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

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Grasslands of the world
Paspalum notatum . Following Lemaire (1997), DM production of vegetative
grasslands depends mainly on three morphogenic variables: Leaf Appearance
Rate (LAR ), Leaf Expansion Rate (LER ) and Leaf Life Span (LLS ). These
variables are genetically defined, but modified by environmental factors. The
combination of these variables determines the structural characteristics of the
grassland, such as tiller density, leaf size and number of leaves per tiller, and the
integral outcome gives the Leaf Area Index (LAI ). Boggiano (2000) verified
that LER and the number and size of leaves and tillers were very sensitive
to N effects and defoliation management , leading to different recovery rates
of Paspalum notatum after grazing. LAR was 5.5–7.0 days, influenced by N
and FO , suggesting that lighter grazing increases sheath length, the expansion
period and the size of new leaves. With low N applications, there is an increase
in the number of small leaves and a longer LLS . Average LLS varied from 21
to 31 days, increasing with FO and with reducing N. It was evident that LAR
and tiller density increase with higher defoliation intensities.
Tiller density depends on LAR and increases with lower FO , which
provoke higher grazing frequency (more defoliations). At the same time, N
increases LAR and stimulates higher tiller densities under lower FO, while
contributing to low tiller number in high FO conditions, where defoliations
are less frequent. N × FO interaction alters compensatory relationships, with
a trend towards higher tiller density on high N–low FO swards. N reduction
decreases tiller density and increasing N raises the weight per tiller. This gives
us the parameters for an area covered by Paspalum notatum , modelling a compact
sward with low production. It differs from a productive grazing pasture ,
with fewer, bigger tillers, in a higher sward profile that ensures high DM accumulation
rates and it is more favourable for animal production.
Average final length of the laminae is more dependent on previous defoliation
intensity (residual LAI ) than on nitrogen supply. Bigger tillers have greater
LER (Table 5.8). It seems to be desirable to promote management practices
that promote larger tillers. LAI is the main factor determining interception of
incident solar radiation, which has direct effect on the DM accumulation rates
TABLE 5.8
Estimated LAI values for Paspalum notatum as a function of surface response models for residual
LAI and after 33 days of regrowth (Boggiano, 2000).
FO (percent of LW) N (kg/ha) LAI after grazing LAI after 33 days regrowth
4.0 0 1.992 3.956
4.0 100 1.555 4.093
4.0 200 0.644 4.144
9.0 0 0.962 2.536
9.0 100 1.462 3.675
9.0 200 1.614 5.924
14.0 0 1.480 2.816
14.0 100 3.045 6.153
14.0 200 4.130 9.404
SOURCE: Adapted from Boggiano, 2000.