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Agronomy Journal 92:121-127 (2000)
© 2000 American Society of Agronomy

SMALL GRAINS

Dryland Winter Wheat as Affected by Previous Crops

Charles A. Norwooda

a Southwest Research-Extension Center, 4500 E. Mary, Garden City, KS 67846 USA

cnorwood{at}oz.oznet.ksu.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
The dryland winter wheat (Triticum aestivum L.)–grain sorghum [Sorghum bicolor (L.) Monech]–fallow (WSF) rotation is suitable for large areas of the U.S. Great Plains. Other crops such as corn (Zea mays L.), sunflower (Helianthus annus L.), and soybean [Glycine max (L.) Merr.] can be substituted for grain sorghum, but the effects of these crops on the subsequent wheat crop have not been extensively documented. A study was conducted near Garden City, KS from 1992 through 1998 to determine the effect of these four crops on soil water at planting (SWP), yield, and yield components of the subsequent wheat crops. Wheat was grown following conventional tillage (CT), reduced tillage (RT), and no tillage (NT). On average, amounts of SWP following sunflower and soybean were 19.9% and 9.3% lower, respectively, than those following corn and sorghum. Grain yields following sunflower averaged 0.85 Mg ha-1 less than those following corn and sorghum in two of six years at all tillage levels, 0.72 Mg ha-1 less in one year with CT and NT, and 0.60 Mg ha-1 less than those following corn in one year with CT and RT. Yields following soybean were 0.61 Mg ha-1 less than those following RT corn in one year and 0.61 Mg ha-1 less than CT corn and CT sorghum in one year. Most of these yield reductions were caused by fewer heads m-2, but fewer kernels head-1 and lower kernel weight occasionally contributed. Sunflower and soybean may cause reductions in subsequent wheat yields, but they provide diversification and may prove beneficial when the whole cropping system is considered.

Abbreviations: CT, conventional tillage • NT, no tillage • RT, reduced tillage • SWP, soil water at planting • WF, wheat–fallow • WSF, wheat–sorghum–fallow


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
DRYLAND crop yields in the U.S. Great Plains are limited by low precipitation and high evaporative potential. Wheat is the major crop grown throughout the area and fallowing has been used traditionally to store water for the following crop. About 1.4 million ha of winter wheat typically are grown in western Kansas (Byram, 1997), mostly in the wheat–fallow (WF) and WSF cropping systems. Reduced tillage and no tillage in the wheat–fallow system have resulted in increased storage of precipitation and increased grain yield (Fenster and Peterson, 1979; Fenster and Wicks, 1982; Smika, 1990; Norwood et al., 1990). However, WF allows only one crop in two years and is inefficient in terms of land use, water use, and yield. The WSF system allows two crops in three years and has proven suitable for the central and southern Great Plains (Fenster and McCalla, 1971; Nilson et al., 1985; Norwood et al., 1990). The efficiency of the WSF system, particularly when combined with RT or NT, is due to more intensive cropping and more efficient use of precipitation. No till results in water storage deeper in the profile than does CT (Smika, 1990; Eck and Jones, 1992; Norwood, 1994). Unger and Weise (1979) reported that 35% of the precipitation was stored with NT prior to sorghum in the WSF system compared with 23% with sweep tillage. Jones and Popham (1997) found that NT management of wheat residue increased soil water content for the next crop by 22 mm with WSF, 15 mm with WF, and 29 mm with continuous wheat. Norwood (1994) found that twice as much water was stored by NT in WSF at sorghum planting than at wheat planting. Peterson et al. (1996) reported that three-year systems such as WSF and wheat–corn–fallow increased water use efficiency by 28% over that of WF.

