WAEC SSCE - Agricultural Science - 1997 (Essay)

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(a) Hybridization

Hybridization is the crossing of two genetically different plants, usually different varieties of the same species, in order to obtain hybrid offspring which combine desirable inherited characters of both parents.

(b) Aims of crop improvement

1. To produce crop varieties with higher yield.
2. To improve the quality of produce, such as size, taste, nutritive value and keeping quality.
3. To produce varieties resistant to pests and diseases.
4. To produce varieties adapted to adverse or varying climatic and soil conditions, for example drought or poor soils.
5. To produce early-maturing, uniform varieties with good harvesting and processing qualities.

(c) Monohybrid cross and Mendel's Law of Segregation

A monohybrid cross is a genetic cross involving only one pair of contrasting characters. Mendel's Law of Segregation states that the two alleles controlling a character separate during gamete formation, so that each gamete receives only one allele. The alleles unite randomly again at fertilization.

For example, let T represent the allele for tallness and t the allele for shortness. Tallness is dominant over shortness.

In the parental generation, a pure tall plant, TT, is crossed with a pure short plant, tt. Every first filial generation plant receives T from the tall parent and t from the short parent; hence all the F₁ plants are Tt and tall.

When two F₁ plants are selfed, Tt × Tt, the alleles segregate during gamete formation. Each parent produces two types of gamete, T and t. Random fertilization gives:

• F₂ genotypic ratio: 1 TT : 2 Tt : 1 tt.
• F₂ phenotypic ratio: 3 tall : 1 short.

The reappearance of short plants (tt) in the F₂ generation shows that the alleles did not blend in the F₁ generation; they segregated into separate gametes.

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(a) Communal land tenure

Essential features:

• Land is owned collectively by the whole community, village or extended family.
• It is controlled and administered by the community head, chief or family head.
• Individual members are allocated portions to use but cannot sell or permanently own the land.
• Rights of use are passed down through generations, and every member has access to land.

Effects on agriculture:

• Insecurity of tenure discourages farmers from planting permanent/tree crops or improving the land.
• Land cannot easily be used as collateral to obtain loans.
• Fragmentation of holdings discourages mechanization, though it does guarantee land access for all members.

(b) Individual land tenure

Essential features:

• Land is owned by an individual who has full rights over it.
• The owner can use, sell, lease, mortgage or bequeath the land.
• Ownership is usually backed by a title deed or certificate of ownership.

Effects on agriculture:

• Security of ownership encourages long-term investment, permanent crops and land improvement.
• The land can be used as collateral to secure agricultural loans.
• Consolidated holdings encourage mechanization, though the system may lead to landlessness for the poor and to land speculation.

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Explanation of the terms:

(a) Salvage value: This is the estimated amount of money that an asset, such as a machine or building, is expected to be worth at the end of its useful life, that is, its scrap or disposal value after it has been fully used or depreciated.

(b) Farm inventory: This is a complete list or record of all the property, goods and resources on the farm, together with their money values, taken at a particular date (usually the beginning or end of the year). It includes land, buildings, livestock, machinery, stored produce and cash.

(c) Farm assets: These are all the valuable properties and resources owned by the farm business that have money value and can be used to generate income. They include fixed assets such as land, buildings and machinery, and current assets such as growing crops, livestock, stored produce and cash.

(d) Point of diminishing return: This is the point in production at which the addition of one more unit of a variable input to fixed inputs begins to yield a smaller (less than proportionate) increase in total output. Beyond this point, each extra unit of input adds less to total product than the unit before it.

(e) Depreciation: This is the gradual loss or reduction in the value of a durable farm asset (such as a tractor, implement or building) over time due to wear and tear, age and obsolescence. It is treated as a cost of production and spread over the useful life of the asset.

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The sheep is a ruminant herbivore, whereas poultry is a non-ruminant bird. The sheep is adapted for microbial digestion of fibrous feeds and chewing of cud; poultry is adapted mainly for rapid digestion of grains and other concentrated feeds.

