Subject: Agricultural Science
Topic: Crop Improvement (Methods)
Lesson Objectives: At the end of the lesson, learners should be able to:

  1. State and explain the methods of crop improvement,
  2. State the advantages and disadvantages of the different methods of crop improvement,
  3. State and explain common terms used in genetics.

Lesson Discussion

In the previous lesson we discussed the definition, history and aims of crop improvement. In this lesson, we will be discussing the methods of crop improvement.

Methods of Crop Improvements

Plant breeders use mainly introduction, selection and cross breeding (hybridization) to bring about crop improvement.

(1) Crop Introduction

This involves taking a plant from its centre of origin to a new place where perhaps, it had not been in existence but where the ecological conditions favour its growth. Some of the crops being grown in Nigeria today were introduced by travellers and traders. For instance, onions, pineapple, groundnut, cocoa, rubber, tobacco, cassava, maize, cocoyam etc were introduced into West Africa especially in the 19th century.


Merits of Introduction

The introduced crops usually have desirable characters and may be used in cross breeding and upgrading of poor local varieties. Crops with desirable traits could be selected from the introduced crop varieties and then planted out. The introduced crops may be planted directly into the field with a view to realizing some economic returns. Other advantages of crop introduction include:
i. Using the introduced crop to produce new and superior crop varieties that are high yielding, disease resistant, have improved quality and show adaptation to adverse climatic conditions.
ii. Using the introduced crop to experiment on whether the improved crops can adapt to the climatic and soil conditions of the country to which they are introduced.
iii. Supplementing stock already available in an area or country with the introduced varieties.

Disadvantages of Crop Introduction

i. It can lead to the introduction of crop disease pathogens that are alien to the new country.
ii. Introduction leads to gradual loss of indigenous crop varieties.
iii. Cost of crop introduction is rather high
iv. Introduction of improved stock requires skilled manpower
v. It does not create variety in the genetic make-up of the crops being introduced.
However, it is recommended that crops to be introduced should be guaranteed and certified free of pests and pathogens before they are finally introduced and used.

(2) Selection

Plants inherit certain traits from their parents or the parent stock. Selection makes use of only those qualities that are inheritable and desirable. It does not create genetic variation in the progeny. Crops which are high yielding, disease resistant, early maturing, have strong stems, high oil content, high sugar or protein content are selected and are then planted out.
Selection is based on the performance of the crop ancestors and the individual crop plants under the prevailing edaphic (soil) and climatic conditions

Types of selection

i. Individual selection: This involves a selection of an individual crop that has desirable characters. Such a crop seed is planted out and the F1 generation that are desirable are selected for subsequent planting and selections.
ii. Mass selection: Many plants that have desirable traits are selected and are then planted out. The progeny of these mass selected crops that are phenotypically desirable are also selected for subsequent planting and selections.
iii. Pedigree/Ancestral selection: When the selection of a crop is based on the performance or the desirable traits in the crop ancestors, it is referred to as pedigree selection.
Selection is a continuous exercise and may be made from among plants produced by the seeds originally selected.

Advantages of selection

(a) only species with desirable characteristics are allowed to grow whether artificially or naturally.
(b) it results in the selection of vigrous species that are adapted to the environment.
(c) it leads to the selection of disease and pest resistant crops.
(d) it results in a retention of high quality varieties
(e) it is used for crop improvement

Disadvantages of crop selection

(a) selection reduce the number of population of crop varieties in an area
(b) if the selection was poorly made, it can lead to rejection of crops with good traits
(c) cost of operation is high
(d) selection requires skilled manpower
(e) it does not create genetic variation in the offspring

(3) Cross Breeding

The products of crop selection are cross bred to produce crops with hybrid vigours. Cross breeding (hybridization) involves crossing two different parents of a crop’s species to produce a genetically improved and superior crop variety. The two different parents have certain desirable traits and these characters are enveloped in the hybrid progeny during hybridization. For instance, if;
i. High and low yielding abilities of crops are represented by YY and yy respectively and
ii. Resistance and susceptibility to diseases are represented by RR and rr respectively; on hybridization the progeny of crops A and B will be both high yielding and disease resistant.

The first filial generation has a hybrid vigours and is genetically more superior to either of the parents. There is always a continuous selfing of the progeny of various filial generations and subsequent selection of the desired offspring of each generation.
It is important that these methods of crop improvement be integrated with seed testing and the use of improved cultural practices to achieve the desired results.


Common Terms Used in Genetics

i. Gene: The gene is the fundamental unit of function of heredity and variation and is located on chromosomes. The genes can replicate themselves, undergo changes (mutation) and can influence phenotypes.
ii. Chromosomes: The chromosomes carry the genes and are usually in pairs in plants and animals.
iii. Genetics: The science that studies inheritance and variations in living organisms.
iv. Homozygous genes: Identical genes in an organism that control specific traits, e.g TT for tallness and tt for dwarfism.
v. Double Recessive: An individual homozygous for a recessive gene e.g yy representing recessive genes for low yielding abilities; rr representing recessive genes for resistance
vi. Genotype: The genetic constitution or make up of an organism.
vii. Phenotype: An outward appearance of an organism. For instance, if an organism is tall, it means that it exists in homozygous condition and has the genotype TT, or in heterozygous condition and has the genotype Tt.
viii. Alleles: Two varieties of genes that exist together in an organism and control contrasting characters.
ix. Homologous Chromosomes: The members of a pair of chromosomes. Usually, the members of a pair of chromosomes are alike in structure and composition. In every species, there is a constant number of chromosomes e.g

Organism———————Chromosome Number

Garden pea—————————- 14

Maize———————————— 20

Rice————————————— 24

Tobacco———————————- 48

Man————————————— 46

Sheep————————————- 54

Rat—————————————– 60

x. Heterozygous genes: When an organism contains two dissimilar genes in this case, the dominant gene (in capital letter) expresses itself in the phenotype. The recessive gene represented by small letter does not express itself in F1 generation. A recessive gene can express itself when it is in the homogenous condition.

Done studying? See previous lessons in Agricultural Science

Take a quick test for this lesson

  1. State and explain the methods of crop improvement?
  2. State the advantages and disadvantages of the different methods of crop improvement.
  3. Mention and explain the terms used in genetics.

Questions answered correctly? Bravo!

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