Evaluation of the effects of different tillage systems , plant patterns and plant densities on grain yield and yield components of corn ( Zea mays L . cv . sc 704 ) in North of Iran

In order to investigate the effect of tillage system and plant density in two plant patterns on kernel yield and its component on corn (Zea mays L.cv.sc704), an experimental design, randomized complete block in a strip factorial was used, treatments arrangement were within four replications in north of Iran in 2011. The main plot was subjected to tillage system in three levels: Rotary System (RS); Disk System (DS) and Plow and Disc System (PDS). Other factors were plant density in three levels (60000, 70000 and 80000 plant/ha) and planting pattern were in conventional row (linear) and new two-rows (zigzag). The results indicated that the grain yield and biological yield, which were affected by tillage system, did not show any significant difference, while the grain yield and biological yield, which were affected by plant density showed significant differences. With the increase of density from 60000 to 80000 plants/ha, grain yield and biological yield increased by an amount of 10.04 and 20.53 t/ha respectively. Biological yield which was affected by plant pattern showed significant differences and new two-rows pattern increased biological yield (18.9t/ha) related to linear (18.8t/ha).


INTRODUCTION
Corn (Zea mays L.) is the most important grain-forage crop in Iran.The average grain yield of corn is more than 8 t/ha and it increases annually.In order to optimize the use of moisture, nutrients and solar radiation, corn seeds must be planted under optimum density and tillage system.Intensive production of field crops practiced until recently to achieve high yields, required intensive tillage and application of other high-technology inputs.This concept, however, implies a number of problems, among which relationship between product quality and quantity are in the foreground, along with increased crop production which shows an important ecological sustainability.Above all, farmers approach production in terms of the cost effectiveness of the applied system (Kisic et al., 2010).
The use of mechanized processes is considered the main factor contributing to the total energy inputs in agricultural systems.Tillage represents half of the operations carried out annually in the field.Consequently, there is a potential to reduce energy inputs and production costs by reducing tillage (Osunbitan et al., 2005;Ozturk et al., 2006).
Tillage practices are needed to increase agronomic stability and productivity while enhancing the environment *Corresponding author.E-mail: MOHSENI1337@yahoo.com.(Hatfield et al., 1998).Since land preparation for doublecropping systems requires timeliness, especially when a moldboard plow is used, reduced tillage, mainly NT systems, are becoming widespread.
Beneficial effects of the crop residue maintenance on the soil surface includes: a reduction of soil erosion and runoff, increased soil water conservation and soil aggregation; and a lesser use of fossil fuel is not a direct effect of crop residue management (Nakamoto et al., 2006).
In order to combat soil loss and preserve soil moisture, more attention has been focused on conservative tillage involving soil management practices that minimize the disruption of the soil structure (Samarajeewa et al., 2006).
Benefits of residue cover include improved soil water storage, enhanced soil organic matter content, nutrient recycling and protection against water and wind erosion (Lopez et al., 2003).
So with the selection of desired plant density, appropriate yield can be produced.Corn is among the least tolerant of crops to high plant population density.Crop growth rate is directly related to the amount of radiation intercepted by the crop.Therefore, the response of grain yield to narrow rows can be analyzed in terms of the effect on the amount of radiation intercepted at the critical periods for kernel set.In some cases, full radiation interception during these periods may not be achieved with wide rows.Andrade et al. (2002) found that corn yield response to decreased row spacing was negatively correlated to radiation interception at pollination time with the wider spacing.Widdicombe and Thelen (2002), however, found that higher yields were attained for corn grown in narrow rows vs. wide conventional rows irrespective of hybrids and plant populations tested in Indiana and Michigan.Corn grain yield typically exhibits a quadratic response to plant density, with a near-linear increase across a range of low densities, a gradually decreasing rate of yield increase relative to density increase, and finally a yield plateau at some relatively high plant density (Duncan, 1984;Ottman and Welch, 1989;Thomison and Jordan, 1995).Higher plant density combined with narrower row spacing results in a more equidistant planting pattern that is expected to delay initiation of intraspecific competition (Duncan, 1984); while early crop growth is increased (Bullock et al., 1988).Although the optimum row spacing varies among plant genus, yields will generally be maximized by sowing in rows that result in an equidistant spacing among plants (Sharratt and McWilliams, 2005).Narrow-row corn has been advocated in recent years as a technique to enhance grain yield (Orchard, 1998).These differences in yield associated with row spacing appear to be accentuated for corn grown at more northerly locations within the U.S. Corn Belt (Sharratt and McWilliams, 2005).Paszkiewicz (1997), for example, found that corn grown in narrow rows to the north of Interstate 90 (44_ N latitude) resulted in an 8% higher grain yield while, those grown in narrow rows to the south of Interstate 90 resulted in a 4% higher grain yield compared with corn grown in wide conventional rows.Crop row spacing can also influence soil water utilization (Sharratt and McWilliams, 2005).

