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Southwest Research-Extension Center, 4500 E. Mary, Garden City, KS 67846
* Corresponding author (cnorwood{at}gcnet.com)
Received for publication January 29, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: D1 and D2, planting dates in mid-April and early May • H1, H2, H3, H4, and H5, corn hybrids having relative maturities of 75, 92, 98, 106, and 110 days, respectively • P1, P2, and P3, 30000, 45000, and 60000 plants ha-1 • WUE, water use efficiency
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Corn must be properly managed to tolerate the low precipitation and high temperatures that characterize the central Great Plains. Management practices include the selection of appropriate hybrids, planting dates, and plant populations. Most research on dryland corn in the USA has been done in the Corn Belt and the southeast. Planting dates in these areas typically have been late April or early May (Lauer et al., 1999; Benson, 1990; Swanson and Wilhelm, 1996; Herbek et al., 1986; Imholte and Carter, 1987). In the northern Corn Belt, planting full-season hybrids early is recommended because the entire growing season can be used (Lauer et al., 1999).
In the western Great Plains, however, low precipitation and high temperatures are the factors most limiting yields. The perception in this part of the country is that dryland corn should be planted early so that it can be pollinated before high midsummer temperatures and drought stress occur. Little research has been done to confirm or dispute this hypothesis. In central and northeastern Kansas, Staggenborg et al. (1999) found planting corn in early April and early May produced similar grain yields, but reduced grain yields occurred with an April planting at one location–year because of low rainfall amounts in June. In southwestern Kansas, Norwood and Currie (1996) compared early-, mid-, and late-May planting dates, and concluded that corn should usually be planted in early- to mid-May. However, they did not study earlier plantings.
Soil temperatures are typically lower at earlier planting dates. Colder soils reduce root and shoot weights (Kasper et al., 1987), require more days for the growing point to reach the soil surface (Swan et al., 1987), and reduce leaf area (Bollero et al., 1996). Most dryland corn in the western Great Plains is planted no-till, meaning that the soils are probably colder than tilled soils, particularly in April. Imholte and Carter (1987) found cold soils under no-till conditions were associated with reduced corn emergence, delayed silking, and increased harvest moisture.
Corn yields also depend on hybrid maturity and plant populations. In the central Corn Belt, populations from 54000 to 69000 plants ha-1 are considered optimum, but somewhat higher populations are needed in the northern Corn Belt to maximize yields (Benson, 1990). In southwestern Kansas, Norwood and Currie (1996) found that the population of a 105-d hybrid should not exceed 45000 plants ha-1. In the driest year of their study; however, a population of 28000 plants ha-1 produced the highest yield. Staggenborg et al. (1999) found that a full-season hybrid generally produced more grain than a short-season hybrid when planted early, if growing season length was not yield-limiting. A short-season hybrid yielded as much or more than a full-season hybrid at later planting dates. Major et al. (1991) stated that shorter-season hybrids may be necessary in areas of low or variable rainfall in southern Alberta. Larsen and Clegg (1999) found a full-season hybrid to produce maximum yield at 85000 plants ha-1, if there were no stress, in central and eastern Nebraska. They recommend that populations be reduced to 45000 to 65000 plants ha-1 under unfavorable environments. Allesi and Power (1974) recommended optimum populations of 30000 to 40000 plants ha-1 in North Dakota.
