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Published in Agron J 99:773-778 (2007)
DOI: 10.2134/agronj2006.0113
© 2007 American Society of Agronomy
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Phosphorus Management

Yield and Economic Responses to Phosphorus Fertilizer Placement in Dual-Use and Grain-Only Wheat Production Systems

J. W. Sija,*, W. E. Pinchaka, D. P. Malinowskia, D. L. Robinsona, S. J. Beversb, T. A. Baughmanb and R. J. Gillc

a Texas Agric. Exp. Stn., TAMU Agric. Res. & Ext. Ctr., P.O. Box 1658, Vernon, TX 76385
b Texas Coop. Ext., TAMU Agric. Res. & Ext. Ctr., P.O. Box 2159, Vernon, TX 76385
c Texas Coop. Ext., 1229 N. US Hwy 281, Stephenville, TX 76401

* Corresponding author (jsij{at}ag.tamu.edu)

Received for publication April 12, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Wheat (Triticum aestivum L.) production in the southern Great Plains is a unique enterprise that provides both high-quality forage and a grain crop within the same growing season. However, information on fertility management programs to maximize forage and beef production in a dual-use wheat production system is lacking. A 3-yr, field-scale production study was initiated on a Tillman clay loam near Vernon, TX, in 1999 to (i) determine the influence of P fertilizer and P fertilizer placement on forage, beef, and grain production from dual-use wheat, and (ii) compare economic costs and returns of dual-use and grain-only wheat production systems. Varying numbers of stocker cattle (Bos spp.) were placed in each pasture based on forage availability. Beef-to-forage allowance among pastures was kept relatively constant by adjusting cattle numbers monthly. Applying 20 kg P ha–1 yr–1 increased soil test P in the upper 15 cm two- to threefold, forage production 18 to 54%, and animal gains ha–1 27 to 29% compared with no P. With respect to forage and subsequent beef production, surface-applied P was generally equal to or better than injected P. Average return between the graze-plus-grain and graze-out systems was significant (P < 0.0001) but not among fertilizer treatments (P = 0.26), although surface-applied P resulted in numerically higher returns each year. There was no significant system x fertilizer treatment interaction. However, during the study period, the graze-plus-grain system was clearly superior to the graze-out system in generating higher net returns ($94 vs. $29 ha–1).

Abbreviations: ADG, average daily gain • BW, body weight • DM, dry matter


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NEARLY 8 MILLION hectares of hard red winter wheat are grown in the semiarid southern Great Plains. Dryland wheat production in this winter-active agricultural system is unique and versatile compared with other wheat-producing regions in the USA because of the common practice of utilizing wheat forage in stocker cattle grazing systems, with the option to terminate wheat grazing and still produce a grain crop. The use of winter wheat as a dual-use crop is a vital component of the agricultural economies of Texas, southern Kansas, eastern New Mexico, Oklahoma, and southeastern Colorado (Pinchak et al., 1996; Ralphs et al., 1997; Redmon et al., 1995; Shroyer et al., 1993). Furthermore, the wheat-stocker industry has a comparative advantage in this region because of the proximity of feedlots.

The grazing value of winter wheat forage has been recognized since the 1930s (Schlehuber and Tucker, 1967). More recently, it has been estimated that, annually, 30 to 80% of the wheat planted in the southern Great Plains is grazed to varying degrees (Krenzer et al., 1992; Pinchak et al., 1996; True et al., 2001). Farmers and ranchers tend to utilize wheat entirely as a forage crop (graze-out) if cattle prices are high relative to wheat grain, whereas they tend to remove cattle before the onset of the reproductive stage and allow the wheat to develop grain if wheat prices are high relative to cattle (graze-plus-grain). This occurs if cattle are removed from wheat in late winter at the first hollow stem stage of development (Redmon et al., 1996), allowing wheat to produce grain.

