determined by combining two 9.2-m long rows and measuring seed moisture content. Nitrogen content of the corn grain and stover was determined by grinding subsamples to pass a 1-mm sieve and analyzing for total N (Technicon Industrial Method, no. 325-74W September 1974; Ammoniacal Nitrogen/BD Acid Digests; Technicon Industrial Systems, Tarrytown, NY 10591).

The experiment was conducted under ambient precipitation. Daily precipitation data were collected at a site 500 m from the experimental plots and were summarized from 1987 through 1993 (Table 3). The same site was used to collect the soil temperature (T) data (bare soil) shown in Table 2. Soil heat units (SHUs) for each day were calculated as:

[1]
and were accumulated between the date of fall N application and 1 December when average soil temperatures ranged between 0 and 1.6°C.


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  		Table 3. Temperature and precipitation departures from normal for the period 1987–1993.

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Climatic Conditions
Soil temperature at the 15-cm depth on the date of fall N application was <=10°C in 5 of 7 yr (Table 2). Only in 1989 did soil temperature markedly exceed 10°C, which is the temperature used as a threshold to guide fall application of N in Minnesota. Fall application is not recommended at soil temperatures >10°C (Randall and Schmitt, 1993). In the 10-d period following fall N application, soil temperature averaged between 4.4 and 9.4°, indicating that soil temperatures did not warm up late in October and in fact were cooling to minimize nitrification. Cumulative SHUs ranged between 85 in 1992 and 208 in 1990, indicating an average temperature of 2.6°C in 1992 and 5.9°C in 1990 in the period between N application and 1 December. Minimum nitrification of AA would be expected under these cool conditions. With the exception of 1987 and 1990, these soil temperatures are similar or cooler than the long-term (1976–1998) temperatures (175 SHUs) from 25 October to 1 December at this site (G.W. Randall, unpublished data, 1999).

Air temperature averages and precipitation totals for May, June, and the April–September growing season shown in Table 3 indicate marked differences among the 7 yr. Air temperatures were above normal in 1987 and 1988, especially in May and June, whereas precipitation was below normal. Very dry conditions persisted again in 1989, but temperatures were near normal. Air temperatures were slightly above normal in 1990 and 1991, whereas precipitation was 27 and 56% above normal, respectively. Cool temperatures prevailed in 1992 and 1993; however, precipitation during the 1993 growing season was excessive (62% above normal).

Corn Production
Statistically significant differences (P < 0.10) in corn grain yield existed among the four N treatments when averaged across years, and the Year x Treatment interaction was significant (Table 4). Seven-yr average grain yields were lowest with fall N without NP, intermediate and equal for fall N with NP and spring N, and highest for the split N treatment (Table 5). The 0.39 Mg ha-1 (6 bu acre-1) yield increase with the split treatment compared with the spring preplant treatment was not expected and contradicts results of a 4-yr study at 33 sites that showed no yield difference between these two application times (Randall and Schmitt, 1994). Statistical differences in grain yield among the treatments for individual years only occurred in 1 of 7 yr (1991). However, absolute yields were lowest for fall N without NP and highest for split N in 5 of 7 yr. In 1991, when May rainfall was 112% above normal and growing season rainfall was 56% above normal, grain yield for fall N without NP was 1.3 Mg ha-1 (21 bu acre-1) less than when NP was used with fall-applied AA. Spring and split application of N produced the highest yields, but they were not significantly (P < 0.10) different from the fall N with NP treatment. In the cool and wet year (1993) when May and growing season rainfall were 70 and 62% above normal, respectively, grain yields were much lower than in 5 of the previous 6 yr, and yield differences among the treatments were not statistically significant. It should be noted, however, that grain yield for fall N without NP was about 0.65 Mg ha-1 (10 bu acre-1) less than the average of the other three treatments. Grain yields were also very low in 1988 (a very hot and dry year) and were not greater than for the check (no N) plots. These findings are in agreement with those reported by Killorn and Zourarakis (1992), who concluded that continuous corn response to time of N application in Iowa was site specific and positively related to growing season rainfall.


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  		Table 4. Analysis of variance for corn grain and silage yield, soybean yield, and N uptake.

