Evaluation Of 36 Genotypes Of Maize (Zea Mays L) For Agronomic Characterizatics In (Abakaliki) Agricultural Zone

Maize (Zea mays l.) belongs to the family of grasses Gramineae(Obi, 2006) now known as Poaceae. It is a cereal crop adapted to a wide range of environmental conditions and is cultivated in all agro-ecological zones of west and central Africa (WCA)( Baffordet al., 2012). It originated from Central America notable Mexico from where it was introduced into Nigeria probably in16th century by the protugese (Future Harvest, 2004).

Maize is a monoecious plant i.e. the male and female flowers are borne separately on the same plant. It has 2-3 m tall single stem with single leaf at each node. The leaves are arranged on the principal stalk in two opposite ranks (Obi, 2006).
Maize shows greater degree of phenotypic diversity than other cereal crops.
Morphologically or architecturally or physiologically, maize is equipped with many desirable features like sparse leaf arrangement allowing maximum light interception and minimum of mutual shading (Mandal, 2014). Maize has a wide-ranging maturities from 70 to 210 days with little or no tillering capacity and a chromosome number of 2n = 20.

Maize is considered to be indigenous to the Americas particularly Southern Mexico and it spread northward to Canada and southward to Argentina and to other parts of the world. It has been domesticated about 8000 years ago and does not survive in its wild form (Mandal, 2014). Maize is considered queen of cereals and has become a leading agricultural crop in many parts of the world, not only in temperate regions but also in tropical and sub-tropical areas. Maize, wheat and rice together make-up three-fourths of the world grain production. Maize has attained the status of major cereal in the world market and is the leading world cereal in terms of both total production (695 million tonnes) and per unit area yield (4815 kg ha-1) (FAOSTAT, 2008).

Loamy or silty loam soil or silty clay loam is the ideal soil type for its cultivation and a pH of 6.5 to 7.5 is the most preferred for its growth. It has wider adaptability and is grown from 58oN to 40oS in latitude, from sea level to 3808 m above sea level, and under 25.4 to 1016 cm rainfall (Mandal, 2014) and with a growing cycle ranging from 3 to 13 months (CIMMYT 2000). However the major maize production areas are located in temperate regions of the globe. The United States, China, Brazil and Mexico account for 70% of global production. In Nigeria, maize is grown mostly by small scale farmers, generally for subsistence as part of mixed agricultural system.

Maize is an important staple food crop for millions of people both in developed and developing countries. It is relatively high yielding, easy to digest, easily processed and cheaper than other cereals. Maize is not only important as human food, but also a basic element of animal feed and raw material for manufacture of many industrial products. The products include corn starch, maltodextrins, corn oil, corn syrup and products of fermentation and distillation industries. It is also being recently used as bio-fuel. It is both the primary crop in majority of the farming systems and the staple food of both the rural and urban population.

According to IITA (2001) report, maize contains 80% carbohydrate, 10% protein, 3.5% fiber and 2% mineral. Iron and Vitamin B are also present in maize. In Nigeria, green maize (fresh on the cob) is eaten parched, baked, roasted or boiled and plays an important role in filling the hunger gap after the dry season and maize has a variety of uses (Khawaret al, 2007). Its grain is a rich source of starch, vitamins, proteins and minerals. The starch extracted from maize grain is used in making confectionary and noodles. Corn syrup from maize contains high fructose and act as sweetener and retains moisture when added to certain.

In Africa, maize production is constrained by a number of stress factors including a complex of pest and diseases that significantly reduces the quality and quantity of production (Akande and Lamidi, 2006) insects (e.g. stem borer) and mites are among the most important (Bosque- Perez, 1995).
Yields have been observed to be dependent on many factors mostly level of pest and disease infection, soil fertility, genetic potential of the variety as well as environmental conditions (Kim et al. 1990). Grain yield in maize increases with increase in grains per cob and number of ears per plants (Soliman et al., 1999). Plant height and ear height increase with increasing plant density, but leaf area, ear length, grains per row and thousand grain weight decrease with increase in plant density, while number of leaves per plant, number of leaves above main ear and number of rows per ear are not affected by plant density (Hassan, 2000).

