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Published in Agron J 99:630-636 (2007)
DOI: 10.2134/agronj2006.0058
© 2007 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
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Tillage

Tillage System by Planting Date Interaction Effects on Corn and Soybean Yield

Mario Perez-Bidegain, Richard M. Cruse* and Allan Ciha

Agronomy Hall, Iowa State Univ., Ames, IA 50011-1010

* Corresponding author (rmc{at}iastate.edu)

Received for publication February 22, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Spring soil temperature and soil water content can be influenced by tillage system. If a tillage system and planting date interaction exists, planting on a single date, as is done in most tillage trials, could bias yield results. We tested for this interaction by comparing corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields using strip tillage, no tillage, and disk-chisel tillage systems with planting dates determined by soil temperature and water content conditions within each tillage system. A 3-yr study (2002–2004) was conducted on a site near Newton, IA that had three soil types: Cumulic Hapludolls, Aquic Hapludolls, and Cumulic Haplaquolls. A split-plot design was used with tillage as whole plots arranged in four randomized complete blocks. Crops in all tillage system treatments were planted on three dates that comprised the split-plots. The criteria to determine the planting dates were soil temperature (>10°C for corn and >13°C for soybean for 12 consecutive hours) and soil water content (less than or equal to the lower plastic limit for any of the tillage treatments) at the 0.05-m depth. A planting date occurred for each of the tillage systems as these criteria were met. For both crops, the earliest date having these soil conditions occurred simultaneously for disk-chisel and strip tillage. The no-tillage plots exhibited these conditions between 4 and 28 d and between 6 and 15 d later for corn and soybean, respectively, than for the other tillage treatments. Corn planted with disk-chisel tillage yielded 0.8 Mg ha–1 more than the mean of the other two tillage treatments across years. Planting date affected corn yield only in 2003. For soybean, planting date affected yield. Soybean planted at the early planting date yielded 0.16 Mg ha–1 more than the mean of the other planting dates across years. There was no interaction of tillage x planting date for yield of either crop. This research indicates that recommendations derived from existing tillage research using a common planting date are valid.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
FARMERS often select planting times based on soil conditions, understanding that date of optimum (or even acceptable) field conditions for planting normally varies due to the tillage system used. Optimum planting date typically refers to the earliest spring date when soil water content and soil temperature conditions are concurrently suitable for planting and when the threat of a killing frost is minimal. The soil temperature and water content conditions are used as the sole measures of optimum planting date in this paper. In designing tillage research projects, researchers have typically ignored the implications that these tillage-induced soil and resulting planting date differences create (Elmore 1987, 1990; Imholte and Carter, 1987; Oplinger and Philbrook, 1992). That is, all planting occurs at a time when soil conditions are acceptable across a wide range of tillage treatments, even though this may not correspond to the optimum planting date for one or more of the systems as defined previously. If an interaction between tillage and planting date exists, planting on a given date could bias the data and resultant tillage recommendations that are based on a single planting date, making the recommendations invalid or misleading.

Plants respond to changes in the soil environment. It is often reported that soil conditions are modified by tillage. Soil water content has been observed to increase as surface residue increased in a dry spring, but it was not affected by surface residue when frequent rain events occurred (Erbach et al., 1986; Vetsch and Randall, 2002). Horton et al. (1994) reported that tillage affected the amount and distribution of surface residue, which ultimately affected soil temperature. Kaspar et al. (1990) observed a 4.2 to 6.8°C reduction in maximum daily soil temperature at the 0.05-m depth between the last week of May and second week of June in a clay loam soil when a mulched surface was present. They added that this outcome was dependent on the type of mulch used and year. Johnson and Lowery (1985) reported that the maximum soil temperature at 0.05 m ranged from 4.2 to 5.9°C lower in no tillage compared with moldboard plow tillage for a silt loam soil. By the end of June, those tillage differences were reduced to <1°C. Soil compaction can limit root growth, root distribution, seedling emergence, and nutrient uptake (Bowen, 1981; Garcia et al., 1988). Drury et al. (2003) reported that moldboard plow tillage in a clay loam soil reduced cone index (averaged over the 0- to 0.2-m depth) compared with no-tillage in a dry year. In a wet year, cone index in no tillage did not differ from moldboard plow tillage and zone tillage.