 

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  		Table 5. Corn grain yield as affected by time of N application and nitrapyrin (NP).

 
Relative efficiency of fall and spring applications vs. split application of N was calculated using the method described by Welch et al. (1971). The yield increase relative to the no-N check with fall N without NP, fall N with NP, and spring N was divided by the yield increase with the split N treatment (Table 5). Relative efficiency of N averaged across the 7 yr was 73% for fall N without NP and 88% for the fall N with NP and spring preplant treatments. These values compare to a 3-yr average of 91% for fall vs. spring application of 134 kg N ha-1 to corn following soybean in Illinois (Welch et al., 1971). At a continuous corn site in Illinois where N was applied at 168 kg N ha-1 they reported 4-yr relative efficiency averages of 79 and 97% for fall and spring application relative to sidedress application. The relative efficiency for each specific year ranged from 49 to 88% for fall N without NP, 77 to 96% for fall N with NP, and 74 to 97% for spring, preplant N. Relative efficiency of N was not calculated for 1988 because yields of the N-treated plots were not greater than for the check plots. The considerable variation from year to year, especially for fall-applied N without NP, demonstrates the great effect that weather-induced N-loss can have on N management strategies. In 1991, when May precipitation was 112% above normal and June rainfall was below normal, fall N without NP and fall N with NP had relative efficiencies of 49 and 84%, respectively, compared with 97% for spring application (21 May). The extremely wet conditions in May decreased efficiency of fall-applied N with NP relative to spring-applied N; however, in 1993 when spring N was applied on 5 May and May and June precipitation was 68% above normal, relative efficiency of fall N with NP (96%) was equal to spring N (93%) compared with only 78% for fall N without NP. These data suggest that both timing and amount of excess rainfall in conjunction with timing of spring N application are important when evaluating time of application and nitrification inhibitor strategies, predicting N losses from fertilizer N, and making supplemental N recommendations for corn. In addition, our findings support the inclusion of a nitrification inhibitor with fall-applied AA when the 15-cm deep soil temperatures are <=10°C.

Averaged across 7 yr, corn grain N concentration was influenced by the N treatments; however, a Year x Treatment interaction existed (Table 4). Examination of individual years shows no influence of the treatments in 4 of 7 yr (Table 6). Grain N concentrations were statistically different among N treatments in the two wet years (1991 and 1993) and the very dry year (1988). In 1991, grain N concentrations for both fall N treatments were about 20% lower compared with the spring and split treatments. In 1993, grain N was 1.0 g kg-1 (9%) higher for the split N treatment compared with the other three treatments. The opposite was true in 1988 when grain N concentration was greatest for fall N without NP and least for the split treatment.


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  		Table 6. Corn grain N concentration as affected by time of N application and nitrapyrin (NP).

 
Nitrogen uptake in the corn silage (total aboveground dry matter) was also influenced by the N treatments when averaged across years (Table 4); but because the Year x Treatment interaction was significant, years will be discussed separately. Uptake of N was increased significantly by the fall with NP, spring, and split N treatments compared with fall N without NP in 1989, 1991, 1992, and 1993 (Table 7). This was due to a combination of higher N concentrations in the plant tissue and greater yields, especially in the two very wet years (1991 and 1993). In 1991, N uptake for fall N with NP was lower than for the spring or split treatments. It should be noted, however, that N uptake was least for the fall N without NP treatment in all years except 1988.


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  		Table 7. Nitrogen uptake in corn silage as affected by time of N application and nitrapyrin (NP).

 
Apparent N recovery from the fertilizer, calculated from total aboveground N uptake of each N treatment minus that from the check (no N) treatment divided by the N rate (150 kg ha-1) is shown in Table 8. Except for 1988 when N recovery was <=10%, apparent N recovery always was lowest for the fall N without NP treatment. Averaged across 7 yr, apparent N recovery ranged from a low of 31% for fall N without NP to 44% for the split N treatment. Little difference in recovery was seen between the fall N with NP and spring preplant treatment. Apparent N recovery reached a maximum of 65% in 1987 when yields were very high (>11 Mg ha-1), and the check yield was only 6.7 Mg ha-1.