The production of maize is not sufficient to meet the continuous increase of consumption. However, there is need to select varieties with high yield and other desirable traits in other to meet the demand of the teeming population. Therefore, attempts to increase maize production are of great importance.
Maize genotypes are routinely evaluated in various agro-ecological zones for adaptation, yield potential and disease reactions, to identify genotypes that can replace existing cultivars and as part of the requirements for releasing suitable varieties for cultivation in farmers’ fields.

Objectives of the study are:
1.    toevaluate 36 genotypes of maize for agronomic characteristics.
2.    to compare the two years result using student t-test analysis.
3.    to establish relationship between the agronomic characteristics using correlation analysis.

 

CHAPTER TWO
LITERATURE REVIEW

Botany of Maize
Maize belongs to the tribe Maydeae of the grass family Poaceae, a classification it shares with many other important agricultural crops, including wheat, rice, oats, sorghum, barley, and sugarcane (Obi, 1991). One main difference between maize and other cereals is that it bears seed heads, ears, that are larger than any other grass. The other Zea sp., referred to as teosintes, is largely wild grasses native to Mexico and Central America (Doeblay, 1990). The number of chromosomes in Zea mays is 2 n = 20.
Maize plant has 2-3 m tall single stem with single leaf at each node (Obi, 2006). The number of leaves on a plant may ranges from eight to forty-eight leaves and multiple ears. Each stalk produces ears that contain many rows of kernels that grow off of the cob of the ear and are enclosed by a leafy husk. Maize is considered queen of cereals. It has C4 photosynthetic pathway which is more efficient than C3 pathway under high temperature and dry-land conditions. C4 plants are most productive in terms of food nutrients produced per unit land area, per unit of water transpired, and per unit of time under conditions suitable for C4 plants (Mandal, 2014). It is a monoecious plant, i.e. the sexes are partitioned into separate pistillate (ear), the female flower and staminate (tassel), the male flower. It has determinate growth habit and the shoot terminates into the inflorescences bearing staminate or pistillate flowers (Dhillon and Prasanna, 2001). The whole structure (ear) is enclosed in numerous large foliaceous bracts and a mass of long styles (silks) protrude from the tip as a mass of silky threads.

According to Poehlman (1987), a single tassel may produce as many as 25,000,000 pollen grains or an average of over 25,000 pollen grains for each kernel on an ear with 800-1000 kernels. Pollen grains are very small, barely visible to naked eye, light in weight, and easily carried by wind. A unique characteristic of maize is that unlike most plants the kernels are completely enclosed by the outer layer known as the husk or shuck. These are leaf sheaths that tightly surround the kernels en masse. The number of leaves in the husk is a direct result of the number of joints on the corn stalk (Mandal, 2014). These husks prevent seed dispersal by wind, birds, or other natural means.

Origin, Evolution and Distribution of Maize
Maize is considered to be indigenous in Americas particularly Southern Mexico. It has been domesticated about 8000 years ago and does not survive in its wild form (Mandal, 2014). The centreof origin for Zea mays has been established as the Mesoamerican region, now Mexico and Central America (Doebley, 1994). There was evolution of high yielding dent kernel cultivars adapted to the Central Corn Belt and the Eastern and the Southern regions of the USA by American farmers in early days. Simultaneously, early maturing flint kernel type cultivars evolved in the Northern USA. It is generally now accepted that the teosinte (Zeamexicana), the nearest known relative of maize, has been progenitor of maize. Opinion is still divided as to whether maize originated by a single domestication from the basal branching teosinte subsp. Zea mays L. spp. Parviglumisor from the lateral branching subsp. Zea mays L. spp. Mexicana, or by a dual domestication from two subspecies. Transformation of maize from teosinte involved evolutionary forces such as mutation, hybridization, genetic drift and selection aided by selective and useful interventions by human beings, who selected useful variants out of large populations of teosinte and concentrated them into isolated evolutionary pools (Poehlman and Sleper, 1995). This resulted in differentiation of maize into over 300 races.

