Agriculture Reference
In-Depth Information
Galium aparine were lower following alfalfa than following cereal crops. This
effect was attributed to alfalfa's competitive ability and to the cutting regime
used for it,which largely prevented seed production by the weeds.In contrast,
densities of Taraxacum officinale and Thlaspi arvense were greater in cereals fol-
lowing alfalfa than in continuous cereal production.Success of Taraxacum offic-
inale in the alfalfa/cereal system was attributed to its prostrate growth habit,
which allowed it to avoid defoliation during hay harvest, and to its extended
period of germination in untilled soil, which permitted seedling establish-
ment after the soil surface was exposed by hay removal in the summer or by
crop dormancy in the fall.Success of Thlaspi arvense in the alfalfa/cereal system
was attributed to its winter annual habit: it germinated in the fall when alfalfa
was dormant and noncompetitive, and it resumed growth early in the spring
prior to the resumption of alfalfa growth. The absence of tillage or herbicides
during alfalfa production allowed both Taraxacum officinale and Thlaspi arvense
to survive and produce seeds,and led to increased infestations of the two weed
species in succeeding cereal crops.
Certain perennial grass weeds, such as Elytrigia (Agropyron) repens , are partic-
ularly well adapted to survival in sod environments (Sheaffer et al ., 1990). For
this reason, their densities should be reduced as much as possible during
annual crop phases, before forages are sown. Tillage and herbicides used to
terminate the growth of forage stands are critical factors regulating the persis-
tence of E. repens and other perennials in subsequent annual phases of a rota-
tion.Impacts of different tillage practices on perennial weeds are examined in
Chapter 4.
Cover crops within rotation systems
As discussed in Chapter 5, cover crops can be grown during periods
when main crops are absent to suppress weed germination and growth
through resource competition, allelopathy, and other processes. In effect,
cover crops fill gaps in a cropping system that would otherwise be occupied by
weeds. This type of niche preemption is illustrated in data from McLenaghen
et al . (1996), who sowed five winter cover crops or let ground lie fallow after
fall-plowing sod. The quantity of ground cover produced by weeds was
inversely proportional to that produced by the cover crops (Figure 7.4). In the
fallow treatment without a cover crop, weeds covered 52% of the ground area.
In contrast, a white mustard cover crop produced 92% ground cover and
reduced weed cover to just 4%.Rye produced 85% ground cover and permitted
only 9% weed cover. At least part of the weed suppression observed in this
study may have been due to competition for nitrogen, since most of the cover
crops captured soil nitrate effectively (McLenaghen et al ., 1996).
Although this and other short-term experiments indicate that cover crops
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