Dual-use wheat production is more complex and requires a higher level of management than wheat grown strictly for grain or forage. Cultivar selection, tillage, planting date, seeding rate, soil fertility management, and pull-off date are crucial to successful implementation of a dual-use winter wheat system. For example, Oklahoma research has shown that not all wheat cultivars are suitable for both grain and fall forage production (Krenzer et al., 1992). In an effort to identify cultivars more suitable in dual-use wheat systems, MacKown and Carver (2005) used grazing pressure as a cultivar selection tool. Unfortunately, in terms of fall forage production and total N and nitrate, this method had no clear advantage over that of a traditional grain-only system method of selection in generating populations from which to develop new wheat cultivars. Producers should plant cultivars based on net return per unit land area, since selecting a cultivar based solely on forage yield or grain yield rarely provided the greatest economic return (Krenzer et al., 1996). Other factors to consider include seedling emergence under early planting dates, grazing tolerance, vegetative regrowth, grazing duration, and timing of first hollow stem. When managed properly, dual-use systems provide grain with bread quality characteristics comparable with that of ungrazed wheat (Khalil et al., 2002).

Nitrogen and P are essential in maximizing forage and grain production in nutrient-deficient soils (Schlehuber and Tucker, 1967). A soil deficiency in either or both nutrients can result in significantly lower forage and grain yields. Unfortunately, there is little information on production practices, including fertility programs, in dual-use wheat/stocker systems (True et al., 2001). Nitrogen requirements for these systems can be readily estimated (Krenzer, 1994). However, the amount and placement of P to be applied is less precise and can be affected by soil characteristics (Krenzer, 1994). For acid soils, Phillips et al. (2000) reported that dual-band applications of P and gypsum increased grain and forage yields compared with P banded without gypsum or P banded and gypsum broadcast. Kaitibie et al. (2002) showed that for acid soils in the southern plains of Oklahoma, a broadcast application of lime before the initial season and 73 kg ha–1 of diammonium phosphate in the seed furrow each year was the optimum economic strategy for dual-use wheat production across a 5-yr period (to amortize the cost of lime). In wheat trials in the southern Great Plains of Texas, Miller (1998) reported that in five of eight site-year comparisons, deep placement of P + N increased forage yields 50% more than surface-incorporated P + N and 45% more than wheat fertilized with the same rate of deep-placed N but no P fertilizer.

Our research hypothesis was that application of P to P-deficient soils would increase both wheat grain and forage yield, and subsequently, animal gain per unit area, and deep-placed P would provide additional efficiency and economic returns to wheat/stocker operations. The objectives of this study were to (i) determine the influence of P fertilizer and P fertilizer placement on forage, beef, and grain production from dual-use wheat, and (ii) identify economic costs and returns associated with P fertilizer and P placement methods in dual-use wheat production in the Texas Rolling Plains.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Operational
The winter wheat pasture study was conducted from mid-September through May during the growing seasons of 1999–2000, 2001–2002, and 2002–2003 on the Smith/Walker Research Unit of the Texas Agricultural Experiment Station {approx}13 km south of Vernon, TX (34°03' N, 99°15' W; 350 m elevation). Wheat planted in 2000 was delayed until December because of early fall precipitation, and insufficient forage was produced to conduct the grazing research component of the trial. Forage and grazing data were not collected from the 2000–2001 growing season when only grain was produced. Therefore, production data are reported for only 3 yr. However, it should be noted that the fertilizer treatments were imposed in the fall of 2000.

The soil in the pastures is predominately Tillman clay loam (fine, mixed, superactive, thermic Vertic Paleustolls) of 0 to 2% slope and a pH between 6.8 and 7.0 in the surface 5 cm of soil and 7.3 in the subsequent lower 10 cm. The Tillman series consists of very deep, well drained, but slowly permeable soils. Results from random soil sampling before initiation of the study indicated that the soils at the research site were generally low in P, and the commercial soil testing laboratory recommended up to 20 kg P ha–1 based on moderate (2200 kg ha–1) grain yields for the site. Soils were subsequently sampled in August each year for nutrient analysis before fertilizer applications, except 2000. Soils were randomly sampled to a depth of 15 cm in four different locations in each pasture using a 2.5-cm soil probe. At each location, 10 random soil cores were separated into the upper 5 cm and lower 10 cm and composited by soil depth. Nutrient analysis was run on each composited sample (four 5-cm and four 10-cm composites per pasture). Hence, soil nutrient analysis was based on 40 soil cores per pasture per year during 3 yr of the study. Fertilizer was applied in liquid formulations by a commercial applicator each year between 1 and 20 September. Application rates were achieved by blending appropriate amounts of urea-ammonium-nitrate (28–0–0 N–P–K), ammonium polyphosphate (10–15–0 N–P–K), and ammonium thiosulfate (12–0–0–26 N–P–K–S) solutions. Three P fertilizer treatments were employed: 0 P, 20 kg P ha–1 as a broadcast spray on the soil surface, and 20 kg P ha–1 injected about 15 cm deep with 50 cm between injection points. Treatments were replicated three times in a randomized complete block design using nine 10-ha pastures as experimental units.

