Abbreviations: CT, conventional-till • MT, minimum-till • NT, no-till • PAW, plant-available water • SW, spring wheat • TPAW, total plant-available water • WW, winter wheat • SF, sunflower
INTRODUCTION
IN the semi-arid northern Great Plains, plant-available water (PAW) and soil erosion are major factors limiting agricultural production. Therefore, farmers need to manage crop residues and tillage to control soil erosion and effectively store and use the limited precipitation received for crop production. No-till and minimum-till systems are effective steps in efficiently saving more precipitation for crop production (Aase and Schaefer, 1996; Halvorson, 1990b; Peterson et al., 1996; Tanaka and Anderson, 1997).
The traditional crop–fallow system of farming uses water/precipitation inefficiently as evidenced by the development of dryland saline-seeps in the northern Great Plains (Halvorson and Black, 1974). The solution to the saline-seep problem is to crop more intensively with efficient use of precipitation for crop production (Halvorson, 1990a). Saline-seep areas have been controlled and returned to crop production by growing alfalfa (Medicago sativa L.) and/or by annual cropping of the seep recharge area (Halvorson, 1984; Halvorson and Reule, 1980).
Deibert et al. (1986) suggested that farmers in the northern Great Plains need to use more continuous cropping and less crop–fallow to attain more efficient use of limited water supplies. Peterson et al. (1996) and McGee et al. (1997) point out that MT and NT fallow systems have a high percentage of the soil water in the profile recharged by the first spring following harvest. Continuing the fallow period for an additional 5 to 12 mo is very inefficient and costly. Therefore, cropping more intensively than crop–fallow is needed to efficiently use the water stored by NT and MT systems. Improved precipitation-storage efficiency with MT and NT allows producers the option of cropping more intensively than with crop–fallow (Halvorson and Reule, 1994; Peterson et al., 1996). Black et al. (1981) reported more efficient water use with more intensive cropping systems. Halvorson and Black (1985) reported crop yields that were generally >80% of 2-yr SW–fallow yields when grown in an annual cropping system with adequate N and P fertilization. Aase and Reitz (1989) and Aase and Schaefer (1996) reported that annually cropped SW with NT was more profitable and productive than SW–fallow in a 356 mm precipitation zone in northeast Montana.
Hall and Cholick (1989) reported varying responses of SW cultivars to tillage systems and a need to select cultivars for use under NT conditions. Most SW cultivars developed for use in the northern Great Plains have been developed using crop–fallow systems and low residue, CT environments.
Information is limited on the successes of more intensive dryland cropping systems in the northern Great Plains that include MT and NT management systems and a deep rooted crop, such as SF, in the rotation. In addition to more efficient water use, more intensive MT and NT cropping systems have the potential to be more profitable (Dhuyvetter et al., 1996) and reduce soil erosion potential (Merrill et al., 1999). This study was undertaken to determine the effects of tillage system (CT, MT, NT), N fertilizer rate (34, 67, and 101 kg N ha-1), and cultivar (Butte86 and Stoa) on SW grain yields within a dryland SW–WW–SF rotation.Methods and materials
The study was initiated in 1984 on a Temvik–Wilton silt loam soil (fine-silty, mixed, superactive, frigid Typic and Pachic Haplustolls) located near Mandan, ND. Surface soil pH was 6.4 , soil organic carbon was 21.4 g kg-1, and soil test P was 20 to 26 mg kg-1 in the spring of 1984 (Black and Tanaka, 1997). Data collection was from 1985 through 1996. An annual cropping rotation, SW–WW–SF, was managed under three tillage systems, CT, MT, and NT (Halvorson et al., 1999a, b). Nitrogen fertilizer was applied in early spring each year as a broadcast application of NH4NO3 at rates of 34, 67, and 101 kg N ha-1, except for 1991 and 1992 when no N was applied because of a build-up of residual soil NO3–N due to drought conditions and low yields from 1988 to 1990. Phosphorus fertilizer was applied broadcast at a rate of 40 kg P ha-1 at the beginning of the study in October 1983. Soil test P levels in the 0- to 15-cm depth averaged 16 mg kg-1 in 1991 and 11 mg kg-1 in 1996. Two SW cultivars with good yield potential, Butte86 and Stoa, were used throughout the study. Each main block of the study was 137.2 by 73.1 m in size. Tillage plots (45.7 x 73.1 m) were oriented in a north–south direction, N plots (137.2 x 24.4 m) in an east–west direction across all tillage plots, and cultivars (22.9 x 73.1 m) in a north–south direction within tillage plots and across all N plots. The smallest plot with the combination of all variables was 22.9 by 24.4 m. Triplicate sets of plots were established to allow all phases of the rotation to be present each year. Experimental design was a strip-strip-split plot, with tillage and N rate treatments stripped and cultivar as subplots with 3 replications.
