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Agronomy Journal 92:295-302 (2000)
© 2000 American Society of Agronomy

SOIL MANAGEMENT

Wheat and Maize Yields in Response to Straw Management and Nitrogen under a Bed Planting System

Agustin Limon-Ortegaa, Kenneth D. Sayrea and Charles A. Francisb

a CIMMYT, A.P. 6-641, Mexico D.F. 06600, Mexico
b Univ. of Nebraska–Lincoln, 225 Keim Hall, Lincoln, NE 68583-0949 USA

k.sayre{at}cgiar.org


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
In the Yaqui Valley, northwest Mexico, the crop sequence that is becoming more common consists of planting wheat (Triticum aestivum L.) as a winter crop on a raised bed followed by maize (Zea mays L.) as a summer crop. In this area, straw of both winter and summer crops is commonly burned. The consequences of burning crop residues on crop yields in the Yaqui Valley have not previously been documented, and alternative practices have not been proposed. A 5-yr study was conducted at the CIANO (Centro de Investigaciones Agrícolas del Noroeste) experiment station in Sonora, Mexico, to compare the effects of burning with other straw management strategies on wheat and maize yields. We tested two tillage systems (conventional-tilled bed, CTB, and permanent bed, PB), five straw management treatments (incorporated with CTB and straw as stubble, partly removed, removed, or burned with PB), and seven N treatments, five applied preplant (0, 75, 150, 225, and 300 kg N ha-1) and two at the 1st node stage (150 and 300 kg N ha-1) of wheat. Maize following wheat received a uniform application of 150 kg N ha-1. The combination of PB and straw as stubble produced superior maize and wheat grain yields in high-yielding environments; in low-yielding environments, PB–straw burned produced greater wheat grain yields. Nitrogen fertilizer application of 150 and 300 kg N ha-1 at the 1st node stage of wheat increased grain yields compared with preplant N fertilizer applications. Permanent beds combined with retaining all crop residues in the soil as stubble have the potential to increase both wheat and maize yields in the Yaqui Valley.

Abbreviations: CTB, conventional-tilled bed, raised-bed system • CTD, canopy temperature depression • PB, permanent raised beds • SDD, stress degree day


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
THE YAQUI VALLEY is located in the state of Sonora in northwest Mexico. About 255000 hectares of agricultural land are irrigated, 60% of which are used for winter wheat (Triticum aestivum L.) production. Local plant breeders have developed maize (Zea maize L.) varieties that grow during the summer and are harvested before the winter wheat is planted in the fall. This development allows farmers to produce two crops per year. Wheat is planted in two to three rows, 15 to 20 cm apart, on top of raised beds, 75 to 90 cm apart. Irrigation water is applied in the furrows between the bed. This planting system facilitates mechanical weed control, increases water-use efficiency, reduces crop lodging, and allows lower seeding rates (Moreno et al., 1993). Other common farming practices in the area use intensive land preparation for each crop, with up to six machinery passes and burning all crop residues after each harvest. Burning crop residues in areas like the Yaqui Valley may be an effective management tool; because crop production is dependent on furrow irrigation, residues should be removed to facilitate irrigation (Hooker et al., 1982) and to aid in proper seed placement (Campbell et al., 1991). Additionally, the time saved by burning is important since the turnaround time between winter and summer crops is only about 30 d. Recent concern about environmental pollution due to smoke from burning motivates the study of alternative tillage practices and straw management. On the other hand, farmers typically apply all of the P fertilizer and nearly 80% of the N fertilizer before planting. The remaining 20% of N is applied after planting, with the first and second irrigation (Meisner et al., 1992).

Generally, tillage methods are dictated by the amount of crop residue remaining on the surface after planting (Gebhardt et al., 1985). However, there are some strategies of crop residue management whose tillage method can not be easily defined. Such is the case when soil is not tilled and crop residues are removed by other means. This practice would not be neither conventional nor conservation tillage. To avoid this ambiguity, tillage–straw residue management should be discussed separately. Alternatively, in this document, the term beds will be used instead of ridges to differentiate from the ridge-till system practiced in the U.S. Corn Belt.

