|
 Main subject - may/june 2006
Hybrid Mayze for High Yield
Prof. Dr. Claudio M. Mundstock
cmmundsttock@ufrgs.br
Prof. Dr. Paulo Regis F. da Silva
paulo.silva@ufrgs.br
Average yield of maize grain in Brazil has continuously evolved, especially from middle last century. The main factors for increasing yield were: the use of materials with greater yield potential, larger use of fertilizers and defensive, more efficient agricultural machines, and the adoption of the system so called (direct planting, with crop rotation and succession). The use of those tools was only possible by means of the generation of technical information, thanks to the efficient research system performed by several public institutions and private companies.
Beginning colonization, grains yield rarely overpassed 1.5 ton/ha and only in the first decades of last the century a yield increase was noticed due to the introduction of more productive materials.
The combined adoption of improved cultivars, suitable inputs and cultivation techniques caused crops yield to become increasingly higher, decade after decade. The evolution of the culture techniques, crop management, and use of suitable cultivars is summarized in Table 1, with the average obtained from the best crops in each decade.
What is the yield potential for maize?
Grain high yield results from the success in using environmental factors with maximum efficiency, minimizing adverse causes to crop development. Such complex equation depends mainly of three weather factors: solar radiation, air temperature, and water availability.
Getting the most of environmental resources use can only be expressed in suitable cultivars or hybrids. Creole populations do not show a good yield potential, since they have been selected due their fitting to consortium systems and tolerance to adverse factors. With the genetic improvement, initially with the development of synthetic cultivars and later the hybrids, many materials have raised which were able to efficiently use environmental factors as well as tolerating higher plant densities.
Differences on grain yield potential of improved open population, synthetic, double hybrid, simple hybrid, when cultivated under high level management conditions are on Table 3, which shows the genetic evolution used in maize improvement programs.
Our experimental station has been performing studies on potential yield on a same environment for four decades. Such trials showed that even in the 1970's there were high yield potential hybrids. Since then, has been noticed a stagnation on yield potential. Higher yield hybrids have only appeared in the market in the 1990's in face of producers demand due to soil conditions improvement (fertility increase), the use of crop rotation and succession with direct planting, the use of higher planting density, and the reduction of row spacing.
Main changes in crop systems
Cultivation system comprises the complex of techniques adopted in each crop in their interactions with other ones (crop rotation and succession), with crop residues management and soil preparation. It is the most complex component in the designation of grain yield and their effects are visible only some time after adopting a system. Crop system is the main determinant for obtaining high grain yield, and it must be kept with no changes from one year to another because it has a cumulative effect as to crop benefits.
The current crop system, initiated late in the 1970's but it was only fully adapted in the 1990's beginning, is based on straw direct planting, without revolving the soil, and on adopting suitable crop rotation and succession systems. There has been a drastic reduction in soil erosion as well as a progressive improvement of soil physical and chemical conditions. With this, it was possible to use other crop techniques more efficiently, which increased grain yield, as, for example, the use of higher yield potential hybrids, higher plant densities, and reduced row spacing.
Crop rotation and succession are the key for grains production system with straw direct planting. The adoption of such system allowed the increase of grain yield, for the first time, overpasseding 10 ton/ha.
Main changes on plant genetics
Genetic potential of a given maize hybrid or cultivar is crucial for obtaining the highest yield, along with improving environment and crop management conditions. Maize genetic resources available to the farmer, have varied significantly along the past decades. The main landmarks of such change were:
a) replacement of creole by improved populations (from 1930's to 1950's);
b) adoption of the double hybrid, in the 1950's and 1960's; and
c) use of simple and triple high yield hybrids, from the 1990's on.
Some plant features which have determined grain yield increase will be presented in each of the three following stages:
1º Introduction of improved populations - The beginning of 20th century was marked by a strong exchange of genetic material among researchers. Brazil, at that time, has also introduced other countries cultivars, especially from United States, which based the development of higher yielding population when compared to the previous creole populations.
Such genetic exchange has incremented grain yield due to the improvement of plant morph-physiological features. The main one was the greater uniformity among plants, especially height, insertion height, and cob size. Those populations tolerated plants densities on a higher level than the previous ones, which caused grains yield to increase.
2º Introduction of double hybrids - In the 1950's one of the greater genetic improvement revolutions has happened through the commercial introduction of maize double hybrids. The main feature of the double hybrid determining high yields were tolerance to higher plant density and greater uniformity among the plants, in cob size, plant height, and cob insertion height. The increase in the number of grains per cob (bigger cobs) and plants densities were the main factors responsible for the increment on grain yield potential, verified by the introduction of double hybrids. Many double hybrids are still being commercialized and their use may be a good option for achieving high yield and economical profits, in cases of specific management.
3º Introduction of simple hybrids - Simple and triple high yield potential hybrids were introduced in the second half of the 1990's. The first simple hybrids, were introduced late in the 1980's, however it did not show more advantages over the good double hybrids available at that time. The change on companies' strategic view when introducing simple hybrids adapted to environmental conditions, especially soil types, water and temperature stresses, responsive to chemical fertilization and plants high densities, has made it possible to obtain grain yields over than 10 t/ha.
The evolution on genetic improvement evidences that, potentially, there has been a genetic gain over time, and such gain is expressed even under low level management conditions, in opposition to a general concept that, under stress conditions, open populations would be the best option.
The new concept of "high yield" necessarily passes by the adoption of a "high potential" hybrid, whatever the final yield achieved. Such change in the conceptualization is fundamentally due to the incorporation of hybrids with more efficiency features on the use of environmental factors, in the transformation of its dry mass into grains, and the higher tolerance to environmental stresses.
Basic yield factors are the maximum use of solar radiation combined to temperature and water suitable availability. For that, it is necessary to adopt high density plants in order to get a suitable foliar area for fast capturing the incident radiation and keeping it longer. Radiation is a fundamental factor for photosynthesis CO2 fixation and for producing the dry mass which is efficiently converted into grains. Plants high density is only recommended in case the hybrid tolerates an aggressive competition among the plants, and is still able to form the cob and produce grains. Such plant morph-physiological aspects were the features which have most changed over the last hybrid generation. Generally speaking, they have been under changes while time passed along, which allowed the increase on plants density. However, besides the higher tolerance to the competition among plants, it was noticed that there has happened the incorporation of a good productive capacity in situations which the crop is submitted to other types of stresses, such as water or nutrient stress.
page 2 ->> Hybrid Mayze for High Yield
|
|
............
|