MATERIALS AND METHODS
The study was conducted at the Agricultural and Natural Resources Research Center of Mazandran, Qarakheil, Qaemshahr, Iran (31°28' N, 52°35' E) in 2011.The weather in this zone had an average temperature of 24.8°C per month and receives rainfall of 419 mm from May through October.Weather conditions in the experiment site are summarized in Table 1.
The soil type was classified as silty clay-loam.Some of its properties are as follows: 25, 25 and 50 g kg-1, clay, silt and sand, respectively; organic matter, 4.3 g kg-1; pH 7.2.
Available N, P and K, were 0.21, 28.1 and 64 respectively.Before seeding, soil available N, P, and K were determined for depths (0 to 30 cm).
This experiment was laid out in strip-factorial on the basis of randomized completely block design with four replication.Tillage systems were carried out in three levels: Rotary system (RS); Disk system (DS); Plow and Disc system (PDS).Other factors were plant density in three levels (60000, 70000 and 80000 plant/ha) and plant patterns were conventional row (linear) and new two-rows (zigzag).The previous crop at the site was canola.NPK fertilizers were applied according to yield potentials and soil test level to the site.Fertilizers used as N. P. K (200-100-100) were made from urea, triple super phosphate and potassium sulfate.Hand weeding was introduced to control weeds.Plants from each plot harvested in an area of 9 m 2 and the moisture content was adjusted to 14%.Cultivar corn was a single cross hybrid (Z.mays L. cv.singel cross 704) that was popular among growers in Iran.Plots were seeded on 31 st of May.The site was irrigated with water using a sprinkler irrigation system.Plants were cut at the surface from the central of the four middle rows in the plots (area of 9 m 2 ).All plots were harvested, on 3rd October, 2011.Ears were separated, weighed and the plant dry weights of forage were measured, grain corn moisture content was also determined.Data were analyzed using the MSTAT-C procedure to develop the ANOVA for a factorial design.The DMRT procedure was used to make tests of simple and interaction effects by MSTAT-C, all differences reported are significant at P 0.05 unless otherwise stated.

Tillage system
Tillage system had a significant effect on plant height and ear height at 0.01 probability level (Table 4).Also, tillage system had significant effect on harvest index and leaf yield at 0.05 probability level (Table 2).The highest harvest index was obtained in plow and disk system and harvest index was obtained in rotary system, same as disk tillage system (Table 3).The highest leaf yield was obtained in rotary system (Table 3).The highest plant height and ear height were obtained in plow and disk system.Ahmad (2007) in an experiment on the wheat plant reported that the maximum wheat plant height  resulted from minimum tillage (70 cm) and minimum wheat plant height resulted from no-tillage system (58 cm).Wang and Partners (2006), in an experiment on wheat during 6 years with three tillage system concluded that, average grain yield under tillage system was at least 8% more than the grain yield under conventional tillage system at 5% probability level.Tillage treatment had at least the highest yield (5604 kg/ha).

