The Agricultural Economics Society's 87th annual conference contributed paper
proposal:
Reducing the impact of sclerotinia disease by determining optimum
crop rotations using dynamic programming
November 2012
Bouda Vosough Ahmadia*, Fiona J. Burnetta, Caroline S. Youngb, Mark C.
Quilkenanda and Alistair W. Stotta
a
b
SRUC, King's Buildings, West Mains Road, Edinburgh, EH9 3JG, UK.
ADAS Consulting Ltd., ADAS Wolverhampton, Woodthorne, Wergs Road,
Wolverhampton, WV6 8TQ, UK.
*
Research Economist at SRUC; E-mail address: [email protected]
Abstract
Sclerotinia rot is a disease caused by the fungus Sclerotinina sclerotiorum which
affects a wide range of crops including oilseed rape, peas, spring beans, potatoes,
lettuce and carrots. The pathogen causes major yield and economic losses in
susceptible crops in the UK. Sclerotinia survives in the soil as sclerotia for up to 10
years. Crop rotation is an important strategy for minimising the impact of the disease.
A range of trade-offs between state of the land in terms of disease and nutritional
values, severity of sclerotinia and financial impacts as a result of different cropping
decisions can be identified. A dynamic programming (DP) model of crop rotation was
developed to study these trade-offs. The objective of the DP was to find the cropping
decision sequence that maximises the net present value of cropping on one hectare of
land over a short and long time horizon. Land was represented by 25 states including
5 sclerotial density levels and, 5 break crops representing the number of years since
the last grain crop decision. Results show that DP method provides a useful
framework to explore the trade-offs between long and short term gains of crop
rotation in relation to plant diseases and economic benefits in arable agriculture that
are at the heart of sustainable food production and land use.
Outline of Paper
Introduction
Sclerotinia rot is a disease caused by the fungus Sclerotinia sclerotiorum which affects
a wide range of arable and horticultural crops including oilseed rape, peas, spring
beans, potatoes, lettuce and carrots. The pathogen causes major yield and economic
losses in susceptible crops (Kora et al., 2003). Sclerotinia survives in the soil as
sclrotia (resting bodies) for up to 10 years, so high level of inocula built up in the soil
in one crop can have a significant impaction on subsequent susceptible crops in the
rotation. Crop rotation can be used to minimise the impact of the disease. Crop
rotation is defined as planting different crops on the same piece of land in sequential
seasons. Crop rotation gives many benefits, including maintaining soil structure and
fertility, reducing agricultural chemical usage, reducing flood losses and avoiding
build up of pathogens and pests.
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Long term and short term management decisions such as crop rotation have an
impact on epidemiology of the disease and therefore on farm economics. Reducing
sclerotinia disease while maximising profit is more complex than simply lengthening
rotations for susceptible crops, hence this study. Bio-economic models provide useful
frameworks to investigate the trade-offs between the state of the land, severity of
sclerotinia and financial impacts as a result of different cropping decisions. We
therefore developed a dynamic programming (DP) model of crop rotation decision
problem to study these trade-offs.
Materials and methods
A DP model (Bellman, 1957) was developed using Microsoft Excel and was run
separately using general purpose DP software (GPDP, Kennedy, 1986). The objective
of the DP was to find the cropping decision sequence that maximises the net present
value (i.e., current value of returns from one hectare of farming land expressed as an
annuity) of cropping on that land over the short term (i.e. 5 years) and long time
horizon. Land was represented by 25 states including 5 sclerotinia states (based on
numbers of sclerotia in soil) and, 5 break crop states representing the number of years
since last grain crop decision. In total 6 cropping decision options were included in
the model. The DP decides which combination and sequence of susceptible crops or
break crops are required to be included in the optimal solution in minimising
sclerotinia and therefore maximising the profit.
Susceptible crops considered were: carrots, oilseed rape, beans, peas, lettuce
and potato. It was assumed that growing susceptible crops raises the number of
sclerotia in soil but subsequent break crop (non-susceptible) decisions will reduce it at
differential rates. The build up and decline curves of sclerotinia in soil as a result of
cropping decisions and their impacts on marketable yields were obtained from the
literature, previous experiments and/or from expert opinion. Stage returns were based
on the gross margin of the current cropping decision but with a yield and variable cost
adjustment function dependent on the state of the land at the current stage. By
changing key parameters in the DP and re-optimising, the impact of alternative
assumptions and crop rotations could be explored.
Example of results
Continuous susceptible cropping, whether treated or untreated, will lead to financial
losses in the long term (Fig 1). However, one break crop in the rotation will prevent
long term build up of sclerotia in land and major financial losses (Fig 1). The financial
outcome of three cropping decisions for land states 1 (i.e., worst state for sclerotinia)
to land state 25 (i.e., best state for sclerotinia) are presented for the last year of a 5year time horizon (Fig 2). Despite a profitable outcome for continuously growing
carrots in some land states in early years, the model confirms that major losses would
be expected for the majority of land states in year 5. However, including one winter
wheat break crop in the rotation generated the highest return for all states in year 5.
Rotation gives the greatest financial benefits when sclerotinia pressure is higher, but it
is also the best financial strategy for land with low sclerotinia (Fig 2). These
examples show that DP method provides a useful framework to explore the trade-offs
between long and short term gains of crop rotation in relation to plant diseases in
arable agriculture that are at the heart of sustainable food production and land use.
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Figure 1. Effect of management decisions of: growing only susceptible crop (SC),
susceptible crop and applying treatment and, susceptible and break crops on
financial outcomes of carrots and oilseed rape for an infinite time horizon.
Figure 2. Financial outcomes of three management decisions for land states 1
(worst for sclerotinia) to 25 (best for sclerotinia) for the last year (year 5) of a 5year time horizon. Presented decisions include: growing only carrots (black solid
line), carrots and applying treatment (grey solid line) and, carrots and winter
wheat as a break (dashed line).
References
Bellman, R. (1975). Dynamic Programming. Princeton University Press, Princeton.
Kennedy, J.O.S. (1986). Dynamic Programming: application to agriculture and
natural resources. Elsevier, Amsterdam.
Kora, C., McDonald, M.R., Boland, G.J., 2003. Sclerotinia Rot of Carrot: An
Example of Phenological Adaptation and Bicyclic Development by Sclerotinia
Sclerotiorum. Plant Disease 87, 456-470.
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