Because of the large experimental units, blocking was designed to take into account slight differences in slope, drainage patterns, and associated soil types. Variability encountered in this experiment reflects the complexity of soils and slopes along the extended length of the study area. Blocking was insufficient to overcome in situ inconsistency associated with this study area. Variability is inherent to conducting integrated systems research at the scale chosen. No pretreatment data were collected that could have been used as a covariate.

Fertilizer treatments were randomly assigned to pastures within blocks the first year of research in 1999 and maintained in the same pastures for the duration of the experiment. Rotating the treatments among pastures during the 4 yr of the study was precluded by obvious residual P effects that are inherent in P applications. Nitrogen and S fertilizers were applied at rates of 73 kg N and 22 kg S ha–1. In the P treatments, N and S were placed with the P because N has been shown to increase wheat yields and plant P concentration more when N was applied with P than when N was separated from P in dual applications (Leikam et al., 1983). Separating N and S from P by surface application where P was injected would have favored the surface-applied P where all three nutrients were applied together. Furthermore, positional availability of N and S is not considered an issue because of their much greater mobility in the soil relative to P. Where no P was applied, the N and S solution was broadcast on the surface. All pastures were lightly tilled with a field cultivator immediately after fertilizer applications to incorporate the fertilizer, control weeds, and prepare a seedbed for planting wheat. Other off-season cultural practices included chisel-plowing 15 cm deep at the end of the wheat growing season and chemical fallow with glyphosate [N-(phosphonomethyl)glycine] through the summer to control weeds. The wheat cultivar Lockett was drill-seeded on 19-cm spacing between 18 and 30 September each year at the rate of 84 kg ha–1 of seed. Various herbicides were applied uniformly across all pastures to the wheat as needed to control winter annual broadleaf and grassy weeds. To enhance long-term soil conservation, a residue goal of 1000 kg ha–1 following the grazing period was established.

One week before animal placement, forage dry matter (DM) was determined for each pasture by harvesting all forage to ground level with hand shears in five randomly-selected 0.5-m2 quadrats. Subsequently, seven 1.7 m2 circular cages were randomly placed throughout each pasture. Two of the seven cages in each pasture were not moved during the grazing period so seasonal forage production could be determined. Before each cattle weigh date, forage was harvested inside and in randomly-selected grazed areas outside the remaining five cages to determine forage production and forage availability. Following clipping, the five cages were randomly repositioned within each pasture, and the procedure repeated before each weigh date. Samples were dried at 50°C for 72 h in a forced-air oven to determine moisture content and dry weight.

Grazing was initiated on 13 and 17 December and 15 January during the 3 yr of the study, utilizing beef steers and heifers with body weights (BWs) that initially averaged 197 to 224 kg hd–1 and were {approx}10 mo of age. Steers were not implanted with growth hormones and heifers were not spayed. During the 1999–2000 growing season, all animals were heifers from British x continental crosses with less than one-eighth B. indicus influence. In subsequent years, cattle came from a ranch in Lordsburg, NM, and were brangus x angus x saler crosses with less than one-fourth B. indicus influence. Cattle in all years had been weaned, vaccinated, and preconditioned for at least 45 d before arrival at the research unit. The number of animals initially placed in each pasture was based on a DM forage allowance of 12 to 18 kg DM (100 kg BW)–1 d–1. Animal numbers at the beginning of the season ranged from 8 to 15 calves per pasture during the 3 yr. Animals were weighed (early morning without fasting) and forage yields were measured on {approx}28-d intervals throughout each season. At weigh dates, cattle were added to or removed from each pasture to reestablish forage allowances of 12 to 18 kg DM (100 kg BW)–1 d–1. These forage allowances were chosen as a reasonable balance between maximizing individual animal growth [based on 19 kg DM (100 kg BW)–1 d–1] and animal production ha–1 (Pinchak et al., 1996).

