Working Group 2: Prioritizing Risk Reduction and Management Strategies for Cadmium

The working group considered priorities for reducing dietary cadmium and its bioavailability.

Control the risk of cadmium in paddy rice to subsistence consumers in Asia
Improved water management (keeping soil anaerobic as long as possible during the growth period) can minimize cadmium movement to rice grain. Inadequate water supply can substantially increase the risk of soil cadmium to rice consumers.
Breeding new rice cultivars which accumulate lower levels of cadmium in grain is also a priority. Because the bioavailability of cadmium in polished rice is related to the poor supply of Fe and Zn, it is critical that the density of bioavailable Fe and Zn in polished rice be increased at the same time as minimizing grain cadmium.
For areas with high soil cadmium, limiting the selection of rice cultivars to those having low cadmium accumulation could reduce existing risks from these soils.

Manage contaminated sites
There are areas around the world where soil cadmium has been increased by past mining and smelting activities. These areas could be used for the production of non-food crops to minimize the risk of movement of cadmium into the food chain. For example, production of industrial crops, growth of low cadmium transfer crops (such as grapes or fruits), use of land for recreational purposes, or growth of plants suitable for phyto-extracton.
Inclusion of soil cadmium levels as a criterion for land selection for set-aside programs could be considered so that higher cadmium risk soils are removed from food production, rather than a farmer having to take a specified portion of land out of production.

Produce cultivars with low(er) bioavailable cadmium for sites with high phytoavailable soil cadmium
Research is needed on inheritance of the cadmium accumulation trait, and for identification of lower cadmium germplasm for each crop species where such breeding would provide benefit. Breeding lower cadmium accumulating cultivars has provided a practical solution for limiting cadmium levels in durum wheat grain and sunflower kernels; evidence of the potential for development of lower cadmium rice cultivars (and peanut, potato, soybean, etc.) has been published. Such low cadmium rice is badly needed for extensive areas in Asia where soils have been contaminated by historical industrial discharge.
It is important to combine breeding for lower cadmium with maintained or improved bioavailable Fe and Zn in the edible product. Where crop cadmium limitation is needed, the field measured crop cadmium accumulation (and where possible the bioavailable crop cadmium) for new cultivars could be included in the variety registration criteria.
Where aluminium-tolerant cultivars are being bred for use on acid soils, it is important to ensure that cadmium accumulation in the new cultivars is not increased.

Correct/prevent of Zn deficiency in soils and crops to minimize cadmium in grain
Zinc deficiency enhances cadmium movement to grain and other storage tissues. Correction of Zn deficiency on low Zn soils can reduce cadmium concentration of grains, while potentially improving both crop yields and nutritional status for consumers. Further, higher crop Zn may reduce the bioavailability of crop cadmium in foods.
Breeding higher grain Zn cultivars should aim to keep grain cadmium low even though this may increase the cost of the Zn improvement breeding program.

Control soil acidification
At least in cadmium-enriched soils, raising soil pH reduces cadmium uptake by most plant species. Under conditions where soil acidity has increased cadmium in crops, the application of limestone can reduce cadmium transfer to foods and diets. With grain cadmium limits for trade, liming soils could be an important tool to achieve the required limits.
In the long term, the use of fertilisers increases soils acidity and regionally, acid rain exacerbates soil acidity and increases cadmium in crops.

Manage salinity on soils with chloride-based salinity
Chloride in soil enhances the plant accumulation of cadmium. Therefore, practices to reduce chloride-based salinity can be effective in reducing the accumulation of cadmium in crops. Use of high chloride irrigation water on staple crops with the potential for cadmium accumulation should be avoided.
Development of salinity-tolerant cultivars of staple crops should be mindful of the need to keep crop cadmium low at the same time.

Reduce cadmium level in phosphate sources and soil amendments
Reducing the level of cadmium in phosphate sources and soil amendments should reduce the rate of long-term accumulation of cadmium in soils.
Low cadmium fertilisers can be produced from low cadmium ores. To produce low-cadmium fertiliser from higher cadmium phosphate ores, appropriate decadmiation technology is needed. Research to develop and demonstrate such a technically feasible and economically viable technology is needed before this approach could be applied in commercial production.
One method industry can use to produce lower cadmium products at competitive costs is to blend ore sources. This approach is complicated by the many other properties of ores which influence the costs of fertiliser production. It may also be possible to blend phosphoric acid or processed fertilisers to lower cadmium concentration in the final product but there are costs involved.
Green technologies have been increasingly successful in producing high quality products with lower environmental costs. New approaches for phosphate production which involve lower costs and risk from wastes or by-products may be possible, or may even generate products with lower potential for accumulation of bioavailable crop cadmium.

Optimize nutrient management and other agricultural practices for both crop yield and nutrient use efficiency
The efficiency of P-fertiliser use by plants may still be improved by creative research. The source, placement, timing, and rate of application may each affect P fertiliser efficiency in crop production. Lower P-fertiliser rates will supply less cadmium regardless of the limits established. However, if soils have inadequate P phytoavailability for maximum crop yield, applied N may be released to the environment rather than used by the plant to maximise yield.
Besides the use of lower cadmium crop cultivars, management which reduces acidification or chloride accumulation, as well as the use of crop residue and mycorrhizal management, may help reduce crop cadmium regardless of fertiliser cadmium levels.

Regulate crop cadmium based on the bioavailable level in the food consumed
Because crop Zn, Fe, and phytate may influence crop cadmium bioavailability, further research is needed on bioavailable cadmium in the crop portion actually consumed, not simply the total cadmium concentration in the raw agricultural commodity.
It may be useful to consider bioavailable nutrient ratios, e.g., Cd:Zn, Cd:Fe in evaluating cadmium limits in foodstuffs.

Target low cadmium P-fertilisers to cadmium-sensitive crops or production systems
It may be socially responsible to target low cadmium fertilisers to areas where food crops are grown on land that has already been enriched with cadmium.