The CT treatments were generally not tilled in the fall after SF harvest but were disked once to a depth of 8 to 12 cm in the spring prior to SW planting. Surface residue cover was usually <30%> not tilled in the fall after SF harvest but were undercut once in the spring with a sweep plow at a shallow depth (<7.5 cm) prior to SW planting. Surface residue cover was usually 30 to 60% after planting. No-till treatments were not tilled following SF harvest and received one application of glyphosate [N-(phosphonomethyl)glycine] herbicide just prior to SW planting in 1985, 1986, 1989, 1990, 1993, and 1994. Surface residue cover was generally >60% after planting. No preplant herbicides were applied in the other years. Residue cover estimates were based on visual observations using experience with photographic measurements made of residue cover in adjacent SW–fallow plots (Merrill et al., 1995). Spring-applied herbicides were used to control broadleaf and grassy weed species within the growing SW crop. Tillage treatments, agronomic operations, and herbicides applied to the SF and WW crops are described by Halvorson et al. (1999a, 1999b).
The SW was generally planted in early May at a seeding rate of about 3.2-million seeds ha-1 with a NT disk drill with 17.8-cm row spacing. The plots were harvested in mid- to late- August each year by hand cutting SW samples for grain yield determination from two 1.5-m2 areas within each plot (1985–1993). In 1994 through 1996, grain yields were determined from a 50-m2 area with a plot combine. Grain yields are expressed on a 120 g kg-1 water content basis.
Soil samples, one 3-cm diameter core per plot, were collected for gravimetric soil water and NO3–N analyses from one cultivar plot for each tillage and N fertilizer treatment each spring (April) before N fertilization. Samples were collected in 30-cm increments to a depth of 120 cm. Soil NO3–N was determined for each depth increment by autoanalyzer (Lachat Instruments, 1989; Technicon Industrial Systems, 1973) on a 5:1 extract/soil ratio using 2 M KCl extracting solution (1985–1992) and a 0.01 M CaSO4 extracting solution (1993–1996). Volumetric soil water content was estimated from gravimetric soil water measurements using a soil bulk density of 1.42 gm cm-3 for the profile (Black and Tanaka, 1997). Total plant available water was estimated as the sum of spring soil PAW in the 0- to 120-cm profile plus growing season precipitation (April through August). Spring soil PAW was estimated by subtracting the lowest measured soil water content (152 mm) in the 0- to 120-cm profile following SW harvest during the 12-yr study from soil water contents in the 0- to 120-cm soil profile each spring. Precipitation was measured from April through October each year with a recording rain-gauge at the site. November through March precipitation was estimated from the U.S. Weather Bureau measurements made at the Northern Great Plains Research Laboratory at Mandan, ND, which was located approximately 5 km northeast of the site.