At first hollow stem ({approx}1 March), two grazing exclosures (2.5 x 4.9 m) were installed in each pasture for the remainder of the season to allow grain production, and thereby simulate a dual-use, graze-plus-grain system. Also, at this growth stage, animal numbers in each pasture were increased to utilize the increased forage production anticipated during March and April. During this phase, animal numbers ranged from 10 to 26 head per pasture. Grazing was terminated on 08, 08, and 15 May during the 3 yr, creating a full-season graze-out system. Residual forage was measured when grazing was terminated. Wheat grain yields were determined by harvesting a 1.8- by 4-m-long strip within each exclosure with a small-plot combine. Grain yield was determined for each exclosure and pasture yield was the average of the two harvested plots.

The experimental design comprised repeated measures and pastures as experimental units. Year was considered a random variable when comparing the two production systems. Data were analyzed using the Proc Mixed model procedure of SAS Institute (1996). Treatment effects were considered different at P ≤ 0.05.

Economic Analysis
An economic analysis was completed for each treatment based on grain yield and cattle gain for each experimental unit to compare economic costs and returns within each grazing management system. The economic results are reported as returns to indirect costs, land, and management. Table 1 is an example of revenue and expenses typically associated with one of the graze-plus-grain management systems used in the current study, and typical of wheat production budgets for the Southern Rolling Plains except that chemical costs were above average for control of winter annual grasses (Bevers and Slosser, 1992). Indirect costs were not included.


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  		Table 1. Economic costs and returns from surface-applied N and P in a graze-plus-grain management system during 3 yr at Vernon, TX.

 
Enterprise budgets were generated for each pasture each year. Gross income in the enterprise budgets was determined by multiplying treatment yields times prices. Because of annual fluctuations of both grain and cattle prices, a grain price of $0.106 kg–1and cattle lease contract price of $0.727 kg–1 of gain were used each year of the trial. The wheat price of $0.106 kg–1 represents the Wilbarger County, Texas, wheat loan rate for the government farm program during the 2002 Farm Bill. The cattle lease contract price of $0.727 kg–1 of gain was the rate that was paid by the cattle owner over the projected time period. Also, government payments were included each year. As a part of a national disaster aid program, in 1999–2000, the government doubled the usual payment made to wheat producers. The government payment was made regardless of grazing system.

Actual direct expenses associated with each production practice were used to determine total expenses per treatment and grazing system. Costs of wheat seed, fertilizer, and chemicals applied were actual expenses. Each year, some form of crop insurance was purchased. For the 2001–2002 and 2002–2003 seasons, crop revenue coverage was purchased for both grazing systems; however, crop insurance premiums and coverage were lowered for the graze-out system. In 1999–2000, catastrophic coverage was purchased, which results in a dramatically smaller cost than in subsequent years when Multiple Peril Coverage Insurance was purchased. Repair costs were based on an average-size farm operation in the Texas Rolling Plains. Tillage operations were recorded and were used to calculate fuel, lube, and tillage labor requirements. A fuel price of $0.25 L–1 was used while tillage labor costs were calculated using a wage rate of $8.00 per hour. Interest costs were calculated at an 8% annual rate. Harvest costs were calculated using a base cost of $32.12 ha–1 and $0.005 kg–1 of yield. Yield above 1.34 Mg ha–1 incurred an additional charge of $0.005 kg–1.

