In Mauritius sugar cane production is sustained through the use of non-renewable resources such as the 360 tonnes a.i. of herbicides and 65 000 tonnes chemical fertilizers containing 11 000 tonnes nitrogen, 5 000 tonnes P₂ 0₂ and 14 000 tonnes K₂0. These resources, in bringing success to sugar cane production, have also become since the 1970s, a focus of concern to the community on account of their possible negative effects on the environment. Indeed, with the detection limits that have become possible with sensitive instruments such as high performance liquid chromatography (HPLC), trace levels of persistent pesticide residue or phosphate and nitrate in surface and ground waters have turned out to be commonplace.

Faced with this public concern, the Mauritius Sugar Industry Research Institute (MSIRI) endeavoured in the 1990s to acquire a detailed knowledge of the impact of sugar cane production on the environment, particularly on water quality. In this context, potable water was monitored regularly every month for nitrate from 1990 to 1994 at 25 locations in Mauritius. The data acquired showed that the amount of nitrate (NO₃) in potable water, as expected, fluctuated with time but it never exceeded the maximum permissible level of 50 mg NO₃ L^-1 recommended by the World Health Organization. In fact, more than 70% of the more than 1000 water samples analyzed contained less than 15 mg NO₃ L^-1 and concentrations higher than 25 mg NO₃ L^-1 were found in less than 10% of the water samples.

Furthermore, more than 60% of the 750 groundwater samples (50% for the 1000 river waters) analyzed during 1995 and 1996 did not contain any detectable level of herbicide residue. Even when present, of all herbicides available for use in sugar cane cultivation, only residues of atrazine, hexazinone and diuron could be detected, with their concentrations being most often in the lower limits (0.05-0.5 ppb) measurable on the high performance liquid chromatograph and far below the maximum limit of 3 ppb atrazine, 14 ppb diuron and 210 ppb hexazinone recommended for drinking water by the United States Environment Protection Agency.

Degradation of the environment by sugar cane production is in fact a concern common to all sugar producing countries and is best dissipated by international research linkages. In this context, a large scale collaborative study between MSIRI and the Queensland Department of Natural Resources with support from the Australian Centre for International Agricultural Research (ACIAR) was undertaken to measure and predict the movement of agrochemicals in tropical sugar cane cultivation. This collaborative study which lasted four and a half years enabled the export of sediment and agrochemicals from sugar cane plantations to surface and groundwaters to be quantified. Thus, despite the undulating topography often encountered in Mauritius with 70% of the yearly rainfall (which may exceed 4000 mm in the superhumid areas) occurring as high intensity downpours between April and January, not more than 1 Kg phosphorus ha^-1 yr^-1, 10 Kg nitrogen ha^-1 yr^-1, 0.2% of the herbicide applied and 2 tonnes sediment ha^-1 yr^-1 would be transported by surface/subsurface runoff to rivers and lagoons.

The impact of an annual load of 10 Kg N ha^-1 and of the 0.2% of herbicides applied on the concentration of nitrate and herbicide residues in river waters can be seen by the fact that, as opposed to the generally undetectable level of nitrate and herbicide residues in the river still under the pristine natural forest environment at Black River, sugar cane cultivation raises nitrate concentrations in streams and rivers to not more than 2 mg NO₃-N L^-1 and results in herbicide residue concentrations not exceeding 1 µg L^-1 atrazine or diuron. Apart from the higher concentration of dissolved P observed during storm events, the dissolved P concentration as a result of sugar cane cultivation would not deviate from that observed under the pristine conditions at Black River.

The extensive science-based data set that has been gathered by MSIRI should comfort both the planting community and the public in showing that sugar cane production as practised now, even under the superhumid conditions in Mauritius, do not pose a threat to the environment as agrochemicals and soil moved offsite are agronomically insignificant and environmentally inconsequential. Based on visual observation of crystal clear watercourses becoming loaded with sediment during or after a heavy rainfall event, the perception of the general public has been that large quantities of sediment and agrochemicals are transported to reservoirs, rivers and lagoons in Mauritius. The data at hand showed that this perception is not founded.

