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 Main subject - may/june 2006
continuation: Hybrid Mayze for High Yield
It would be interesting to analyze the fashionable concept of using open population cultivars or low potential hybrids in medium or low grains yield crops, justifying that they are "rustic" materials, with high tolerance to environmental stresses. The use of simple, contemporary, high potential hybrids in such situations show that they are more suitable and more tolerant to such stresses than the old populations. The higher seed cost of these hybrids can be compensated by the increment obtained on grain yield.
Tab. 1 - Main technological innovations in maize associated to high yield fields, in Brazil
Main changes on plant nutrition
High grain yieldings are achieved when the soil is able to supply plants nutritional needs and store enough water to avoy that the plants are damaged by short periods without rain.
Taking into consideration that most nutrients is absorbed between emergence and flowering, its release from soil must be fast and plentiful. Such quantity is hardly ever supplied by soils, in case there is no fertilization, either chemical or organical type. The soils of the main maize producing areas are, generally, poor in phosphorus and do not have enough organic matter to supply the nitrogen needs for that crop.
Recommended fertilization doses have been increasing over the past couple years due to the increase on grain yield. In order to achieve such high yield it is necessary to make nutrients available to plant since its sowing. The system of straw direct planting has changed the fertilization recommendation since it takes into consideration not only the expected results but also the contribution of the previous crop, especially in nitrogen fertilization.
The great change in fertilization lies on the interaction between recommended nutrient dosages and the species and dry mass yielding of the crop preceding maize. Such interaction is essential for promoting high yield. The increase of maize grains yield is only obtained when cultivated after a non grass species. Such interactive effect is not well known, so that it can not be explained only by the nutrients supplied by the previous species.
Main changes on planting time
Producers choose planting times based on the risks of water deficiency during critical periods of low temperatures and frost as well as on the crop rotation and succession system. With this, plantings are seen in up to seven months in higher temperature areas.
Wide planting band is generally adopted when the yielding is not high. As grains yielding is supposed to be improved, air temperature and solar radiation must be further observed, which must be high during pre-flowering to milky grains state. With this, the best time for maize planting in the Rio Grande do Sul, for example, is during October so that the flowering occurs in December and grains filling occurs in January and February (southern hemisphere). Such recommendation must be adopted in areas with low water deficiency risk in December, January, and February, or under supplementary irrigation conditions.
When the risk of water deficiency in summer is high or depending on the sequence of crop adopted, producer sows early in the season or late in the period in which crop establishment is possible. In one or other cases, in both times, the crop does not benefit from all solar radiation and yields are potentially lower.
The effects of water deficiency are very drastic on maize crop, resulting in severe restrictions to grains yield. The periods of water deficiency are occasional and not well defined as to their occurrence during the year. It is noticed that there is a sowing concentration in a clearly defined period for each area. The decision is usually taken in face of the water deficiency during crop cycle. Plantings early in the season (usually in August) are less likely to suffer water deficiency. The harm from less radiation available to the plants early in their cycle is partially compensated by the high sun stroke verified in December / January, late in the cycle, which benefits grains filling. Plantings during the second crop (December/January) present lower potential, since flowering will take place early in March, when solar radiation is low, with less grains filling in March/April. Besides, plants are more susceptible to diseases attacks at that late time.
Grains yields over 10ton/ha have being currently achieved in August/September plantings. It shows that hybrids potential can still be higher if planting is performed during October, when there is no risk of water deficiency.
Tab. 2 - Maximum maize grain yield obtainned in research areas with high level cropping technology
Main changes on plants arrangement
High maize grain yields can only be obtained with the perfect fitting of the number of plants by area unity. The ideal plants number per area is determined according to the hybrid used, the way of using maize, soil fertility level, and expected fertilization, and the risk of water stress during the cycle.
Under the best environment conditions, current hybrids are recommended with densities from 70,000 to 75,000 plants/ha, for achieving crop yields between 12 and 13 t/ha. Density recommendation must be reduced in less favorable environments, such as under soil low fertility or water deficiency.
Plants density has been gradually increasing over the past 50 years due to the appearance, in the market, of tolerant hybrids to the stress posed by the higher competition between plants, but in a more effective way, in the past 10 years.
Maize plant has a peculiar feature as to the reaction to the increase on plants density. The capacity of releasing cobs and stigmas emergence, depends mainly on the stress tolerance of each material has to that stress. Old creole populations and the first hybrids launched in the market did not show such capacity, and the maximum densities recommended have hardly ever overpassed 50,000 plants/ha. Discontinuing the focus on searching materials with this feature was only broken during 1990's, when some simple, high plant density tolerant hybrids arose. Maize response to high densities varies, besides hybrid, water and nutritional soil conditions, with row spacing.
Rows spacing has became smaller over time, from 1.0m to 0.50m. Larger spacings, which were used to make consortium system and the weed control, were maintained early in the mechanization period in order to make weeding easy. The use of straw direct planting has prevented the mechanized weeding by using selective herbicides, and the distance between rows could be decreased.
Row spacing of 0,45m
The smaller spacing between rows allows arranging plants in a more equidistant way. Its advantage are fast soil coverage, better use of solar radiation and less water loss. It is only maintained in situations of high grain yield in which the use of solar radiation is the limiting factor to yield. Another benefit from using reduced row spacing is the more efficient weed control due to soil fast shadowing. Usually, when crops have severe restrictions which cause grain yield to be lower, there are no benefits from using smaller row spacing. Usually, benefits from using reduced row spacing on grain yield are small, up to 10%.
Why is it necessary to obtain high yield?
Maize higher economy is usually obtained through management, which results in grain high yield. Adopting such concept has guided the discussion on the real need of achieving high productivity with more results per hectare of crop. The two essential points in such discussion are: probability of weather risks, and producer's investment capacity.
Weather risks refer, mainly, to frequent drought times in unpredictable moments. In this way, a thought states that in areas which are more likely to such risks, crops must receive less input investments and use lower potential hybrids. In this case, if water deficiency occurs, economical losses would be less. However, with such strategy, high yields will not be expected if weather conditions are favorable during growth season. At long term, profit will be small because, although the losses in the dry years are smaller, the gains in the favorable years will not be large.
Another thought, by which the producer is able to invest in inputs, proposes investments aiming at high profits, even facing risks due to the likely water stress but betting on larger and compensating gains in favorable years. Research has shown that the materials, which are managed to potentialize grains yield, are more water stress tolerant than those which received less investment. Plants grow more, which allows successfully tolerating stress periods. Producer success comes from realizing that there is only profit from maize if the productivity is high, because profit margins are only made viable in this situation.
Differential costs between an average and a high yielding crop are in the seed and other inputs costs, as fertilizers and chemicals. Increments on grains yield have been as high that they can cover costs increase and make crop viable.
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