The races gradually got adapted to different agro-climatic regions in the Americas away from the centre of origin. Some scientists suggested multi-centre origin of maize on the basis of wide diversity with respect to cytological, 15 morphological and physiological characteristics and their early widespread distribution. On the basis of morphological data, there have been suggestions that there has been extensive gene flow between maize and teosinte and that the genetic constitution of teosinte has been greatly altered by maize germplasm. Molecular studies have confirmed that there is a two-way gene flow, but at a low level. Consequently, maize and teosinte maintain distinct genetic constitutions despite sporadic introgression (Doebley, 1990).

Archaeological records suggest that domestication of maize began at least 6000 years ago, occurring independently in regions of the southwestern United States, Mexico, and Central America (Mangelsdorf, 1974). The Portuguese introduced maize to Southeast-Asia from the America in the 16th century. Then maize was introduced into Spain after the return of Columbus from America and from Spain it went to France, Italy and Turkey. In India, Portuguese introduced maize during the seventeenth century. From India it went to China and later it was introduced in Philippines and the East Indies.  There has been much controversy as to when maize was introduced to Africa. However, one fact stands out clearly in the controversy, that is, no one has proposed that maize originated from Africa. Although the proposal by van Eijnatten (1965) that maize was introduced to Nigeria around 1478 A.D. after Christopher Columbus discovered the Americas seems quite reasonable. There is evidence that maize was already a food crop in Nigeria, specifically at Ile-Ife, about 1000 A.D. Goodwin (1953) described potsherds from Ile-Ife that were decorated by rolling maize cobs over wet clay and Stanton (1963), who was then a Research Officer at the West African Maize Research unit (WAMRU), Moor Plantation, Ibadan illustrated the potsherds. Jeffrey (1953) dated the potsherds and concluded that the maize was introduced into the region about 1000-1100 A.D.

Taxonomy of Maize
The scientific classification of maize
Kingdom:     Plantae
Order:     Poleas
Family:     Poaceae
Sub family:     Panicoideae
Tribe:     Andropogoneae
Genus:    Zea
Species: Zea mays

The genus Zeacontains, according to its latest taxonomic treatment has six distinct taxa classified into four species given below;
a) Zea mays (2n = 2x = 20) – Maize
b) Zeamexicana(2n = 2x = 20) – annual teosinte
c) Zeaperennis(2n = 4x = 40) – perennial tetraploidteosinte
d) Zeadiploperennis(2n = 2x = 20) – perennial diploid teosinte

Uses of Maize
Maize is a multipurpose crop because every part of its plant has economic value. The grain, leaves, stalk, tassel and cob can all be used to produce a large variety of food and non food products (IITA, 2001). The use of maize varies in different countries. In USA, EU, Canada and other developed countries, maize is used mainly to feed animals directly or sold to feed industry and as raw material for extractive/ fermentation industries (Galinat, 1988; Shaw, 1988;Mexico, 1994; Morris, 1998). In developing countries, use of maize is variable. In Latin America and Africa, the main use of maize is for food while in Asia it is used for food and animal feed. In fact, in many countries, it is the basic staple food and an important ingredient in the diets of people. It is utilized in more than 2000 forms and the most diversified food, feed and industrial crop. In industrialized countries, maize is largely used as livestock feed and as a raw material for industrial products, while in low-income countries, it is mainly used for human consumption (IITA, 2001). It has three key uses; as food for human, as feed for livestock and as raw material for industry.

