Man initially existed on the natural produce of land, sea and rivers where fruits, wildlife, shellfish, etc., were fetched. He was not satisfied with his way of life then, because people were increasing, and his daily search for food was not promising. Green light for the civilised life came when man discovered that fruits and crops were growing around his shelter. The seeds he had been throwing about were growing to produce fruits. With less food problem now to think of, man devoted more of his time and energy to developing his agriculture. Continuous increase in population has forced man to develop better means of increasing food supply. This desire is expressed in man’s ability to cultivate improved varieties of cocoa, oil palm, rubber, cassava, maize, etc. The use of these varieties will ensure healthy plants, which will yield well and increase the farmers’ profit.
Most of the crops grown for either domestic consumption or commercial purpose possess one or more desirable, or undesirable traits. Improving upon a particular variety of crops means transferring superiority to it from others of the same species and eliminating undesirable characteristics. The great work is often undertaken by plant breeders and the science of improving the crop is known as plant breeding. This assignment, in Nigeria, is mostly done in Agricultural Research Institutes. Some of such Institutes include National Root Crops Research Institute, Umudike. National Cereals Research Institute – Badeggi. International Institute of Tropical Agriculture. Institute of Agricultural Research -Ahmadu Bello University, Samaru, Zaria.
Aims of Crop Improvement
The major targets of the crop breeder while embarking on the task of improving crops, include achieving the following:
(i) Increased Yield: The attention of the plant breeder is focused on producing fast growing crops with very high yields per hectare of land, for instance, a hybrid maize may produce as much as 6 tons/ ha compared to local variety with about 1.2tons/ha.
(ii) Improved Quality: The breeder considers palatability, nutritive value, low fibre content, high sugar content, clean seeds/grains, etc., his goal as he develops new varieties of crops.
(iii) Adaptability: The breeder also thinks of developing varieties of crops that are able to withstand extreme conditions of cold, drought, wind, and other adverse climatic conditions.
(iv) Resistance to Pests and Diseases: The breeder also aims at producing varieties that have resistance for pathogens, insects, nematodes, etc.
(v) Improved Harvesting Quality: To facilitate harvesting of some crops with machines, the production of dwarf instead of tall crops is targeted by plant breeders e.g. dwarf sorghum, oil palm, coconut, etc.
Methods of Crop Improvement
The aforementioned aims are largely achieved through the contributions of the science of genetics, which enables the breeder to transfer desired traits from one plant to another. He uses any or a combination of the three methods below:
(i) The Introduction Method
Any crop grown outside its home of origin is an introduced crop. Many crops grown in Nigeria now were brought from other countries by travellers. Cassava and cocoa for instance, originated from South America, while maize originated from Central America. It should be noted that any crop taken into a country is quarantined to make sure it is pest- and disease-free.
Advantages of introduction method of crop improvement
(i) It is useful in starting research work.
(ii) Desirable traits of crops can be quickly obtained.
(iii) It is easier to practise compared to pureline selection or cross breeding.
Disadvantages of introduction method
(i) There is no change in traits.
(ii) Desirable traits may deteriorate with time because of cross pollination with undesirable traits.
(iii) Introduced crops may find it difficult to adapt to new environment.
(iv) Diseases and pests may be introduced along where quarantine is not enforced.
There may be some observed variations in appearance, size, weight, etc., in the seeds of crops even within the same stand of plant or the same variety of crop in the farm. The performance, growth rate or yield of these crops may not be the same. Since the farmer aims at raising fast growing crops with high yields, he, therefore, has the option of selecting the outstanding types and discarding those with undesirable traits. Individual or mass selection of planting materials should be carefully and scientifically carried out to achieve the desired objective.
The plants are not identical in genetic characters of the parent plants. Seedlings with undesirable characters may be produced not withstanding how favourable the environmental factors are. Explanations to these variations are offered by the science of Genetics. Genetics is the science of passing on characteristics, or traits, from parents to offspring. The man who worked to develop the knowledge of Heredity is Gregory Mendel. It is confirmed that genes, which are the hereditary factors (i.e. determinants of the characters), are in the nucleus.
Mendel, in 1866 stated two laws of inheritance as follows:
(i) The law of segregation: The law states that any character is controlled by a pair of genes but that they are independently passed on through a gamete to the offspring. The separation of genes during the formation of reproductive cells is referred to as segregation. A plant for instance, which has a pair of genes (7) Tallness and (t) Shortness would transmit only one of these through its gamete to its offspring.
Mendelian Law of segregation
AA – Homozygous tall Pea
(ii) The law of Independent Assortment: The law states that each pair of character behaves as a separate unit and is inherited independently of any other character. A plant for instance, possessing a pair of genes with Tall and Short character for shape and Yellow and Pink character for stem colour, would have the pair for shape transferred as an independent unit during cell division.
Relevance of genetics in agriculture
It should be noted that the principles of genetics are the same for both plants and animals. Breeders have exploited the facts of heredity to improve crops and animals to meet the needs of the growing population. When a particular wild seed shows a new character and that character leads to better
yield, man crossed it with existing varieties. This has been the practice with most crops like mango, oil palm, orange, cassava, maize, etc.
1. Crop improvement is a means to strengthen and transfer desirable traits in plants while eliminating the undesirable characteristics.
2. Plant breeders in Research Institutions and Higher Institutions of Learning in Nigeria undertake the Crop Improvement exercise.
3. Crop Improvement aims at:
(a) increasing yield of crops.
(b) improving quality of crops.
(c) developing crops adaptable to climatic conditions.
(d) producing disease and pest-resistant crops.
(e) Improving the harvesting of quality crops.
4. Crop improvement can be achieved by means of:
(c) Cross breeding (Hybridization)
5. Desirable traits can quickly be obtained using Introduction and Cross breeding methods while selection could not lead to establishment of new traits.
6. Products of Hybridization (Hybrids) are fast growing, high yielding, disease and drought resistant.
7. Genetics is the science of passing characteristics of traits from parents to offspring.
8. Genes are determinants of characters and are located in the chromosomes of the nucleus.
9. Gregor Mendel, the originator of the science of Heredity developed two laws as follows: –
(a) The first is the Law of Segregation of genes which allows for independent transmission of genes to offspring.
(b) The second is the Law of independent Assortment of genes which describes the control of the behaviour of a character by a separate unit or a pair of genes.
10. Most crops like cocoa, oil palm, mango, etc., are improved through facts exploited from Genetics.