Whereas much has been published in support of crop rotations more intensive than wheat fallow, little research has been conducted concerning the effects of the previous crop on wheat yield. The 1996 Farm Bill removed crop restrictions, meaning that farmers can diversify and base their selection of crops on what they believe is most profitable. Crops other than corn or sorghum have potential for inclusion in the wheat–summer crop–fallow rotation. Guy and Gareau (1998) found mustard (Sinapis alba L.), dry pea (Pisum sativum L.) and lentil (Lens culinaris Medik.) to be beneficial rotational crops prior to winter wheat. Norwood (1999) compared corn, grain sorghum, sunflower, and soybean and found all four crops suitable during the period of study. Peterson et al. (1997) reported that the 3-yr WSF and wheat–corn–fallow rotations and the 4-yr wheat–corn–proso millet (Panicum miliaceum L.)–fallow and wheat–sorghum–sorghum–fallow rotations increased annual grain production by 72% compared with WF. A perception exists that crops such as sunflower deplete more water from the soil profile and reduce yield of following crops. This depletion of water was confirmed (Hattendorf et al., 1988; Jaafar et al., 1993; Norwood, 1999), but its effect on the following crop has not been studied extensively. Therefore, the objective of this study was to determine the effects of four row crops, corn, grain sorghum, sunflower, and soybean, on soil water at winter wheat planting and on the yield and yield components of winter wheat grown with CT, RT, and NT.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Research was conducted at the Southwest Research-Extension Center near Garden City, KS from May 1990 (first row crop planting) through June 1998 (last wheat harvest). The soil was a Ulysses silt loam (fine-silty, mixed, superactive, mesic Aridic Haplustoll) with a pH of 7.8 and an organic matter content of 15 g kg-1. Long-term average climatic data for Garden City are 455 mm precipitation, 12°C mean temperature, 1808 mm open pan evaporation (April–September) and a 170-d frost-free period. Precipitation during the study period is presented in Table 1 .


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  		Table 1 Precipitation during study period

 
The wheat–row crop–fallow system is a rotation of two crops in 3 yr with a 10- to 11-mo fallow period preceding each crop. The plot area originally was a production field of annually cropped grain sorghum for several years prior to the study. The plot area was divided into four replications and each replication was divided into thirds so that data could be collected from wheat, row crops, and fallow in each year. The experimental design was a strip-plot (Gomez and Gomez, 1984). Tillage treatment was the vertical effect. Wheat planted in the row crop stubble across tillage treatments was the horizontal effect. The tillage treatments were 18 m wide by 36 m long, whereas the wheat plots were 9 m wide by 18 m long. Corn, grain sorghum, sunflower, and soybean were planted in May or June of each year and harvested in September or October. Wheat was planted in mid-September of the following year into the stubble remaining from the previous row crops and harvested in late June. The sequence began again with row crop planting into wheat stubble in May of the next year. The portion of the study presented in this paper began with tillage and herbicides applied to the stubble remaining from the row crops harvested in 1990. Only the wheat phase is discussed here because results of the row-crop phase were published previously (Norwood, 1999). Soybean and sunflower were destroyed by predators in 1991, so no wheat yields are reported for 1993.

The crops were grown under CT, RT, and NT conditions. The CT plots received three or four tillage operations during fallow to control weeds. Tillage was performed with a sweep plow equipped with three 1.52-m 75° V-blades. Herbicides were applied to the RT and NT plots during fallow to control weeds and conserve residue. Cyanazine (2-[[4-chloro-6-(ethyl-amino)-1,3,5-triazin-2-yl]amino]-2-methly-propanenitrile) was applied to the RT and NT plots (row crop stubble) at a rate of 2.7 kg ha-1 in April or May. When the cyanazine no longer controlled weeds in the RT plots (typically 30 to 45 d after treatment), sweep tillage was used to control weeds. Two tillage operations usually were required. Weeds were controlled in the NT plots with appropriate rates of the postemergence herbicides glyphosate (N-[phosphonomethyl]glycine) and 2,4-D ([2,4-dichlorophenoxy]acetic acid) after the cyanazine degraded. Two applications usually were required. The wheat received fallow applications of 60 kg N ha-1 in each year. The soil was not initially low in P, but 100 kg P2O5 ha-1 was applied at the beginning of the study to prevent any later deficiencies. The silt loam soils in western Kansas are naturally high in K, thus applications were not made. The `TAM 107' wheat was planted in 1991 through 1996 and `TAM 110' was planted in 1997. Seeding rate was 60 kg ha-1. The wheat was harvested with a plot combine, and yield was corrected to 12.5 g kg-1 moisture. The yield components heads m-2, kernels head-1, and kernel weight were determined by sampling two 1-m rows in each plot.

