Producing Food: Chemicals, Breeding and Intensive Farming

1More food from every hectare

In 2022 Rwanda had 13.2 million people, an average of 503 per km². With so many people sharing the land, each hectare must produce more food every year. Farmers do this with fertilisers, pesticides and herbicides, improved crop varieties and animal breeds, large fields of a single crop, and animals kept in sheds and fed. Every one of these methods increases food production; every one can also harm ecosystems if it is used carelessly. This unit weighs up how human activities affect ecosystems, and how we can protect them.

2Fertilisers: food for crops, a threat to lakes

Crops remove mineral salts from the soil every harvest, and fertilisers replace them (Unit 3). Used carelessly they cause harm:

• Eutrophication: nitrate and phosphate washed into lakes cause algal blooms that use up the oxygen, as you learned in Senior 2.
• Drinking water: nitrate leached into wells and springs can make water unsafe, especially for babies.
• Soil: repeated heavy use of some fertilisers makes soil more acidic.

Good practice: test the soil, apply the right amount at the right time (not before heavy rain), add manure or compost, and grow legumes in rotation.

3Pesticides and biomagnification

Pesticides protect crops and livestock from insects, fungi and ticks. The problems:

• They also kill non-target species, including bees that pollinate crops and the ladybirds and spiders that eat pests.
• Pests exposed again and again can evolve resistance, so stronger doses are needed.
• Persistent pesticides such as DDT are not broken down in the body; they are stored in fat. Each animal eats many smaller ones, so the poison becomes more concentrated at each step up the food chain (Figure 1). Top predators can be poisoned even when the water contains only a trace.

Figure 1: Biomagnification of a persistent pesticide along an aquatic food chain (illustrative concentrations in parts per million, ppm).

4Herbicides and monocultures

Herbicides kill weeds that compete with crops for light, water and minerals (Unit 1). But they also remove wild plants that feed insects and birds, and spray drifting in the wind can damage nearby crops and hedges.

Monocultures, such as tea estates, large eucalyptus plantations and consolidated fields of maize or rice, make planting, weeding and harvesting efficient. But:

• they replace a habitat with many species by one species, so biodiversity falls;
• a pest or disease of that crop can spread from plant to plant across the whole field;
• the same crop removes the same minerals year after year, so more fertiliser is needed.

5Selective breeding

For thousands of years farmers have chosen the best animals and plants as parents. The steps are:

1. Decide on the feature wanted: more milk, resistance to a disease, larger grains, tolerance of drought.
2. Choose parents that show the feature most strongly, and breed them.
3. From the offspring, choose those with the feature most strongly, and breed them.
4. Repeat for many generations.

Benefits: higher yields, crops that resist pests and diseases (so less pesticide is needed), animals suited to local conditions. Problems: breeding from a few closely related parents reduces the variety of genes, so a new disease can strike a whole breed or variety at once; some high-yielding breeds need more food, water and veterinary care.

6Intensive livestock production

Keeping cows in sheds with cut fodder ("zero grazing"), or chickens and pigs in houses, produces a lot of milk, eggs and meat from little land, protects soil from overgrazing, and makes it easy to collect manure. But:

• large amounts of manure and urine can wash into streams and cause eutrophication if they are not stored and used properly;
• crowded animals spread diseases quickly (density-dependent, Unit 2), and heavy use of antibiotics can breed resistant bacteria;
• animals fed on grain use food that people could eat: as Unit 1 showed, about 90% of the energy is lost at each step of a food chain.