Several studies have shown the effects of planting date on corn yield. Imholte and Carter (1987) obtained the highest corn grain yield when corn was planted between 27 April and 5 May for the moldboard plow and no-tillage treatments. Nafziger (1994) reported that maximum corn grain yield when planting took place between 20 April and 11 May, but he did not report the tillage system used. Lauer et al. (1999) reported that the maximum corn grain yield occurred when planted between 1 May and 7 May with no tillage in southern Wisconsin. Using minimum tillage, Ramsel (2001) reported similar planting dates as Wisconsin for maximum corn yield response for Central Iowa. Oplinger and Philbrook (1992) reported that a reduction in soybean grain yield occurred when planting date changed from mid-May to mid-June, and they found a reduction in soybean grain yield with no tillage compared with fall moldboard plus spring cultivation. Pedersen and Lauer (2003) reported that a soybean grain yield reduction occurred between the first week and the last week of May for fall chisel plow tillage and no tillage. The studies by Oplinger and Philbrook (1992) and Pedersen and Lauer (2003) were conducted in a silt loam soil in Wisconsin. Contrary to those two soybean studies, Turman et al. (1995) and Elmore (1990) observed no planting date effect on soybean yield for planting dates ranging from the middle of May until the first week of July for studies conducted on silt loam soils.

Low soil temperature is associated with early planting. These conditions slow the onset of corn seed germination and reduce the speed of shoot elongation before emergence (Miedema and Sinnaeve, 1980). Low temperature may cause different types of physiologic damage in corn, like chilling injury and chlorosis (Miedema, 1982), hence affecting plant population. Cold and wet conditions after planting may reduce plant stand due to soil fungal damage in corn and soybean (Nyvall, 1999). On the other hand, early-planted (between 24 April and 8 May) corn has a longer growing season, allowing it to achieve physiologic maturity before the first frost (Lauer et al., 1999). However, these authors suggested that the corn planting decision before 20 April should be based on soil temperature.

Different tillage systems have been reported to have different effects on soil properties that are associated with soil heating and drying processes. Thus, tillage systems can affect soil conditions that determine planting date and influence early-season plant performance. Eckert (1984) detected an interaction between tillage and planting date for corn yield response, but the interaction did not follow any particular pattern. Eckert concluded there was no advantage in delaying no-tillage planting date to allow for warmer soil temperature in a well drained soil. In contrast, Herbek et al. (1986) observed a trend for increased corn yield under no tillage when planting date changed from late April to mid-May in a poorly drained soil. In cool and wet springs, they suggested that delaying no-tillage corn planting 2 wk after conventional tillage would not cause corn grain yield losses. Contrary to the Herbek et al. (1986) observation, Dwyer et al. (2000) observed that planting corn under cool and wet conditions using no tillage or reduced tillage reduced corn yield. Elmore (1990) and Turman et al. (1995) did not detect an interaction between tillage type and planting date on soybean yield.

The objective of this study was to test the interaction between tillage system and planting date for corn and soybean. Three tillage systems (strip tillage, no tillage, and disk-chisel tillage) were evaluated over three planting dates, with each planting date coinciding with the optimum soil conditions (established by soil temperature and soil water content criteria) for each tillage system.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Two experiments, one involving corn and the other soybean, were established on soils of the Nodaway-Zook-Nevin association south of Newton, IA, in 2002, 2003, and 2004. Three soils were present on the experimental site: a Nodaway silt loam (fine silty, mixed, mesic Cumulic Hapludolls), a Nevin silty clay loam (fine silty, mixed, mesic, Aquic Hapludolls), and a Zook silty clay loam (fine, montmorillonitic, mesic, Cumulic Haplaquolls). The Nevin soil and the Zook soil are poorly drained, whereas the Nodaway soil is moderately well drained (USDA, 1979). Both experiments were blocked to accommodate a single soil type within a given block. Monthly rainfall and mean air temperature for the three growing seasons are shown in Table 1.


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  		Table 1. Monthly rainfall and mean air temperature during April through September of 2002, 2003, and 2004, in Newton, IA.

