Maize is the most important grain crop in South Africa and is produced throughout the country under diverse environments.
Successful maize production depends on the correct application of production inputs that will sustain the environment as well as agricultural production. These inputs are, inter alia, adapted cultivars, plant population, soil tillage, fertilisation, weed, insect and disease control, harvesting, marketing and financial resources.
In developed countries, maize is consumed mainly as second-cycle produce, in the form of meat, eggs and dairy products. In developing countries, maize is consumed directly and serves as staple diet for some 200 million people. Most people regard maize as a breakfast cereal. However, in a processed form it is also found as fuel (ethanol) and starch. Starch in turn involves enzymatic conversion into products such as sorbitol, dextrin, sorbic and lactic acid, and appears in household items such as beer, ice cream, syrup, shoe polish, glue, fireworks, ink, batteries, mustard, cosmetics, aspirin and paint.
Approximately 8,0 million tons of maize grain are produced in South Africa annually on approximately 3,1 million ha of land. Half of the production consists of white maize, for human food consumption. Maize needs 450 to 600 mm of water per season, which is mainly acquired from the soil moisture reserves. About 15,0 kg of grain are produced for each millimetre of water consumed. At maturity, each plant will have consumed 250 l of water. The total leaf area at maturity may exceed one square metre per plant. 2 The assimilation of nitrogen, phosphorus and potassium reaches a peak during flowering. At maturity the total nutrient uptake of a single maize plant is 8,7 g of nitrogen, 5,1 g of phosphorus, and 4,0 g of potassium.
Each ton of grain produced removes 15,0 to 18,0 kg of nitrogen, 2,5 to 3,0 kg of phosphorus and 3,0 to 4,0 kg of potassium from the soil. No other crop utilises sunlight more effectively than maize, and its yield per ha is the highest of all grain crops. At maturity, the total energy used by one plant is equivalent to that of 8 293 15 W electric globes in an hour.
The number of kernel rows may vary between four and 40, depending on the variety. Up to 1 000 kernels may be produced by a single plant. In spite of only one pollen grain being required to produce one kernel, each tassel produces some 25 000 000 pollen grains, i. e. 25 000 grains for each kernel. As a result, up to 40 % of the tassels in a planting may be lost without affecting pollination, other factors remaining optimal.
MORPHOLOGY, GROWTH AND DEVELOPMENT
Root system
The plant has a profusely branched, fine root system. Under optimal conditions, the total root length, excluding the root hairs, can reach 1 500 m. If root growth is not restricted, the root system of a mature plant extends approximately 1,5 m laterally and downwards to approximately 2,0 m or even deeper. The permanent root system has adventitious and prop roots.
Adventitious roots 3 develop in a crown of roots from nodes below the soil surface. Normally four to six adventitious roots are formed per band. After tasselling, prop roots develop into bands from the first two to three aerial nodes. These roots are comparatively thick, pigmented and covered with a waxy substance. Prop roots have the dual function of providing support to the plant and taking up nutrients. Numerous root hairs occur on young plants. Root hairs increase root surface area that is exposed to the soil, and play an important role in absorption of water and nutrients.
Leaves
The eight to 20 leaves that may form are arranged spirally on the stem, and they occur alternately in two opposite rows on the stem. The maize leaf is a typical grass leaf and consists of a sheath, ligules, auricles and a blade. The leaf blade is long, narrow, undulating and tapers towards the tip and is glabrous to hairy. The leaf is supported by a prominent mid-rib along its entire length. Stomata occur in rows along the entire of the leaf surface. More stomata occur on the underside of the leaf than on the upper surface. On the upper surface motor cells are present. These large, wedge-shaped cells occur in rows, parallel to and between the rows of stomata. During moist conditions, these cells rapidly absorb water, become turgid and unfold the leaf. During warm, dry weather, the cells quickly lose their turgor with the result that leaves curl inwards exposing a smaller leaf surface to evaporation.
Stem
The maize stem varies in height from less than 0,6 m in some genotypes to more than 5,0 m (in extreme cases) in others. The stem is cylindrical, solid and is clearly divided into nodes and internodes. It may have eight to 21 internodes. The internodes directly below the first four leaves do not lengthen, whereas those below the sixth, seventh and eighth leaves lengthen to approximately 25, 50 and 90 mm, respectively. Tillers may develop from nodes below the soil surface. The lateral shoot bearing the main ear develops more or less from the bud on the eighth node above the soil surface. The five or six buds directly below the bud give rise to rudimentary lateral shoots of which one or two develop to produce ears.