Additionally, the rate of progress in the Yaqui Valley for increasing wheat yields using new genotypes (about 0.88% per year) is becoming more difficult to maintain (Sayre et al., 1997). These factors, along with N fertilization, must be considered to develop new farming practices that can increase grain yields. Our objectives, therefore, were (i) to evaluate alternative wheat and maize straw management practices combined with a permanent bed-planting system in comparison with a conventional-tilled bed and (ii) to assess N management effects in wheat in response to different tillage–straw treatments.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
The experiment was initiated in 1992 at CIANO (Centro de Investigaciones Agrícolas del Noroeste) research station, near Ciudad Obregón in Sonora, Mexico (27.33° N, 109.09° W; 38 m above sea level). The soil type was a coarse sandy clay, mixed montmorillonitic Typic Calciorthid, low in organic matter (0.76%) and slightly alkaline (pH 7.7). The climate is dry; average day-time temperatures during the grain-filling stage are moderate (18°C) for wheat and hot (31°C) for maize. This report presents results obtained in five wheat seasons and four maize seasons between 1993 and 1997.

The experiment comprised two crops in rotation: wheat as a winter crop (planted in late November to early December and harvested in early May) and maize as a summer crop (planted in June and harvested in October). Soybean [Glycine max (L.) Merr.] was planted in alternate years with maize. White fly (Bemisia sp.) infestation on soybean in the Yaqui Valley allowed its production only in 1994 (data not shown). Crops were produced using a bed-planting system with wheat in two rows 20 cm apart and maize in one seed row on top of each bed, 75 cm apart, in plots consisting of eight raised beds 13 m long. The experiment included three replications in randomized complete block design with a split plot treatment arrangement. Main plot treatments (Table 1) were combinations of two tillage systems (conventional-tilled bed, CTB, and permanent bed, PB) with five straw management strategies (incorporated with CTB, left as stubble, partly removed, all removed, and all burned with PB).


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  		Table 1 Description of the tillage–straw management treatments and their abbreviations

 
Split plots consisted of seven N treatments applied to wheat; five were a single preplant application of fertilizer at rates from 0 to 300 kg N ha-1 in 75 kg N ha-1 increments. The sixth and seventh N treatments were 150 and 300 kg N ha-1, both topdressed at Zadoks stage 31 (1st node stage; Zadoks et al., 1974). All plots received twice a year, before sowing crops, an application of 46 kg P ha-1 banded in furrow and incorporated through cultivation when reshaping beds. When maize reached the V8 growth stage (Ritchie et al., 1997), all plots received a uniform application of 150 kg N ha-1 irrespective of the previous N treatment to wheat. In both crops, urea fertilizer was banded in the furrows, followed by irrigation to move fertilizer into the soil. Previously, plots were cultivated to reshape beds for irrigation and to reduce the contact of fertilizer with crop residues.

Plots in the winter season were planted to the bread wheat cultivar Rayon 89 from the Patronato group at a seeding rate of 100 kg ha-1, resulting in a number of plants that varied over seasons from 56 to 113 plants m-2. Maize H-431 single-cross hybrid from the Patronato group was planted at a seeding rate of 23 kg ha-1, resulting in 6.7 plants m-2 (except in 1996, when a different planter configuration was used to obtain 12 plants m-2). Crops were planted using an Aitchison-SeedMatic 2112C drill (Aitchison Industries, New Zealand) equipped with planter units modified with residue-cutting coulters. Plots were irrigated at 10 to 15 d before planting, to allow crop volunteers and other weeds to emerge. Chemical weed control in both crops was administered 1 or 2 d before planting through the application of 1.4 kg a.i. ha-1 glyphosate [N-(phosphonomethyl)glycine]. Mechanical weed control was made through cultivation at the 1st node stage of wheat and the V8 growth stage of maize, before N application and irrigation. In-season irrigations were made when the average of available water, determined gravimetrically in plots selected randomly, was depleted by 50% in the top 60 cm.

Wheat was mechanically harvested from four beds (33 m2) when the crop reached 160 to 189 g kg-1 seed moisture content. Final grain yield of wheat was adjusted to 120 g kg-1 moisture content. The final area harvested for maize after maturity in each plot was two beds (19.5 m2), and yield was adjusted to 155 g kg-1 moisture content. Maize yield in 1993 was not determined for three N treatments applied previously to wheat (75 preplant, 225 preplant, and 150 kg N ha-1 at the 1st node stage), since no differences were expected due to N treatments at initial stages of research.