Gravimetric SWP was measured in CT and NT in each year. Two subsamples per tillage treatment in each replication were taken in 0.3-m increments to a depth of 1.8 m and the water is reported as available soil water (i.e., the amount of water held between 0.03 and 1.5 Mpa). The Ulysses silt loam in this study can hold 330 mm of available water in a 1.8 m profile (Harner et al., 1965).

Analysis of variance was conducted for individual years and across years. Means were separated by Fisher's protected LSD. Linear regression was used to further define the effects of the yield components.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
Climatic Conditions
Annual precipitation and fallow precipitation were above average in all years (Table 1). Precipitation was near or above average in each growing season (September through June) except for the period ending in 1996, when the total of 263 mm was 51 mm below average and resulted in reduced yields (discussed below). Most of the shortfall occurred from October 1995 through April 1996 when only 61 mm (80 mm less than average) of precipitation fell. The most growing-season precipitation (367 mm) occurred during the period ending in 1994. Precipitation during fallow (October through September) was greatest (557 mm) for the period ending in 1993 and least (437 mm) for the period ending in 1997. Temperatures did not deviate much from average (data not shown). Temperatures of -3°C on 10 April and -8°C on 11 April, however, reduced yields in 1995.

Soil Water at Planting
Average levels of SWP following sunflower and soybean were 49 mm (19.9%) and 23 mm (9.3%) less than the average for corn and sorghum. Average SWP was 16 mm (7.3%) greater in NT than in CT (Table 2) . However, crop and tillage differences were not consistent across years. Average SWP for the 1992 wheat was only 158 mm, less than 50% of field capacity. Soil water content was 31 mm lower in NT than in CT. Although fallow precipitation was above average during the 1990 through 1991 fallow period prior to the 1992 wheat (Table 1), distribution was poor. Not evident in Table 1 are periods of low rainfall and high temperatures that occurred during the summer of 1991, apparently causing higher evaporative water loss in NT than in CT. The amount and distribution of residues remaining from the previous crops were not very effective in retaining soil water. Although evaporative losses in CT are usually higher because of tillage and the resulting loss of crop residue, the CT plots were not tilled as often in 1991, because the low rainfall resulted in fewer weeds. A dust mulch may have protected the soil surface in CT and retarded evaporation (Hammel et al. 1981; Tanaka, 1985). Soil water at planting was lowest in the sunflower plots in 1992, but it did not differ significantly from that of the other treatments. It averaged 11 mm lower in the sunflower and soybean plots than in the corn and sorghum plots in 1994, because the sunflower and soybean residues were less effective in conserving precipitation and reducing evaporation. No differences among tillage treatments occurred in 1994.


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  		Table 2 Soil water at wheat planting as affected by crop and tillage. Garden City, KS, 1992–1998