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  		Table 8. Apparent N recovery from the fertilizer N as affected by time of N application and nitrapyrin (NP).

 
Annual economical return to fertilizer for each of the application times and NP was calculated based on average input costs (Table 9). These data clearly show greatest return for split application ($239.40 ha-1 yr-1) and lowest return for fall-applied N without NP ($166.70 ha-1 yr-1). Spring preplant application resulted in $18.50 ha-1 yr-1 greater return than fall N with NP, due to the cost of the NP. But many farmers in the northern latitudes of the USA would choose to fall-apply N rather than delaying application to the spring when wet soil conditions, weather, and time constraints can be significant factors.


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  		Table 9. Annual economic return to fertilizer N as affected by time of application and nitrapyrin for the 7-yr period.

 
Soybean Production
Soybean yields were taken in the year following corn to determine if carryover from the N treatments affected soybean production. Soybean yield averaged across 7 yr ranged from 2.92 to 2.97 Mg ha-1 (43.5–44.4 bu acre-1) and was not affected by the N treatments, and no interaction with year occurred (Table 4).


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Nitrogen uptake efficiency, corn yield, and economic return can be greatly affected by time of N application and NP based on the results from this 7-yr study. Although differences in corn yields among N treatments were statistically significant in only 1 of 7 yr, the 7-yr average showed corn yields to be increased by 5, 5, and 10% by the fall N with NP, spring, and split N treatments, respectively, compared with fall-applied N without NP. Nitrogen uptake in the aboveground plant tissue (silage) was affected by N treatments in 1989, 1991, 1992, and 1993. Averaged across these 4 yr, N uptake was increased 13, 24, and 29% by the fall N with NP, spring, and split N treatments, respectively, compared with fall N without NP. Apparent recovery of N in the aboveground tissue averaged across all years ranged from 31% for fall N without NP to 44% for the split treatment. Differences between the fall N with NP and spring preplant N treatments for grain yield and apparent N recovery were minimal. However, economic return was greatest for the spring and split treatments due to higher grain yields and/or less input expense when NP was not used. Because spring preplant applications of N often encounter wetter soil conditions, potentially greater soil compaction, and greater time, labor, and equipment constraints for both the farmer and the fertilizer dealer, many growers likely would choose to apply N in the fall rather than waiting until the following spring as a preplant or split (spring–early summer) application. These data strongly support the inclusion of NP in AA applied in late October in Minnesota when soils have cooled to 10°C and below.

Based on soil temperature data in this study, we conclude that corn yield response and N uptake are not related to soil temperature at the time of fall N application or temperature decline patterns until freeze-up as long as soil temperature at the time of application is about 10°C or less. Greatest yield responses in our study occurred in years following a warmer-than-normal late fall (1990), following a late fall with normal temperatures (1988), and following a much colder-than-normal late fall (1992). Thus, we are drawn to the conclusion that spring and growing season rainfall have a much greater influence on N management strategies for corn production than does late fall soil temperature.

Growing season precipitation, especially in May and June, had a tremendous influence on the data obtained in this study and its subsequent interpretation. For instance, corn yield, N uptake, and profitability were not enhanced by inclusion of NP in fall-applied N or from spring applications in years with normal to below-normal growing season precipitation. However, in years with above-normal growing season precipitation, yield, N uptake, and profitability were increased greatly by the use of NP with fall-applied N or by spring or split applications of N. This was especially true in 1991 (May rainfall was 112% above normal) when corn grain yields were increased by 17, 23, and 25% for the fall N with NP, spring, and split N treatments, respectively, compared with fall-applied N without NP. For the 7-yr period, corn grain yield and profitability were greatest for the late fall N with NP, spring, and split applications. Thus, we recommend all three of these N application strategies as best management practices (BMPs) for poorly drained, tiled, Mollisols in the northern Corn Belt.


   ACKNOWLEDGMENTS
 
The authors thank the soils field crew for the collection of the data, Brian Anderson for the annual tabulation and analyses of the data, Arielle Balak for preparation of this manuscript, and Neil Hansen and Michael Schmitt for their constructive comments on an earlier draft. Partial funding of this research was provided by Dow AgroSciences and is greatly appreciated.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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