Effect of plant pattern
The plant pattern had significant effect on biological yield (Table 2).The highest biological yield was obtained in two-row pattern with 18874 kg/ha (Tables 3).Biological yield of one row pattern was 18800 kg /ha.Plant pattern had no significant effect on other traits in this study (Tables 2 and 4).Ottman and Welch (1989) have reported a positive response in yield to growing corn in narrower rows.Pedersen and Lauer (2003) found an 11% lower yield for corn grown in 0.19 m rows vs. 0.38and 0.76 m rows in Wisconsin; while Farnham (2001) found a 2% lower yield for corn grown in 0.38 m rows vs. 0.76 m rows in Iowa.Farnham (2001) observed significant hybrid row spacing interaction among six hybrids grown in narrow and wide conventional rows in Iowa.Westgate et al. (1997) however, reported that light interception was not affected by corn row spacing; they found no yield advantage to growing corn in narrow (spacing of 0.38 m) rows vs. conventional (spacing of 0.76 m) rows over two growing seasons in Minnesota.

Plant density
Plant density had a significant effect on grain yield and biological yield at 0.01 probability levels (Table 4); also plant density had significant effect on 1000 seeds weight, stem yield and leaf yield at 0.05 probability levels (Tables 2 and 4).The highest grain yield (10.04 t/ha) and biological yield (20.53 t/ha) were produced in 80000 plants/ha.Plant density had no significant difference in 60000 and 70000 densities on grain yield and biological yield (Table 3).With an increase in density, 1000 seeds weight decreased, however, increase in density from 70000 to 80000 plants/ha, the grain yield and biological yield had 8.8 and 12.6% increase respectively (Table 3).The highest stem yield and leaf yield were obtained from the density of 80000 plants/ha (Table 3).With the increase in the density of 80000 plants/ha (Table 5), yield and yield component of corn varieties in 2 densities of 55.000 and 110.000 plants/ha of 21 hybrid single cross and 13 Inbreed line with a commercial witness were significantly affected by plant density (Rodrigues et al., 2003).Shakarami and partners (2009) in investigating three plant density (7, 10 and 13 plants m 2 ) of corn, recognized that the highest grain yield, harvest index, number of grain rows and number of grain ears was produced in 10 plants m 2 and the highest biological yield obtained from 13 plants m 2 .Kisic et al. (2010) in the study of crop yield and plant density under different tillage systems found that the plant density and yields of maize, soybean, oilseed rape, winter wheat and spring barley point to the conclusion that high density crop (winter wheat, spring barley and oilseed rape) are suitable for growing under reduced tillage systems.Yield of low density spring crops (maize and soybean) obtained under the no tillage system were not satisfactory, especially in climatically extreme years.
Tetio- Kagho and Gardner (1988) reported that with the increase of plant density and herbal density causes resonance rivalry between plants for absorption of environmental resources and so stem diameter which is

Interaction effects
The statistical analysis of the data shows that there were no significant differences in grain yield due to different tillage system × plant density × plant pattern interactions (Table 2).

Table 1 .
Weather condition in experiment site during corn growth stages.

Table 2 .
Mean square effects of tillage systems, Plant pattern and plant densities on Grain yield, Biological yield ,Stem yield, Leaf yield and Harvest index

Source of variation DF Grain yield Biological yield Stem yield Leaf yield Harvest index
*,** and ns significant at the 5% , 1% and non significant respectively.Table 3. Means comperison effects of tillage systems , Plant pattern and plant densities on Grain yield, Biological yield, Stem yield, Leaf yield and Harvest index.

Table 4 .
Mean square effects of tillage systems, plant pattern and plant densities on Number of grain row, Number of row per ear, 1000seeds weight, plant height and ear height *,** and ns significant at the 5% , 1% and non-significant respectively.

Table 5 .
Means comperison effects of tillage systems , Plant pattern and plant densities on Number of grain row, Number of row per ear, 1000seeds weight, Plant height and Eear height.
a affected, decreases.