|By goGreen | September 6, 2011|
Guidelines for selecting maize cultivars
Cultivar choice, if correctly planned, can make a great contribution to risk reduction and should constitute an important part of production planning. Cultivars differ from one another with regard to a variety of characteristics. Therefore, every cultivar has its own adaptability and yield potential. These differences between cultivars leave a producer with alternatives that can be utilised fully. The producer should, however, first verify the reaction of new or foreign cultivars before abandoning proven cultivars.
Yield potential and adaptability
As a result of a wide diversity of conditions under which maize is produced in South Africa, it is essential that cultivars that are eventually planted should be adapted to specific production conditions. Cultivars with wide adaptability can be used to stabilise yields under variable weather conditions of South Africa. Together with adaptability, stability of cultivars should also be considered. Greater stability of a cultivar, leads to predict ability of yield reaction at a specific potential.
Length of growing season
The length of the growing season of cultivars plays an important role, especially in cooler production areas. The cooler environment causes large differences among cultivars in the period from planting to flowering and physiological maturity and accentuates differences among the various growing periods.
Cultivars differ in their susceptibility to diseases such as ear rot, maize streak virus disease, grey leafspot, rust, cob-and-tassel smut, stemrot as well as root rots. Cultivars with the best levels of resistance or tolerance to a disease should be selected for planting where a specific disease occurs.
Lodging of plants has a financial implication for the producer, because a number of ears may be laying on the soil, making it uneconomical to be picked up by hand. Good progress has been made with cultivars showing less lodging, but differences among cultivars still occur.
Prolificacy can be described as the potential of a cultivar to produce more than one ear at maturity. This characteristic is linked to the adaptability of a cultivar and can be used to good advantage where lower plant populations are required or where it develops spontaneously unintentionally.
FERTILISATION OF MAIZE
It is of the utmost importance that the correct soil sampling methods be used when submitting samples for laboratory analysis.
The following rates of application, band-placed at planting 50 mm to the side of the seed and 50 mm below the seed, should not be exceeded:
0,9 m rows: not more than 40 kg N/ha
1,5 m rows: not more than 30 kg N/ha
2,1 m rows: not more than 20 kg N/ha
N and K applications should not exceed 70, 50 and 30 kg/ha for the respective row widths. Larger quantities can, however, be applied, provided that these are placed 70 to 100 mm to the side and below the seed. N should always be included in the fertiliser plant mixtures, but weather conditions and residual N in the soil will determine when most N should be applied.
Deficiencies in nitrogen give rise to young plants that are pale, light green or yellow. During later stages older leaves begin yellowing, showing at first a characteristic inverted V-shape.
The general practice is to band-place P at 50 mm to the side and 50 mm below the seed. Deficiency symptoms usually occur on young plants, especially under cool, wet conditions.
Leaves are dark green with reddish-purple tips and margins.
The general practice is to band-place K, 50 mm to the side and 50 mm below the seed in a fertiliser mixture at planting. The following application rates should not be exceeded:
0,9 m rows: not more than 40 kg P/ha
1,5 m rows: not more than 30 kg P/ha
2,1 m rows: not more than 20 kg P/ha
K and N applications should not exceed 70, 50 and 30 kg/ha for the respective row widths. Larger quantities can, however, be band-placed, provided that these are located 70 to 100 mm to the side and below the seed. A potassium deficiency is initially noted as yellow or necrotic leaf margins, beginning at the lower leaves and spreading to the upper leaves. Mature plants lodge easily when suffering a K deficiency, because stems are predisposed to stalk rot under such conditions.
The microelement, zinc, is mostly applied, as it is included in many fertiliser mixtures. A deficiency is characterised by light streaks or bands between the veins from the leaf base to the tip. Under cool overcast conditions, these symptoms suddenly appear but disappear just as quickly once the sun appears.
Successful cultivation of maize depends largely on the efficacy of weed control. Weed control during the first six to eight weeks after planting is crucial, because weeds compete vigorously with the crop for nutrients and water during this period.
Annual yield losses occur as a result of weed infestations in cultivated crops. The annual yield loss in maize because of weed problems is estimated to be approximately 10 %. The loss occurs as a result of weed competition for nutrients, water and light.
The presence of weeds during harvesting may slow the process, pollute grain with seeds, transmit odours to grain, causing downgrading, or incur additional costs for removal of seeds. Certain seeds, such as those of the thorn apple (Datura), may be poisonous when consumed by animals or humans.
Methods of weed control
Weeds can be removed mechanically, by implements or by hand. Dense stands of weeds may be burnt as an emergency measure.
Ploughing during winter or early spring is an effective method of destroying the majority of weeds. To control weeds during the season, crops may be planted in wide rows for mechanical control. Certain problem weeds in maize can be controlled by an alternative crop where crop rotation is practised.
Chemical liquids, granules or gases are used to kill germinating or growing weeds, or even weed seeds. Control of nut grass with pre-emergence herbicides is not effective when applied after emergence. It is important to cultivate fields before applying herbicides.
PRINCIPLES OF PEST CONTROL
Integrated pest management
Integrated pest management is a system whereby various strategies are used to protect crops by suppressing the insect population and limiting damage. These management practices incorporate all practical methods of pest control in a pest management system. These measures include chemical control, biological control, plant resistance and cultivation control.
