Effect of Irrigation System, Tillage System, and Seeding Rates on Wheat ( Triticum aestivum L.) Growth, Grain Yield and Its Water Consumption and Efficiency

A field trial was conducted at a private farm in AL-Hashimiya district Babylon Governorate—the republic of Iraq during the 2016-2017 and 2017-2018 growing seasons. This study was conducted using two irrigation methods, sprinkler and surface irrigation, for each of them had three Tillage methods (zero-tillage, medium-tillage, deep-tillage) and each tillage system had four seeding rate of wheat yield (120, 180, 240, 300) kg∙ha −1 . Results indicated that the consumptive water use was 557.5 and 535.9 mm for surface irrigation and 460.9 and 442.6 mm for sprinkler irrigation in the 2016-2017 and 2017-2018 growing seasons. Sprinkler irrigation significantly increased the flag leaf area with no significant effect on plant height. However, the minimum tillage and seeding rate (240 kg∙ha −1 ) significantly increased the plant height and flag leaf area in both growing seasons. For the grain yield, the sprinkler irrigation, minimum tillage, and seeding rate in the wheat grain-yield of, less water use, water


Introduction
A major challenge in crop production is to achieve the goal of increasing both yield and resource use efficiency. Irrigation water is a scarce and expensive resource constraining wheat production in semi-arid and arid regions. Globally, agriculture uses almost 70% of all freshwater withdrawals for irrigation [1]. In Iraq, water has become a limiting factor for growing most crops due to the shortage of water in both Tigris and Euphrates rivers and scarcity of rains, especially in the northern parts of Iraq where almost two-thirds of the wheat crop are grown in the so-called rainfed areas. Iraq is facing a serious water shortage problem for the first time in its long history. Wheat is a major cereal crop widely grown in Iraq that is in the first rank as the highest acreage is specified to the wheat each year. In the 2016-2017 growing season, the total cultivated area under wheat, total production, and productivity in Iraq were 1,054,000 ha, 2,974,000 t, and 1822 kg•ha −1 , respectively (Ministry of planning, directorate of agriculture statistics, 2017). This reflects the importance of this vital crop as a source of proteins and calories locally and globally, with an annual production of about 730 million tonnes globally [2].
When water is not easily accessible in the required amounts for plants, it will adversely affect growth and plant development in terms of anatomy, morphology, physiology, and biochemistry, leading to a reduction in plant leaf size and yield [3] [4] [5] [6]. Therefore, research centers and universities should pay more attention and interest in dealing with such problems by adopting suitable approaches and agronomic practices, including using sprinkler irrigation, tillage system, and seeding rates to develop drought-resistant crops to water shortage and using water efficiently. Sprinkler irrigation is one of the methods used to supply plants with the required amount of water efficiency and reduce water loss [7]. It is widely applied globally due to its advantages [8]. However, in Iraq, it is still used at a limited level. The process of preparing the soil for growing crops is of great importance, particularly tillage, which makes a significant change in the soil structure [9]. Tillage systems affect soil properties such as temperature, moisture, bulk density, particle aggregation, organic matter content, and plant properties, such as root density. Improving soil properties is an essential part of the sustainable intensification of crop production systems [10].
There are three tillage practices, conventional tillage, minimum tillage, and zero tillage, each with its advantages and disadvantages. No-tillage or minimum tillage is an important, valuable strategy for improving grain yield in arid regions [11] [12] [13]. Furthermore, the minimum tillage-based conservation agriculture is a crucial strategy for crop intensification, especially when combined with crop residue retention, which can substantially improve soil properties [14]. The seeding rate (S) is one of the essential agronomic practices relevant to the irrigation and tillage system, as it is the crucial factor for plants in their efficient exploitation of available resources (water, air, radiation, fertilizer, and anything affecting plant growth) [15]. Therefore, the present study aims to investigate the effect of three interrelated factors: the irrigation system (sprinkler irrigation and surface irrigation), tillage system (zero tillage, minimum tillage, and deep tillage), and seeding rates (120, 180, 240, and 300 kg•ha −1 ) on wheat (Triticum aestivum L.) growth, grain yield and its water consumption and efficiency.

Study Area
A field experiments were conducted at Al-Hashimya District, Babylon Province, Iraq ( Figure 1). The landform is a plain area about 25.2 m above sea level, characterized by alluvial soils and classified under the super group "Typic Torrifluvent". Among the climate characteristics of the region, some are a subtropical climate with an average air temperature of 25.6 Co, an average annual rainfall of about 135 mm, evaporation above 2122 mm, and an average wind speed of 3.8 m•sec −1 with a relative humidity of 38%. Figure 2 shows climate data in at Al-Hashimya District (Iraqi Ministry of Agriculture, 2017-2018). The texture class is clay loam and soil pH of (7.31) with a medium-high land type, as shown in Table 1.