Analysis of variance procedures were conducted using SAS statistical procedures (SAS Institute, 1991) with years treated as a fixed variable. All differences discussed are significant at the P = 0.05 probability level unless otherwise stated. A least significant difference (LSD) was calculated only when the analysis of variance F-test was significant at the P = 0.05 probability level.Results
Soil Nitrate Nitrogen
Spring soil NO3–N levels varied significantly with tillage system, N rate, and year with significant tillage x year, N rate x year, and tillage x N rate x year interactions. Spring soil NO3–N levels (0- to 120-cm depth) associated with the tillage x N rate x year interaction are reported in Table 1 . No differences were observed in spring soil NO3–N levels among tillage treatments when compared over N rates and years from 1985 through 1988. In 1989, spring soil NO3–N levels were significantly greater with MT than with NT and CT for the highest N rate. In 1990, spring soil NO3–N levels were greater with CT than NT at the highest N rate. In 1991, CT and MT had a higher level of soil NO3–N than NT at the low N rate. At the medium N rate, CT had a higher soil NO3–N level than MT and NT. At the high N rate, MT had a higher level of soil NO3–N than CT and NT. In 1992, CT had a higher level of soil NO3–N than MT and NT at the low and high N rates. At the medium N rate, soil NO3–N was greater with CT than MT. In 1993, there were no differences in soil NO3–N among tillage treatments at the low N rate, with CT having higher levels than NT at the medium N rate and CT and MT having higher levels than NT at the highest N rate. In 1994, MT had a higher level of soil NO3–N than NT at the lowest N rate, with CT and MT having higher levels than NT at the medium N rate and CT having higher levels than MT and NT at the highest N rate. In 1995 and 1996, no differences were observed among tillage treatments for each of the N rates. The data in Table 1 show that spring soil NO3–N in the soil profile had increased considerably following the drought years of 1988 through 1990, which experienced poor WW and SF yields and reduced N requirements (Halvorson et al., 1999a, b). The trend was for higher levels of soil NO3–N with increasing N rates from 1990 through 1994, with spring soil NO3–N levels in 1995 and 1996 approaching levels similar to those in 1985 at study initiation.
The cultivar x year interaction effects on grain yields are shown in Fig. 7 . Grain yields were greater for Butte86 than for Stoa in 1985, 1986, 1991, 1992, 1993, and 1995 (6 out of the 12 years of this study). Stoa grain yields equaled those of Butte86 during the other six years. In this study, Butte86 generally reached the heading and grain filling growth stages about 6 d earlier than Stoa, thereby escaping some of the effects of plant diseases and late summer drought stress. This may partially explain the greater yields with Butte86 over Stoa in some years. The overall impact of SW cultivar on grain yields in this study was small.
Summary
The results of this study show that SW grain yields following SF in rotation are generally enhanced using MT and NT systems compared with CT during most years with adequate N fertility. Grain yields tended to be greatest with NT compared with CT during those years with <400> treatments generally produced greater SW yields than NT, particularly at low N rates. Leaf spot disease pressure was greater during the wetter years and at the low N rate (Krupinsky et al., 1997, 1998). The results show that during extremely dry years (e.g., 1988 through 1990), reduced tillage treatments did not store enough additional water to significantly enhance yield potential over that of the CT system. Responses to N fertilization were insignificant during these dry years, which resulted in increased residual spring soil NO3–N levels. Responses to N fertilization were similar for both cultivars during the average and wetter years. Butte86 grain yields were greater than those of Stoa in 6 out of 12 years, three of which were years with >400 mm TPAW and two years with 300 to 400 mm TPAW. Grain yields were similar for both cultivars for all other years. The highest 12-yr average grain yield was obtained with the highest N rate and NT.
Spring wheat yield responses in this study are in agreement with annual cropping SW yields reported by Aase and Schaefer (1996) using NT, Halvorson and Black (1985), and Black et al. (1981) in northeastern Montana. In 6 out of the 12 years in this study, spring wheat yields exceeded the average 2-yr, SW–fallow yields (1737 kg ha-1) reported for five southcentral North Dakota counties near the study site for 1989 through 1996 (Beard and Hamlin, 1995; Beard and Waldhaus, 1997). In two years, yields were 77 and 87% of SW–fallow yields; in two years, yields were 54% of SW–fallow yields; and in two years, yields were about 34% of SW–fallow yields.
These results indicate that farmers in the northern Great Plains can successfully produce SW following SF in annual cropping rotations that do not include a fallow period. During production periods with low soil water recharge following sunflower in rotation, farmers may want to consider producing a crop with lower water-use requirements than spring wheat to avoid uneconomical spring wheat yields. Fallow following sunflower should be considered as the last alternative because of a high soil erosion potential.SAS Institute Inc 1991
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