Maize is a special type of popcorn (zea mays evarta) that expands when exposed to heat, sources of historical information indicate that maize. It is probably the oldest of the corn primroses, which survived thanks to genetic improvement, some of them through selection over thousands of years reached their present shape and size [1].

It can be said that the production rates of this crop are still below the level of global production, because of the dependence of Iraqi agriculture on open pollinated, compound and synthetic varieties, poor field management, and lack of studies on yield and its relationship to the size of the split, which is considered one of the most important determinants of the spread of maize among farmers and consumers, in addition to the reluctance of farmers to plant it because of its low productivity and the lack of a price policy for this crop that encourages farmers to grow it, therefore, Iraq resorts to imports to fill the shortage in the local market. The sources indicated that there are several factors that determine the spread of this crop, including the negative relationship between plant yield and size of shoots [2]. Some of the researchers pointed out the possibility of improving the two characteristics, by following the methods of iterative selection between genetic communities and producing new genetic communities with a high genetic stock for the two characteristics of high yield and size of fission, then elicit genotypes or strains involved in the production of crosses to reach an acceptable compatibility between the two traits [3], or the tendency to use more efficient statistical methods to know the direct and indirect impact, such as path analysis [4]. Also, genetic stability may have an effect on the size of the cleavage, Rangle et al. [5] were able to improve the relationship between the two traits by following an appropriate method to elicit phenotypes from maize. The size of cleavage increased by 50% for single crosses compared to synthetic and open pollinated varieties. Researcher Rangle et al. [6] added to the possibility of harnessing statistical and genetic measures, to find out the superior compositions in plant yield and fission size, another determinant of maize production is fungal diseases, that reduce the size of the diastasis [7]. The grain shape and size influence the size of the cleavage, in addition to the effect of the size of the splitting on the harvest date, the percentage of moisture in the grain at harvest, and the mechanical damage resulting from the harvesters, as well as storage conditions of heat and humidity [8].

Junior et al. [9] showed that the balanced use of environmental resources such as planting dates, plant densities, cultivation methods, and fertilization levels would improve GP and PE traits. 

With the following steps:

• Cultivation of two communities, and in each community, self-pollination of selected plants is carried out, as well as cross-breeding with individuals from the second community, and the self-pollination seeds are preserved

• The seeds resulting from the crosses are taken, and their offspring are cultivated and evaluated, and the general and specific abilities are identified

• Cultivation of preserved seeds of self-pollinating plants of superior plants of both general and special abilities

• Crossings between them are carried out with all possible possibilities in each community separately and the seeds obtained from the two sources are mixed and planted in slabs, and the cycle is repeated as required for the ripening of these two communities

One of the important methods of genetic development among open pollinated plant communities is the method of frequent sibling selection, for the purpose of breeding genotypes such as compound, synthetic, open pollinated varieties or commercial hybrids. The program includes three phases: selection, the second is evaluation of the apical crosses, and the third phase is recombination. Hallauer [10,11] and Lonnqusit and Williams [12] independently stated that the reproductive cycle could be completed with fewer seasons when genotypes were used, both self-pollinated seeds and cross-crossed seeds can be obtained from them. Garay et al. [13] reported that this method is proposed to improve the performance of general and specific recombination susceptibility in genetic populations.

Russel and Eberhart [14] indicated that the repetitive and mutual selection program brought about a qualitative evolution in mixed-breeding societies. Jenweerawat et al. [15] showed that a number of lineages can be selected in each cycle of mutual iterative selection without the lineage being related to each cycle. Holthaus and Lamkey [16] confirmed that iterative selection of S2- progenies for six cycles, it produced genetic progress, increased the performance of the two populations, and maintained phenotypic variability by increasing the frequency of preferred and desirable genes. The iterative selection method has been studied by a number of researchers, including Freitas et al. [17], who indicated that iterative selection is effective in eliciting superior genotypes or crosses by breeding progeny from each cycle, the intensity of selection differed in the iterative selection programs according to the researcher's desire.

Holthaus and Lamkey [16] used an intensity of selection for the repeat selection program of 9.7% for the half-sibling program (Half-sib), 15.6% for the S2-progeny program, and 12.1% for the reciprocal selection program (Ful-sib), while Dona et al. [18] used a selective intensity of 15% for the phenotypic frequency selection programme, Santos et al. [7] used selection intensity between 10-20% to screen lineages inferred from iterative selection, it is clear from the researchers' data that the intensity of selection depends on the genotypic present in the population and on the level of sifting determined by the researcher, as high selection intensity gives greater progress than low selection intensity gives.