Although preventive control measures may often seem to be “effective”, this effectiveness can be ascribed to relatively low population levels of
specific insects during certain years, when epidemic outbreaks do not occur. If very high infection levels do occur during epidemic years, preventive control will be ineffective and large
losses may be experienced. It is therefore important to determine the infestation level before spraying or seed treatments are applied.
This implies that pest populations are suppressed by cultivation practices, which are detrimental to the pest. These practices include soil cultivation during winter, eradicating volunteer plants, cultivar choice and adapting planting times.
Natural control of pests occurs continually in fields where natural enemies attack all the life-stages of insect pests. Aphids and hibernating larvae of stem borers in particular are killed by natural enemies. The natural enemy complex can be protected to a certain extent by using insecticides which are more environmentally friendly, and which are not very toxic to nontarget organisms.
The use of plants which are resistant to insects, is extremely beneficial, because of both short and long-term benefits. The short-term benefit of plant resistance is that it limits pest damage, whileeconomic threshold values are often not reached.
Plant-parasitic nematodes are present in all production areas of South Africa. A progressive yield loss over a number of seasons is usually the only indication of nematode infestation. Yield loss is normally a response to damage to the root system of the maize plant.
Economic control of nematodes in maize is difficult, mainly because of the high cost of nematicides. Maize price fluctuations, different cultivation practices and differences in production potential must be taken into account to determine economic justification of chemical nematode control.
When infestation levels are high, chemical control in irrigated maize can readily be recommended from an economic point of view.
Yield increases as a result of chemical nematode control are very erratic, the exception being with the presence of high infestations of rootknot nematodes in dryland maize. Rootknot nematode populations may, under favourable conditions and within one season, increase to such an extent that economic yield losses are incurred. In such cases, it is essential to control the nematode population, even though the control measures when regarded over one season, are not economically justifiable.
Economic nematode control is therefore a complex strategy, which cannot be effectively applied without proper knowledge of the infestation levels and considering various other factors.
Maize can be regarded as an important grain crop under irrigation, as it produces very high yields. It can produce from 80 to 100 tons/ha green material and 16 to 21 tons/ha of dry material within a relatively short period (100 to 120 days). It is therefore one of the most efficient grain crops in terms of water utilisation. Maize is usually produced under full irrigation in order to obtain the highest yields.
Maize is predominantly harvested mechanically, although exceptions do occur in the case of hand harvesting.
The entire plant can be cut and placed into stacks (stook) while still green. Once it is dry, the ears can be picked and threshed, or the entire plant with the ear can be utilised as maize hay. Alternatively, the plants can be left in the field to dry and the ears harvested.
In South Africa, maize is usually left in the field until moisture percentages of 12,5 to 14,0 are reached before it is harvested and delivered to a silo.
PRODUCTION MANAGEMENT GUIDELINES
Growth stage 0: from planting to emergence
Planting depth affects the period from planting to emergence, because the seedlings of deeplyplanted seed will take longer to emerge than shallowly-planted seed. If planted too deep, the mesocotyl may open below the soil surface and cause the seedling to die off. The seedling obtains its nutrients mainly from seed reserves. Primary roots may be in contact with bandplaced fertiliser even before emergence. Too much fertiliser close to the seed may cause burning.
Growth stage 1: four leaves unfolded
Some of the problems encountered during this stage should not have a permanent effect on yield, provided the problems are rectified promptly.During this stage plants are very susceptible to drift-sand damage. Hail and light frost may damage the exposed leaves, but because the growth point is still below the soil surface, damage should be negligible. Waterlogging at this stage may be harmful to the seedling, because the growth point is still below ground level. Tilling close to the plants may harm the roots, which will put the plants under stress and detrimentally affect yield.
Growth stage 2: eight leaves unfolded
Nutrient deficiencies will restrict leaf growth. If necessary, this is the correct stage to apply a fertiliser as side dressing. Nitrogen should, however, be applied to moist soil and roots should damaged as little as possible. Defoliation by hail or other factors may cause a yield loss of 10 to 20 %.As long as the growth point is still below ground level, waterlogging may cause damping off of plants. Flooding at later stages, when the growth point stays above the water, is not as detrimental.
Growth stage 3: twelve leaves unfolded
Stress as a result of water or nutrient deficiencies during this stage will affect the ultimate size and yield of ears. Plants, breaking below the growth point, will not recover.
Growth stage 4: Sixteen leaves unfolded
Hot soil surfaces may affect the development of prop roots. The tassel begins to show in the calyx. Water and nutrient deficiencies may detrimentally affect silk development and therefore the number of kernels per ear. Hail damage may detrimentally affect yield.
Growth stage 5: appearance of silks and pollen shedding
Planting dates should be chosen so as to ensure that this stage coincides with normally favourable growing conditions. Water supply is important because wilting of plants (water stress) early inthe morning detrimentally affects pollination.
Growth stages 6 and 7: hard dough
Denting of kernels begins and this is the right stage to make silage.D
Growth stage 9: Physiological maturity
Monitor moisture content of grain regularly to start harvesting as soon as possible (below 14 %) to reduce grain losses.