Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 79

cantly change seed density in the soil, but they concluded that manure did
have a potential for spreading weeds. In particular, Abutilon theophrasti was
probably introduced onto many New York farms during the last 30 years in
contaminated feed, and then spread from field to field in manure once it had
established. Weed density in manure may be greater in less-developed coun-
tries where animals are regularly grazed on weedy stubble after crop harvest.
Dastgheib (1989) estimated that farmers in Iran were sowing nearly 10
million weed seeds per hectare per year with the sheep manure used to ferti-
lize a wheat/paddy rice double-cropping system.
Mack (1981) documented the spread of Bromus tectorum in western North
America. From a few initial introductions, probably in contaminated wheat
seed, the weed first spread along rail lines and cattle trails, and then outward
from these corridors to become a dominant species on much of the rangeland
between the Cascade-Sierra Nevada and Rocky Mountains.Apparently,dispo-
sal of manure and bedding from cattle cars was particularly effective in creat-
ing secondary points of introduction.Since cattle are today often raised in one
location, finished in another, and slaughtered at a third, the potential for
spread of weeds during cattle transport remains large.
Several strategies can be used to reduce the risk of spreading weeds with
manure.One is use of clean concentrates and fodder.Seeds cannot occur in the
manure unless they are first present in the feed. Intensive pasture manage-
ment can prevent weeds from going to seed (Chapter 9) (Sharrow & Mosher,
1982; Popay & Field, 1996). Mowing is also effective in this regard.At the very
least, mill screenings should not be fed to animals unless they are first heat-
treated to kill weed seeds. Ensiling is highly effective at killing seeds of most
weed species (Zahnley & Fitch, 1941; Takabayashi, Kubota & Abe, 1979;
Blackshaw & Rode, 1991). Consequently, ensiling forage that is contaminated
with a problem weed may be preferable to direct feeding. However, a substan-
tial percentage of Abutilon theophrasti , Convolvulus arvensis , and Polygonum con-
volvulus seeds can survive ensiling (Zahnley & Fitch, 1941; Blackshaw & Rode,
1991). High-temperature composting, anaerobic fermentation, or oven
drying can greatly reduce the number of viable weed seeds in manure (Kirk &
Courtney, 1972; Takabayashi, Kubota & Abe, 1979; Bloemhard et al ., 1992;
S ˇ arapatka, Holub & Lhotská, 1993; Tompkins, Chaw & Abiola, 1998).
However, the outside of a compost pile will not heat sufficiently to kill seeds,
so simply piling the manure for a few weeks without turning may leave high
densities of viable seeds (Cudney et al ., 1992). Finally, manure from off the
farm should be evaluated for weed seeds before transport.