Male Flowering

Pollen release in the field is the last stage of vegetative growth, at which the plant height stops, which is one of the characteristics affecting the plant yield, affected by the surrounding environmental conditions, especially high temperatures, levels of fertilization, plant densities, and all breeding methods focused on early flowering and short-lived varieties [19]. Gozubenli and Konuskan [20] indicated that there were no significant differences between the duration of male flowering, wen testing one of the genotypes of yellow corn under four levels of nitrogen fertilizer and four plant densities. Ahmad et al. [21] confirmed when comparing 14 genotypes of yellow corn that there were significant differences in the number of days of male flowering, which ranged between 43-53 days.

Female Flowering

Female flowering is one of the critical periods that affect plant yield, the number and weight of grains is a function of the rate of growth of the crop during the flowering period [22]. Kheibari et al. [23] indicated that there were significant differences between the duration of female flowering for three genotypes of maize with two densities, as the flowering duration ranged between 74.33 and 85.58 days.   Sarjamei et al. [24] indicated that there were no significant differences between the duration of female flowering for the genotype of maize KSC704 when cultivated with three cultivation methods and with three plant densities.

Plant Height

Plant height in maize is considered a quantitative trait related to yield, it is one of the important characteristics not only for the specifications of modern varieties, but also for the production of green and dry matter alike.   The height of the plant depends on the number of nodes and the distance between the nodes, and in general. Plant height can be between 0.3 and 7.0 m depending on the cultivar and the prevailing environmental conditions, in addition, the early cultivars are shorter in length than the later cultivars [25]. The results of Dona et al. [18] showed that there were significant differences in plant height for the first and second cycle of axis repeat selection. Mashreghi et al. [26] did not find significant differences in plant height when cultivating one cultivar with three cultivation methods and with three plant densities, it can be inferred from the researchers' results for the characteristic of plant height, the opinion of some researchers tends to obtain the high height of the yellow corn plant, if the genotype is polyploidy, but if the plant bears one cone, the best is the short plants.

Maize Ear Height

An expert, de Souza et al. [27] showed that there were significant differences between individual crosses produced from repeated cross-crossing, as the ear height ranged between 92 and 130 cm. Ahmad et al. [21] obtained significant differences when comparing 14 genotypes of yellow corn. The ear height ranged between 70.17 and 104.33 cm. Bello et al. [22] indicated that there were significant differences in the average ear height when comparing ten genotypes of maize, as the average ear height ranged between 34 and 37 cm.

Leaves Number

The results of Murtza et al. [28] showed that there were significant differences in the number of leaves between fourteen genotypes. Sarjamei et al. [24] confirmed that there are significant differences in the number of leaves in plants, when cultivating one of the genotypes with three cultivation methods and three plant densities. The interaction between cultivation methods and plant densities did not significantly affect the number of leaves. Kheibari et al. [23] demonstrated when testing three maize genotypes with three plant densities, there were significant differences in the number of plant leaves above the upper cone of the plant.

Leaf Area Index

The leaf area index has an important role among the number of factors affecting crop growth and plant yield, The leaf area is expressed as the photosynthetic surfaces that absorb sunlight and accumulate dry matter. Al-Khazali et al. [29] indicated that there are significant differences between plant densities, density achieved 71.4 thousand plants. Hectare-1 had the highest leaf area index, reaching 3.32 and 3.25, respectively, for the two seasons, spring and autumn. Abdul Hassan and Wahib [30] found an increase in leaf area and its evidence for the selection program by self-pollination for four seasons of the first year for all selected structures by 16 and 18% for the spring and 15 and 19% for the autumnal, the leaf area index increased by 11, 9 and 4% for the spring and 13, 5 and 7% for the fall season in the second year, as the selection was effective in the fall season for both years by increasing the leaf area index ratio compared to the spring season.

Ear Rows Number

The number of ear rows is one of the secondary yield components, which has a significant impact on plant yield, should be given great importance in breeding programs to improve grain yield, and in most studies for this trait it was found that it has a significant correlation with the yield [31]. Mosa [32] obtained significant differences for the number of rows in the ear when crossing six pure strains and three scouts and crossing them, the highest rate was 17.10 rows for one of the multiplications, and the lowest rate was 11.07 rows for another multiplication. The results of Ilker [33] showed that there were significant differences in the number of ear rows between eight genotypes of sweet corn, as the number of ear rows ranged between 15.0 and 17.7 rows.