Soil Preparation
Three tillage methods were used to prepare the soil by plowing it, including zero    [13]. After that, the soil was leveled and divided into plots, each with a dimension of 4 × 5 m 2 . Then the sprinkler irrigation system was installed by laying the main pipes with a spacing of 10 m. The distance between the sprinklers was 10 m. As for the surface irrigation system, plastic tubes with a diameter of 2 inches were used, and a meter was attached to measure the amount of irrigation water to be added to each plot. A soil separation distance of 3 m was left between the two systems to prevent the impact of the sprinkler irrigation system on the surface irrigation system [16].

Agronomic Practices
Seeds of wheat were sown on 1 st December in the 2016 and 2017 growing seasons, respectively. Seeds were planted in rows with a spacing of 20 cm. The experimental units were fertilized according to the agricultural extension recommendations of the experiment region by adding 100 kg•ha −1 P 2 O 5 46% once at the soil preparation, and 200 kg•ha −1 nitrogen (urea 46%) as top dressing three times at the beginning of tillering, stem elongation, and booting stages [18]. The

Irrigation Water Supply
The irrigation process was performed after depletion of 50% of the available water. The amount of supplied water was measured each time, depending on the soil moisture content calculation before irrigation. The irrigation was carried out based on moisture depletion of the 0 -10 cm layer from the sowing to the end of the vegetative growth phase. The irrigation depth was increased up to 10 -20 cm for the flowering stage based on moisture depletion and 20 -30 cm to the end of physiological maturity to reach moisture content near field capacity [8]. Soil field capacity and permanent wilting point were measured using a pressure plate apparatus, while available water content was calculated using Equation (1) [19].
where, D = Available water depth (%), FC = Field capacity, PWP = Permanent wilting point, BD = Bulk density (Mg/m 3 ), D = Soil depth (cm) The water balance Equation (2) was used as a direct method for calculating the wheat crops actual water consumption [20].
where: ΔS: Change in storage soil moisture, I: Water applied by irrigation, P: Rainfall, C: Capillary rise.
When ΔS = zero because the soil moisture storage is the same at the beginning and end of the season; P = zero to block rain by covering, and D = zero because irrigation is conducted with a drainage limit of 50% of available water and a certain depth of the soil layer 0 -30 cm. Therefore, Equation (4) becomes Equation (3):

Test Water Use Efficiency
The efficiency of field water use was calculated according to Equation (4) [21].
where: WUEf = Efficient use of field water (kg•m −3 ), Y = Grain yield (kg), and WA = The amount of water added in the irrigation process (m 3 •Season −1 ).

Growth Characteristics
Plant height was measured from the soil surface up to the awns ends for ten plants randomly chosen from each experimental unit. Flag leaf area (cm 2 ) was calculated using Equation (5) [22]. Grain yield and its components was conducted by an average weight of 1000 grains was randomly taken from the yield from each experimental unit and weighed by a sensitive electronic scale [23]. Grain yield was tested by an area of 1.2 m 2 was harvested in each plot (four rows in the middle with 2 m in length). Grains were separated from the straw, weighed, and transformed to the t•ha −1 at 14% moisture content of grains [24].

Plant Height
It is clear from Figure 4 and Figure 5 that the irrigation system had no significant effect on plant height in both growing seasons. However, the tillage system and seeding rate significantly affected this characteristic in both growing seasons. For the tillage system, minimum tillage gave the highest average of plant height (82.29 and 82.25) cm compared with the lowest average of 74.00 and 74.08 cm in both growing seasons. Concerning the interaction, all bi and triple interactions were significant in both growing seasons. The highest average of plant height 84.44 cm was under the sprinkler irrigation I1, and the high seeding rate 240 kg•ha −1 (S3) compared with the lowest one 72.00 cm under surface irrigation I2 and for the lowest seeding rate (S1) in both growing seasons. The same effect of S3 seeding rate was obtained under the minimum tillage T1, in both    [15], on different wheat cultivars as a single factor. Combining the seeding rate with other factors as it was in this study may be a good approach to understanding the role of the seeding rate. Figure 6 and Figure 7 show the effect of study factors on the flag leaf area. It is clear that all factors and their interactions significantly affected this characteristic in both growing seasons. Concerning the irrigation system, sprinkler irrigation gave the highest average of 37.87 and 36.84 cm 2 compared with 33.92 and 33.39 cm 2 for surface irrigation in both growing seasons. This may be because sprinkler irrigation supplies plant vegetative growth with uniform and even quantities of water (i.e., uniform distribution), which makes water readily available for leaf growth, which, in turn, helps in cell division and expansion of flag leaf. Similar findings were obtained by [1], where a significant increase in the     [24]. Figure 6 and Figure 7 also show that all interactions were significant. Concerning the interaction between the irrigation system and seeding rate, sprinkler irrigation recorded the highest average 43.56 and 42.34 cm 2 at S2 seeding rate .03 cm 2 ) obtained under the surface irrigation (I2), zero tillage (T0) and the highest seeding rate (300 kg•ha −1 ) in both growing seasons, respectively. This means that sprinkler irrigation was in favor of plant growth with the help of seeding rate (medium one) under minimum tillage [26] [27]. The response of the flag leaf area under the influence of interaction was better than the response to each factor as single.

