Effects of tillage on growth, yield and root lodging of six maize hybrids in upland fields converted from paddy fields in Andosol

ABSTRACT We investigated the effects of tillage on growth, grain yield and root lodging of maize hybrids in upland fields converted from paddy fields in Andosol of northern Tohoku. Six hybrids were grown with rotary tilling or plowing (chisel plowing plus power harrow) in 2016 and 2017. Soil penetration resistance was higher in plowing than in rotary tilling under a soil depth of 5 cm. There were no significant interactions between hybrid and tillage method in plant height, SPAD value, grain yield, and yield components, indicating that growth and grain yield of the six hybrids were unaffected by tillage method. Maize hybrids had similar culm length and ear height, while horizontal pulling resistance was higher in plowing than in rotary tilling. In addition, root lodged plants caused by typhoons were fewer in plowing than in rotary tilling in 2017. There was a negative correlation between horizontal pulling resistance and the number of root lodged plants. The highest horizontal pulling resistance and the least number of root lodged plants were found in a maize hybrid KD641 grown by plowing. The results suggest that root lodging can be alleviated by growing root-lodging-resistant maize hybrids such as KD641 with plowing. Graphical abstract


Introduction
Farmland consolidation is progressing, and paddy fields larger than 1 ha account for 9.3% of the total paddy area in Japan (Ministry of Agriculture, Forestry and Fisheries [MAFF], 2016), while the number of core farmers is decreasing (MAFF, 2018). Therefore, core farmers need to manage large-scale farms with small numbers of workers. Rotary tilling, a conventional tillage method in paddy fields after the 1960s (Naka, 1981), operates at speeds of 2-3 km h −1 . Higher work efficiency is thus required for large-scale farm management. A tillage system of chisel plowing plus power harrowing offers a higher speed tillage method (Otani, Sekiya, Kanmuri, Nakayama & Saito, 2013), and the system is expected to be introduced in upland fields converted from paddy fields Otani, 2015;Shinoto, Matsunami, Otani & Maruyama, 2019). Carter and Barnett (1987) and Wall and Stobbe (1983) reported on the different performance of maize hybrids grown with conventional and no-tillage systems. Carter and Barnett (1987) reported that some hybrids differed in yield according to tillage method; yield of some hybrids ranked lower under conventional tillage among hybrids, whereas ranked middle in no tillage among hybrids. Herrera, Verhulst, Trethowan, Stamp and Govaerts (2013) noted that there is a need to evaluate genotypes developed under no tillage and extend the research on genotype performance under no tillage. Therefore, grain yield might be different according to tillage methods. On the other hand, other investigations failed to detect a difference in maize hybrid performance among several tillage methods (Duiker, Haldeman & Johnson, 2006;Hallauer & Colvin, 1985;Imholte & Carter, 1987;Karlen & Sojka, 1985;Kaspar et al., 1987;Mock & Erbach, 1977). This discrepancy suggests that the maize hybrid response to tillage method is affected by environmental factors such as weather conditions and soil types. There have been few reports, however, investigating the maize hybrid response to tillage systems in upland fields converted from paddy fields in northern Tohoku. In addition, maize hybrids are provided by private companies in Japan, and so the replacement of maize hybrids is faster when compared with other major crops such as rice and wheat. It is thus important to investigate the interaction between hybrid and tillage methods in order to extend the plowing method in upland fields converted from paddy fields.
The objective of this study was to evaluate plant growth, grain yield and root lodging of maize hybrids grown with rotary tilling and plowing in upland fields converted from paddy fields in Andosol of northern Tohoku. Six cultivars were selected from major seed companies that are widely used in the northern Tohoku region.
The experiment design was a strip plot design with three blocks. Each block was divided into six subplots. The two tillage systems were used on the main plots, and the hybrids comprised the subplots. The area of each subplot was 24 m 2 (3 m × 8 m) in 2016 and 48 m 2 (6 m × 8 m) in 2017. Conventional tillage was rotary tilling (RT) to a depth of 20 cm using a 2.6 m-wide rotary tiller (LXR2610, Matsuyama Plow MFG. Co., Ltd, Nagano, Japan) with 75-80 horsepower tractors. Plowing tillage (PT) was conducted to a depth of 20 cm using a 1.9 m-wide chisel plow (MSC6PSQLK, Sugano Farm Machinery MFG., Co., Ltd., Ibaraki, Japan) with a 75-85 horsepower tractor and harrowing was to a depth of 5 cm using a 2.0 m-wide power harrow (BETA230P, Sugano Farm Machinery MFG., Co., Ltd, Ibaraki, Japan) with 75-80 horsepower tractors. Experiment fields were different in 2016 and 2017, because of crop rotation.
Two to three seeds per hill were planted on 2 June 2016, and on 6 June 2017, with 75 cm row spacing and 18 cm hill spacing with a seeder (TDR, Agritecno YAZAKI Co., Ltd., Hyogo, Japan). Plants were thinned to one plant per hill around V2 stage. Basal dressing of N, P 2 O 5 , and K 2 O at the rates of 15, 20, and 15 g m −2 (150, 200, and 150 kg ha −1 ), respectively, was applied as a blended fertilizer (N: P 2 O 5 : K 2 O = 15:20:15). For weed management, herbicides were applied at just after sowing (Dimethenamid-Linuron) and at the V6 stage (Topramezone). Pesticide was not applied.
Plant height and leaf color were measured for five continuous plants in each replication in each field at about 10-day intervals. Leaf color was measured at the middle part of the topmost fully expanded leaves with a chlorophyll meter (SPAD-502 Plus, Konica Minolta Inc., Tokyo, Japan), and at top ear leaves at 74 days after sowing (DAS) at the silking stage.
At harvest, only ears were obtained from a 3-m 2 (2 rows × 2 m) area in each replication. After measuring ear length, the ears were shelled. After shelling, the kernels were equalized to 300-400 g, and then counted. After counting kernel number, 100 grain weight was calculated and grain yield were determined. Grain moisture was measured with a moisture meter (PM650, Kett Electric Laboratory, Tokyo, Japan). Grain yield and 100 grain weight were calculated as 15% moisture content.
Traits of root lodging were measured with 10 continuous plants in the same row in each replication, except for plants that could not be measured for culm length. An iron tool was attached to bottom of plants. A digital force gauge (DS2-50N, Imada Co., Ltd., Aichi, Japan) was attached to the iron tool, and then the part of iron tool at 1 m-height from the soil surface was pulled until it was 30°from vertical (Koinuma, Ikegaya & Ito, 1998). The value of the digital force gauge was regarded as horizontal pulling resistance (HPR). HPR measurements were conducted 25 days after silking in 2016 and 25-32 days after silking in 2017. Culm length and ear height were also measured after measurements of HPR in 2016 or 12 days before the measurement of HPR in 2017. Horizontal pulling resistance-value (HPR-value) was calculated using the following formula (Koinuma et al., 1998).
The lower HPR-value, the higher root lodging resistance. Ishige, Yamada and Shiga (1983) evaluated resistance to lodging of maize by using discriminant function, in which three parameters were adopted; pulling resistance, plant weight and center of gravity. Tani (1963) showed that the moment of above-ground weight was calculated by the product of plant weight and the center of gravity. Koinuma et al. (1998) defined the moment of above-ground weight as the product of culm length and ear height, which correlates with above-ground weight and center of gravity, respectively. They defined HPRvalue as the square root of product of culm length and ear height divided by horizontal pulling resistance.
In 2017, root lodging and stalk lodging occurred due to typhoons on September 18 and on October 23. Root lodging and stalk lodging were determined in 10 plants, which were used for the measurement of HPR at 160 DAS. Plants inclining more than 30°toward ground level were regarded as root lodging, and plants bending sharply under a node, including just above the node bearing the uppermost ear, were regarded as stalk lodging (Yoshimura, 2004). Plants regarded as both root lodging and stalk lodging were categorized as stalk lodging (Yoshimura, 2004). Plants that were damaged while measuring HPR were not included in root lodging and stalk lodging results.