Canopy temperature depression (CTD) is defined as the difference between canopy temperature and air temperature. Vapor pressure deficit is a measure of the drying power of the air. Canopy temperature depression was measured in wheat plants in 1997 at five growth stages (Zadoks stages 41–43, 45, 49–53, 65, and 73; Zadoks et al., 1974) simultaneously with vapor pressure deficit with a hand-held infrared thermometer (IRT) [Model 510B AG, Everest Interscience, Tucson, AZ]. The IRT field of view was 15°, with emissivity fixed at 0.98. Both measurements were taken simultaneously starting at about 1300 h on clear, sunny days by holding the IRT about 35° from horizontal to avoid any shading of the target. Four random measurements were taken within the harvest area.

Treatment effects were compared through an analysis of variance using SAS software (SAS Inst., 1989) on both wheat and maize, with year effects as random (Carmer et al., 1989) and tillage–straw and N fertilizer as fixed. Main effects and interactions of maize were analyzed over 4 yr and four N treatments. The treatment x year interactions were tested using stability analysis by regressing wheat and maize yields for each treatment (tillage–straw and N rate) on the annual mean grain yield of all treatments (Raun et al., 1993). The tillage–straw x N interaction was assessed by the Mitscherlich function (Overman et al., 1994) regressing the grain yield of each tillage–straw treatment over preplant N treatments and tested with an approximate F-test (Mead et al., 1993). Grain yield response to applied N in the Mitscherlich model is given by

(1)
where Y is grain yield, Mg ha-1; N is applied N, kg ha-1; A is maximum yield, Mg ha-1; b is the intercept parameter for yield; and c is the N response coefficient, ha kg-1. Applications of N at the 1st node stage and preplant were compared through orthogonal contrasts.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Grain Yield
Wheat and maize grain yield were affected by inter-actions of tillage–straw x year, N x year, and till-age–straw x N (Table 2) . Wheat grain yields ranged from 4.14 to 6.25 Mg ha-1, and maize yields from 2.26 to 6.17 Mg ha-1. These values are comparable to or above commercial grain yields for these two crops in the region. The low maize yields in the 1996 trial (Fig. 1) could have resulted from the high plant population (43% greater than normal) that occurred when a different planter configuration was used in an attempt to compensate for the low plant population measured in former years, mainly in treatments with straw residues.


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  		Table 2 Mean squares of wheat and maize grain yield as affected by year, tillage–straw residue management and N level at CIANO (Mexico), 1993–1997

 


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  		Fig. 1 Linear regressions of individual tillage–straw treatments of wheat (5 yr) and maize (4 yr) grain yields on environment mean yields each year at CIANO, Sonora, Mexico (1993–1997)

 
Tillage–straw treatments averaged over years had significant effects on wheat grain yield at P = 0.059 but not on maize yield (Table 2). Comparison of tillage–straw treatments showed that, over the 5 yr of the study, the highest wheat yield occurred in PB–straw burned and PB–straw as stubble treatments (5.52 and 5.57 Mg ha-1; Table 3) . Numerical comparison of maize yields indicated that CTB–straw incorporated resulted in the lowest maize yield average (3.2 Mg ha-1) of the tillage–straw treatments.


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  		Table 3 Wheat and maize grain yields for five tillage–straw treatments at each N level.{dagger}

 
Tillage–Straw x Year Interaction
Because any crop management x year interaction can be difficult to explain, the application of stability analysis has been suggested by Raun et al. (1993) and Guertal et al. (1994) as an alternative way to interpret such interactions. Regression coefficients from stability analysis for the tillage–straw interaction were significantly different from zero (Table 4) , ranging from 0.81 to 1.18 for wheat and from 0.89 to 1.21 for maize (Fig. 1). This result is an indication that tillage–straw treatments responded differently across environments. Wheat grain yield produced in CTB–straw incorporated remained relatively constant in high-yielding environments reaching a plateau at of 5.5 Mg ha-1. This result was an indication that the incorporation of crop residues does not always have beneficial effects on grain yield as reported by Biederbeck et al. (1980) and Sidhu and Beri (1989).


