Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 241

where X is the residue biomass in g m 2 , and k is a fitted constant (Figure 5.8a)
(Teasdale & Mohler, 1993; C. L. Mohler, unpublished data). Although this
equation adequately describes the reduction in light by any given type of
residue, the coefficient, k , varies between residue types, and through time as
residue decays. A somewhat more general description of the relation between
light level and amount of residue is given by the same equation, but with
residue quantified as surface area per unit of ground area (Figure 5.8b) (i.e.,m 2
m 2 - Teasdale & Mohler,2000).Crop canopies differentially filter out the ger-
mination promoting red wavelengths (see Chapter 2), but dead mulch materi-
als have only a minor effect on light quality (Teasdale & Daughtry, 1993;
Teasdale & Mohler, 1993).
Although the mean light transmittance through crop residue is highly pre-
dictable, light level at the soil surface under residue varies greatly. Relatively
high light levels are found at some locations even under a thick layer of mulch
(Table 5.3) (Teasdale & Mohler, 1993). Thus, cover crops used as mulch can
reduce the light-cued germination of many surface and shallowly buried
weed seeds, but sufficient light to stimulate germination of some seeds will
penetrate through all but the heaviest mulch layers.Seeds of most crop species
do not require light for germination and are capable of high percentage ger-
mination beneath mulch.
In those microsites where weed seedlings are shaded by residue, seedlings
may become etiolated as they attempt to extend photosynthetic surfaces
above the mulch layer. Consequently, weed seedling growth in mulch may be
slowed by shading effects on photosynthesis, as well as increased metabolic
costs of extra stem material. In contrast, the larger seed size of most crops rel-
ative to weeds conveys a greater ability to grow up through mulch without
exhausting seed reserves or becoming excessively etiolated.
Residue effects on herbivores and pathogens
A wide diversity of organisms potentially damaging to weeds can be
promoted by crop residue. Mollusk populations increase under crop residue
at the soil surface due to decreased desiccation and increased food supply, and
may create problems for crop production in no-till, high-residue systems
(Edwards, 1975). However, damage to weed seedlings by mollusks in high-
residue conditions can also be considerable. C. L. Mohler (unpublished data)
measured seedling survival of three weed species in tilled and untilled maize.
In one of the two years of the study,survival of Digitaria sanguinalis from emer-
gence until flowering was 60% in conventionally tilled maize, but only 16% in
no-till maize. Systematic recording of the presence of slime trails and the
types of damage sustained by each plant prior to death indicated that mollusk