As human food
Another factor that helped maize to win favor with humans is its nutritional value. Though it is economically and agriculturally defined as a grain, it has the nutritional qualities of a vegetable, for it is deficient in the lower B vitamins that are characteristic of grains. Maize contains many vitamins including A, C, and E; though white corn is lower in vitamin A. It also has high carbohydrate content and therefore is a good source of calories and thus energy (Obi, 2006). In fact, because of its elevated fat content, corn is superior to other grain as a source of energy. In addition, since the grain itself, the ear, and the stalk possess nutritional value, it exceeds all other plants for digestible nutrients per hectare. The maize grain can be prepared for food in many different ways (fried, grilled, in a salad or soup). Processing maize can also produce a wide range of products such as corn flour and corn meal.
Maize grains have great nutritional value as they contain 72 % starch, 10 % protein, 4.8% oil, 8.5 % fibre, 3.0 % sugar, 1.7 % ash (Chaudhary, 1983) and many vitamins including A, C, and E; though white maize is lower in vitamin A. It also has high carbohydrate content and therefore is a good source of calories and thus energy. Maize is an importance source of protein accounting up to 60% of the daily human protein supply (Musiliaet al., 2000, Sofieet al., 2009).

As Raw Material for Industry
Studies in maize production in different parts of Nigeria have shown the increasing importance of the crop amidst growing utilization by food processing industries and livestock feed mills (Ogunsumiet al., 2005). The meal from maize can be obtained by manual or mechanical milling. The other products include: tortillas, maize flours, chips and several types of snack, breakfast cereal, thickners, pastes, syrups, sweeteners, grits, maize oil, soft drinks, beer, whisky, etc. Mhaize is the most important raw material for industrial starch. Maize starch is a maize product and it is employed in the manufacture, ceramics, dyes, plastics, oil, paper and paper boards, textiles, cosmetics and pharmaceutical industries. The derivatives of maize starch include glucose or corn syrup, corn sugar, dextrin and industrial alcohol, which are employed in different industries. Edible oil is extracted from seeds, which is an all purpose culinary oil. Ethanol obtained from maize can be used as a biomass fuel. Stigmas from female corn-flowers, known as corn silk, can be used as herbal supplements.

As livestock feed
Maize has found its way into most products on supermarket shelves and has become a fundamental ingredient in livestock feed. Due to its richness in fat and high caloric content, maize has quickly become the favored feed-grain for livestock across America, many other countries still favor grass-fed practices. Proof of this fact is demonstrated by the overwhelming statistic that 95% of all maize grown is dent maize, which while white dents are a preferred food in Central America and southern Africa, yellow dents which are prevalent in the United States are the varietals used for livestock feed.
Another contributing factor to its popularity is its high-starch and low- fiber levels that make maize easier for livestock to digest than other grains. Yet, the maize is still altered from its raw form to create the feed which is generally made with whole, cracked, or steam-flaked grains enhanced with vitamins and minerals to meet the specific needs of the animal it is intended for.

Types of Maize
Maize variation may be artificially defined according to kernel type. Types of maize are flint maize, dent maize, flour maize, sweet maize, popcorn and pod maize. Except pod maize all these types of maize are differentiated based on the quality, quantity and pattern of endosperm composition in the kernel and are not indicative of natural relationships (Obi, 2006).

Dent maize
Dent maize is characterized by the presence of corneous, horny endosperm at the sides and back of the kernels, while the central core is a soft, floury endosperm extending to the crown of the endosperm where, upon drying, it collapses to produce a distinct indentation. Degree of denting varies with the genetic background. Dent maize is used primarily as animal food, but also serves as a raw material for industry and as a staple food. Most dent corn is yellow or white in colour and is used primarily as livestock feed (Obi, 2006). Though white dents are a preferred food in Mexico, Central America, the Caribbean, and southern Africa.
Flint maize
The flint maize has a thick, hard, vitreous (glassy) or corneous endosperm layer surrounding a small, soft granular center. The relative amounts of soft and corneous starch, however, vary in different varieties. Generally, the kernels are smooth and rounded, and the ears long and slender with a comparatively small number of rows or kernels. In temperate zones, flint maize often matures earlier, germinates better, have more spring vigor, more tillers and fewer prop roots than dent strains.

