Over the 10,000 years since agriculture began to be developed, peoples everywhere have discovered the food value of wild plants and animals, and domesticated and bred them. The most important crops are cereals such as wheat, rice, barley, corn, and rye; sugarcane and sugar beets; meat animals such as sheep, cattle, goats, and pigs or swine; poultry such as chickens, ducks, and turkeys; animal products such as milk, cheese, and eggs; and nuts and oils. Fruits, vegetables, and olives are also major foods for people. Feed grains for animals include soybeans, field corn, and sorghum. Separate articles on individual plants and animals contain further information.
Tillage operations that prepare the soil for planting and control weeds also expose bare soil to possible erosion by wind and water. Erosion removes fertile soil and contributes to problems of air and water pollution. Several techniques are used to combat erosion. Crop farmers are shifting toward reduced-tillage techniques to prepare their seedbeds. Many keep a mulch cover of residues to shield their soil from wind and water erosion. Crop rotation and cover crops are also being increasingly used to hold soil in place between plantings. Still, many small-seeded crops require a finely worked, pulverized seedbed, and soil erosion cannot be completely eliminated.
Fertilizers from both natural and manufactured sources can also contribute to water pollution. Soluble-nitrogen forms can leach into groundwater, and all fertilizers, including animal manures, can be carried along in surface runoff. Crop farmers are paying more attention to reducing fertilizer loss, partly for environmental reasons but also because of fertilizer prices. The amount of water pollution contributed by fertilizer is generally low compared to other sources, and it can be reduced by avoiding excessive fertilization and by reducing soil runoff. Despite some pollution risks, the use of fertilizers, both organic and manufactured, is essential to enrich heavily used croplands.
Crop farmers must continually battle the insects and diseases that attack their plants. When used correctly and with care, chemical insecticides and fungicides are helpful in improving the quality and dependability of food supplies, but some have had detrimental effects on the environment, and several pest species have developed resistance to once-effective pesticides. Crop farmers are currently turning toward integrated programs of pest management, involving the use of biological and chemical controls to improve the timing and effectiveness of pest control and to minimize environmental risk.
A wide diversity of crops are grown by the subsistence farmers of the world, but larger commercial farms frequently concentrate on a few crops or even a single crop that bring them greatest returns. The continuous growing of a single crop can be financially efficient but can lead to a concentration of associated pests. These may present only minor problems that can be easily controlled, or they may build to devastating levels that cause widespread loss. Most crop farmers strive for rotation and crop diversity, but soils, rainfall, and economics may limit their choices.
Geneticists are concerned that the gene pool for some crops, the hereditary stock from which plants are chosen for breeding, has become narrowed, often centered on a few varieties selected for superior production under todays conditions. Farmers generally choose to grow the best available strains. The superiority of a small number of improved varieties has caused wholesale abandonment of thousands of local strains of crops. This reduction of the gene pool may make it more difficult to meet future needs of changing climate, soil, and challenges from unforeseen pests. In an effort to preserve the size of the gene pool, international centers and experimental stations scattered throughout the world are beginning to develop collections of genetic samples of these local crop strains. Much irreplaceable material has been lost, but an increasing commitment by funding agencies and scientists offers hope that this genetic waste will cease.
Accurate statistics on world crop production are difficult to assemble. Much crop food is consumed privately or sold in local markets for which statistics are not gathered, and many countries do not perform regular crop surveys. Several organizations, however, have developed good systems for gathering data and estimating national and world food production, and geostationary satellites that monitor croplands by means of sensors have enormously improved annual forecasts and surveys.