In the more arid areas of the Great Plains, a better understanding of the effects of planting date, hybrid, and plant population on yield and water use of dryland corn is needed. The data reported in this article is the second portion of a study conducted from 1996 through 1999. A discussion of yield and yield components as affected by hybrid, planting date, and plant population was previously reported (Norwood, 2001). As stated in that article, later planting resulted in greater yield in each year. The reasons for the yield increases were probably associated with cold soil temperatures at the first planting date and less favorable climatic conditions for that date. Climatic conditions during the study (above average, well-distributed rainfall) favored the higher yield potential of the later-maturing hybrids. Due to the favorable climatic conditions the higher populations usually produced more grain. The objective of the portion of the study reported here is to determine the effects of planting date, hybrid maturity, and plant population on soil water depletion, water use, WUE, and yield of dryland corn.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Corn was planted in a wheat–corn–fallow rotation, which consists of two crops in 3 yr with a 10- to 11-mo fallow period preceding both crops. Corn was planted with a Buffalo1 slot planter into the stubble remaining from the preceding wheat crop in mid-April and early May of each year. Experimental treatments consisted of two planting dates (16 April and 8 May 1996, 17 April and 6 May 1997, 15 April and 12 May 1998, and 21 April and 6 May 1999), five corn hybrids differing in relative maturities, and three plant populations (30000, 45000, and 60000 plants ha-1) arranged in a randomized complete block design with a split-split plot arrangement and four replications. Planting date was the main plot, hybrid was the subplot, and plant population was the sub-subplot. Each sub-subplot was 3 m wide (4–75 cm rows) by 9 m long. Only three of the five hybrids were planted in 1996. Hybrids were Pioneer Brand 3984, 3860, 3737, 3514, and 3394, with relative maturities of 75, 92, 98, 106, and 110 d, respectively. The two earliest hybrids were selected to represent hybrids with earlier relative maturities. The three latest hybrids were selected based on their adaptation to the area. The 98- and 106-d hybrids have been successfully grown under dryland conditions in the area. The 110-d hybrid was probably more suited for limited irrigation, but was included because it represents the upper end of maturity for dryland. Because little research on different dryland corn hybrids has been conducted in southwest Kansas, selection was based on recommendations from Pioneer Hi-Bred International. The hybrids were planted at a rate of about 87 000 seeds ha-1 and hand-thinned after emergence to the appropriate three populations. Seed destruction by rodents in 1996 resulted in a plant population of 55 000 rather than 60 000 for P3 in 1996. The corn was no-till in 1996, 1997, and 1998 but tillage was used prior to planting in 1999 to destroy the rodent habitat.
Herbicides used in each year were atrazine [6-chloro-N-ethyl-N-(1-methlyethyl)-1,3,5-triazine-2,4-diamine] applied at a rate of 2.2 kg ha-1 in July after wheat harvest, followed by 1.1 kg ha-1 atrazine + 1.5 kg ha-1 dimethenamid (2-chloro-N-[(1-methyl-2-methoxy)ethyl]-N-(2,4-dimethyl-thien-3-yl)-acetamide) in early May for in-crop weed control. The plots received 90 kg ha-1 N before planting in each year. The soil P level averaged about 45 kg ha-1, about the middle of the medium soil test range (Whitney and Murphy, 1969). Nonetheless, 100 kg ha-1 P2O5 was applied at the beginning of the study to eliminate any potential deficiencies.
Gravimetric soil water contents at planting and harvest were taken in each year. One sample from each sub-subplot was taken at 0.3-m increments to a depth of 1.8 m and the water 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). Water use efficiency was calculated as (soil water at planting - soil water at harvest + rainfall). The plot area was flat (<0.5% slope), so runoff was not measured. Deep percolation was not measured either, because it is usually not considered important under the dryland conditions of western Kansas. Ears from the center two rows were counted and hand harvested; yield was corrected to 155 g kg-1 moisture. The corn was harvested as it matured, usually at a moisture content of 150 to 200 g kg-1. A few dropped ears and lodged plants occurred. Although these numbers were not recorded separately, all ears were harvested.
Statistical analysis was done using PROC ANOVA (SAS Inst., 1998) with mean separation by Fisher's protected LSD.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Interpretation of Data
Data for soil water, yield, water use, and WUE are included in Tables 2 through 8. There were no significant three-way interactions; thus, individual planting date x hybrid x population combinations are not presented. The results varied from year to year and interactions or averages are presented in accordance with the statistically significant results that occurred in each year. Data are presented for significant two-way interactions, but if no interactions occurred, only averages are presented.