 
A split-plot design was used for each experiment, with whole plots arranged in four randomized complete blocks. Three tillage treatments (disk-chisel, strip tillage, and no tillage) were applied to whole plots. Three planting dates, with each date based on soil temperature and water content, comprised the split plots. The whole plots were 13.68 m wide and 30 m long and had 18 rows with 0.76-m spacing. Each split plot was 4.56 m wide and 30 m long and included six crop rows. In both experiments and for all 3 yr, strip tillage was conducted in November of the year preceding planting. Strip tillage was conducted with a Progressive 1300 strip-till implement (Progressive Farm Products, Hudson, IL) that had 0.76-m row spacing and was equipped with trash wipers in front of the individual units and 0.46-m trash disk sealers to hold the soil in the ridge. Strip tillage was applied at a soil depth of 0.25 m and created a managed strip area over each row that was approximately 0.46 m wide. A Landoll 1550 In Row Ripper (Landoll Corporation, Marysville, KS) pulled at the 0.25-m depth was used to establish the disk-chisel tillage treatment. It was equipped with a 0.5-m coulter in front of each shank that had a parabolic shape with a 0.06-m wide shank-fin point. The disk-chisel tillage plots received subsequent field cultivations (0.10-m depth) in November 2001 and again in April 2002. For the 2003 and 2004 studies, the disk-chisel tillage treatment was done during the preceding November each year and was followed the next spring with an April field cultivation.

Corn and soybean planting date treatments were determined using soil temperature and soil water content measurements at the 0.05-m depth in the managed strip area for strip tillage and in the general plot area for no-tillage and disk-chisel tillage systems. Soil temperature was monitored every hour beginning 4 Apr. 2002, 12 Apr. 2003, and 9 Apr. 2004, respectively, and continued until the planting date using a soil temperature sensor that was buried horizontal to the soil surface at the 0.05-m depth in each whole plot. Soil temperature was recorded with a WatchDog Data Logger Model 125 (Spectrum Technologies, Plainfield, IL). The lower plastic limit soil water content was selected as the soil water criterion for acceptable planting (Mueller et al., 2003) and was determined by feel methodology according to McBridge (2002). Tests for the lower plastic limit measurement began once a given tillage treatment met the temperature criterion. Soil temperature records and soil water content were checked daily at 1:00 PM. Planting for each tillage system was done on the first spring day within each tillage treatment that the soil temperature at 0.05 m was greater than our temperature criterion for 12 consecutive hours (10°C on corn plots and 13°C on soybean plots) and soil water content was less than or equal to the lower plastic limit. The temperatures were selected per germination temperature requirement for corn (Shaw, 1977) and soybean (Hatfield and Egli, 1974). The lower plastic limit soil water content was selected as the optimum soil moisture content for planting per Mueller et al. (2003).

Monitoring of soil temperature and soil water content continued in the tillage systems that had not yet met the criteria. Once one of the remaining tillage treatments met the criteria, another round of planting occurred across all tillage treatments. Only two planting dates (Table 2) were identified using this approach. It had been anticipated that conditions caused by each tillage system would result in three unique planting dates based on the selected criteria. However, strip tillage and disk-chisel tillage had common planting dates every year. To create our third split plot for planting dates, an additional arbitrary date was selected after the two dates identified for the three tillage systems used in this study.


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  		Table 2. Planting dates for corn and soybean based on soil temperature and soil water content criteria for this study collected from the 0.05-m depth.

 
Asgrow brand RX 30 RR/YG hybrid corn was planted with a Kinze 3500 planter equipped with a free-floating, 0.30-m diameter coulter wheel attachment with 12 curved tines in interlocked mountings and a rubber "V" closing wheel (Kinze Manufacturing, Williamsburg, IA). Seeds were planted at a planter setting of 76 860 seeds ha–1 and at a depth of 0.05 m. The number of emerged corn plants was assessed every other day from first emergence to the end of emergence. A date was determined to have occurred when two consecutive emergence assessments resulted in no change in plant populations. The count was made in three different locations that were randomly selected in the two central rows of each split plot with the first count, and the same locations for each split plot were then used for the subsequent counts. Counts were made on a 3-m row length. The percentage of plants emerged was calculated by dividing the number of plants emerged on a given day by the final number of plants that emerged and multiplying by 100. Estimates of number of days for 50% emergence were made by interpolation.