Inflorescence
Male and female flowers are borne on the same plant as separate inflorescences. Male flowers are borne in the tassel and female flowers on the ear.
Maize ear
The maize ear (the female inflorescence) terminates one or more lateral branches, usually halfway up the stem. Bracts enclose the ear. The silk of the flowers at the bottom appear first and thereafter those on the upper part of the ear. It remains receptive to pollen for approximately three weeks but after the tenth day, receptivity decreases.
Maize kernel
The maize kernel consists of an endosperm, embryo, a pericarp and tip cap (Fig. 1). The endosperm contains the main carbohydrates. The embryo contains the parts that give rise to the next generation, while the pericarp and tip cap enclose the entire kernel.
The endosperm contains approximately 80 % of the carbohydrates, 20 % of the fat and 25 % of the minerals, while the embryo contains about 80 % of the fat, 75 % of the minerals and 20 % of the protein found in the kernel. The starch part of the kernel is used in foods and many other products such as adhesives, clothing, and pharmaceutical tablets and in paper production.
The starch can be converted into sweeteners and used in products such as soft drinks, sweets, bakery products and jams, to name but a few. The oil from the embryo is used in cooking oils, margarine and salad dressings. The protein, hulls and soluble part of the maize kernel are used in animal and poultry feed.
Kernels can be of the dent or flint (round) types. Dent kernels have a dented crown, which is formed during drying when the softer starch in the middle of the kernel shrinks faster than the outer more translucent sides.
The dent kernel has two flat sides opposite each other and the one side contains the embryo. The embryo contains all the parts that give rise to the next generation. Flint kernels can be round or flat in appearance and contain mainly translucent starch, with only a small part of soft starch in the middle, hence the name. The pericarp and tip cap enclose the entire kernel.
Maize with a high percentage of translucent of hard endosperm is preferred by the dry milling industry, because it produces more of the popular high-quality and high-value products sought after than does soft maize.
Growth and development
Different growth stages are numbered 0 to 10.
Growth stage 0 lasts from planting of the seed up to when the seedling is just visible above the soil surface. Growth stage 10 is reached when the plant is biologically mature.
Growth stage 0: from planting to seed emergence
During germination, the growth point and the entire stem are about 25 to 40 mm below the soil surface. Under warm, moist conditions seedlings emerge after about six to 10 days, but under cool or dry conditions this may take two weeks or longer. The optimum temperature range for germination is between 20 and 30 ºC, while optimum moisture content of the soil should be approximately 60 % of soil capacity.
Growth stage 1: four leaves completely unfolded
The maximum number of leaves and lateral shoots is predetermined and a new leaf unfolds more or less every third day. The growth point at this stage is still below the soil surface and aerial parts are limited to the leaf sheath and blades. Initiation of tasselling also occurs at this stage.
Growth stage 2: eight leaves completely unfolded
During this period, leaf area increases five to 10 times, while stem mass increases 50 to 100 times. Growth stage 1 9 Ear initiation has already commenced. Tillers begin to develop from nodes below the soil surface. The growth point at this stage is approximately 5,0 to 7,5 cm above the soil surface.
Growth stage 3: twelve leaves completely unfolded
The tassel in the growth point begins to develop rapidly. Lateral shoots bearing cobs develop rapidly from the sixth to eighth nodes above the soil surface and the potential number of seed buds of the ear has already been determined.
Growth stage 4: sixteen leaves completely unfolded
The stem lengthens rapidly and the tassel is almost fully developed. Silks begin to develop and lengthen from the base of the upper ear.
Growth stage 5: silk appearance and pollen shedding
All leaves are completely unfolded and the tassel has been visible for two to three days. The lateral shoot bearing the main ear as well as bracts has almost reached maturity. At this point demand for nutrients and water is high.
Growth stage 6: green mealie stage
The ear, lateral shoot and bracts are fully developed and starch begins to accumulate in the endosperm.
Growth stage 7: soft dough stage
Grain mass continues to increase and sugars are converted into starch.
Growth stage 8: hard dough stage
Sugars in the kernel disappear rapidly. Starch accumulates in the crown of the kernel and extends downwards.
Growth stage 9: physiological maturity
When the kernel has reached its maximum dry mass, a layer of black cells develops at the kernel
base. Grains are physiologically mature and only the moisture content must be reduced.
Growth stage 10: drying of kernels
(biological maturity)
Although grains have reached physiological maturity, they must dry out before reaching biological maturity. Under favourable conditions, drying takes place at approximately 5 % per week up to the 20 % level, after which there is a slowdown.