The ever-increasing World population means that there is constant pressure on food.
In a world of limited size, there is thus great incentive to produce improved varieties of crop plants.
Man has been able to make progress in this direction through the study of genetics, which enables him to transfer desired traits from one plant to another.
The plant breeder utilizes the discoveries of the geneticists, together with those of other branches of science, particularly cytology, taxonomy, physiology, anatomy, and biochemistry, to achieve improvements in crops.
Aims of crop improvement in Agriculture
To increase yields. The primary purpose of plant breeding is to develop varieties that use of nutrients. thus increasing yields without loss of quality.
To meet particular needs of growers. Improvements in farming methods may make particular characteristics in plants desirable.
For example many growers prefer dwarf varieties of sorghum which can be harvested by machine instead of tall ones that can be harvested only by hand.
To meet the needs of consumers. The demands of food processing and consumers create pressure for new qualities in plants. For example, maize with more protein and less chaff, and tomatoes with higher vitamin content.
To adapt to climatic conditions. It is of great importance to develop varieties that are able to withstand extreme conditions of cold, drought, and wind.
Frequently this has been accomplished by adjusting the growth cycle of the variety to suit the growing season.
To increase resistance to pests and diseases. The development of crop varieties that have resistance to pathogenic organisms and insects has provided the only feasible control for certain diseases and pests of some crop plants.
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Means of crop improvement
Crop improvement practices which can be used to produce more from crop plants in less time or by a smaller number of people can be directed towards genotypic and phenotypic factors.
Crop improvements brought about by breeding involve genetic change and can therefore be transferred from one generation to another.
Some of the methods which have been used to achieve these include introduction, selection, hybridization, and mutation.
One species or varieties of crops with desirable qualities can be imported into areas where they have not existed before. This is possible if the environmental conditions in such areas are similar to those of the plant’s original home: for example, cocoa was introduced to West Africa from Brazil in South America.
Alternatively, pollen can be collected and shipped to other areas for artificial pollination. Plants are allowed into most countries only after they have passed quarantine tests.
From earliest times. man has recognized variability in inherent productivity among individuals. This means that within any species of plant or animal, some individuals do better than others. It is also known that ‘like begets like’ This principle is used in Selective breeding.
This is done artificially by picking, for breeding, individuals with certain characteristics which are most favoured by the environment. In the natural environment, those not favoured will die off, this is natural selection.
It is possible to discern by observation of fundamental differences in the quality of plant material and to select and increase the desirable types.
Since selection cannot create variability but can only improve that already in existence, selection is only effective on heritable differences.
This is the development of plants by transferring heritable qualities from one plant to another through the fusion of male and female gametes.
This is the most important of all crop improvement methods because the desirable
characters can be transferred from parents to their successive generations. Hybridization can occur in animals as well as plants.
This involves crossing plants of different varieties. It is used to upgrade species of plants which have qualities such as high yield, diseases resistance, and tolerance to heat, drought, and wind.
The pollen grains from plants with such desirable qualities can be collected and transferred manually to the stigma of the flower of another plant of the same species which might be lacking in such character, but have its own special traits such as adaptation to a particular environment.
When fertilization occurs, the offspring will
combine the good qualities of the male plant with that of the female plant. The offspring with the desirable qualities will be selected for propagation.
A plant which has been selectively fertilized or crossed continuously with closely related species (inbred) for many generations so that the desirable qualities it possesses does not change from generation to generation, is
called a pure line. It will always reproduce itself with great precision. If pollen from a pure line variety is used to pollinate another plant of the same species, then various grades of the improved variety result.
If a pure line species is crossed with another pure line species whose dominant characters are markedly different, a hybrid variety will emerge. This will be accompanied
by great vigour or heterosis (hybrid vigour).
The less closely related the parents in such a
cross, the greater will be the heterosis.
Crop susceptibility or resistance to any
disease is controlled by a specific set of genes.
To produce a variety resistant to a certain disease, the gene number or arrangement must be changed. The new arrangement is called a mutant.
Mutants may be produced naturally or artificially. In artificial mutation, mutagens such as X-rays or colchicine are used to destroy the chromosomes responsible for disease susceptibility. In this way, a diseases resistant variety is produced.
This involves the development of the crops physical appearance. This can be achieved by the adoption of modern agronomic practices which provide an opportunity to improve crop production. These practices include using improved soil tillage systems, special soil.
Conservation and soil fertilization methods planting at the right time and adoption of scientific planting operations. This may also involve such practices as nursery and transplanting techniques for suitable crops and efficient weed, disease, and pest control
Improvement methods of crop
A poor variety of plant cannot be made to produce well, however perfect the cultural methods adopted.
It is therefore important that the variety of crop chosen for propagation should be of high genetic quality.
Genetic improvement methods should therefore be integrated with good agronomic practices and effective means of utilization of agricultural products through better methods of preservation, storage and transportation to achieve high productivity
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