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

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Grasslands of the world
TABLE 5.6
Pasture parameters and radiant energy conversion efficiency on a natural grassland in the Central
Depression of Rio Grande do Sul, with dominance of P. notatum and different forage offer (FO)
levels (5-year average; 21.6 TJ/ha of incident PAR).
Dry matter on offer – percent liveweight
Parameters
4.0 8.0 12.0 16.0
Accumulation Rate (AR) kg DM/ha/day 11.88 15.52 16.28 15.44
DM Production kg/ha 2 075 3 488 3 723 3 393
Primary Aerial Productivity (PAP) MJ/ha 40.877 68.714 73.343 66.842
PAR/PAP Efficiency % 0.20 0.34 0.36 0.33
Daily liveweight gain (DLWG ) kg/head 0.150 0.350 0.450 0.480
Animal-day/ha no. 572 351 286 276
Stocking rate kg LW/day/ha 710 468 381 368
Residual DM kg/ha 568 1 006 1 444 1 882
Liveweight gain (LWG ) kg/ha 80 120 140 135
Secondary production (SP) MJ/ha 1 880 2 820 3 290 3 173
PAR/SP Efficiency % 0.009 0.0015 0.017 0.013
PAP/SP % 4.48 4.53 4.66 4.10
SOURCE: Adapted from Maraschin et al., 1997; Nabinger, 1998.
them. With plant development and dead matter accumulation at the base of the
plants, there seems to be a reduction in AR and also a liveweight gain inhibition,
as the treatment with an FO of 16 percent of liveweight suggested. The parallelism
of both development curves suggests liveweight gain variations are closely related
to AR (kg DM/ha/day), as both showed optimal response at an FO of 13 percent
liveweight.
Grazing intensity effects can be evaluated on the energy flow of the system,
as a function of FO levels. Forage availability (kg DM/ha) and liveweight gain
(kg LW/ha) are multiplied by 19.7 and 23.5 MJ/kg, respectively (Briske and
Heitschmidt, 1991). Based on normal global radiation and photosynthetically
active radiation (PAR), Nabinger (1998) determined conversion efficiency
indexes, which represent the quotient between the energy values considered,
multiplied by 100 (Table 5.6). With an FO of 4.0 percent LW, conversion
efficiency of PAR to DM is estimated at close to 0.20 percent. Conversion
increased to 0.34 percent with an FO of 8 percent LW, peaked at 0.36 percent
with an FO of 12.0 percent LW and declined to 0.33 percent with an FO of
16.0 percent LW, as a consequence of age and senescence of natural grasslands,
influenced by FO levels. Primary production of natural grassland increased
markedly (+80 percent) when FO was 12 percent.
The grazing animal prefers green grass to dry , leaf to stem, and the upper half
of young leaves; this shows clearly what forage is desirable and what should therefore
be encouraged in the sward. It is necessary to distinguish between the total
aerial biomass of plants and the available DM for the grazing animal; the former
is the forage technically within reach of the animal, while the latter is the forage
selected. It has to be clear what should be the optimum height of the pasture after
grazing to maintain regrowth capacity (Maraschin, 1993). Understanding this