Conservation tillage systems can be an important part of a sustainable agricultural system, in that they can be used to decrease soil erosion losses ordinarily associated with typical conventional agricultural practices. It is important to remember that anything that is done to decrease erosion losses also decreases need to add as much fertilizer and water soils, given that top soil generally contains most organic matter. Conservation tillage also, ideally, decreases water pollution (via decreasing soil erosion), saves fossil fuel energy and thus decreases CO₂ emissions, compared to conventional tillage systems. Because soil organic matter tends to increase  under   conservation   tillage,   as  compared  to conventional plowing, soils are also more effective to carbon storing.

Conservation tillage systems include a variety of techniques, including "no-till" "minimum till" "ridge till" "chisel plow" and "mulch till". The Soil Conservation Service (now called the Natural Resources Service) refers to these systems as "residue management". Conservation tillage is basically, any system of cultivation that reduces soil or water loss when compared to conventional moldboard plowing, which turns over the soil completely. Most definitions specify that at least 30% of the crop residue must remain on the soil surface at the time of planting. It was designed to conserve soil, water,  energy, and protect water quality (Mitchel et al., 2015). Soil compaction can cause unfavorable soil physical, chemical and microorganism conditions in subsoil, which hinder root growth and crop yield (Mosaddeghi et al., 2009).

According to survey results from Conservation Technology Information Center (CTIC) (Sare, 2014), most operations in Midwest that use a cover cropping system do so in tandem with no-till practices or organic production, to help mitigate potential negative effects with these particular systems in comparison with traditional tillage or non-organic methods, respectively. Soil tillage also modifies mineralization rates of nutrients, which feeds back on soil carbon input (Barré et al., 2010). No-till is a growing practice for soil conservation (Horowitz et al., 2010).

Soil compaction had negative effect on soybean production (Acuña and Villamil, 2014). Penetration of plant roots through compacted soils is difficult (Chen and Weil, 2010). Corn (Zea mays L.) silage production is very important in winter in north of Iran that producer need to forage, but deceasing temperature and solar energy in delay sowing date resulting in low silage yield because farmers used from common plant density, row spacing and plant pattern.

Corn (Z. 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 increase annually. In order to optimize moisture use, nutrients and solar radiation and corn seeds must be plant 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 increase crop production, which shows an important ecological sustainability. Above all, farmers approach production in terms of cost effectiveness of applied system (Kisic et al., 2010).

Use of agricultural mechanization was considered the main factor contributing to total energy inputs in agricultural system. Tillage represents half of operations carried out annually in field. Consequently, there is a potential to reduce energy inputs and production costs by reducing tillage (Ozturk et al., 2006). Since land preparation for double-cropping 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 soil surface include a reduction of soil erosion and runoff, an increase soil water conservation and soil aggregation, and a less use of fossil fuel is not direct effect of crop residue management (Nakamoto et al., 2006).

In order to combat soil loss and preserve soil moisture, a more attention has been focus on conservative tillage involving  soil   management    practices    that   minimize disruption of soil structure (Samarajeewa et al., 2006). Soil compaction of agricultural soils is a global well recognized problem (Hamza and Anderson, 2005) due to deteriorated soil environment and adverse effects of intensive use of farm machinery on crop yield (Hamza et al., 2011). In China, subsoil compaction was also cause by inappropriate tillage, traffic and field operations on poor time (Zhang et al., 2006). Tillage is one of most effective ways to reduce soil compaction (Daniells, 2012).

Therefore, with selection of desired plant density, appropriate yield can be produced. Corn is among crops least tolerant to high plant population density. Roekel and Coulter (2011) determined a close relationship between maize yield and plant density. The studied hybrid produced maximal yield by a plant density of 81700 plants ha^-1 or even higher.

On basis of their research, Berzsenyi and Lap (2005) have found that optimal plant density varied between 67483 and 70161 plants ha^-1 regarding the average of the involved hybrids. Total dry matter increases from 6 to 40% when plant density increases from about 79000 to 165000 plants ha^-1 in some studies (Turgut et al., 2005; Yilmaz et al., 2007).

According to Pepó and Sárvári (2013), maize is a plant with individual productivity; therefore, plant density determines yield significantly. This experiment was conduct to determine best plant density and tillage system in North of Iran.

The study was conduct at Agricultural and Natural Resources Research Center Station of Dashte-Naz in MazandranIran for two years 2012-2013 (36°37' N, 53°11' E). The weather in this zone had an average temperature of 24.46°C per month. Receives average rainfall of 231.1 mm from May through October for two years. Weather condition in the experiment site are summarized (Tables 1 and 2).

The soil type was classified as clay loam, with pH 7.2. This experiment was laid out in strip-plot on randomized completely block design basis with four replications. Tillage systems were in three levels: 1. Plow and Disc system (PDS). 2. Disk system (DS) - 3. No tillage (NT). Another factor was plant density in four levels (70000, 80000, 90000 and 100000 plants ha^-1).

The previous crop at site was wheat. NPK fertilizers were applied according to yield potentials and soil test levels. N P K (200-100-100) fertilizer used was applied as urea, triple super phosphate and potassium sulfate according to soil test. Hand weeding was used to control weeds. Plants from each plot were harvested in an area of 9 m². Cultivar corn was a single cross hybrid (Z. mays L. cv. single cross 704) that was popular among growers in Iran. Ear height, plant height, ear length, row number, kernel number in row, ear diameter, wet and dry silage yield, wet and dry ear weight, wet and dry stem weight, wet and dry leaf weight were measured. The site was irrigated with water using a sprinkler irrigation system. Plants were cut at surface from central of four middle rows in plots (area of9 m²).

Data were analyzed using the MSTAT-C procedure to develop the ANOVA for a split-split plot design. The Duncan^’s Multiple Range Test (DMRT) procedure was applied to make tests of simple and interaction effects by MSTAT-C, all differences reported are significant at P 0.05 unless otherwise stated.

The authors have not declared any conflict of interests.