Weather data
Meteorological data was obtained from the weather station at the Tohoku Agricultural Research Center and from the Morioka Meteorological Observatory of the Japan Meteorological Agency. The normal values for daily mean temperature and precipitation were averaged from 1981 to 2010. The normal values for the duration of sunshine were averaged from 1997 to 2010.

Statistical analysis
Analysis of variance was computed for data from each year. Statistical analysis was carried out using analysis software (JMP 11.2.0, SAS Institute Inc., Cary, NC, USA).  Figure 1 shows the soil penetration resistance. Soil penetration resistance in RT was 0.1-0.3 MPa at a soil depth of 1 to 20 cm. Soil penetration resistance in PT was slightly higher than that in RT at the surface soil, but it rapidly increased to more than 0.4 MPa at a soil depth of 5 to 20 cm.  Table 2 shows the effects of hybrid and tillage on silking day, grain yield and yield components. Silking in PT was 1 day earlier than that in RT in 2016. The interactions between hybrid and tillage for grain yield and yield components were not significant in 2016. Tillage and the interactions between hybrid and tillage for all traits were not significant in 2017. Table 3 shows root lodging traits. There were significant differences among hybrids for all the traits in 2 years. SH3786 had the lowest, and KD641 had the highest HPR among hybrids. HPR was higher in PT than in RT (p < 0.1). There were significant differences in ear height between tillage treatments in 2016. The interaction between hybrid and tillage for HPR was significant in 2016; however, no significant interaction was found in any traits in 2017. HPR-values did not have normal distribution according to Shapiro-Wilk test (2016: p = 0.0004, 2017: p = 0.0087), therefore, statistical analysis was not carried out. In 2017, the number of root lodged plants was significantly different among hybrids; SH3786 had the most root lodged plants, and KD641 had the least root lodged plants. Tillage had a significant effect on the number of root lodged plants; the number of root lodged plants was smaller in PT than in RT. The interaction between hybrid and tillage was significant in root lodging; among hybrids, the number of root lodged plants was reduced more for P2088 and KD641 and less for SH3786 in PT than in RT. In contrast, the number of stalk lodged plants was larger in PT than in RT. Figure 3 shows relationships between the number of root lodged plants and HPR or HPR-value in 2017. There was a significant negative relationship between HPR and the number of root lodged plants. A significant positive relationship was also observed between HPR-value and