Row Grain Number

Holthaus and Lameky [16] reported that the number of row grains decreased by an average of 0.5 after six cycles of selection in the S2-progeny programme, while the increase was significant in the repetitive and mutual selection program. Beiragi et al. [34] obtained significant differences in the number of row grains when testing 18 genotypes of maize. Hoshang [35] showed that there were significant differences for grain grade.

Grain Number per Plant

Plant yield in maize and some cereal crops is positively correlated with the number of grains per plant or the number of grains per unit area, the fact that this trait is under the influence of a large number of pairs of genes is a reflection of most of the environmental and genetic variables on the genotype. Wahib [36] between the presence of significant differences between parents and their factors in the number of grains per plant, one of the crosses achieved the highest rate of 672.0 seeds per plant compared to the rest of the crosses and the parents included in the experiment. Increasing plant density for two seasons and two levels of nitrogen decreased the number of plant seeds by 29% [37].

Ear Number per Plant

The ears number per plant is one of the main components of grain yield, which all education programs aim to improve. Daif et al. [38] confirmed that there are no significant differences between single and triple crosses for this trait, its average ranged between 1.07 and 1.35 ears per plant. Yousef et al. [39] explained when comparing the two cultivars of maize, Safa and Babel, and planting them with three plant densities of 53, 106, and 160,000 plants. ha^-1 and two sites. Dona et al. [18] obtained significant differences in the number of cotyledons, as it ranged in the first cycle between 0.88 and 1.13 I in the plant, and in the second cycle between 0.78 and 1.06 ear in the plant. Okporie et al. [40] obtained significant differences in the ear number when comparing seventy-one genotypes after one cycle of iterative phenotypic selection, the average number of ears for the first cycle was 2 ears per plant, compared to the parent population, which amounted to 1.7 ears. The results of Abdul Hassan and Wahib [30] showed the effectiveness of selection under the lack of irrigation by increasing the number of earwigs for one of the selections by 8% compared to the original in the spring season. Kheibari et al. [23] showed that there were significant differences between the three used genotypes and the overlap between the genotypes, as for the effect of plant densities, it had no significant effect on the trait.

Grain Weight

Okporie et al. [40] indicated that there were significant differences in grain weight between genotypes after one cycle of iterative phenotypic selection, with 39.4 g for the parent population and 48.7 g for the first cycle, one of the selections excelled in terms of grain weight by 12% from the original under the lack of irrigation, another selection increased by 7 and 16% over the original for the two agricultural seasons.

Grain Filling Duration

The grain yield depends on the total dry matter accumulation in the grain from the period of grain development after fertilization to physiological maturity. Grain filling includes three stages, the first is rapid division and differentiation, and the second stage is linear. The dry matter accumulates quickly, and about 90% of the dry matter accumulation occurs in this stage and the third stage (maturation drying) [41]. The linear phase is called the Effective Grain Filling Duration (EGFD), while the percentage of dry matter accumulation in the linear phase is called Grain-Filling Rat (GFE), therefore, the duration of the grain filling is considered one of the traits on which indirect selection takes place through the high heritability and relatedness ratio, the duration of seed filling can be determined after 50% female flowering until physiological maturity [41]. Golezani and Tajbakhsh [42] showed that there were no significant differences between the duration of grain filling for five maize genotypes, as it ranged between 39 and 49 days.

Day Number to Physiological Maturity

The physiological maturity of maize can be determined in the field after completing 50% flowering until the field loses 50% of the green color of the plant [43], or by determining the grain moisture in the field with a field moisture measuring device, when the moisture content reaches 30-32%, after this degree, there is no increase in the accumulation of dry matter, and the harvesting process can be carried out, physiological maturity can be inferred from the appearance of the black layer at the point of contact of the seed with the ear. Shrestha [44] indicated that there were significant differences in the number of days to physiological maturity when cultivating one of the genotypes with five levels of nitrogen fertilizer and with three plant densities, the comparison treatment achieved the shortest physiological maturity period of 130.44 days, and the plant density was 55555 plants. ha^-1 has the shortest ripening period (131.46 days) and the longest period (132.66 days) for a density of 83333 plants. ha^-1.