Labelled diagrams of the alimentary canals

Differences between the digestive systems

Feature	Sheep	Poultry
Type of feeder	Ruminant, adapted to grazing and roughage.	Non-ruminant bird, adapted mainly to grains and other concentrated feeds.
Mouth and teeth	Has teeth for biting and grinding food. It later regurgitates and rechews cud.	Has a beak and no teeth. Food is swallowed without chewing.
Crop	No crop.	Crop present for temporary storage and softening of food.
Stomach	Has a four-compartment stomach: rumen, reticulum, omasum and abomasum.	Has a glandular stomach, the proventriculus, followed by a muscular gizzard.
Grinding of food	Food is ground by molar teeth during mastication and rumination.	Food is ground in the thick muscular gizzard, often with swallowed grit or small stones.
Cellulose digestion	Micro-organisms in the rumen digest cellulose and other fibrous materials.	Cellulose digestion is very limited; only slight fermentation occurs in the paired caeca.
Caeca	Has one caecum.	Has two caeca.
Terminal opening	The alimentary canal ends at the anus.	The digestive, urinary and reproductive tracts open into the cloaca, which opens at the vent.

Food pathway in sheep: mouth → oesophagus → rumen/reticulum → omasum → abomasum → small intestine → large intestine → anus.

Food pathway in poultry: beak → oesophagus → crop → proventriculus → gizzard → small intestine → paired caeca → large intestine → cloaca → vent.

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(a) Types of credit in agricultural production:

• (i) Short term credit: This is a loan granted for a short period, usually up to one year, used to meet running or recurrent expenses such as buying seeds, fertilizer, agro-chemicals and paying labour. It is repaid after harvest and sale of produce.
• (ii) Medium term credit: This is a loan given for a moderate period, usually between one and five years, used to buy items of medium durability such as work animals, small implements, dairy stock and to fence or improve the farm.
• (iii) Long term credit: This is a loan granted for a long period, usually above five years, used for heavy capital investment such as buying land, tractors and machinery, erecting farm buildings and establishing plantations or orchards.

(b)(i) Four sources of agricultural credit:

1. Commercial and agricultural (development) banks
2. Government agencies and schemes (for example the Agricultural Credit Guarantee Scheme)
3. Co-operative societies and thrift societies
4. Money lenders, friends and relatives

(b)(ii) Four reasons why farmers find it difficult to get loans from banks:

1. Lack of collateral security: Many small farmers have no land title, buildings or other acceptable assets to pledge against the loan.
2. High risk of farming: Because farming is exposed to drought, flood, pests and diseases, banks fear the farmer may not be able to repay.
3. Illiteracy and poor record keeping: Many farmers cannot prepare the loan applications, feasibility reports and accounts that banks demand.
4. High interest rates and short repayment periods: The cost of borrowing is high and repayment terms often do not match the farmer's harvest season, discouraging both the farmer and the lender.

(Other reasons include remoteness of banks from rural areas and cumbersome loan procedures.)

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Short notes on four of the listed terms:

(a) Deforestation: This is the clearing, felling or removal of trees and forest vegetation from an area of land without replacing them. It is done to obtain timber, fuel wood and farmland or to build settlements. It exposes the soil to erosion, reduces rainfall, destroys wildlife habitats and lowers soil fertility, and it is therefore a harmful practice when uncontrolled.

(b) Regeneration: This is the natural or artificial renewal or re-establishment of a forest. Natural regeneration occurs when trees reseed themselves from fallen seeds or coppice from stumps, while artificial regeneration is achieved by deliberately planting seedlings. It restores the tree cover on land that has been logged or degraded.

(c) Afforestation: This is the deliberate planting of trees to establish a forest on land that was previously bare, grassland or farmland that had no forest before. It is practised to check erosion and desertification, provide timber and fuel, serve as windbreaks and improve the environment.

(d) Selective exploitation: This is a system of forest harvesting in which only mature trees of desired species and size are felled, while young and immature trees are left standing to continue growing. It conserves the forest, allows continuous yield and maintains forest cover, unlike clear felling.

(e) Taungya system: This is a form of agro-forestry in which farmers are allowed to raise arable food crops in between newly planted forest tree seedlings on the same land. The farmer tends both the crops and the young trees; he harvests the food crops while the trees grow, and moves on when the canopy closes. It combines food production with forest establishment and reduces the cost of raising the forest.

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Farm power refers to the energy or force applied to perform agricultural operations such as ploughing, harvesting, pumping and processing. Four sources are discussed below.