Development of sound agronomic practices

The current little impact of sugar cane production on the environment owes a lot to the sound agronomic practices adopted by the planting community and developed through research. Thus, harvesting sugar cane inevitably removes nutrients from the soils and if they are not replaced, all cropped soils will ultimately run short of available nitrogen, potassium, phosphorus and other essential elements to become ultimately incapable of sustaining a profitable sugar cane production. Mineral fertilizers and their management are therefore key issues in sugar cane production. A balanced approach to mineral fertilizer use to ensure, on the one hand, that soils are not depleted of nutrients while on the other hand, avoiding excesses to minimize their detrimental impact on the quality of ground and surface waters is practised by the agricultural community through

• Site-specific fertilization using soil testing to prevent over-fertilization or depletion of soil reserves, and

Judicious use of fertilizers particularly through appropriate time of application. With the rapid degradation of the herbicides in the surface 2.5 cm layer or the fast transformation of fertilizers in the top 15 cm soil, little herbicide residue or fertilizer remains in the surface soil after a month for transport into streams, rivers and aquifers. Water in Mauritius is thus at risk from herbicides and fertilizers only if runoff or leaching from the sugar cane fields occurs within a month from the time of applying the agrochemicals. In this context, in every climatic or geographical zone of Mauritius, the bulk of herbicides and fertilizers is applied to sugar cane during the second half of the year (July-December) when high intensity rainfall is rarely encountered. More than 70% of rainfall in Mauritius is in effect received from January to April. The bulk (more than 70%) of the herbicides and fertilizers in sugar cane are consequently used when the risk to contaminate fresh water resources is at its lowest.

The delicate desirable balance that has been achieved between sugar cane production and environmental cleanliness could not have been possible without the effort to maintain the topsoil quality and quantity. Indeed preserving the surface soil where the organic matter is located is a sound practice not only from the agronomic but also from the environmental viewpoint. Its preservation keeps it from contributing to the world's greenhouse warming problem and from contaminating the freshwater resources through erosion. Though as a perennial crop, sugar cane covers the land extensively thereby minimizing soil erosion, nevertheless, the risk of sediment and associated agrochemicals being carried into water courses is reduced even further through the following soil conserving or cultural practices adopted by the planting community :

• Planting of barrier crops such as Vetiveria on edges and banks of sugar cane fields,

Establishment of riparian buffer zones along streams and rivers,

Contour planting and minimum tillage on slopes to minimize erosion,

Extension of minimum tillage practice to flat lands to avoid soil disturbance,

Development of green cane harvesting not only to ultimately eliminate burning and its deleterious consequences, e.g. emission of smoke, greenhouse gases and particulate matter, to the atmosphere, but also to increase the plant litter acting as a mulch for protecting the soil surface against the beating action of raindrops and for reducing herbicide use.

Development of sound agricultural practices would not be complete if management of the water needs of the sugar cane crop is not considered. In fact, the most critical issue regarding resource is water because its management is the only effective measure we have against an uneven spatial and seasonal distribution of rainfall. Water use efficiency in sugar cane production in Mauritius is being constantly improved to reduce losses and thus pollution of the environment. The improvement in water management has to-date implied the following :

• Complete replacement of surface irrigation by more efficient techniques such as drip and centre pivot systems,

Resort to prescription (and now to deficit) irrigation based on crop water requirements.

The current harmony between sugar cane production and the environment in Mauritius, it must be recognized, is not due solely to sound agronomic management. Research and development to attain that harmony have spanned the whole production cycle, starting from crop improvement through cultural practices and disposal of factory wastes after the sugar manufacturing processes to technology transfer to the planting community. Indeed, the contribution of other disciplines in the protection of the environment may be gauged through the following :

Crop improvement

• Only sugar cane varieties resistant to diseases and pests are released for commercial production to avoid the need for fungicides and insecticides during cultivation.

• Insecticides are not used in sugar cane production, except in special circumstances, such as, during the 1997/1998 cropping season when 230 L of synthetic pyrethroids were applied over 455 ha to control an outbreak of locust epidemic. Biological control within a strategy of integrated pest management or cultural practices are preferred in pest control in sugar cane. As an example, scale insects are controlled by thrashing the sugar cane.

• Trash blanketing is practised in the subhumid zones to reduce, if not to eliminate, herbicides use in sugar cane. Already the amount of herbicides used has decreased from 700 tonnes a.i. in the 1990s to less than 400 tonnes a.i. in 2003. Going hand in hand with the endeavour to reduce herbicide application in sugar cane, are efforts to improve the techniques of applying the herbicides, and efforts to screen for herbicides which are safer and non-toxic to the environment.

• Treatment of factory effluents in wastewater ponds and by other techniques to meet environment standards and to enable re-use of the wastewaters in irrigation.

Extension

• To enable producers to fulfill their role not only in support of productivity but also in the protection of the environment, they are constantly kept informed of the more efficient and environmentally sound technologies with respect to use of agrochemicals, adoption of high-yielding varieties, cultural operations, crop protection, natural resource management, and so on.

In commercial agriculture, regulations or standards for environmental protection are perceived by the agricultural community to be associated with increased costs and are considered a hindrance to productivity. Research and development on sugar cane in Mauritius have accomplished the feat of bringing sugar cane production and environmental protection from their diametrically opposed positions, to one where they are in synchrony with each other. Sugar cane production in Mauritius has in fact become a vivid example of how profitable agriculture can be pursued while keeping the environment clean and beautiful.

 

October 2003

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