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Soil Water Depletion
The amounts of water remaining at harvest in the 0.3-m increments of the 1.8-m profile for the various treatment combinations are shown in Fig. 1, and the total amounts remaining in the entire profiles are shown in Table 3. Although there were sometimes date x hybrid interactions in the whole profile, the interactions were cumulative and were rarely statistically significant at the individual depths. Thus, for simplicity, the data for individual hybrids in the upper portion of Fig. 1 are averaged across dates and populations, and any interactions for the whole profile are presented in Table 3. There were depth interactions between date and population, so the profiles in the lower portion of Fig. 1 illustrate the date x population interactions averaged across hybrids. Less water was removed in 1996 (Fig. 1, Table 3) than in the other years because frequent rainfall replenished the profile during the growing season. There were date x hybrid interactions in 1998 and 1999. The interactions occurred because the hybrids differed in the amounts of water removed between D1 and D2. There were no statistical differences in the amounts of water removed by H1 and H5 for the two planting dates in 1998, but H2, H3, and H4 removed more water for the second vs. the first planting date (Table 3). In 1999 H1 and H3 removed more water following D2. Although the reasons for these interactions are unknown, they are probably incidental. However, the amount of water removed by any hybrid for D1 was never more than was removed for D2.
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Higher populations usually removed more water. There were date x population interactions in 1997 and 1998 (Table 3). Population had no effect on soil water depletion following D1 in 1997, but following D2, P3 removed more water than P1. In 1998 P2 and P3 removed more water following D1 than did P1, and there were significant differences in removal by all three populations following D2. In 1996, P3 removed more water than did P1 and P2. In 1999 all three populations differed. In both 1997 and 1998, the increased depletion with population was greater at 1.8 m following D2 than following D1 (lower portion of Fig. 1). In 1997, there was little difference in depletion by the populations at 1.8 m following D1 and no difference between P2 and P3 following D1 in 1998. In contrast, there were greater differences in depletion by P3 at the lower depths following D1 than D2 in 1996, although there were no statistical differences in depletion between dates. Population effects are discussed further in the last section of this article.
Yield, Water Use, and Water Use Efficiency
Yield usually increased with hybrid maturity and plant population in 1996 (Table 4), except that H4 yielded less than did H3 when planted on D2. Unlike H3 and H5, H4 did not yield significantly more for D2. The date x population interaction was present because P2 resulted in a greater yield increase following D2 (2.12 Mg ha-1, 31%) than did the other populations. Corn planted on D1 used an average of 40 mm more water than corn planted on D2, and P3 used 24 mm more water than did P1 or P2. However, hybrid water uses did not differ significantly. Since yield was greater from D2 and water use was greater from D1, WUEs were correspondingly higher following D2. Water use efficiency also increased significantly with population.
Water and heat stress in 1997 resulted in corn yielding substantially less following D1 than D2. Yields from D2 in 1997 (Table 5) ranged from 2.44 Mg ha-1 (82%) more for H3 to 4.23 Mg ha-1 (160%) more for H5 than yields from D1. Averaged across all hybrids, D2 yield was 97% more than D1 yield. However, yields of H1 did not differ significantly between planting dates. Yields of hybrids planted on D1 did not differ significantly. This was the only year that H1 did not yield less than all other hybrids. The year 1997 (both planting dates) was also the only year in which H2 did not yield less than the later-maturing hybrids. Thus, in 1997, maximum yield was obtained with the 92-d hybrid. The date x population interaction was due to D1 yields decreasing as population increased from P1 to P2, but D2 yields increasing as population increased from P1 to P2. There was no difference in the yield of P2 and P3 from either date.
As in 1996, water use exhibited no interactions. Water use was 6% higher following D2 than D1, and 1997 was the only year that corn planted on D2 had higher water use. Water use increased with hybrid maturity up to H4, but there was no difference in water use due to population. Water use efficiency of all hybrids was higher following D2 than following D1. Because of the low yield following D1, water use efficiencies were the lowest of the 4-yr period. The average WUE of D2 was 88% greater than that of D1.
Significant yield increases with maturity occurred for all hybrids in 1998 (Table 6), but there was no difference in WUE between H3 and H4. Date x population interactions occurred for yield, water use, and WUE because P2 and P3 differed following D2, but not D1. Although hybrid averages are presented for yield and WUE, the date x population interactions indicate higher yields and WUEs following D2. Yield and WUE differences between dates widened as population increased. The H5 yield was 178% higher than that of H1, and H5 WUE was 113% higher. Water use was greater following D1, as in 1996. Water use from both dates increased with hybrid maturity up to H4.
