Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 243

Table 5.3. Frequency of locations under mulch that received photosynthetically active
radiation at various levels relative to full sunlight

Hairy vetch Rye

Mulch biomass a (g m 2 ) 319 638 246 738

Bare ground (%) 19 4 37 3

Mean PPFD b (%) 24 7 34 5

Radiation relative to full sunlight
0.1% 0 25 0 6

0.1%-1% 0 19 0 42

1%-10% 40 31 25 35

10%-25% 27 19 19 10

25%-50% 17 4 31 6

50% 17 2 25 0

Notes:
a Biomass levels correspond to 100% and 200% ofthe biomass produced by the winter cover crop.
b Photosynthetic photon flux density.
Source: Teasdale & Mohler (1993).

grazing was the primary cause of mortality. However, the effect was insignifi-
cant in the drier year of the study, and Amaranthus retroflexus and Abutilon theo-
phrasti were not significantly affected by mollusks in either year. In another
study,survival of D.sanguinalis was not affected by residue (Mohler & Callaway,
1992).Thus, the effect of mollusks on weed seedlings can be sporadic.
Earthworms remain closer to the soil surface under crop residue due to
cooler, wetter soil conditions.Earthworms have been shown to consume grass
seeds, and only part of these are egested in a viable condition (McRill & Sagar,
1973; McRill, 1974; Grant, 1983). Earthworms can also move weed seeds
downward in the soil profile (van der Reest & Rogaar, 1988), making seedling
emergence less likely. They also kill seedlings of both grass and broadleaf
species by pulling young shoots into their burrows (Shumway & Koide, 1994).
Other weed seed consumers that may be promoted by crop residue include
carabid beetles, ants, crickets, and small mammals (Chapter 8) (Lund &
Turpin, 1977; Risch & Carroll, 1986; Brust & House, 1988).
Since all of the organisms discussed above are generalist feeders that can
threaten crops under some circumstances, careful management is required to
exploit their weed control potential. Nevertheless, a species that finds a crop
and a weed equally palatable may have a more damaging impact on the weed
if it has a smaller seed than the crop.The larger seed size of the crop allows fast
growth through the mulch layer,where herbivory is most intense.In contrast,