Although agronomic data from
experiment station trials demon-
strate a clear link between timely
weeding and enhanced maize
yields, economic data from on-farm
trials do not always confirm the
profitability of "improved" weeding
practices. On-farm weed control
experiments are notoriously diffi-
cult to manage, since weed growth
varies across sites and between sea-
sons, and defining a standard
baseline "farmer practice" is not
easy. Economic analysis of trial
data is complicated by the difficulty
of estimating the opportunity cost
of labor (often family labor), which
may change throughout the season.
Many African farmers in fact adjust
their weeding practices according
to the level of weed infestation and
labor availability, which suggests
they appreciate the necessity of
weeding and allocate labor to
weeding when it pays to do so.



Maize grain yield (t/ha)
 4


 No e
 3



 2



 1-



 0 _
 Management Low High
 Land Hand hoe
 preparation


Where weeding competes for labor
with other income-generating ac-
tivities, herbicides have often been
investigated as an alternative form
of weed control. As with many
other improved technologies,
returns to herbicide use are highly
sensitive to site-specific factors.
Work done in Zambia suggests that
adoption of chemical weed control
in maize is more likely to be profit-
able for large-scale farmers with
access to tractors than for small-
holders (Vernon and Parker 1983).

Striga, also known as witchweed, is
an indigenous parasitic weed that
attacks the traditional crops of the
African savanna, including maize,
sorghum, pearl millet, groundnuts,
and cowpeas. As maize has become
more widely cultivated, striga has
become an increasingly important
maize parasite. Screening methods
to assess striga resistance are being
developed and several inbred lines


Low High Low High
 Ox plow Tractor


and hybrids tolerant to one or more
species of striga have been identi-
fied, but much additional research
on the ecology and control of this
pest is needed.

Insects
Stem borers, termites, rootworms
and cutworms, and storage insects
are a major cause of low maize
yields and grain losses after har-
vest. The trend in sub-Saharan
Africa toward greater cropping in-
tensity, the increasing use of
minimum or conservation tillage
practices, and the growing practice
of incorporating crop residues all
contribute to rising insect popula-
tions. Although many insect pests
can be controlled effectively with
chemicals, pesticides are often
difficult to obtain, particularly for
smallholders who lack access to
production credit. On the other
hand, chemical control is increas-
ingly viewed as undesirable by
many policy makers, since improp-
erly used pesticides threaten
human safety and damage the
environment. Concern for small-
scale farmers is particularly great,
given their generally poor knowl-
edge about pesticide safety. In view
of the implications for human and
environmental health, it is likely
that the use of toxic chemicals for
pest control will eventually be
reduced (Mihm and Renfro 1987),
and breeding for insect tolerance
and resistance is now seen as an
integral part of controlling insect
pests.

In Africa, where maize grain is
frequently stored before it is
properly dried and/or without
insecticide treatment, insect dam-
age to stored grain can cause losses
of 50% or more. Problems with
storage pests typically develop as a
result of high temperatures and


Source: Rodriguez (1989).

Figure 16. Effect of different soil management practices on maize grain
yield, Kamboise, northern Nigeria, 1981.