Discussion
Maize growth was largely unaffected by tillage methods in the present investigation. Although there were significant differences in the early stages of growth between RT and PT, the differences were not significant in other stages (Figure 2). The results agreed with the previous report . It is possible that tillage method affects soil temperatures, as soil temperature in no-tillage is lower than that in conventional tillage due to covering the soil surface with crop residue (Carter & Barnett, 1987;Wall & Stobbe, 1983). A previous report (Shinoto, Maruyama, Matsunami & Otani, 2018a) indicates that there is no significant difference in soil temperature between RT and PT at a soil depth from 15 to 30 cm, as both methods remove crop residue from the soil surface.
Silking day was earlier in PT than in RT (Table 2) due to the accelerated early growth by PT, which was similar to the previous report . The tassel and final ear primordia are formed after the final leaf primordium is formed (Nielson, 2002), which is after V6 stage. The interactions between hybrid and tillage were not significant for SPAD values from V6 stage (36 DAS) Notes: Bars represent standard error (n = 3). ** and *, significant difference at p < 0.01 and p < 0.05, respectively. ns: not significant (p < 0.05).Rotary: Rotary tilling.
to the silking stage (75 DAS) (Figure 2). Therefore, responses of tassel and ear formations to tillage treatments were similar among hybrids. The interactions between hybrid and tillage were not significant for plant height until the silking stage ( Figure 2). This result suggested that vegetative growth among hybrids did not differ between tillage treatments. Final kernel number and grain weight are associated with crop growth around bracketing silking (Cirilo & Andrade, 1994;Jones, Schreiber & Roessler, 1996). There were no significant differences in the interaction between hybrid and tillage in plant height and SPAD value at silking (Figure 2). Therefore, the potentiality of kernel number and grain weight of RT might be similar to that of PT. Furthermore, grain yield was not significantly affected by hybrid or tillage (Table 2), indicating that hybrid performance was similar in RT and PT in upland fields converted from paddy fields in northern Tohoku. These results will enable farmers to apply the plowing method to grow new maize hybrids in these fields. Nevertheless, further experiments with hybrids and tillage methods are needed, when PT is applied to the other soil types.
PT improved root lodging resistance of maize as compared with RT. Although maize hybrids had similar culm length and ear height in both the tillage treatments, HPR was higher (P < 0.1) and HPR-value tended to be lower in PT than in RT in 2016 and 2017 (Table 3). In 2017, two typhoons caused root lodging in maize hybrids in the present investigation. The number of root lodged plants was lower in PT than in RT. Harada et al. (2009) suggest that root anchorage in no-tillage plants is stronger than in RT, because roots elongate in the hard soil surface. Soil penetration resistance at 5 to 20 cm was higher in PT than in RT (Figure 1). The root system is shallower in PT than in RT (Shinoto et al., 2018a). Therefore, it is likely that root lodging was alleviated in PT due to the root system distributed in hard soil. Root lodging tended to be more severe in RT than in PT due to heavy rain and strong winds, because root anchorage was inferior in RT than in PT. Stalk lodging was more delete in PT than in RT, after root lodging was alleviated by the strong root anchorage in PT. The previous report (Shinoto et al., 2019) indicates that root lodging is less in PT than in RT in upland fields converted from paddy fields both in Andosol and Gleysol.
Among hybrids, KD641 had the highest HPR in 2016 and 2017 and the lowest number of root lodged plants  Notes: **Significant at the 0.01 probability level. *Significant at the 0.05 probability level. ✝Significant at the 0.1 probability level. ns, nonsignificant at the 0.1 probability level. a Relative maturity, b horizontal pulling resistance. HPR-value indicated data ± standard error. Rotary: Rotary tilling.

The number of root lodged plants
The number of root lodged plants and lower HPR-value had lower numbers of root lodged plants (Figure 3). These results indicate that maize hybrids having high root lodging resistance grown by the plowing method are more resistant to heavy rain and strong winds, such as in typhoons, in upland fields converted from paddy fields. The interactions between hybrid and tillage in plant growth and grain yield were not significant in the present investigation. Maize hybrids grown by plowing showed similar characteristics as those grown by rotary tilling in upland fields converted from paddy fields in northern Tohoku. Nevertheless, the present investigation confirmed that root lodging resistance of maize was higher in plowing than in rotary tilling. In addition, there were differences in root lodging resistance among maize hybrids. The results suggest that growing maize hybrids with high root lodging resistance by the plowing method alleviates root lodging under heavy rain and strong winds.