Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 324

pesticides and fertilizers, crop varieties responsive to chemical inputs, and
larger, more costly machinery. Diversity has fallen as crop and livestock pro-
duction have become separated onto different farms or into different regions,
and as government programs have subsidized the production of some crops,
but not others (Power & Follett, 1987). A similar decline in crop diversity has
taken place in some developing countries through the expanded use of agro-
chemical inputs, “high response” crop varieties, and agricultural machinery
(Chambers, 1990; Pretty, 1995). As in the industrialized countries, govern-
ment policies and subsidies, extension education, and lending practices have
contributed to increased crop specialization and, concomitantly, greater reli-
ance on purchased production inputs.
In contrast to the low levels of crop diversity that have become characteris-
tic of most industrialized farming systems,many farmers in developing coun-
tries choose to maintain traditional production methods characterized by
high levels of crop diversity. Much of this diversity is the result of intercrop-
ping and multiple cropping practices, which are used to raise and stabilize
crop yields with minimal reliance on purchased fertilizers, pesticides, and
machinery (see section “Why farmers plant crops in mixtures” below).
Farmers in the Latin American tropics plant more than 40% of their cassava,
60% of their maize, and 80% of their bean in intercrop mixtures (Francis, Flor
& Temple, 1976; Leihner, 1983). West African farmers plant intercrops on
more than 80% of the cultivated land (Steiner, 1984) and may sow as many as
12 crop species in a 7-m 2 area (Okigbo & Greenland, 1976). A survey of six vil-
lages in India indicated that up to 91% of the cropped area was sown in mix-
tures that contained as many as eight crop species (Jodha, 1981).
Intercropping and multiple cropping are widely practiced in the Himalayan
region (Ashby & Pachico, 1987) and have been described as “almost universal”
in Chinese vegetable production systems (Wittwer, 1987).
Agroforestry practices can also increase crop diversity dramatically. Kebun-
talun agroforestry systems in Java may contain 100 or more plant species used
as ornamentals, medicines, cash crops, building materials, firewood, spices,
vegetables, fruits, and other foods (Christanty et al ., 1986). Huastec farmers in
northeastern Mexico manage mixtures of more than 300 species of herbs,
shrubs, and trees used for construction materials, fencing, fuel, medicines,
basketry, tools, soaps, dyes, musical instruments, feeds, fruits, vegetable
greens, and cash crops, especially coffee (Alcorn, 1984). Such systems feature
large numbers of woody perennial species, minimal soil disturbance, leaf
litter accumulation, and long-term successional changes in plant species
composition.
In industrialized countries, a high degree of crop diversity may still be