Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 66

competition increases (Légère & Schreiber, 1989; McLachlan et al ., 1993;
Knezevic & Horak, 1998). Differences in branching of C. obtusifolia emerging
on different dates and located at various distances from the soybean row
allowed that species to maintain a nearly constant proportion of leaves above
the crop,regardless of the timing of competition (Smith & Jordan,1993).Since
morphological flexibility allows some weed species to overtop even competi-
tive crops like soybean, either the tactics discussed above need to create a sub-
stantial size differential between the crop and weed or else tall weeds will
need to be dealt with by other means.
Plants compete for nutrients and water as well as for light.This is apparent
from the many studies in which perennial cover crops growing beneath a
main crop reduced yield less in a wet year or when irrigated, even though the
cover crop was kept short (Chapter 7) (Mohler, 1995; Teasdale, 1998).
Similarly, a short weed like Taraxacum officinale can reduce yield of a tall crop
like sweet corn in a dry year (Mohler, 1991), even though shading by the crop
reduces transpiration by the weed.
In annual cropping systems,the soil is unoccupied by roots at the beginning
of the season, and usually nutrients and water are relatively abundant. Under
such conditions, the outcome of below-ground competition between the crop
and weed depends primarily on the rate at which the two species occupy the
soil with roots, and their relative rates of uptake. Andrews & Newman (1970)
showed that root density is critical in competition for nutrients.Because small-
seeded species tend to have a higher rate of root elongation (Table 2.7) (Seibert
& Pearce, 1993), weeds tend to rapidly occupy the soil volume to the detriment
of the crop. Probably because weeds are adapted to exploit the brief pulse of
nutrient availability that follows disturbance, they also usually have substan-
tially higher macronutrient concentrations in the shoot than do the crops with
which they compete (Vengris et al ., 1953; Alkämper, 1976; Qasem, 1992). They
thus sequester nutrients that would otherwise be available to the crop. Given
that weeds have this double competitive advantage, fertilization often favors
weeds more than crops (Chapter 5) (Vengris, Colby & Drake, 1955; Alkämper,
1976; Lawson & Wiseman,1979; Dyck & Liebman,1994).Consequently,direct-
ing water and nutrients toward the crop rather than the weeds is a critical com-
ponent of weed management.This can be achieved by fertilizing and irrigating
directly in the crop rooting zone,timing the application of fertilizer or manure
to correspond to the needs of the crop,and using organic materials that inhibit
root growth of weeds (Chapter 5).
Below-ground competition works differently in perennial systems where
the soil is permanently occupied with roots (Grubb, 1994). In perennial com-
munities, the competitive dominant is usually the species that can deplete the