 
Crop by tillage interactions occurred in each year from 1995 through 1998. Contents of SWP after corn or sorghum were usually similar. Soil water at planting after sunflower was less than that after corn and sorghum in each year except for NT sorghum in 1995 and CT sorghum in 1998. Soil water at planting following soybean was sometimes greater than that following sunflower but did not exhibit any particular pattern. However, SWP after sunflower was never greater than that after soybean. Across years, amounts of SWP were similar following corn or sorghum, less following soybean, and least following sunflower. The distribution of SWP in the soil profiles (Fig. 1) indicates that accumulation of water decreased with depth and that most of the differences in water content in the whole profile were from differences in the lower portions. Less water occurred deeper in the profile following sunflower and, to a lesser extent, soybean, than following corn or sorghum. The amounts of SWP in the third through sixth increments were usually smaller following CT sunflower and CT soybean than following corn and sorghum and those differences widened with depth. Conventionally tilled sunflower resulted in less SWP in the sixth increment than any crop in four of six years. Although differences in profile SWP occurred between crops for NT as well as CT, NT resulted in smaller differences, particularly in the third through sixth increments. Most of the differences in amounts of SWP following sunflower and soybean versus corn and sorghum were eliminated with NT. Further analysis of the data in Table 2 reveals that SWP following CT sunflower averaged 67 mm less than the SWP following CT corn and sorghum (averaged) during 1995 through 1998. No-till SWP was 27 mm less with sunflower than with corn and sorghum. Water accumulated faster in the NT sunflower and soybean plots than in the other NT plots. Although soil water content was determined only at the end of fallow, it was probably greater following corn or sorghum than following sunflower or soybean by spring (April), because the corn and sorghum residues trapped more snow. Further evidence of snow trap can be found in the discussion of the row-crop phase of this study (Norwood, 1999). Depletion patterns of sorghum and sunflower were similar to each other, as were depletion patterns of corn and soybean. Sorghum and sunflower removed more water from the profiles than did corn and soybean, particularly in the lower portions, and sunflower removed slightly more water than did sorghum. However, the data in Table 2 and Fig. 1 indicate that SWP was usually lower after sunflower and soybean than after sorghum and corn, indicating the ineffectiveness of the residue remaining from sunflower and soybean in trapping snow. Significant snow occurred in all years except the one prior to the 1997 wheat (discussed below). The sunflower and soybean profiles were probably drier in the spring, particularly in the lower portions, than the corn and sorghum profiles. Thus, in the spring and summer, water accumulated faster in the lower portions of the NT sunflower and soybean profiles. This was due to strong suction gradients in the drier soil that augmented downward movement caused by a gravitational gradient, thus redistributing the additional water (Hillel, 1998). Less water accumulated in the CT profiles because of evaporative loss after tillage. Fewer differences in SWP occurred in the corn and sorghum profiles because the residues of both crops were equally effective in trapping snow. Fewer crop and tillage differences occurred among all crops in 1994 because of a record snowfall of 1518 mm during the previous winter. The snow contained 162 mm of water, enough to equalize soil water content between treatments. Conversely, no measurable snow fell during fallow before the 1997 wheat. That was the only year when the NT profiles of corn and sorghum retained more water than did the CT profiles. Further analysis of the data in Table 1 shows that precipitation during the October through April portion of fallow prior to the 1997 wheat totaled only 61 mm, the lowest of the six fallow periods. Above average, well-distributed rainfall during the remainder of fallow resulted in more water in the NT corn, sorghum, and soybean profiles. Water was lost from the CT profiles because of tillage. This is the opposite of what occurred prior to the 1992 wheat, discussed above, when the dust mulch resulting from tillage apparently reduced evaporation in CT. The reason for less SWP in the NT sunflower profile in 1997 compared with CT is unknown, but an examination of the raw soil water data indicates more variation among subsamples in the sunflower plots than in the other plots.



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  		Fig. 1 Available soil water at wheat planting as affected by previous crop, tillage, and depth. Bars indicate LSD at P < 0.10

 
Yield
Wheat yields ranged from a low of 1.11 Mg ha-1 after CT sunflower in 1996 to a high of 4.42 Mg ha-1 after RT corn in 1997 (Table 3) . Yields were reduced by a freeze on 10–11 April 1995 during the jointing stage and by dry weather and insects in 1996. The previous crop had no effect on wheat yields in 1994 and 1995. Wheat yield following sunflower was less than yields following the other crops in 1992. Crop by tillage interactions occurred from 1996 through 1998. Wheat yields in the corn and sorghum plots differed only in 1996, when yield following sorghum was less in RT. Yields following sunflower were less than those following corn and sorghum at all tillage levels in 1997 and in CT and NT in 1998. The crop by tillage interaction in 1996 was confounded by the yield reduction from low precipitation and insects and resulted in more plot to plot variation, perhaps eliminating some potential differences. Note that all wheat yield means following sunflower in 1996 were lower than those following the other crops and that wheat yields following both CT and RT sunflower and soybean were significantly lower than those following corn, but not sorghum. Considering all years, the largest wheat yield reduction following sunflower was 1.44 Mg ha-1 with CT (using the average wheat yield following corn and sorghum) in 1997, which also was the largest percentage reduction (36.2%). Strangely, the smallest yield reduction following sunflower also occurred in 1997, 0.54 Mg ha-1 (12.4%) for RT, whereas the yield reduction for NT was 0.94 Mg ha-1 (22.7%) Yields following soybean in the 1996 through 1998 period never differed from yields following corn or sorghum in NT, but were less than those following RT corn in 1996 and CT corn and sorghum in 1997. Yields after sunflower were less than those after soybean in CT and NT in 1997 and 1998. Yields after sunflower never exceeded those following soybean. Across years, no difference occurred between yields following corn or sorghum, but yield following soybean was lower and yield following sunflower was least. Anderson et al. (1999) conducted a study during 1994 through 1997 and found sunflower reduced wheat yield during dry years. This is logical because of higher water use by sunflower (Anderson et al. 1999, Norwood 1999), making growing-season precipitation more important. In my study, the yield reductions following sunflower in 1992, 1997, and 1998 followed one or more months of below normal precipitation in the spring of each year, although precipitation for the entire growing season was near or above average (Table 1).


