Grasslands of the World.pdf - Disasters and Conflicts - UNEP

S.G. REYNOLDS
Grasslands of central North America 243
Plate 6.18
Crested wheatgrass (Agropyron cristatum ).
ing season (Nyren, 1979; Vallentine, 1989). In northern mixed prairie, early
application of nitrogen may stimulate the aggressive cool-season species
western wheatgrass (originally Agropyron smithii , now Pascopyrum smithii )
at the expense of warm-season species (Nyren, 1979). High nitrogen fertilizer
rates late in the growing season to benefit warm-season species such as blue
grama may stimulate cool-season species the next spring (Wight, 1976). In
southern areas, cool-season introduced species such as Kentucky bluegrass
(Poa pratensis ) may be stimulated (Owensby, 1970; Rehm, Sorensen and
Moline, 1976; Vallentine, 1989).
Power (1972) argued that on many northern Great Plains rangelands,
inorganic nitrogen is immobilized when nitrogen is added as fertilizer and
sufficient nitrogen must be added to overcome that immobilized. He stated
that the system could be maintained if annual fertilizer additions plus mineralization
equals immobilization plus irreversible losses.
Even though substantial responses in herbage yield can be accomplished
with range fertilization in grasslands, the practice may be only marginally
feasible economically. In blue grama rangeland in south-central New Mexico,
despite doubling of herbage and cattle production from annual additions of
40 kg/ha, economic returns were marginal (Chili et al., 1998). The economics
of range fertilization depend mostly on cost of fertilizer and livestock
prices.