Yields were generally lower in 1999 (Table 7) than in 1998, and D2 yields were similar to D2 yields in 1997. Average D2 yields were 5.55 Mg ha-1 in 1997, and 5.71 Mg ha-1 in 1999. However, yields from D1 were not reduced as much as in 1997 because of more rain in July (Table 1). All hybrids except H1 yielded more following D2. Significant yield increases with maturity occurred for all hybrids from D1, but there were no differences between H3 and H4, nor H4 and H5 following D2. The hybrid x population interaction indicates that yields of H1 through H4 increased with population, but the P2 and P3 yields of H5 were similar. As with yield, WUE usually increased with population. Planting date had no significant effect on the water use of H1 through H4, but the water use of H5 was greater following D1. Hybrid water use tended to increase with maturity and population.
On average, all hybrids except H1 yielded significantly more when planted on D2 (1997 through 1999, Table 8). Average yields from D2 were 42, 28, 26, and 32% higher than from D1 for H2 through H5, respectively. Including 1996, there were only 3 out of 36 date x hybrid combinations when the D2 yield was not greater than D1. The yield for D1 never exceeded that of D2. The amount of water used increased with hybrid maturity, but there were no statistical differences in water use between H2 and H3 or between H4 and H5. The low population averaged the least water use, but there was no difference in the water use of P2 and P3. Water use efficiency of all hybrids was higher following D2. Average WUE following D2 were 43, 45, 29, 30, and 37% higher than from D1 for H1 through H5, respectively. Average yield and WUE tended to increase with population, but not all differences were statistically significant.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Yield and WUE of dryland corn was higher than was expected when this study was begun. It is important that dryland crops in the more arid regions of the Great Plains use all of the water in the soil profile, and make efficient use of that water. It was thought at the beginning of the study that all hybrids would deplete the profile, and that the later maturing hybrids in the study might use all the water before maturity, decreasing yield. However, an increase in yield with maturity generally occurred, because the amount of profile water removed increased with hybrid maturity, with earlier hybrids not extracting all of the available soil water. Differences were greatest between the 75-d hybrid and the other hybrids. Higher populations were not detrimental and, in fact, usually resulted in higher yield, except following D1 in 1997, when the most stress occurred. Except for D1 in 1997 there was no evidence that later-maturing hybrids, including the 110-d hybrid, should be planted at lower populations than earlier hybrids.
Higher populations resulted in more water depletion than lower populations, usually resulting in increased yield and WUE. In 1997 and 1998, the amount of water removed from the lower portion of the profile increased more with increased population, particularly from P2 to P3, following D2 than D1 (lower portion of Fig. 1). A suggestion is that root growth at the higher populations for D1 was less than D2, resulting in less water uptake and yield. Richner et al. (1996) found that relatively small temperature reductions of 2 to 3°C when the soil temperature was around 15°C, reduced root development in corn seedlings sensitive to low temperatures. Soil temperatures were not measured in the plot area, but average soil temperatures at our weather station during the last half of April were 16.5, 14.7, 13.1, and 11.7°C for 1996 through 1999, respectively, vs. 20.3, 18.1, 17.1, and 14.7°C in the first half of May (data not shown). Thus, soil temperatures during the last half of April in 1997, 1998, and 1999 were less than 15°C, while temperatures in the first half of May exceeded 15°C in 1996, 1997, and 1998. Temperatures following both dates were lower in 1999 than in the other years and there was not as much difference between dates in soil water depletion from the lower portion of the profile. In contrast, 1996 soil temperatures for both planting dates were the highest of the 4-yr period, resulting in more favorable conditions for germination and early growth of both plantings.
The yield differences between planting dates may also be related to leaf area development. Bollero et al. (1996) found grain yield to decrease linearly with decreasing soil temperature. They believe that increased grain yield with increasing soil temperature was due to larger leaf surfaces in the upper portion of the canopy. In this study, the plants were shorter following D1 than D2 and probably had less leaf area, although it was not measured. Thus, increased yield following D2 could be due to both root growth and increased leaf area. Differences in yield between dates were large but differences in water depletion were relatively small, and reasons for yield differences from different planting dates need further research.
In summary, earlier planting decreased yields in 33 of 36 date x hybrid combinations. The highest yields and WUEs were achieved with the second planting date, combined with later-maturing hybrids and higher plant populations.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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1 Mention of a trade name does not imply endorsement by Kansas State University over comparable products. 
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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