Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 244

smaller-seeded weeds may have their shoot apexes exposed to intense herbi-
vory for several days while they attempt to accumulate sufficient photosyn-
thate to complete growth through the mulch.
Grain straw and killed legume cover crops have been shown to promote
populations of several disease-causing organisms, including species of
Pseudomonas, Rhizoctonia , and Pythium (Stroo, Elliot & Papendick, 1988; Rickerl
et al ., 1992; Rothrock et al ., 1995), and these can pose some threat to crops.
Attack by damping off fungi sometimes also destroys substantial numbers of
weed seedlings in high-residue systems (C. L. Mohler, personal observation).
Whether disease-causing organisms promoted by residue can be manipulated
to selectively control weeds without damaging crops remains to be deter-
mined. Factors that could be manipulated include type of mulch, crop plant-
ing date, direct seeded versus transplanted crops, and choice of crop cultivar.
Separating the effects of reduced photosynthesis, etiolation, and increased
humidity on disease susceptibility of particular weeds and crops could
provide mechanistic insights into how to use these organisms successfully for
weed management.
Residue effects on weed and crop performance
Data from studies investigating the combined chemical,physical,and
biological impacts of crop residue on weeds and crops lead to two general con-
clusions. First, residue has the potential to suppress weeds while having a
neutral or positive effect on crops,though this outcome is by no means univer-
sal. Second, weed responses to residue depend on the quantity of residue
applied, whether or not it is incorporated into the soil, and the biology of the
particular species involved.
An example of the successful use of soil-incorporated residue for weed
management was reported by Boydston & Hang (1995), who measured weed
growth in potato following bare fallow, sudangrass, and rapeseed green
manure treatments. The sudangrass treatment was tilled in the fall, whereas
fallow and rapeseed treatments were tilled the following spring,several weeks
or several days before planting potato. Weed density and biomass (mostly the
broadleaf annual species Chenopodium album and Amaranthus retroflexus ) were
strongly reduced in the rapeseed treatment compared with the bare fallow
and sudangrass treatments (Figure 5.9). Total yield of potato tubers was 10%
to 18% higher when the crop grew after rapeseed rather than after fallow,
whether or not weeds were present. Sudangrass green manure raised potato
yield 13% in one year, but tended to reduce yield in a second year, an effect the
investigators attributed to leaching losses of N that occurred before potato
could use nutrients released from the incorporated residue.