Agriculture Reference
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species, such as Aethusa cynapium , were most abundant in spring-sown crops
(barley and potato), but fall-germinating weeds, such as Poa annua , were most
abundant in fall-sown crops (winter barley, wheat, and oat).
Whereas continuous monocultures and short rotations of similar species
select for weeds adapted to particular temporal niches, rotation of crops with
different planting dates, contrasting growth periods, and differently timed
management practices can disrupt selection for adapted weed species. In
experiments conducted in Sweden, HÃ¥kansson (1982) observed that Allium
vineale was an important weed in winter cereal crops sown repeatedly on the
same land, but that it was unimportant in spring-sown cereals. The weight
of new bulbs produced by A. vineale in spring-sown cereals was only 1% to 5%
of that in winter cereals. This difference was attributed to different times of
tillage for seedbed preparation coupled with seasonal changes in the weed's
susceptibility to mechanical damage.Spring tillage preceding spring planting
was most effective in killing A. vineale because at that time the plants had few
reserves with which to recover from damage; tillage later in the year, after A.
vineale had accumulated reserves, provided less effective control. The impact
of spring tillage was persistent and created a rotation effect. When spring-
sown crops occurred in more than 40% of a rotation, A. vineale populations
were greatly reduced in subsequent winter cereals.
In the short term, the use of particular crops and management practices
within a rotation can contribute to general reductions in weed density.
Dotzenko,Ozkan & Storer (1969) found,for example,that weed seed density at
the end of a four-year cropping cycle was lower when bean followed sugar beet
than when maize or barley followed sugar beet. Much of this effect was attrib-
uted to differences among the crops in tillage and planting dates.Because bean
had the latest planting date of the three crops preceding sugar beet, it allowed
the highest proportion of weeds to emerge and be killed during seedbed prep-
aration,and was most effective in limiting weed seed production.Whether this
effect would have continued in future years is unknown, however. If weed
species preadapted for late germination migrated into the field, or if resident
weed species evolved late-germinating genotypes,differences among rotations
in weed community composition might have become evident, and the general
reduction in weed density obtained by rotating sugar beet with bean might
have disappeared. Long-term experiments tracking the population dynamics
of different weed species within the context of different crop rotations are
needed to address this issue. In addition to fixed rotation treatments, such
experiments might include flexible rotation treatments that would change in
response to shifts in weed community composition.
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