Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 297

have been narrowed down to a few promising candidates, actual testing
against weeds becomes more practical.
Choice of the weed or weeds against which the crop lines are to be tested is
a critical issue. If the crop responds similarly to most of the weeds with which
it occurs, with differences in response primarily due to weed biomass rather
than taxon,then the choice of weed species is not especially critical and a stan-
dard “tester” species can be chosen. However, the limited evidence currently
available indicates that crop response to weeds is taxon dependent
(McWhorter & Hartwig, 1972; Monks & Oliver, 1988; Wilson & Wright, 1990;
Bussan et al ., 1997). Fortunately, statistical procedures are available that allow
efficient choice of selection environments, in this case weed taxa (Zobel,
Wright & Gauch, 1988; Crossa, Gauch & Zobel, 1990).
Phenology
Weed-free periods and weed infestation periods
The timing of crop emergence, growth, and maturation ( phenology )
relative to competing weeds has a large impact on crop production. Both
empirical studies (Mann & Barnes, 1947; Bowden & Friesen, 1967;
Håkansson, 1986) and simulation models (Kropff et al ., 1993) have demon-
strated that an advantage of even a few days can greatly shift the competitive
balance between crops and weeds.Early in the life of an annual plant,biomass
and leaf area tend to increase exponentially (Shinozaki & Kira, 1956).
Consequently, a head start of a few days can greatly affect the relative sizes of
the two species at the point when they grow into competitive contact. If a
slightly larger size allows the crop to shade the weed at the time their canopies
meet, then this size difference early in the season is likely to compound
further.
Many studies in a wide range of annual crops have used this principle to
demonstrate that an initial weed-free period is sufficient to obtain full crop
yield (Zimdahl, 1980). Characteristically, as the initial weed-free period grows
longer, yield rises to an asymptote (Figure 6.8). Thus, after some threshold
date, weeds no longer have a measurable effect on the current crop. As dis-
cussed in Chapter 2, seed production by the weeds may cause problems in
future years, but conceptually that is a separate issue.
Minimum initial weed-free periods result from the preemptive nature of
competition between plants. Once the crop begins to shade the weeds, weed
growth rate will be reduced. If the crop's head start is sufficiently great, the
weeds will remain suppressed in a subcanopy position and will be unable to