Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 222

Despite the fact that many weed species are more responsive to fertilizer
than are crops, the phenomenon is not universal. Tollenaar et al . (1994)
reported, for example, that increasing the quantity of N fertilizer applied to
maize under weedy conditions in Ontario resulted in less weed biomass and
greater maize yield. When weeds (mostly Amaranthus retroflexus, Chenopodium
album , and Setaria viridis ) were allowed to establish at the three- to four-leaf
stage of maize development, weed competition reduced crop yield by an
average of 31% at low N application rates (10-80 kg N ha 1 ),but by only 13% at
high N rates (130-200 kg N ha 1 ). McKenzie (1996) found that increasing the
quantity of N fertilizer applied to perennial ryegrass pastures in South Africa
reduced weed tiller density and weed relative frequency. During late summer,
when weed growth was greatest, weeds were present in 82% of sample quad-
rats in plots receiving 120 kg N ha 1 yr 1 , whereas they were present in only
about 45% of the quadrats in plots receiving 360 kg N ha 1 yr 1 . McKenzie

(1996) attributed this result to increased leaf area production and shading of
weeds by perennial ryegrass at high N rates.
The possibility that fertilization practices might be harmonized with weed
management is attractive. However, given the potential for increases in nutri-
ent availability to exacerbate rather than diminish weed problems, there is a
considerable need to better understand and predict the effects of fertility con-
ditions on weed-crop interactions. Modeling is one potentially useful
approach to address this issue. Models can be used to predict the performance
of plant species in mixture based on knowledge of how the individual species
respond to variations in environmental conditions when grown in pure
stands. Models can also be used to generate testable hypotheses for experi-
mental work.
Kropff and his co-workers examined weed-crop competition using the
intercom model, which incorporates information concerning nutrient
supply and uptake, light interception, photosynthesis, and root and shoot
growth (Kropff & van Laar, 1993). Under high nutrient conditions, the model
indicated that height growth and leaf area production are critical factors
determining the outcome of competition between species such as sugar beet
and Chenopodium album (Kropff et al ., 1993). Under low nutrient conditions,
the model predicted that competitive dominance of one species over another
is favored by morphological features that confer greater rates of nutrient
capture (e.g., longer and denser root systems) and physiological features that
confer greater rates of biomass production per unit of captured nutrient (e.g.,
photosynthetic C assimilation via the C 4 rather than C 3 pathway) (Kropff,
1993).Similar effects have been predicted by the allocate model developed
by Tilman (1988).