Abstract:
Sugarcane is a high-biomass producing crop and often requires substantial amounts of
nitrogen (N) fertiliser to achieve optimal yields. Nitrogen fertiliser represents a significant
input cost for the sugar industry. This nutrient is highly challenging to manage due to its
susceptibility to various kinds of losses following application, for example, leaching and
denitrification. In addition to reduced profitability, N losses potentially lead to
environmental degradation, for example, through eutrophication of water bodies. As a
result of the complexities of N dynamics in sugarcane cropping systems, mechanistic
crop models are now more commonly being used to help understand these various N inand
outflows, and to inform better management practices. These models first require
extensive calibration, testing and validation with measured data in order to gain
confidence in their performance, however. In this study, a historical dataset from a
fertigation trial conducted in Komatipoort, Mpumalanga, was firstly used to assess the
ability of DSSAT-Canegro (with a newly included N subroutine) to simulate cane and
sucrose yields as well as aboveground N mass in response to different N fertiliser rates
over five consecutive seasons (2003-2007). Cane and sucrose yields, as well as
aboveground N mass were adequately simulated in response to various N rates under
drip irrigation. In addition to this, cane yield, aboveground N mass and soil water N
concentrations were monitored for model testing purposes as part of this study in trials
conducted in Pongola (irrigated), Mount Edgecombe (rainfed), and Inanda (rainfed), all
in KwaZulu Natal. For the Mount Edgecombe trial, N fertiliser treatments that took soil N
levels into account before deciding when to fertilise were included, and the potentially
reduced leaching loads were investigated by the model. In most cases, the DSSATCanegro
model simulated N dynamics and cane yield adequately under both irrigated and rainfed conditions. The model was also observed to perform well under contrasting
environmental conditions, such as during periods of drought versus high rainfall. In a
number of cases, significant differences in cane yield between treatments receiving
different rates of N were not observed in the measured data, indicating that the crop
was able to acquire sufficient N from organic matter mineralisation and fertiliser N from
previous seasons applications or the different N rates were all more than the required
amount. Based on the potential implications of reduced cane yield following water stress
and/or N stress, and unwanted N exports to the environment, it is concluded that
DSSAT-Canegro is a useful tool to improve our knowledge of N dynamics in sugarcane
ropping systems and to develop best management practices.