Corn fertilization was limited to nitrogen as urea ammonium nitrate solution applied to the surface of a small trench created with a coulter used to clear surface residue from the application zone. It was applied at 200 kg ha–1 on 31 May 2002 and at 180 kg ha–1 on 13 and 9 June in 2003 and 2004, respectively. Soil tests for P and K were in the high to very high range; therefore, these nutrients were not applied. No visible nutrient deficiency symptoms were observed. Weed control was maintained with the acid equivalent for glyphosate [N-(phosphonomethyl)glycine)] at 1.85 kg ha–1 on 10 May and 15 June 2002 and on 21 May and 17 June 2003. A mix of the acid equivalent for glyphosate at 0.84 kg ha–1, the active ingredient acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide] at 2.3 kg ha–1, and the active ingredient atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] at 1.15 kg ha–1 was applied on 5 May 2004.

Asgrow brand 703 RR soybean seeds, without seed treatment, were planted with the same planter used for corn at a planter setting of 501 600 seeds ha–1 at a depth of 0.025 m. Weed control was maintained with the acid equivalent for glyphosate at 2.14 kg ha–1 and 2.28 kg ha–1 on 17 June and 16 July 2002 and at 1.85 kg ha–1 on 21 May 2003 and 11 May 2004, respectively.

Crops were harvested after physiologic maturity was reached (Ritchie et al., 1997) and grain moisture content dropped sufficiently that mechanical shelling could be conducted without kernel damage. Corn yield was determined for the whole plot (six rows) with a John Deere (Moline, IL) combine equipped with an Ag Leader 2000 yield monitor in 2002. In 2003 and 2004, grain yield of corn was determined by hand harvesting 5.25 m of two central rows. Total ear weight was determined. Whole ear weights were converted to shelled corn weights using tables (Iowa State University, 1965) and converted to 155 g kg–1 moisture content. A sample of six ears was shelled to determine grain moisture with a grain moisture tester (DICKEY-john Corp., Auburn, IL). Soybean yield was determined on 20 m of two central rows with a Nursery Master Harvester (Salt Lake City, UT) equipped with a scale. Soybean yield was adjusted to 130 g kg–1 moisture.

Analysis of variance was performed across years using the GLM procedure of SAS (SAS Institute, 1999) for each experiment when errors of variances were homogeneous among years (Mead et al., 2003). Otherwise, ANOVA was performed by year. Tillage and planting dates were considered fixed effects. Orthogonal contrasts were used to make comparisons among treatments. When there was an interaction of year and treatment, treatment effect was assessed by year using the SLICE option of the LSMEANS procedure. Protected LSDs for variable responses for pooled data were calculated as described by Steel et al. (1997). The level of significance for all statistical tests was 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Several periods of low precipitation occurred during the 3-yr study (Table 1). The driest monthly period occurred during August 2003, when it rained 96 mm less than the 50-yr average. The 2004 growing season received the lowest precipitation of the 3 yr. Mean monthly air temperatures were cooler than the 50-yr mean (Table 1), except for June, July, and September 2002, August 2003, and September 2004. Implications of selected variances from normal are given in the Discussion section.

Corn
A tillage system x planting date interaction was detected for number of days to achieve 50% emergence in 2002 and 2003 (Table 3). In 2004, tillage system (P < 0.0003) and planting date (P < 0.0001) affected number of days to achieve 50% emergence. A year x tillage system x planting date interaction was detected for number of plants emerged (P < 0.0209). Additionally, a tillage system x planting date interaction was detected for number of plants emerged in 2002 and 2004 (P < 0.005 and P < 0.006, respectively).


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  		Table 3. Number of days to achieve 50% emergence of corn plants in response to three tillage systems and three planting dates in 2002 and 2003.

 
The number of days to achieve 50% emergence was greatest at the intermediate planting dates in 2002 and at the early planting date in 2003 (Table 3). Corn planted with no tillage needed 2.3 and 3.7 more days to reach 50% emergence than corn planted under disk-chisel tillage at early and late planting dates, respectively, in 2002. In 2003, corn planted with no tillage reached 50% emergence 2.8 d later than corn planted with disk-chisel tillage for the early planting date (Table 3). In 2004, the number of days to achieve 50% emergence averaged across planting dates was 8.4, 10.4, and 13.6 for disk-chisel tillage, strip tillage, and no tillage, respectively. Corn planted at the early, intermediate, and late planting dates needed 11.3, 15.5, and 5.6 d to achieve 50% emergence, respectively, when averaged across tillage systems.