Cattle expenses included labor, feed, minerals, hay, and veterinarian costs. All cattle expenses were recorded separately for the two grazing systems. These costs were then allocated to treatments based on the number of head d–1 pasture–1. Consequently, the cattle expense is a function of the treatment and the number of cattle that grazed each pasture.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Fertilizer applications of 20 kg P ha–1 yr–1 increased (P < 0.05) bicarbonate-extractable (Olsen et al., 1954) P levels in the soil surface from 9 or 10 mg kg–1 to 23 or 24 mg kg–1 in the surface 5 cm and from 3 mg kg–1 to 8 or 10 mg kg–1 in the remaining 15 cm of surface soil (Table 2). The rapid increase in soil test P indicates that the annual application rate of 20 kg P ha–1 exceeded the long-term fertilizer requirement for this cropping system, and that residual P would be expected to increase subsequent crop yields for several years depending on the soil properties, crops grown, and management practices applied (Halvorson and Black, 1985). Because the sufficiency level of P is about 10 mg kg–1, the 23 mg kg–1 achieved in this study was high and could be considered added value from P applications.


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  		Table 2. Soil test P levels at two soil depths and three fertilizer practices during 3 yr on Tillman clay loam at Vernon, TX.{dagger}

 
Phosphorus fertilizer applications increased wheat forage production each year of the study, with the greatest percentage increases occurring during the first grazing phase or before 1 March. Averaged across the 3-yr trial, P application increased forage production 54% (700 kg ha–1) and 18% (575 kg ha–1) before and after 1 March, respectively (Table 3). Across the entire growing season, P application increased forage production from 16% in 2000 to 47% in 2003 or 29% averaged across all years. Although fall forage production is especially important to stocker cattle programs in the southern Great Plains, the season-long increase in production is also valuable but less recognized. At the time grazing was terminated in mid-May, >2000 kg ha–1 biomass was left in the field (Table 3), indicating an additional 1000 kg ha–1 grazing potential was not captured, and that adequate residue (1000 kg ha–1) remained to minimize soil erosion.


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  		Table 3. Mean forage production in grazed pastures of ‘Lockett’ wheat receiving three fertility practices during 3 yr at Vernon, TX.{dagger}

 
It is apparent that deep placement of fertilizers gave no advantage over broadcast application followed by incorporation, except through February of 2000 (Table 3). The location of knife rows were visible in late fall of each year as evidenced by the somewhat larger, darker green wheat. By late winter, the location of the knife rows could not be identified. In a grain-only system, Peterson et al. (1981) showed that P applied with the wheat seed was more efficient than P broadcast and incorporated to a depth of 15 cm in soils low in P. However, efficiency of the two application methods was similar at medium soil P levels. In our study, the lowest soil P levels occurred during the fall of the first year, resulting in a response to the deep-placed fertilizer. The 20 kg P ha–1 application rate apparently increased soil P levels sufficiently in later seasons to diminish the benefit of banded P, making the surface applications equally effective or better. The darker green wheat strips that were visible above the deep-placed N+P in all years could largely be a response to N in the band.

We anticipated average daily gain (ADG) and gain hd–1 would be similar among all pastures and treatments. Adding or removing animals at 28-d intervals based on forage allowance should reflect the differences in animal gain ha–1 resulting from differences in forage production among pastures. As expected, ADG and gain hd–1 were similar among treatments (Table 4).


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  		Table 4. Beef and grain production from ‘Lockett’ wheat receiving three fertility practices for 3 yr in a graze-plus-grain management system at Vernon, TX.

 
Phosphorus applications (deep- and surface-applied) increased average 3-yr animal gain ha–1 by 22 kg or 27% in the grazing phase before 1 March. Animal gain ha–1 was increased each year of the study, ranging from 21 to 39% higher with applied P. The forage allowance stocking strategy facilitated the capture of 93% (average 27% increase in beef production divided by an average 29% increase in forage production) of increased forage production in additional beef gain.

During the 3 yr, grain yields in the graze-plus-grain system were not increased with the addition of P fertilizer (Table 4). Tillman clay loam is not considered the most productive soil for grain production. A 2000 kg ha–1 yield is average in the region, and the nutrient requirements were apparently adequate for this level of production.

Under the graze-out management system (Table 5), ADG and gain hd–1 were both unaffected by P applications, again indicating that uniform forage allowance was sufficiently maintained throughout the study period. Gain ha–1 was increased only in 2002–2003 and as an average of the 3 yr. During the 3-yr period, animal gain ha–1 was increased 80 kg ha–1 or 34% with surface-applied N+P compared with N alone, and was significantly higher than animal gain where N+P was injected.


