Biology Reference
In-Depth Information
wise crop producers place nutrients where crops have better access than weeds
(Blackshaw et al. 2002, 2005; Kirkland and Beckie 1998). For some species such as
green foxtail and stinkweed, a combination of strategic nitrogen placement and no
till can dramatically reduce weed populations over time (O'Donovan et al. 1997).
Weeds are also relatively susceptible to the negative effects of shade (Mohler
2001). In addition, some weed species require a
light
signal for germination, and
are inhibited by the red-light depleted radiation that filters through crop leaves
(Górski 1975; King 1975; Silvertown 1980). Astute managers are aware of these
vulnerabilities and strive to promote rapid, uniform crop emergence and ground
cover to preempt resources potentially available to weeds. The soil moisture neces-
sary for rapid crop germination and emergence to facilitate preemptive resource
acquisition is more likely to be found in direct-seeding and no-till systems than in
tilled systems.
8.6
Diversified Cropping Systems Promote Crop Health
The most effective and sustainable form of weed management involves diversified crop-
ping systems. Weeds fortunate enough to grow in simple, repeated cropping systems
will continue to have little difficulty adapting and thriving in those systems (Harker and
Clayton 2003). Buhler (1999) suggests that the majority of all current cropping systems
are highly simplified, allowing the best adapted weed species to proliferate.
The weed seed population of the soil is greatly influenced by the type of crop
grown. “Soil conditions being similar, the composition of the flora under continuous
wheat and barley is very much the same, but the relative composition of the constitu-
ent species varies greatly, some being favoured by the wheat crop and others by the
barley” (Brenchley and Warington 1933). “Continuous production of a single crop
and short sequences of crops with similar management practices promote the
increase of weed species adapted to conditions similar to those used for producing
the crops. In contrast, over the course of a diverse rotation employing crops with
different planting and harvest dates, different growth habits and residue characteris-
tics, and different tillage and weed management practices, weeds can be challenged
with a wide range of stresses and mortality risks, and will be given few consistent
opportunities for unchecked growth and reproduction” (Liebman and Staver 2001).
The successful utilization of crop diversity in weed management systems is
mostly governed by the life cycle of the most dominant weed(s) and the life cycle
of the rotational crops. Many crop rotations involve substantial crop species diver-
sity, but lack crop life cycle diversity. For example, if summer annual wild oat is the
dominant weed species, crop producers should not solely grow summer annual
crops in their rotation. Conversely, downy brome (
Bromus tectorum
L.), a winter
annual, is easily managed in winter wheat when the crop rotation includes canola -
a summer annual crop in Canada (Blackshaw 1994).
Using crop rotations with varying crop life cycles is not the only way of intro-
ducing diversity into a cropping system. Diversity can also be introduced by
