Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 422

dynamics under this common practice, grazing could be used to reduce weed
seed production and survival.
Dowling & Wong (1993) studied the effect of grazing and herbicide use in
the final months of annual grass pastures before two successive wheat crops in
New South Wales,Australia.Wheat in this region is rotated with annual grass
pastures.Annual grass seed and seedling densities in the first wheat crop were
reduced 91%-99% by herbicides plus grazing or grazing alone, compared to
no pre-planting weed control. The most weed suppressive treatment was
heavy grazing,which consisted of 10 total grazing days over a six-week period
at a stocking rate of 533 sheep ha 1 . Preseason herbicide and grazing treat-
ments were not effective against broadleaf weeds. Broadleaf weed densities
were inversely proportional to densities of annual grasses, with the no-
grazing-no-herbicide treatment having the most annual grasses and the
fewest broadleafs. The proportion of Bromus and Vulpia spp. declined into the
second crop cycle, whereas the proportion of Lolium rigidum increased, due to
less effective control by preseason treatments, less effective control in the
wheat, and greater seed dormancy. Wheat yields were higher in treatments
with preseason vegetation management for both years of the two-year wheat
sequence.
Anumber of factors contributed to the effectiveness of grazing in reducing
weed densities in this experiment. First, most weed seed production occurred
at the end of the pasture cycle when wheat was absent from the field. In many
annual crop systems, weed seed production occurs in the last part of the crop
cycle before crop harvest,when grazing is not feasible.Second,the seeds of the
principal annual grass weeds,except Lolium rigidum ,maintained little viability
in the soil seed bank, and were therefore highly affected by reductions in
current-year seed production.Seed banks are much longer-lived in some other
annual crop systems (Chapter 2).Third, the weeds to be controlled were palat-
able to sheep. Fourth, large numbers of livestock with maintenance-level
nutrition requirements were available to be concentrated in small areas.These
factors make clear that the successful use of grazing during fallow periods for
reducing weed seed production will require a careful match-up of grazing
rates with crop cycles and weed seed production periods.
On smallholder farms, weeds themselves may make up an important part
of the available forage in aftermath and fallow grazing (Humphreys,1991,pp.
6-12). Farmers may weed selectively during the cropping period with the
express purpose of increasing forage availability during the fallow period. In
coastal Ecuador Nuwanyakpa et al . (1983) found that seven weeds had higher
digestibility and crude protein in the dry season than improved forages.Cattle
readily consumed Alternanthera gullensis , but not other weeds such as Sida spp.