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  		Table 3 Winter wheat yield as affected by previous crop. Garden City, KS, 1992–1998

 
Tillage generally had no effect on yield from 1992 through 1995, except for a reduction in RT in 1994, for unknown reasons. No till or RT usually increased yields in 1996 and 1997. Wheat yields following sunflower averaged 32.9, 18.7, and 16.8% less than yields following corn and sorghum in CT, RT, and NT, respectively, in those two years. Thus, although tillage had no effect when averaged across years, results showed a tendency for RT and NT to lessen the effects of sunflower on wheat yield. The favorable effect of NT, when it occurred, was apparently due to more SWP, as discussed above.

Yield Components
Grain yield is the product of heads m-2, kernels head-1, and kernel weight. Lower yields following all crops in the dry year of 1996 resulted from fewer heads m-2 (Table 4) . However, the yield reduction resulting from the freeze in 1995 resulted from fewer kernels head -1 and lower kernel weight. The r2 values in Table 5 indicate that 75, 85, and 73% of the variations in yield in 1992, 1997, and 1998, respectively, were due to differences in the number of heads m-2. Wheat following sunflower had significantly fewer heads m-2 than the other crops in those three years (Table 4) and yield also was less (Table 3). However, no significant differences occurred in head number in 1996, when yields following sunflower and soybean tended to be less than those following corn. In that year, kernel weight was lower following sunflower and soybean than following corn and sorghum (Table 4), although the regression coefficient (Table 5) was not significant. The reduction in yield following sunflower in 1997 was accompanied by fewer kernels head-1, in addition to fewer heads m-2. A difference in kernel number occurred only in 1997. On average, heads m-2 followed the order sorghum > corn = soybean > sunflower. The reasons for the lower number of heads m-2 following sunflower are not completely clear but appear to be related to SWP and spring rainfall. The lower head numbers following sunflower in 1997 and 1998 were preceded by both lower SWP (Table 2, Fig. 1) and lower than average April rainfall (Table 1). There was no significant difference in profile SWP in 1992 but 3 mm of rain in April and 40 mm in May were the lowest of the study period for these two months. No crop by tillage interactions were observed. More heads m-2 occurred in CT in 1992 and in NT in 1994 and 1996, but on average tillage had no effect. Crop by tillage interactions occurred for 100 kernel weight in 1992 and 1997 and the r2 value was significant at P < 0.10 in 1992 (Table 5). The main reason for the interaction was slightly lower kernel weight for CT sunflower. Kernel weights tended to be lower for sunflower than for the other crops, but the only significant difference was between RT sunflower and RT corn. I have no plausible explanation for the interaction in 1997.


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  		Table 4 Winter wheat yield components as affected by previous crop. Garden City, KS, 1992–1998

 

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  		Table 5 Regression of yield components on winter wheat yield. Garden City, KS, 1992–1998

 
To summarize, wheat yields following sunflower were less than those following corn and sorghum in two years with all tillage levels and in one year with CT and NT and were less than those following corn with CT and RT in one additional year. Wheat yields following soybean were occasionally less than those following corn and sorghum. Averaged across years, however, wheat yields following both sunflower and soybean were less than those following corn and sorghum, and yields following sunflower were less than those following soybean. These yield reductions resulted mostly from fewer heads m-2, but fewer kernels head-1 and lower kernel weight sometimes contributed. Less SWP following sunflower and soybean also was related to the yield reductions. Less SWP probably resulted from lower and less effective amounts of sunflower and soybean residues compared with those remaining from corn and sorghum. No till or RT increased yields in only two of six years. However, NT and RT tended to lessen the yield reduction due to sunflower in those two years. Although sunflower and soybean sometimes reduce subsequent yield of wheat, they allow diversification and may prove beneficial when the yield and economics of the whole cropping system are considered.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Kansas Agric. Exp. Stn. Contribution no. 99-416-J.

Received for publication April 19, 1999.
   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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