(Number of Grains) Spikes-1
The number of grains per spikes is one of the important grain yield components, which is highly influenced by the environmental conditions and crop management. However, it is highly under genetic control [28]. Figures 8-11 represent the effect of study factors and their interactions on this characteristic in both growing seasons. Sprinkler irrigation, although an increased number of grains spikes-1, this increase was not significant in both growing seasons. The same trend was obtained for the number of spikes m-2 Figure 7 and Figure 8 as sprinkler irrigation recorded a non-significant increase over surface irrigation in both growing seasons. For the tillage system, minimum tillage (T1) significantly increased the number of grains pikes-1 (43.40 and 42.07) vs. (37.00 and 34.69) for zero tillage in both growing seasons, respectively. Seeding rates (S2 and S3) tended to give the highest number of grains spikes-1 with no significant differences between them compared with (39.23 and 35.06) for S4 seeding rate in both growing seasons, respectively.
This result was in accordance with the results of Figure 7 and Figure 8. The interaction between the irrigation system and the seeding rate was significant. Sprinkler irrigation (I1) with the S3 seeding rate gave the highest values (44.06    Figure 10. Effect of the irrigation system, tillage system, and seeding rate on 1000 grains weight 2016-2017 season Figure 11. Effect of the irrigation system, tillage system, and seeding rate on 1000 grains weight 2017-2018 season. 28.9) grains spike-1 for surface irrigation, zero tillage, and (S4) seeding rate in both growing seasons, respectively.