1. Human (muscular) power: This is the energy supplied by the farmer and farm labourers using their muscles. It is used for light operations such as hoeing, weeding, planting, harvesting and carrying loads. It is readily available and cheap but is slow, tedious and low in output, making it suitable only for small holdings.
2. Animal (draught) power: This is power provided by work animals such as bulls, oxen, donkeys, horses and camels. The animals are harnessed to implements to plough, ridge, harrow and haul carts. It is cheaper than machines, uses locally available feed and its dung manures the soil, but it is slow and the animals may be attacked by tsetse fly in the humid zones.
3. Mechanical power: This is power supplied by engines and machines such as tractors, combine harvesters, water pumps and processing mills, driven by petrol or diesel. It is fast, does heavy work and can cover large areas within a short time, but it is expensive to buy, needs fuel and skilled maintenance.
4. Electrical power: This is energy supplied through electricity used to drive pumps, mills, incubators, lighting in poultry houses, milking machines and cold rooms. It is clean, convenient and easily controlled, but it depends on a reliable supply and is not always available in rural areas.

Other sources include wind power (windmills for pumping water) and solar power (for drying, lighting and pumping).

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(a)(i) Six uses of lime in agriculture:

1. It reduces soil acidity by raising the soil pH.
2. It supplies the plant nutrients calcium and (in dolomite) magnesium.
3. It improves soil structure by causing clay particles to floculate, giving better aeration and drainage.
4. It promotes the activity of soil micro-organisms and hastens decomposition of organic matter.
5. It increases the availability of other nutrients such as phosphorus and molybdenum in acid soils.
6. It reduces the toxicity of iron, aluminium and manganese in strongly acid soils, and helps to control some soil-borne pests and diseases.

(a)(ii) Four common liming materials:

• Quicklime (calcium oxide, CaO)
• Slaked or hydrated lime (calcium hydroxide, Ca(OH)₂)
• Ground limestone (calcium carbonate, CaCO₃)
• Dolomitic limestone (CaMg(CO₃)₂); wood ash and marl are also used.

(b)(i) Three examples of nitrogenous fertilizer:

• Urea
• Ammonium sulphate
• Ammonium nitrate (calcium ammonium nitrate and sodium nitrate are also acceptable)

(b)(ii) Three examples of a phosphorus fertilizer:

• Single superphosphate
• Triple superphosphate
• Rock phosphate (bone meal and basic slag are also acceptable)

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(a)(i) Definition of weathering: Weathering is the gradual breaking down, disintegration and decomposition of rocks into smaller particles and eventually into soil, in their place of origin (in situ), through the action of physical, chemical and biological agents.

(a)(ii) Two types of weathering:

• Physical (mechanical) weathering
• Chemical weathering

(Biological weathering is a recognised third type.)

(b) Four factors that influence weathering:

1. Temperature: Alternate heating and cooling causes rock minerals to expand and contract at different rates. This sets up stresses that crack and flake the rock, so areas with wide temperature ranges weather faster.
2. Water (rainfall): Water carries out chemical reactions such as hydrolysis, hydration and solution that decompose rock minerals. Rain water also seeps into cracks and, on freezing or through flow, widens them and washes away loosened particles.
3. Living organisms: Plant roots grow into cracks and prise rocks apart, while burrowing animals, lichens, bacteria and decaying organic matter release acids that dissolve and break down the rock.
4. Nature (composition and structure) of the rock: Soft, porous and jointed rocks with easily soluble minerals weather quickly, whereas hard, compact rocks with resistant minerals weather slowly.

Other influencing factors include wind (abrasion and removal of particles) and topography/relief (steep slopes expose fresh rock to rapid weathering).

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Brief explanation of four terms in animal production:

(a) Oestrus cycle: This is the regular reproductive cycle in a mature non-pregnant female animal, being the interval from the beginning of one heat period to the beginning of the next. It is controlled by hormones and includes the phases of pro-oestrus, oestrus, met-oestrus and di-oestrus. In the cow it is about 21 days.

(b) Heat period (oestrus): This is the period within the oestrus cycle when the female is sexually receptive and will readily accept the male for mating. It is the time when ripe eggs are released (ovulation) and is the correct time for natural service or artificial insemination. The animal shows signs such as restlessness, mounting others and a swollen vulva.

(c) Gestation period: This is the length of time an animal carries its young in the womb, from the day of successful fertilization (conception) to the day of birth. It varies with the species, for example about 280 days in the cow, 150 days in the sheep and goat, and 114 days in the pig.

(d) Parturition: This is the act of giving birth, that is, the expulsion of the fully developed young one from the womb of the mother at the end of the gestation period. It is called calving in cattle, lambing in sheep, kidding in goats and farrowing in pigs.

(e) Colostrum: This is the first thick, yellowish milk produced by the mother immediately after parturition. It is rich in proteins, vitamins and antibodies which give the newborn immunity against diseases, and it also acts as a mild laxative to clear the young animal's gut.