Figures 1 and 2 show the number of plants emerged for each tillage system and planting date in 2002 and 2004, respectively. Corn planted under disk-chisel tillage had 14 000 fewer plants per hectare than was observed under no tillage at the intermediate planting date in 2002. Plant populations were at or above targeted populations on selected treatments (Fig. 1 and 2), likely due to the planter dropping more seeds than settings indicated and good soil conditions for germination and emergence.


Figure 1
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  		Fig. 1. Tillage system x planting date interaction for number of corn plants emerged in 2002. The LSD(0.05) value is 7.65 x 103. The early planting date was 11 April, the intermediate planting date was 16 April, and the late planting date was 6 May.

 

Figure 2
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  		Fig. 2. Tillage system x planting date interaction for number of corn plants emerged in 2004. The LSD(0.05) value is 7.65 x 103. The early planting date was 12 April, the intermediate planting date was 16 April, and the late planting date was 3 May.

 
Tillage system affected corn grain yield (Table 4). Averaged over the 3 yr, corn planted with disk-chisel tillage, strip tillage, and no tillage yielded 12.9, 12.3, and 11.9 Mg ha–1, respectively. Corn planted with disk-chisel tillage yielded 0.8 Mg ha–1 more than the mean of corn in strip tillage and no tillage for this period (P < 0.0096). Corn grain yield response to planting date varied among years. In 2002 and 2004, planting date did not affect corn grain yield (Table 5). In 2003, corn planted at the early planting date yielded 1.7 Mg ha–1 more than the average yield of the corn planted at the intermediate and late dates (P < 0.005).


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  		Table 4. Analysis of variance for corn and soybean yield across 3 yr for three tillage systems and three planting dates.

 

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  		Table 5. Corn grain yield in response to three planting dates within 3 yr (2002, 2003, 2004).

 
Soybean
Tillage system did not affect soybean grain yield (Table 4). Soybean grain yield was 2.9 Mg ha–1 for all tillage systems averaged across the three planting dates and the 3 yr. Planting date did affect soybean grain yield (Table 4). Soybean planted at the early, intermediate, and late planting date yielded 3.0, 2.9, and 2.8 Mg ha–1, respectively. Soybean planted at the early planting date yielded 0.16 Mg ha–1 more than the mean yield of the intermediate and late planting dates (P < 0.0025).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The different soil conditions established by the different tillage systems affected the planting date as determined by soil temperature (Fig. 3 exemplifies the tillage effect on soil temperatures) and soil water content criteria used in this study. These planting conditions, when coupled with weather (e.g., the cold and rainy period after the intermediate planting date in 2002) also affected the number of days to achieve 50% corn plant emergence. Our results disagree with Vetsch and Randall (2002), who did not find a tillage response for number of days to achieve 50% emergence of corn planted during May in a soybean–corn rotation. Although a three-way interaction effect was detected for plant population, the plant populations observed were in the optimum range to maximize grain yield (Nafziger, 1994) in 2003 and 2004, suggesting that little impact on final yield is likely. Farnham (2001b) and Widdicombe and Thelen (2002) found no corn grain yield response in the range of plant populations for which we detected the tillage system x planting date interaction (data not shown, year 2004). Corn planted with disk-chisel tillage at the intermediate planting date had a suboptimum plant population in 2002 (Fig. 1) but had the greatest grain yield (Table 5). The reduction in the numbers of plants emerged with disk-chisel tillage is attributed to soil crusting with this treatment (Unger, 1984).


Figure 3
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  		Fig. 3. Soil daily maximum temperature at 0.05-m depth for three tillage systems from 5 April (Day 95) until 30 May (Day 151), 2002. Arrows indicate corn planting dates.

