Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 413

cycle. When grazing was used to destroy the bolting flowerhead of Centaurea
solstitialis , a winter annual weed of California rangelands, plants resprouted
from basal and axillary buds and produced new inflorescences, often in
greater number than without grazing (Thomsen et al ., 1993). Annual, erect-
growing broadleaf species are poorly defended against herbivory, because
they produce few buds which are highly accessible at the base of leaves or
branches. Shrubs and some perennial forbs, both as weeds and forage compo-
nents in rangelands, regrow from aerial axillary buds when the apical bud is
removed. Some species sprout from basal buds or roots. Depending on shrub
architecture, many or few buds may be available for regrowth after grazing
(Orshan, 1989).
The physiological mechanisms for herbivory tolerance include increased
photosynthetic rate after tissue removal, temporary reallocation of photosyn-
thates among shoots and roots, and use of accumulated carbohydrates (Briske
& Richards, 1994). These mechanisms have been studied principally in range
species that have different degrees of grazing tolerance. Richards (1984) com-
pared the response of Agropyron cristatum and A. spicatum to grazing and attrib-
uted A. cristatum 's greater defoliation tolerance to the temporary reallocation
of photosynthates from root growth to shoot growth. Agropyron cristatum
had 50% lower root growth during the recovery period compared with A.
spicatum .
Plants in different growth stages differ in their ability to resist grazing or
tissue loss. Weather conditions also affect a plant's recovery from tissue loss.
Seedlings are often the most vulnerable stage and may be damaged either by
grazing or trampling. Survival of seedlings of the perennial range grasses
Agropyron cristatum and A. desertorum was only 0.4% when grazed or trampled
compared to 11.6% when cattle were excluded in a semiarid Utah environ-
ment (Saliki & Norton, 1987). However, seedling survival of Macroptilium atro-
purpureum ,aleguminous forage in tropical Australia,increased from 0%-1% to
approximately 14% when grazing was increased from 1.1 to 1.7 animals ha 1
(Jones & Bunch, 1987). Trampling and grazing losses were offset by the gains
from improved seedling growth with increased grazing of an associated per-
ennial grass.
Perennial plants,particularly in climates with cold winters or extended dry
seasons, have regular annual cycles of accumulation and use of carbohydrate
and other reserves.These fluctuations influence their response to herbivory or
tissue loss (Caldwell, 1984). Cyclic seasonal fluctuations in carbohydrate
reserves were key to understanding the response of the perennial pasture
weed Pteridium aquilinum to burning which destroys plant tissue similar to
grazing or trampling (Preest & Cranswick, 1978). A midsummer burn right