Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 230

(Hargrove, 1991; Sarrantonio, 1994).Cover crops are often N 2 -fixing legumes,
but other species, such as grasses and crucifers, are also used.
Cover crop residue can be managed in two distinct ways. Incorporation of
cover crops into soil through various forms of tillage is called green manuring ,a
practice that has been used by farmers throughout the world for millennia
(Pieters, 1927, pp. 10-16, 238-311). Alternatively, cover crop residue can be
retained on the soil surface through no-tillage and zone tillage techniques
and used as mulch. Other materials, such as sawdust (Obiefuna, 1986), food
and distillation wastes (Singh, Singh & Singh, 1991), and sewage sludge (Roe,
Stoffella & Bryan,1993),also can be used as mulch,but the costs of purchasing
and transporting them onto farm fields tend to restrict their use to high-value
crops or situations where application to land is less expensive or more envi-
ronmentally benign than alternative disposal methods. Cover cropping pro-
vides a means of inexpensively producing mulch in situ.
Green manures and mulch crops can be grown (i) when land would other-
wise lie fallow, (ii) in mixtures with “main crops” grown for cash, fodder, and
food; or (iii) as substitutes for main crops during normal seasons of crop pro-
duction (Chapter 7) (Sarrantonio, 1992). The approach used depends upon
crop characteristics and the environment.For example,in colder regions,such
as the northeastern and north-central USA, sufficient time is usually available
to establish winter wheat or rye cover crops after harvesting maize grown for
silage, but not after maize grown for grain. Other cover crops, such as hairy
vetch and clover species, must be planted before September if they are to
survive until the following spring. Consequently, in many cropping systems
used in short-season areas, a winter cover crop can be used successfully only if
it is planted into a preceding main crop.
Broadcasting cover crop seeds on the soil surface to establish them during
the growth of main crops generally results in poor stands, though occasional
success has been reported with winter rye (Mohler, 1991; Johnson, DeFelice &
Helsel, 1993). Reasons for poor establishment of surface-sown seeds include
consumption by insects, mollusks, and small mammals (Figure 5.6) (Mohler,
unpublished data), and failure to imbibe sufficient water. To improve estab-
lishment, cover crop seeds can be incorporated into soil during the last culti-
vation of main crops (Scott & Burt, 1987). Burial of cover crop seeds hides
them from seed predators and, by speeding germination and establishment,
shortens the period of vulnerability to herbivores (Figure 5.6). Cover crops
planted at last cultivation rarely compete significantly with the associated
main crops, although cover crop seedlings may die if main crops cast dense
shade.
In temperate regions,cover crops used as green manures typically are killed