Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 412

Whether a plant is grazed at all and the extent of defoliation are influenced
by positive and negative plant quality factors. Herbivores avoid or graze less
on plants with low nutritional quality (protein, digestible dry matter), high
secondary plant metabolites (tannins, alkaloids, nitrates, and oxalates), and
physical defenses (thorns and spines) (Minson, 1981; Norton, 1982;
Launchbaugh, 1996). These factors vary not only by plant species, but also by
plant growth stage. For example, sheep and cattle consume the tender shoots
of Cirsium arvense , but not the mature, spiny foliage (Hartley, Lyttle & Popay,
1984).Only those weeds that are readily grazed without harm to the herbivore
can be controlled by grazing management (Scifres, 1991). Weeds that are
unpalatable or toxic are not susceptible to control by selective grazing,
although they may be weakened by deliberate trampling.
In addition to mechanisms to avoid grazing, plants have compensatory
mechanisms to speed recovery from grazing (Crawley, 1983, pp. 86-110;
Whithman et al ., 1991). This ability to tolerate grazing depends on morpho-
logical and physiological characteristics that come into play depending on the
severity of defoliation.Regrowth will be most rapid after limited defoliation if
leaf growing points are still intact, with cell elongation occurring from leaf
meristems. Regrowth after more severe defoliation is slower, since axillary
buds must first differentiate meristems and then root mass and surface area
must recuperate. Radiotracer studies of forage species after total defoliation
have shown that new growth becomes independent of root reserves within 10
days for alfalfa and 36-48 hours for perennial ryegrass (Smetham,1990).Such
studies on weed species are uncommon.
The more complete and more frequent is the defoliation of a weed species,
the more likely that grazing will be a useful control measure. In a worst-case
scenario, cattle and wildlife do not consume the foliage of the subtropical
weed Solanum viarum and thus cannot be used to reduce weed vigor, but ingest
the fruit,spreading the seed in nutrient-rich bare spots (Mullahey et al .,1998).
This herbivory pattern has accelerated the spread of this recently introduced
weed throughout the southeastern USA.
Both weed and desirable pasture and range species can be classified
morphologically by the location of their apical meristematic tissue (Clements,
1989).Many weeds species have buds that are inaccessible to common grazing
herbivores and consequently recover rapidly from grazing. These include
Imperata cilindrica , Sorghum halapense , Cirsium arvense , and Euphorbia esula , all of
which spread by growth of roots or rhizomes.The growing points of stolonif-
erous and prostrate species are right at the ground surface and are partially
consumed only with the most severe grazing. Rosette growth in the first year
protects certain biennial and winter annual weeds during part of their life