Grain Yield

The grain yield of maize is a complex trait, which plant breeders try to increase through other characteristics such as grain weight, number of ear rows or ear diameter, number of row grains or ear length, and the number of ears. Abuzar et al. [45] obtained significant differences for the yield trait when cultivating one of the genotypes with five plant densities, as the density reached 60,000 plants. Ha-1 has the highest yield, reaching 2604 kg. ha^-1 and the lowest yield was 764.3 kg. Hectare-1 achieved density of 140 thousand plants. ha^-1. Hadi and Wahib [37] showed that the use of four selection criteria in maize increased the yield of the original cultivar for the two seasons and the two levels of nitrogen, but increasing the plant density decreased the yield. Abdul Hassan and Wahib [30] indicated that there was an increase in the yield, as it increased by 19% in the spring season for one of the teams, and another team increased by 20 and 16% for the two seasons.

Male Flowering

Noor et al. [46] found that the heritability in the broad sense of the number of days of male flowering reached 77%. Bektash and Wahib (2003) when comparing six maize genotypes, found differences in the components of phenotypic variance 8.620, 7.135, 1.485, genotype 19.763, 18.719, 1.044, and environmental variance 8.149, 6.012, and 2.138 for the three seasons, respectively.

Female Flowering

It was found from the results of Nataraj that the coefficient of phenotypic and genotypic was 4.361 and 4.268%, respectively, and the heritability rate was 95.88, as for the experiment of Bektash and Wahib [47] when they tested six genotypes of maize for two seasons, the phenotypic and genotypic were 6.306 and 4.899 respectively for the first season, 39.67 and 37.906 respectively for the second season, and 13.906 and 12.008 respectively for the third season.

Plant Height

Freitas et al. [17] indicated that the rate of heritability in the broad sense amounted to 58.488%, the genetic progress achieved at the 5 and 1% levels amounted to 82.95 and 106.30, respectively, and the expected genetic progression rate at the 5 and 1% levels amounted to 35.66 and 45.70, respectively. In the experiment of Hadi and Wahib [37], genotypic constituted a high percentage of phenotypic variation, reaching 98%, and the percentage of heritability in the broad sense was 49%, and the genetic achievement was 11.73.

Ear Height

Wahib [36] tested genetic material introduced from maize using strain x scout cross-crossing, and indicated that the heritability percentage of the strains and scouts used amounted to 96.82 and 94.54%, respectively, as for the phenotypic and genotypic, the strains were 69.4 and 67.16, respectively, while the scouts were 40.47 and 38.26, respectively.

Ear Rows Number

Mostafavi et al. [48] showed that the heritability rate in the broad sense was 76% under irrigation sufficiency and 61% under drought stress. Bekele and Rao [49] confirmed that the heritability rate in the broad sense of a group of genotypes resulting from cross-crossing six pure strains with four reagents was 42%, and that the genetic progression of this trait was 0.857. The results of Nataraj et al. [50] when evaluating thirty-nine genotypes of maize showed that the heritability in the broad sense was 78.0%, the genetic progression rate was 21.697%, and the phenotypic and genotypic were 13.34 and 11.85, respectively.

Grains Number per Plant

Kumar et al. [51] indicated when testing 86 genotypes of maize that the heritability ratio in the broad sense, it reached 93.18%, and the genetic progress achieved at the level of 5 and 1% reached 11.96 and 15.32. The genetic progression rate was 36.88 and 47.26%, the phenotypic and genotypic was 38.82 and 36.17, and the coefficient of phenotypic and genotypic was 19.21 and 18.54%, respectively. Hussain and Ali [52] showed that the heritability in the broad sense of the number of grains per plant was 99 and 96% for the spring and autumn seasons, respectively.

Ear Number per Plant

Mostafavi et al. [48] when testing eighteen individual crosses, reported that the percentage of heritability in the broad sense of the number of cones per plant was 50% under sufficient irrigation and 74% under drought stress. Freitas et al. [17] confirmed that the heritability rate in the broad sense of the trait of the number of cobs was 38.947%, and this indicates the possibility of improving this trait by crossing followed by selection due to low heritability in the broad sense and that the trait is under the influence of the non-host action of the gene pairs.

Day Number to Physiological Maturity

Jadhav and Sarkar [53] found, when testing eight genotypes of yellow maize, that the heritability rate for the 80% stage of physiological maturity reached 69.9%, and the achieved genetic progression was 38 days. 88%, respectively.

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