Popcorn
Popcorns are perhaps the most primitive of the surviving races of maize. This maize type is characterized by a very hard, corneous endosperm containing only a small portion of soft starch. Popcorns are essentially small-kerneled flint types. The kernels may be either pointed (rice-like) or round (pearl-like). Some of the more recently developed popcorns have thick pericarps (seed coats), while some primitive semi-popcorns, such as the Argentine popcorns, have thin pericarps. Popcorn is a relatively minor crop compared to dent corn. It is used primarily for human consumption as freshly popped corn or as the basis of popcorn confections (Obi, 2006).

Flour maize
Floury maize has soft starch throughout, with practically no hard, vitreous endosperm and thus is opaque in kernel phenotype. Kernels tend to shrink uniformly upon drying, so usually have little or no denting. When dry, they are easy to grind, but may mold on the mature ear in wet areas.
Sweet maize

In sweet maize, the sugary gene prevents or retards the normal conversion of sugar into starch during endosperm development, and the kernel accumulates a water-soluble polysaccharide called “phytoglycogen.” As a result, the dry, sugary kernels are wrinkled and glassy. The higher content of water-soluble polysaccharide adds a texture quality factor in addition to sweetness. Sweet maize is eaten in the immature milk stage and is one of the most popular vegetables. Sweet maize is more important economically than its limited commercial production would indicate, because it is consumed directly as human food (fresh market or canned and frozen products) rather than indirectly as livestock feed.
Pod maize
Pod maize (tunicate maize) is more of an ornamental type. The major gene involved produces long glumes enclosing each kernel individually, such occurs in many other grasses. The ear is also enclosed in husks, as with other types of maize. Homozygous pod maize usually is highly self-sterile, and the ordinary type of pod maize is heterozygous. Pod maize may be dent, sweet, waxy, pop, flint or floury in endosperm characteristics. It is merely a curiosity and is not grown commercially.
Waxy Corn: Wax corn has a uniformly dull appearance. Instead of amylose, the starch of waxy corn is amylopectin as a result of waxy (wx) mutation. Ordinary corn consists of about 78% amylopectin (a high molecular weight branched chain starch) and 22% amylose (a low molecular weight straight chain starch).

Maize Floral Biology and Pollination
Maize is monoecious with determinate growth habit and highly cross-pollinated where about95% of the pistillate flowers on a cob receive pollen from nearby other plants (Mandal, 2014). About 5% of the 18 kernels on a cob are produced as a result of self-pollination. Maize is generally protandrous, that is, male spikelets mature earlier than the female spikelets. The pollen shedding normally begins 1-3 days before the emergence of silk and continues 3-4 days after the silks are receptive and ready for pollination. According to Poehlman (1987), a single tassel may produce as many as 25,000,000 pollen grains or an average of over 25,000 pollen grains for each kernel on an ear with 800-1000 kernels. Pollen grains are very small, barely visible to naked eye, light in weight, and easily carried by wind. Wind borne nature of pollen and protandry facilitate cross-pollination, but 5% self-pollination may happen. Pollen grain viability is for about 12-18 hours and thereafter they may be killed in few hours by heat or desiccation. In hot, dry and windy conditions, the pollen shedding may be over early. Under these conditions, the tassel may be injured or the silk may lose the moisture and as a result, barren cobs are produced.
For crossing purpose, the top of an ear before emergence of the silk is cut by a sharp razor and covered with butter paper bag. Simultaneously, the tassel of the desired male parent is covered with tassel bag. The anthesis (dehiscence of anthers) starts from the central shoot of the tassel. It starts from the top and proceeds downwards. The covering of tassel is done on those male parents in whichone-fourth of the tassel has dehisced.
Pollination is done when a uniform growth of silk is visible on the cobs covered with butter paper bag. The tassel bag containing freshly shed pollen is transferred over the cobs after removing the butter paper bags from the cobs.