1000-Grain Weight
Grain weight is considered one of the major yield components in wheat. The final weight of grain depends upon the source strength in exporting photoassimilates, sink capacity, grain filling, and rate and period of filling of photoassimilates from the beginning of flowering until physiological maturity. Figure 12 and Figure 13 indicate the effect of study factors and their interactions on 1000-grain weight. Sprinkler irrigation significantly increased this characteristic in both growing seasons, recording the highest values (40.99 and 41.12) in both growing seasons compared with (38.34 and 39.05) g for surface irrigation, respectively. This may be due to the high values of the flag leaf area caused by sprinkler irrigation [29], Figure 6 and Figure 7. The flag leaf area is playing a vital role in supplying the developing grains with photoassimilates in the last growth stages (i.e., during the grain filling stage) as it supplies almost 80% of assimilates transported to developing grains due to their close position to the spikes [15]. Minimum tillage treatment (T1) increased this characteristic in both growing seasons (42.29 and 42.24 g) compared with (36.3 and 37.12 g) for zero tillage treatment, respectively. However, [25] found no significant differences between zero tillage and conventional one in this characteristic. There was a significant gradual increase in the 1000-grain weight for the seeding rate with an increased seeding rate up to the highest seeding rate (S4), where a significant decrease occurred in Figure 11 and Figure 12. This decline in this characteristic may be due to reduced flag leaf area at the (S4) seeding rate ( Figure 5 and Figure 6). The findings of [15] support this result where increased seeding rates decreased 1000-grain weight. Figure 11 and Figure 12 also indicate that all interactions were significant. Sprinkler irrigation (I1) at the seeding rate (S3) gave the highest average (45.44 and 43.44) g compared with (31.57 and 37.05) g for surface irrigation (I2) at the highest seeding rate (S4) in both growing seasons, respectively. For the tillage system, minimum tillage (T1) at the seeding rate (S3) recorded the highest value of 1000-grain weight (46.95 and 45.6) g compared with (28.70 and 33.14) gm for zero tillage (T0) at the highest seeding rate (S4) in both growing seasons, respectively. Triple interaction shows that sprinkler irrigation (I1) with the minimum tillage (T1) under the seeding rate (S3) gave the highest value (48.8 and 47.27) gm compared with the lowest values (25.5 and 31.23) gm for surface irrigation (I2) with zero tillage (T0) at the highest seeding rate (S4) in both growing seasons, respectively. This means that the sprinkler irrigation favored this characteristic with the assistance of seeding rate (medium one) under the minimum tillage; that is, the response of 1000-grain weight under the influence of this interaction was better than the response to every single factor alone. Figure 12 and Figure 13 show the effect of the study factors (irrigation, tillage, and seeding rate) on the grain yield (t•ha −1 ) and their interactions in both growing seasons. The sprinkler irrigation (I1) recorded the highest averages (4.32 and 4.16 t•ha −1 ) compared with the lowest (3.83 and 3.76 t•ha −1 ) for surface irrigation (I2). This result may be due to the highest average flag leaf area [30]. Figure 6 and  (T0), respectively. This result was due to the increase of the flag leaf area ( Figure  4 and Figure 5), a (number of spikes) m-2 ( Figure 6 and Figure 7), the number of grain spikes-1 (Figure 8 and Figure 9), and an increase 1000-grain weight (g) (Figure 10 and Figure 11) caused by the minimum tillage treatment. For the seeding rate, there was a gradual increase in the grain yield with an increased seeding rate up to the S3 rate and then a decline at the S4 seeding rate in both growing seasons. This result was an outcome of the increased grain yield components: number of spikes/m-2 ( Figure 6 and Figure 7), number of grains spikes-1 (Figure 8 and Figure 9), and 1000-grain weight ( Figure 10 and Figure  11). The present findings were in agreement with the results reported in [31] in wheat, where a similar trend was obtained. All interactions were significant. Sprinkler irrigation (I1) in both growing seasons at the seeding rate S3 gave the highest grain yield (4.58 and 4.48 t•ha −1 ) compared with the lowest values (3.54 and 3.49 t•ha −1 ) for surface irrigation (I2) at the lowest seeding rate (S1) respectively. Minimum tillage (T1) at seeding rate (S3) gave the highest values (4.54 and 4.49 t•ha −1 ) in both growing seasons compared with the lowest values (3.52 and 343 t•ha −1 ) for zero tillage and seeding rate (S1). In triple interaction, the highest grain yield (4.87 and 4.77 t•ha −1 ) was obtained under the influence of sprinkler irrigation (I1), minimum tillage (T1), and (S3) seeding rate in both growing seasons compared with the lowest values (3.27 and 3.23 t•ha −1 ) under the influence of surface irrigation (I2), zero tillage (T0) and lowest seeding rate (S1). Figure 14 and Figure 15 refer to the water balance equation factors for the different irrigation treatments of wheat represented by ETa. It was noted that there were significant differences in the actual ETa values of wheat under different irrigation treatments. The highest water consumptive use of wheat was at the surface irrigation (557.5 and 535.9 mm season-1) compared with (460.9 and 442.6 mm season-1) for sprinkler irrigation in both growing seasons 2016-2017 and 2017-2018, respectively. This may be due to the increased number of irrigations 13 and 14 in both growing seasons for the surface irrigation treatment ( Figure  15). However, the irrigation number under the sprinkler irrigation was 16 and 17 in both growing seasons, Figure 14. The differences in the consumptive water use in both irrigation systems may be attributed to the increased added water quantities at each time due to the decreased irrigation efficiency for surface irrigation compared with sprinkler irrigation. This explanation was supported by the findings reported in [32] [33]. Moreover, the reason for increased water consumption for surface irrigation may be due to the availability of moisture content for plants at depths makes water available for them which, in turn, increases the plant water consumption where paints stomata are widely opened with high loses of water via transpiration [34] [35]. Figure 16 and Figure 17 represent the effect of the irrigation system, tillage system, seeding rate, and their interactions on the field water use efficiency (WUE) in both growing seasons. The average WUE were (1. 10   This may be because sprinkler irrigation gave the highest uniformity of water distribution and less water consumption. Besides, that minimum tillage (T1) gave the best root distribution, improving the growth characteristics. Also, the seeding rate of 240 kg•ha −1 (S3) gave the highest grain yield, described in Figure 11 and Figure 12,

Conclusion
The present study's findings highlighted the important role of the sprinkler irrigation system in the improvement of the growth attributes under the minimum tillage system and the suitable seeding rate. This synergistic action of these factors increased grain yield and its components. Furthermore, wheat production costs might be reduced considerably following sprinkler irrigation and minimum tillage systems with saving water via less water consumption and high water use efficiency.