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  		Table 5. Beef production from ‘Lockett’ wheat receiving three fertility practices for 3 yr in a graze-out management system at Vernon, TX.

 
The enterprise budgets created for each replication of each fertilizer treatment provided the data in Table 6, showing 3-yr average income, direct expenses, and returns from the three fertilizer practices in the graze-plus-grain system. Greatest revenue was generated from surface-applied N+P. Revenue from the grain totaled $229.23 ha–1 while cattle gain generated an additional $72.85 ha–1. Cattle revenue was slightly higher with deep-placed N+P. Expenses were highest where P was applied, representing the added fertilizer costs and additional grain harvesting costs. Expenses for P treatments averaged $30.76 ha–1 higher than expenses where only N was applied. Net returns were highest ($109.89 ha–1) for surface-applied N+P (Table 6). In the graze-plus-grain production system, P added returns to indirect costs, land, and management regardless of application method; however, surface application of P generated the greater returns.


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  		Table 6. Enterprise budgets for three soil fertility practices in a graze-plus-grain management system during 3 yr at Vernon, TX.

 
Table 7 contains 3-yr averaged enterprise budgets for the graze-out management program. Revenues were lower than those in the graze-plus-grain production system. Again, revenues were highest with surface-applied N+P, exceeding values for injected N+P and N alone by $18.39 ha–1 and $57.26 ha–1, respectively. Expenses were also greatest with surface-applied N+P due to the higher stocking rate afforded by increased forage production. Net returns were highest with surface-applied N+P, exceeding $40 ha–1 and nearly double those from N alone. While net returns to indirect costs, land, and management were more favorable where P was applied, the narrow margin of returns in all treatments in the graze-out program was cause for economic concern.


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  		Table 7. Enterprise budgets for three fertility practices in a graze-out management system during 3 yr at Vernon, TX.

 
In each system, returns were highest with surface-applied P. Increases in forage, beef, and grain production due to surface-applied P improved income sufficiently to exceed the cost of P fertilizer by nearly $20 (graze-out) to $30 (graze-plus-grain) ha–1 yr–1 (Tables 6 and 7). Surface-applied P resulted in economic returns to indirect expenses, land, and management that were $69 ha–1 higher in the graze-plus-grain system than with surface-applied P in the graze-out system. Averaged across all fertility treatments, the graze-plus-grain system returned $65 ha–1 more ($94 vs. $29 ha–1, P < 0.0001) than that from the graze-out management system.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Consecutive year applications of P at 20 kg ha–1 yr–1 resulted in elevating soil test P levels from 9 or 10 mg kg–1 to 23 or 24 mg kg–1 in the surface 5 cm. This increase in P indicates that the long-term fertilizer requirement for wheat production in the southern Great Plains environment was exceeded, establishing a soil P reserve that would influence crop yields for several more years. The soil P reserve in this study did not enter into the economics of dual-use wheat systems. Beef production improved proportionally with increased amounts of forage in response to P fertilization, demonstrating that the additional forage can be captured in additional beef gains. Increases in forage, beef, and grain production due to surface-applied P improved income sufficiently to exceed the cost of P fertilizer by nearly $20 (graze-out) to $30 (graze-plus-grain) ha–1 yr–1. Surface-applied P resulted in economic returns to indirect expenses, land, and management that were $69 ha–1 higher in the graze-plus-grain system than with surface-applied P in the graze-out system. Under the prevailing economic conditions, profitable wheat/stocker operations appears closely tied to grain harvest as well as effective utilization of wheat as a forage crop in the Rolling Plains. Over the course of the study, the graze-plus-grain system was clearly superior to the graze-out system, generating $65 ha–1 more in net returns.


   ACKNOWLEDGMENTS
 
We wish to acknowledge the dedicated efforts of the technical support staff from each cooperating project. Appreciation is also extended to the following companies and institutions for financial and material support that enabled us to conduct this extensive research effort: Fluid Fertilizer Foundation, Potash and Phosphate Institute/Foundation for Agronomic Research, Agrium, Poole Chemical Company, Simplot Fertilizer, IMC, and the Texas Wheat Producers Board.


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





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