Important tips in carrying out pollination on maize
a)    Bagging of the tassel should be done one day in advance in previous evening to avoid contamination from foreign pollen.
b)    Date of pollination and name of programme must be written on the tassel bags with water-proof pencil.
c)    Pollination must be completed within 3-4 hrs after removal of anthers to ensure viability of pollen.
d)    Pollination must be completed within one week of silk emergence.

Climatic requirement for maize production

Temperature requirements
The optimum temperature for maize growth and development is 18 to 32 °C, with temperatures of 35 °C and above considered inhibitory. The optimum soil temperatures for germination and early seedling growth are 12 °C or greater, and at tasselling 21 to 30 °C is ideal. However, high temperatures, which can exceed 38 °C may limit crop establishment.
Rainfall requirements

Maize can grow and yield with as little as 300 mm rainfall (40% to 60% yield decline compared to optimal conditions), but prefers 500 to 1200 mm as the optimal range (CARDI and NWS, 2008). Depending on soil type and stored soil moisture, crop failure would be expected if less than 300 mm of rain were received.
Photoperiod requirement
Maize is grown globally from 50°N to 40°S, and from sea level up to 4000 m altitude. Maize is a short-day plant with 12.5 hours/day being suggested as the critical photoperiod. Photoperiods greater than this may increase the total number of leaves produced prior to initiation of tasselling, and may increase the time taken from emergence to tassel initiation (Birch 1997).

Soil requirement for maize production
The preference of most field crops is for fertile, welldrained loamy soils. Maize is relatively well adapted to a wide range of soils with pH 5.0 to 8.0. It is not as acid tolerant as peanut, but is more tolerant to low phosphorus (P) than soybean. Maize is moderately sensitive to salinity, which reduces uptake of nutrients and decreases total dry matter production. (CARDI and NWS, 2008). Improved maize variety can reach maximum expression in yield only when grown on soils well supplied with balanced amounts of plants minerals. Maize requires large amount of minerals and makes the greatest demands and use of nitrogen from about two weeks before tasselling until three weeks after tasselling (Obi, 2006).

Maize Products
Maize Flour
These can be used exactly as wheat flour in making bread, breakfast meals and more. Maize flour, also called corn flour is highly rich in protein, dietary fibre and very low in fat. Maize flour is by far the most widely eaten flour after wheat and rice flour. It is uniquely rich in dietary fibre, protein, vitamin B6, magnesium and omega 6 acids, vital for good heart and fight against infections. Fortified maize flour has been used in the eradication of malnutrition in some parts of the world.

Maize Starch
Maize starch plays a leading role in determining the texture of many foods, which is vital to both the consumer and the food manufacturer, as a major factor that governs the acceptability and palatability of most food productsinclude paper manufacture, textile, adhesives and packed foods, and as the starting material for the manufacture of syrups and dextrose sugar by hydrolysis. The starch obtained from the wet milling of waxy maize, also called “amioca”, which consist mainly of amyl pectin, is non-jelly and has clear, fluid, adhesive properties. Heated and dried maize starch/water slurries yield pre-gelatinized starch, known as “instant starch” as it thickness upon addition of cold water. Glucose and dextrose are used in beer, cider, soft drinks, pharmaceuticals, confectionary, baking and jams. The dextrins are products obtained by the breakdown of the solid dry starch, which is heated with chemical products like mineral acid almost always hydrochloric acid (Meija, 2003). Maize starch is used as an adhesive in pigment coating for paper and paperboard. The most commonly used pigments are clay, calcium, carbonate and titanium dioxide. The primary purpose of coating is to enhance the printability and appearance of the paper.