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  		Table 4 Statistical significance of regression coefficients of grain yields from tillage–straw and N treatments regressed on environmental mean yields for stability analyses

 
Wheat regression coefficients for tillage–straw treatments without straw residues (PB–straw burned and PB–straw removed) were less than 1.0, while treatments with straw retention (CTB–straw incorporated, PB–straw as stubble and PB–straw partly removed) had regression coefficients greater than 1.0 (Fig. 1). The differential response (P = 0.04) of these two groups of tillage–straw treatments indicates that removing straw residues by any means increased wheat grain yield in low-yielding environments. In contrast, treatments consisting of straw retention increased wheat yields in high-yielding environments. For example, in low-yielding environments there was a tendency for the PB–straw burned to outyield the PB–straw as stubble treatment. In high-yielding environments, the opposite occurred: PB–straw as stubble was superior to PB–straw burned. These results can partially explain some of the contradictory effects of burning straw on grain yields reported in the literature. Biederbeck et al. (1980) and Izaurralde et al. (1986) found that grain yields increased as a result of spring burning. In contrast, Sidhu and Beri (1989) reported that the increase in yield due to burning was not substantial.

The response of maize as a function of environments was different from wheat in that treatments with straw retention compared with straw removal by any means was not different (P = 0.46). Permanent bed–straw as stubble demonstrated a distinctive advantage over CTB–straw incorporated (P = 0.04) in high-yielding environments. However, other tillage–straw treatment comparisons had little advantage over CTB–straw in-corporated. This indicated that the tillage–straw treatments, except PB–straw as stubble, resulted in similar grain yield responses across environments over the 4 yr of data for maize.

Nitrogen x Year Interaction
Mean wheat yields increased with increasing N rate with a maximum 5-yr average yield of 6.35 Mg ha-1. Nitrogen management in wheat was not often a limiting factor for the following maize crop, not even in 1995, when maximum maize yields reached nearly 6 Mg ha-1 (Fig. 1). Stability analysis for the N x year interaction was applied to each of the seven N treatments in wheat and to four N treatments in maize where 4 yr of data were available (Fig. 2) . Wheat regression coefficients for the 0 and 75 kg N ha-1 treatments were not different from zero (Table 4) and thus were excluded from the stability analysis. However, grain yields at 0 and 75 kg N ha-1 regressed on years allowed us to see potential residual effects of tillage–straw treatments on wheat grain production as a function of time. In this analysis, the regression coefficients for all tillage–straw treatments were negative and significant at the zero N rate (data not shown) except for PB–straw left as stubble treatment. This result suggests that the potential source of N mineralized from crop residues (Campbell et al., 1993) can, when left as stubble, compensate for the zero N rate and sustain wheat grain yields over time. Regression coefficients for the other tillage–straw treatments indicated that wheat grain yields were reduced at an average rate of 0.28 Mg ha-1 yr-1 during the 5-yr period. On the other hand, none of the tillage–straw treatments at 75 kg N ha-1 level affected wheat yields over time, suggesting that wheat grain yield at this N level was not influenced by a residual effect of straw management.



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  		Fig. 2 Linear regressions of individual N treatments of wheat (5 yr) and maize (4 yr) grain yields on environment mean yields each year at CIANO, Sonora, Mexico (1993–1997). Nitrogen rates indicated for maize were N treatments applied to previous wheat crop

 
Treatments that received at least 150 kg N ha-1 and were regressed on environmental mean yields of wheat had significantly different regression coefficients (Table 4). As the N fertilizer rate increased, irrespective of the time of application, the regression coefficients increased from 0.70 to 1.28 (Fig. 2). In high-yielding environments and as N rate increased, wheat produced greater grain yields. In contrast, in low-yielding environments and high N rates, wheat did not respond to the added N. The regression coefficient for the 150 kg N ha-1 at the 1st node stage indicated that this rate was the most stable N treatment across environments. The highest grain yields were produced at 300 kg N ha-1 level. The benefit of applying 150 kg N ha-1 fertilizer at the 1st node stage rather than preplant was evident in 1994 and 1996, when the grain yield of the former treatment outyielded the latter by about 0.5 Mg ha-1 (Fig. 2). In other years, when N application at the 1st node stage did not result in increased yield, its effect was similar to N preplant applications.

The application of 300 kg N ha-1 did not result in different yield stability for the following maize crop. Since maize yields resulting from both N treatments were similar (Fig. 2), neither fertilizer treatment applied to wheat represents an economical advantage for the subsequent maize crop.

