Agriculture Reference
In-Depth Information
Crop Development
Phasic development describes the duration of different growth phases and the bio-
mass partitioning among different plant organs. Morphological development refers
to organ development during the plant life cycle. Both are affected by temperature
(Table 10.1), as calculated by growing degree-days. Phasic development is also
affected by photoperiod and genetics (Table 10.1). Genetic diversity within a crop
species enables plants to be adapted to diverse settings in different regions of the
world. For example, wheat genotypes are grown from temperate Argentina (latitude
50° S) to Sweden (60° N) and in tropical regions between.
Plant growth rate and duration are equally important in determining potential
crop yields; hence, the accuracy of yield simulation models. Record high yields of
annual crops are always obtained in cooler environments where there is maximal
duration of daylight for plant growth. Warmer climates can equal the total annual
yields of the cooler region yields by growing more than one crop per year.
The principal functional approach used to estimate the duration of crop growth is
based on thermal time calculation (Gallagher 1979). Thermal time (
t
d
) is the accu-
mulation of degree-days (i.e., °C d) above a base temperature and is calculated as
n
(
)
t
=
Σ
1
T
−
T
d
a
b
i
=
where
T
a
is 24-hour daily mean temperature;
T
b
is the base temperature below
which the crop growth ceases; and
n
is the number of days.
T
a
is usually approxi-
mated by taking the mean of daily maximum and minimum temperatures (Ritchie
and NeSmith 1991).
Thermal time to simulate development requires temperature to be measured
close to the growing point of the plant. Ritchie and NeSmith (1991) showed that
using air temperatures to calculate thermal time and to predict the number of leaf
tips and leaf development overpredicted leaf numbers in the CERES corn model,
and required correction using a higher phyllochron value (i.e., duration between
leaf tip appearances) (Vinocur and Ritchie 2001).
Several crop species are sensitive to photoperiod. In general, plants adapted
to grow in shorter day lengths (e.g., corn, sorghum, and soybean) develop more
quickly when exposed to shorter days. Plants adapted to grow in longer day
lengths (e.g., wheat and barley) grow more quickly when exposed to longer
days. In addition to temperature, Ritchie and NeSmith (1991) showed that pho-
toperiod in corn can significantly affect leaf number and the duration of vegeta-
tive stages.
Yield and Yield Components
Simulation procedures for yield estimates differ among crop models. One approach
is to assume a constant fraction of biomass produced at maturity (i.e., the point of
economic yield) or to assume a constant increment of biomass production each day
after grain filling starts. Another approach is to separately estimate the yield com-
ponents (ear number, kernels per ear, and kernel weight).