Poultry and Livestock Feed
The maize grain gives the highest conversion ratio to meat, milk and eggs when is compared with others grains used as livestock feed, this is due its high starch and low fibre content which make it a very concentrated source of energy for livestock production. Although there is not available statistic for maize and livestock use, it is believed that greater maize is preferred for livestock feed and it is used as whole grains, cracked or coarse ground, dry or wet or steamed and generally supplemented with vitamins and others proteins.

Maize Silage
Maize silage is energy rich forage that can be used in the ration instead of expensive concentrates. It is highly palatable and contains fibre for rumination and energy for milk production and body condition. Maize is harvested less frequently than grasses and legumes and maintains consistent quality over a wide harvest window.  Maize is harvesedt directly without the need for in filed wilting. Maize silage has high dry matter and energy, which compliments grass-based rations.

Maize Oil
Maize oil is very important product of the maize milling industry. USA is the largest maize oil production country all over the world with its maize oil production taking up over 50% of the world total production. The maize oil is prone to be absorbed by human bodies and the absorption ratio is as high as 97%. Thus, ıt is an ideal edible oil and healthcare oil. In families of Europe and USA valuing the diet nutrition and health, they generally choose maize germ oil as the major edible oil (Orhun and Korkut, 2011). Industrial uses for maize oil include soap, salve, paint, rust proofing for metal surfaces, inks, textiles, and insecticides. Maize oil and free fatty-acids – industrial uses; chemicals and insecticides, lecithin (for pharmaceuticals, cosmetics, linoleum, printing inks, etc.), paint and varnish, printing ink, rubber substitutes, rust preventative (surface coatings), soluble oil (leather and tanning use), textiles. Maize oil is also one source of biodiesel and used biofuel. Maize oil biodiesel is suitable for use in diesel engines.

Yield and yield attributes of maize
High yield is considered to be the most important trait in any maize breeding programme. Grain yield in maize like in other crops is a complex and quantitatively inherited traits. Maize yield is cumulative effect of action and interaction of several yield components like, number of ears/plant, kernel rows/ear, kernels/row, test weight (kernel weight) and shelling percentage.
In addition to these primary yield components, there are several secondary yield components and these include nutrient uptake, photosynthesis, translocation, sink size, transpiration and respiration.  Increase in yield has been influenced by a combination of genetic and cultural factors, but it has been estimated that approximately 60% can be attributed to genetic improvement (Duvick, 1992). Increased N fertilizer application accompanied early gains in grain yield of single-cross maize hybrids (Duvick and Cassman, 1999).

Maturity duration, standability, and resistance to biotic and abiotic stresses also affect grain yield. It has been generally seen that high yielding hybrids developed have longer duration and grain filling period (early flowering and delayed senescence), rapid grain filling period, increased sink size (more kernels per unit area), larger kernels, reduced bareness, higher harvest and shelling indices, shorter plant and tassel, upright leaves, shorter anthesis-silking interval, better adaptability, superior disease and insect-pest resistance and enhanced tolerance to abiotic stresses. Further, two additional features that contribute towards higher yield in maize are ability to respond to higher levels of nitrogen and suitability for cultivation under high plant population. Good standability contributes enormously towards higher yield in maize.
Well developed root system, strong stem, short plant height, low ear placement, ability to stay green at maturity and resistance to diseases and insects are major factors affecting maize standability (Dhillon and Prasanna, 2001). These characters are of universal applicability. Stability of yield performance is important from viewpoint of having high and stable yield across a wide range of environments.
Breeding for appropriate maturity is an important consideration in maize breeding programme. Maize is a short-day plant and accordingly time of flowering is influenced by temperature and photoperiod. Traits related to maturity are days to flower, brown husk, kernel moisture at harvest and black layer formation. Days to silk are considered to be the most reliable index of days to maturity. Genetically, days to maturity are quantitatively inherited trait and selection at inbred line development has been found to be effective to design parents/hybrids with appropriate maturity. And it should also be kept in mind that there is a negative correlation between days to maturity and grain yield and therefore in view of prevailing agricultural system, the emphasis on early or mid or late duration cultivars should be decided.

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