Tillage–Straw x Nitrogen Interaction
Grain yields of wheat and maize for the tillage–straw treatments at each N level are reported in Table 3. Wheat grain yield increased with increasing N rates over all tillage–straw treatments. Wheat grain yield data for each tillage–straw treatment as a function of preplant N fertilizer application were fitted to the Mitscherlich model. Table 5 contains the parameter estimates of the model for each tillage–straw treatment. The comparison of the A and c parameters among tillage–straw treatments showed no differences, although the intercept parameter b was different among tillage–straw treatments. Intercept parameter b, which reflects the N supplied by the soil, demonstrates that PB–straw partly removed produced the greatest wheat grain yields at the zero N level followed by PB–straw as stubble (Table 5). This result suggests that the effect of crop residues left as stubble on wheat grain yield could have been due to higher rate of mineralizable N in the surface 15 cm (Campbell et al., 1993). Conversely, the N supplied by the soil, as shown by the intercept parameter b, for CTB–straw incorporated indicates that the low grain produced, at low N rates (Table 3), could have been due to a high N immobilization due to straw incorporation. Apparently, the effect of straw management at low N rates on wheat yields not only depended on quantity of straw residue, but also on type of crop residue (wheat only or both wheat and maize) and tillage practices.


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  		Table 5 Parameter estimates and test of the model Y = A/[1 + exp(b - cN)] (Overman et al., 1994) applied to wheat yields in every tillage–straw treatment along with preplant N applications

 
Even though parameters A and c were not statistically different (Table 5), it is interesting to note their numerical difference among tillage–straw treatments. For example, maximum wheat grain yield (parameter A) and response to N supply (parameter c) were greater for PB–straw burned and PB–straw as stubble which is disconcerting since these two management practices are completely different, with opposite effects on soil properties and then on grain yield.

Orthogonal contrasts to compare the wheat grain yield resulting from PB–straw burned with PB–straw as stubble showed that grain yields of both treatments were different only at 300 kg N ha-1 applied at the 1st node stage (P = 0.01). The PB–straw burned treatment produced higher wheat grain yields at preplant application of 300 kg N ha-1 rather than at the 1st node stage (P = 0.002), while PB–straw as stubble produced similar results at this N level. Wheat grain yield increase for delaying the application of 150 kg N ha-1 until the 1st node stage, compared with preplant application, ranged from 0.17 to 0.36 Mg ha-1 for all tillage–straw treatments (Table 3). However, this effect was significant (P < 0.05) only for CTB–straw incorporated and PB–straw as stubble. The grain yield gained through this fertilization practice in these two tillage–straw treatments was 315 kg ha-1 on average (Table 3). Comparing changes in grain yield from 150 to 300 kg N ha-1 showed that the preplant N application for PB–straw as stubble was 0.23 Mg ha-1 greater than N application at the 1st node stage, whereas for PB–straw burned the advantage was 0.57 Mg ha-1 (Table 3). This differential rate of response to N fertilization clearly suggests that the optimum N fertilizer rate is different, at least for these two tillage–straw treatments in a bed planting system.

Maize yield data did not fit the Mitscherlich model. This result indicates that the N fertilizer application to wheat had no influence on maize yield. However, when no N was applied to wheat, maize yield in PB–straw burned was greater by 0.60 Mg ha-1 compared with all other tillage–straw treatments (Table 3). The noticeable advantage for burning, compared with PB–straw removed, suggests that the benefits to maize yield were not only the contribution of root biomass as a source of organic matter (Campbell et al., 1991), but also straw removal method. Grain yields of maize grown at the 0 and 150 kg N ha-1 treatments were not different with PB–all straw burned.

Wheat Canopy Temperature Depression
Vapor pressure deficit values did not vary during measurements of CTD and were not used in the analysis as a covariable to adjust CTD values. The largest differences among tillage–straw treatments were detected when CTD was measured at <=5 d before watering, except in wheat during the grain milk stage (Zadoks stage 73). Tillage–straw treatments with straw retention showed cooler canopy temperatures than treatments with straw removal (Table 6) . This difference can presumably be associated with a high plant water stress before watering. According to Jackson et al. (1988), the higher CTD in the tillage–straw treatments with straw removal suggests that the transpiration rate was less due to lower soil water content than in treatments with straw retention.


