Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 360

Selective weeding is a possible alternative to planting cover crops between
rows of trees. In a five-year study initiated in newly planted coffee, Aguilar et
al . (1997) compared conventional total weed suppression (using machete
slashing and herbicide tank mixes) with selective weeding (using herbicides
plus slashing or just slashing).The objective of the selective treatments was to
suppress potentially competitive weeds in inter-row areas and promote the
growth of weed species, such as Oplismenus burmanii , Commelina difusa , and
Drymaria cordata , characterized by a low, creeping growth habit, and shallow
root systems.These weeds were considered to be benign ground covers.
Results of the experiment indicated that selective weeding was effective in
changing weed community composition, minimizing weed competition
against the coffee crop, and promoting plant cover for soil conservation
(Aguilar et al ., 1997). Before initiation of the experiment, ground cover weeds
constituted 11% of the herbaceous vegetation cover, but by the fifth year they
were 34% of the vegetation cover in the selective herbicide-plus-machete
treatment, and 64% of the selective machete-only treatment. During the dry
season, soil in selectively weeded plots was protected by a mulch of senesced
ground-cover weeds, whereas soil in the conventional plots was largely bare.
Selective weed management resulted in reduced coffee yield in the first year of
production, but for each of the following two years and for the three-year
total, yields did not differ among treatments. As compared with the conven-
tional treatment, the selective herbicide-plus-machete and machete-only
treatments increased labor costs 28% and 135%, but reduced herbicide use by
25% and 100%.
Using additional tree strata to manage weeds in tree crops
Agroforestry systems composed of mixtures of tree species are
common in the humid tropics and often are characterized by multilayered
canopies, very complete use of light, and few weeds (Christanty et al ., 1986).
Such systems mimic the natural forests surrounding them (Ewel, 1986),
and weeds that are able to grow in tree mixtures are most frequently vines and
undesirable tree species rather than herbaceous annuals. Ewel (1986) and
others have suggested that mixed-species tree farming is a sustainable agri-
cultural system for high rainfall, low soil nutrient environments where
farmers lack cash or credit to buy synthetic agrichemicals and other produc-
tion inputs. Coffee-based systems illustrate how tree crops can be grown with
other trees, and how weeds may be affected.
Overstory tree species are planted with coffee primarily to regulate the sun-
light available for photosynthesis by coffee and to stabilize coffee yields over
several-year periods (Carvajal, 1984; Kimemia & Njoroge, 1988). With little or