Farming practices and productivityAQA A-Level Biology: Revision notes

Section 1

Aim: more food energy for humans

Farmers aim to increase the net primary production of a crop, or the net production of livestock, that humans can harvest. Production is affected by farming practices designed to increase the efficiency of energy transfer. There are two main approaches: simplify food webs to reduce energy losses to non-human food chains, and reduce respiratory losses within the human food chain.

Key termsefficiency of energy transfernet primary production

Section 2

Simplifying food webs

In a natural ecosystem, many organisms feed on the crop and compete with it. Each of these takes energy that would otherwise end up in the harvested crop. Farmers reduce these losses by:

Insecticides kill insect pests that feed on the crop, so less energy flows into non-human food chains.
Herbicides kill weeds that compete with the crop for light, water and mineral ions, so the crop grows larger.
Fungicides kill fungal pathogens that take sugars and damage leaves.
Biological agents, such as predators or parasites of pests, can reduce pest numbers.

The result is a simplified food web in which a greater proportion of the energy flows into the crop and then into the human food chain.

Key termssimplified food webinsecticideherbicidefungicide

Exam tip

Say where the energy goes: it is not lost from the crop to a pest, so it stays in the harvested biomass.

Section 3

Reducing respiratory losses

For livestock, the net production is $N = I - (F + R)$ . Farmers increase N by reducing R, the energy lost in respiration:

Restricting movement (for example, keeping animals in pens) reduces energy released in respiration for muscle contraction.
Controlling temperature by housing the animals in warm buildings reduces the respiration needed to maintain body temperature.

A larger proportion of the energy ingested is then converted to biomass, so the human food chain is more efficient.

Key termsrespiratory lossnet production

Common mistake

Saying that animals 'do not respire' when kept still. Respiration still occurs; the respiratory loss is smaller.

Section 4

Worked example and evaluation

Cattle ingest 240 MJ per week. Free-range, they store 24 MJ as biomass: $24 \div 240 \times 100 = 10\%$ . Indoors in a warm barn, they store 48 MJ: $48 \div 240 \times 100 = 20\%$ , so twice as efficient.

When evaluating, balance the gain in productivity against the costs: energy and money for heating and buildings, as well as ethical concerns about animal welfare. Chemical control of pests may also affect wildlife.

Key termsproductivityevaluation

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Farming practices and productivity

A farmer rears broiler chickens for meat. The chickens are kept in warm, closed sheds at a constant temperature, with little space to move around.
Explain how the farmer's practices increase the net production of the chickens.2 marks
A farmer grows wheat. She sprays the crop with a herbicide to kill weeds, an insecticide to kill aphids and a fungicide to kill fungal pathogens.
Explain how the fungicide increases the yield of the wheat.2 marks
Two farmers rear beef cattle. Farmer A lets the cattle roam freely over a large pasture. Farmer B keeps the cattle in a barn at a constant 18 °C with little room to move. Each animal ingests food with a chemical energy store of 240 MJ per week. The animals on farm A store 24 MJ per week in new biomass, and those on farm B store 48 MJ per week.
Calculate the percentage of ingested energy that is stored in new biomass on each farm, and state how many times greater it is on farm B than on farm A.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).