 
In spite of the tillage system x planting date interaction detected for number of plants emerged, the number of plants emerged did not affect the yield response to tillage systems in this study. Modern corn hybrids are more stress tolerant than earlier hybrids (Duvick and Cassman, 1999), which may explain the relatively small yield differences observed between planting dates and tillage-induced soil conditions. Additionally, modern planter systems (Liu et al., 2004) are better designed to achieve adequate plant populations when soil conditions for planting are less than optimum, allowing farmers to plant in less-than-ideal soil conditions and still develop suitable plant populations. The combination of technological capability to plant in suboptimal soil conditions (especially related to soil water content) and stress tolerance of corn hybrids likely reduces yield differences that might have been observed with a combination of older planter design and older hybrids when planting was done under suboptimal conditions.

Managers must consider yield implications associated with excessively early or late corn planting, independent of soil conditions. Approximately 50% of Iowa corn is planted by the first week in May (National Agricultural Statistics Service, 2006). Early plantings are at greater risk of cold or freezing injury. Later plantings have a shorter segment of the growing season for photosynthesis and grain production and are also confronted with pollination that occurs later in the season, increasing the risk of heat and water stress and causing poorer seed set.

Soil conditions and their effects on planting date for different tillage systems do not seem critical when designing tillage studies using modern corn hybrids and soybean cultivars. However, tillage system and planting date selection clearly affected corn grain production in these Central Iowa soils. The significantly higher yield of the disk-chisel system compared with the average of the other two tillage systems points to the importance of soil disturbance on these soils in maximizing corn yield, although other advantages, such as improved soil and water conservation, likely exist with the conservation tillage systems.

The absence of a grain yield interaction of tillage system with planting date suggests that the previous research that compared the tillage effects on grain yield when all tillage systems were planted on the same day is likely valid. Although the dates of planting for all tillage systems were determined using the same criteria, other growth factors that affect later crop growth are critical to crop yield. For example, our study suggests that a lack of precipitation during August 2003 (Table 1) adversely affected soybean and intermediate- and late-planted corn yield. The early-planted corn likely had earlier silking, better pollination, and grain fill that minimized the dry August weather effects.

Our results on corn grain response to tillage system differed from those reported by Herbek et al. (1986). They reported that no-tillage corn yielded more than conventionally tilled corn in a dry year, but they did not find tillage system effects during a wet year. Their planting dates ranged from late April to early June, resulting in a wider range of growing conditions than we observed. Our ranking of corn grain yield according to tillage method agrees with that reported by Vetsch and Randall (2002) for a continuous corn system in southern Minnesota.

Disk-chisel and strip tillage reached optimum soil conditions to plant corn earlier than extension recommendations for optimum planting date (Farnham, 2001a) in 2 of the 3 yr studied. Optimum time for planting corn in Iowa falls between 20 April and 5 May. On the other hand, no tillage reached optimum soil conditions within the period recommended to plant corn in 2 of the 3 yr studied (Farnham, 2001a). Although early planting date did not affect corn grain yield, it is advisable to consider other factors when determining a planting date. Low air temperature early in the spring can cause chilling injury (Miedema, 1982) and consequently loss of plants. These considerations and the possible effect of damage due to soil fungi during a period of cold and wet soil in early spring should be considered when selecting the optimum planting date in tillage systems that leave the row zone without mulch.

Soybean grain yield did not respond to tillage system (Table 4). Our results agree with those of Elmore (1987, 1990) who reported no differences among tillage systems when soil moisture contents were at field capacity at planting. However, Elmore hypothesized that soil water contents different than field capacity at planting might cause a different soybean grain yield response to tillage. Our results do not support the hypothesis presented by Elmore (1987) because our planting dates were selected based on soil temperature and soil water content, and the tillage x planting date interaction was not detected. On the other hand, Vasilas et al. (1988) found that no-tillage systems yielded less than tillage systems that left ≤32% of the soil surface covered by residues.

Soybean had the greatest grain yield for the earliest planting date. Pedersen and Lauer (2003) found the same response. In 2 of the 3 yr for this study, the earliest planting date was the first week of May. Elmore (1990) reported the same trend for soybean grain yield with similar planting dates for no-tillage in a well drained soil. However, tandemp-disk tillage resulted in the highest soybean grain yield when planting was at the end of May.

Our results support the assumption that corn and soybean planted under different tillage systems respond to planting date similarly. However, corn yield is more sensitive to tillage system than soybean yield. To prove the absence of a tillage system x planting date interaction, the methodology applied in this research should be considered with other soils, locations, and possibly hybrids/varieties.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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