Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 428

responses can be developed (DiTomaso, 2000). Euphorbia esula has invaded not
only cattle grazing lands in southern Canada and north-central USA, but also
roadsides and parks (Swiadon, Drlik & Woo, 1998). Solanum viarum has
invaded several hundred thousand hectares of pastures, roadsides, and peren-
nial crops such as citrus and sugarcane in the southeastern USA, especially
Florida, since it was first identified in 1987 (Mullahey et al ., 1998).
Weed impact on grazing land productivity
Weeds in pastures and rangelands interfere directly and indirectly
with the production of animal products.
Direct interference results from either the grazing of poisonous plants,dis-
cussed earlier, or from physical damage to animals or products. For example,
burrs from Bassia birchii in subtropical semiarid regions or Arctium minus in
temperate humid regions contaminate wool. The sharply awned seeds of
Hordeum murinum and H.leporinum perforate sheep pelts,damage meat quality,
and injure young lambs, but represent minimal risks to cattle (Field & Daly,
1990).
Indirect interference by weeds is a product of their effects on the quality
and quantity of forage. Kelly & Popay (1985) found Carduus nutans ground
cover to be 2.8%, 29.7%, and 6.2% over three years in two C. nutans -infested
pastures. They hypothesized that if animal production losses were in direct
proportion to C. nutans ground cover, control would be profitable only in one
year. Rumex obtusifolius , a weed of temperate pastures, has only 65% of the
nutrient value of perennial ryegrass and produces only 55% to 80% as much
biomass (Courtney, 1985). With three to four harvests per year, grass biomass
was reduced up to 70%, depending on weed density. Grass biomass declined
1% for every 1% increase in R.obtusifolius cover.With five to seven cuts per year,
up to 10% R.obtusifolius cover did not affect perennial ryegrass yields,although
with greater cover,losses reached 15%.On the other hand,Meeklah & Mitchell
(1985) found that 13% and 18% ground cover by Bellis perennis caused no yield
reduction in a high-yielding sward, and 28% ground cover resulted in only a
6% decline in forage production. While B. perennis is highly visible in rye-
grass-clover pastures, its prostrate growth habit and low growth rates com-
pared to forage species minimize its impact on production.
Several studies have measured the effect of varying levels of weed infesta-
tions on animal production. Hartley (1983 a ) found that the control of Juncus
spp. infestations of 7% to 11% ground cover resulted in a 16% to 19% increase
in stock-carrying capacity. In another study, Hartley (1983 b ) found that over a
four-month grazing period sheep live-weight gains were reduced 1.7 kg for
each Cirsium vulgare plant m 2 over a range of 0.1 to 1.5 plants m 2 .The use of