Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 249

section “Allelopathy” above), weed emergence through rye straw was no less
than would have been expected for a non-phytotoxic mulch with rye's propor-
tion of solid volume (Teasdale & Mohler, 2000). Rye roots have a dispropor-
tionate percentage of the plant's allelopathic activity (L. A. Weston, personal
communication) and this may partially explain the apparent similarity
between rye straw and other mulch materials in this and earlier studies
(Teasdale & Mohler, 1992).
Most of the variation between plant species in ability to penetrate mulch is
related to seed mass (Mohler, 2000). This allows good emergence of large-
seeded crops like maize and soybean through mulch rates that severely sup-
press small-seeded weeds. Typically 5-10 Mg ha 1 of mulch is required to get
substantial reduction in density of the small-seeded species (

2 mg) that
dominate most weed communities; in some years, even higher mulch rates
may be insufficient (Figure 5.11) (Mohler & Teasdale, 1993; Buhler, Mester &
Kohler, 1996; Vidal & Bauman, 1996). Because fallow season cover crops typi-
cally only produce 2-4 Mg ha 1 of top growth, a mulch grown in situ should
not be expected to provide adequate weed control by itself. Moreover, high
mulch rates are likely to interfere with crop production. Wicks, Crutchfield &
Burnside (1994) found, for example, that 6.8 Mg ha 1 of wheat straw gave
better suppression of grass weeds than lower rates,but maximum maize yield
was obtained at 4.4 Mg ha 1 .These considerations indicate that mulch should
be considered as one component of an integrated weed management strategy
rather than as a direct substitute for cultivation or herbicides.
The many studies using no-till planting into mulches grown in situ have
largely supported these observations.In most of these studies mulch provided
substantial suppression of annual weeds, but some additional weed manage-
ment, usually in the form of herbicides, was necessary to obtain acceptable
weed control and crop yield (Crutchfield, Wicks & Burnside, 1985; Shilling et
al .,1986; Wallace & Bellinder,1992; Johnson,DeFelice & Helsel,1993; Curran,
Hoffman & Werner, 1994; Brecke & Shilling, 1996; Burgos & Talbert, 1996;
Yenish, Worsham & York, 1996). In a few studies where mulch rate was low,
the mulch had little effect on the weeds (Eadie et al ., 1992; Lanfranconi,
Bellinder & Wallace, 1993). However, some studies have found that mulch
alone provided adequate weed control in most or all location-years tested
(Liebl et al ., 1992; Hoffman, Regnier & Cardina, 1993; Masiunas, Weston &
Weller, 1995; Creamer et al ., 1996 a ; Smeda & Weller, 1996). Of these, all but
Hoffman, Regnier & Cardina (1993) used rye or a mixture including rye,
which supports the case for the effectiveness of rye's allelopathic properties
under field conditions. In some cases, good weed control was probably also a
consequence of low weed pressure (Hoffman, Regnier & Cardina 1993), or