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  		Table 6 Canopy temperature depression (CTD) means for tillage–straw and N treatments in 1997, by Wheat Zadoks stage (ZS) and days before irrigation at measurement

 
Canopy temperature depression was consistently warmer at low N rates and cooler at high N rates (Table 6). Comparison of CTD measurements between timing applications of N showed that CTD before Zadoks stage 45 was the same in both N treatments. From Zadoks stage 65 and on, however, for 150 kg N ha-1 at the 1st node stage, CTD was cooler by 11% than the same N rate preplant applied. For 300 kg N ha-1 in both applications, this difference was measured only at Zadoks stage 65, but with opposite results: CTD for preplant N application was lower (-3.33°C) than for N application at 1st node application (-3.0°C).

Stress degree days (SDD), which are the summation of seasonal CTDs, were regressed on wheat grain yield for each tillage–straw treatment; all regression coefficients were negative and significantly different from zero (Table 7) . Diaz et al. (1983) reported similar results. The negative relationship indicates that, as SDD approached zero (higher canopy temperature), the grain yields were lower. Conventional-tilled bed–straw incorporated had the most negative regression coefficient and PB–straw as stubble the least negative of all (Fig. 3) . Those coefficients suggest that the effect of high CTD on grain yield reduction was more severe in CTB–straw incorporated than in PB–straw as stubble.


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  		Table 7 Statistical significance of regression coefficients of grain yields from tillage–straw treatments regressed on stress day (SDD) measured in the 1997 season

 


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  		Fig. 3 Linear regressions of individual tillage–straw treatments of wheat grain yields on stress degree days (SDD) in 1997

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
The two most contrasting straw management strategies (PB–straw burned and PB–straw as stubble) produced the greatest wheat grain yields (5.52 and 5.57 Mg ha-1). The superiority of PB–straw burned to enhance wheat grain yield production over all other tillage–straw treatments was attributed to several conditions. For example, initial wheat and maize stands were better and emergence was reached 1 or 2 d earlier than treatments with straw retention, which may be an advantage for grain yield formation (Gan et al., 1992). Therefore, faster emergence can probably even enhance the grain yield potential of PB–straw as stubble through the removal of crop residues from the row area by the attachment of a row cleaner to the planter (Opoku et al., 1997).

Even though grain yields of wheat and maize over the course of the study were not affected by burning, this practice should be discouraged since undesirable effects in the soil physicochemical properties reported at other locations (Dormaar et al., 1979; Giovaninni and Lucchesi, 1997) can be extended to the Yaqui Valley. Instead, leaving all residues as stubble should be preferred, since grain yields produced under this system by both wheat and maize were comparable to the system of burning residues. Additionally, crop residues as stubble are a potential source of N that compensate for the zero N rates and sustain the wheat grain yields in this cropping systems. This benefit, over the 5 yr of study, was mostly seen at low N rates. However, one of the major inconveniences of leaving all straw residues is that water for irrigation is not easily managed, especially when watering after harvesting to prepare for planting the next crop. The lowest grain yields in both wheat and maize was obtained in the CTB–straw incorporated treatment. Although the reason for this was not studied, it might be associated with plant phytotoxicity observed at early growth stages, especially in maize, as a result of the phytotoxic compounds released during straw decomposition (Cochran et al., 1977). The presence of white grub (Sphenophorus sp. larvae) observed in the 1996 and 1997 might be another reason for the reduced yields of maize in this treatment.

Nitrogen application at the 1st node stage of wheat had excellent potential to produce greater wheat yields. However, the value of N application at this stage should be explored over a wider range of N rates, to estimate an adequate N fertilizer strategy in the PB–straw as stubble treatment. From the present study, an adequate N fertilizer rate for wheat at the 1st node stage should be <300 kg N ha-1.

Canopy temperature depression measurements were found to be useful for monitoring plant water stress as a result of tillage–straw management, especially if measurements were taken at <4 d before watering. The PB–straw as stubble treatment had lower canopy temperatures and higher soil water contents than the other tillage–straw treatments. This difference suggests that PB–straw as stubble treatment had lower water requirements that should be further investigated for irrigation scheduling. The disadvantage of CTD measurements is that, to obtain reliable readings, days should be sunny with clear skies; otherwise, the instrument may not detect differences.SAS Institute 1989


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge the expert assistance of Jaime Cruz and Saul Sánchez.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
The senior author was a graduate student at the Univ. of Nebraska–Lincoln sponsored by the Mexican government through CONACYT (Consejo Nacional de Ciencia y Tecnología).

Received for publication February 4, 1999.
   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
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 Results and discussion
 Conclusions
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