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1980). For example, based on data in Green (1987) and Clements et al . (1995),
two rotary hoeings plus two inter-row cultivations in maize requires 585 MJ
ha 1 of fuel, whereas a typical herbicide program of 2.2 kg ha 1 of metolach-
lor plus 0.56 kg ha 1 atrazine uses 714 MJ ha 1 . A single application of gly-
phosate at 1.68 kg ha 1 uses the equivalent of 763 MJ ha 1 . Flame weeding is
considerably more energy intensive: a 50 kg ha 1 propane flame weeding
uses 2700 MJ ha 1 (Ascard, 1995 a ). In contrast, herbicides that are applied at
low rates probably require relatively little energy. Considering whole crop
production systems, herbicides range from a significant proportion of the
total energy input for Iowa corn (14%, Pimentel & Burgess, 1980) and Ohio
soybean (13%, Scott & Krummel, 1980) to a minor input for Florida cabbage
(1.6%,How,1980) and a trivial input for Kansas wheat (0.15%, Briggle, 1980).
These values would probably change little if mechanical management were
used instead. When weed control represents a substantial energy input,
however,some energy savings are possible with integrated systems (Clements
et al ., 1995).
Directions for future research
The improvement of mechanical weed management requires a broad
research agenda, including biological, agronomic, and engineering studies.
Much work is needed on the mechanisms whereby tillage affects perennial
weed populations. How do the size distributions of root and rhizome frag-
ments compare following tillage with different implements, and how does
this affect a fragment's survival probability and subsequent rate of growth?
Do particular species survive better when buried with the shoot attached or
with the shoot severed from the roots? How do these factors interact with soil
temperature and moisture conditions in determining the degree of control by
tillage?
Knowledge is equally sparse on the mechanisms whereby tillage affects
seed banks. In particular, it is not even clear whether most depletion of seed
banks by tillage is due primarily to additional seedling emergence, as implied
by Figure 4.6b,or whether the tillage-related mortality factors listed in Figure
4.7 play an important role for some weed populations. Ultimately, the emer-
gence models discussed in the section “Effect of the timing of tillage on weed
seedling density”need to be extended to predict not just when emergence will
occur, but how many seedlings will emerge, and how this relates to the tillage
regime.
The destruction of weed seedlings during cultivation also requires
mechanistic investigation. When using tools that disturb only a shallow
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