Subsistence farmers — Northern KwaZulu-Natal

EDSON NCUBE and SYLVIA PHOKANE; ARC-Grain Crops Institute

Aflatoxins are naturally occurring poisonous substances produced by the fungi Aspergillus flavus and A. parasiticus which infect and contaminate maize and groundnut grains.

Aflatoxins are potentially problematic in seasons favourable for plant infection or contamination of stored grains. In unfavourable seasons levels of aflatoxins may be negligible with minor potential mycotoxicotic effects on humans and animals that consume the grain.

A previous study carried out during the 2005/2006 and 2006/2007 seasons on aflatoxin contamination of maize and groundnut samples produced by subsistence farmers in Limpopo, Mpumalanga, KwaZulu-Natal and the Eastern Cape provinces, indicated that aflatoxin contamination is sporadic, but it can reach levels as high as 49 parts per billion (ppb) in a single maize sample and be absent in other samples collected from the same area, e.g. Jozini in northern KwaZulu-Natal.

The maximum allowable limit for aflatoxins in human food in South Africa is 10 ppb (ten parts per billion is the same as one pinch of salt in 10 tons of potato chips or one second in 22 years).

Maize is a major staple food in South Africa with the average daily intake being 300 g or more per capita. Exposure to human food or animal feed contaminated with aflatoxins through ingestion, results in reduced performance, depressed immunity and sickness or death in humans and animals.

High aflatoxin levels can be lethal in livestock, particularly young pigs, pregnant sows, calves and young poultry. Contamination of crops with aflatoxins is often most severe in rural areas where subsistence farmers are unaware of their existence, and follow agricultural practices that might contribute to the production of aflatoxins. The quality of maize consumed, thus determines the quality of human and animal life.

Contamination of maize with Aspergillus fungi and aflatoxin

Aflatoxins are produced during both pre-harvest and post-harvest stages of grain production by Aspergillus species which survive on maize debris and infect maize ears through the silk. The fungus grows from the ear tip (silk) towards the base.

Hot and humid conditions favour the growth of Aspergillus species. Drought stress, temperatures between 26°C - 30°C as well as insect and physical grain damage are factors that contribute to aflatoxin production in the field. Aflatoxin production may also increase during storage where relative humidity, grain moisture and temperature are favourable for aflatoxin contamination.

Subsistence farming, aflatoxin contamination and maize production

Subsistence farmers often lack the necessary resources to ensure production of quality grain from field plantings to consumption.

With restricted access to resistant hybrid seed, fertilisers and pesticides, the yield and product quality of subsistence farmers is often severely affected by poor soil fertility, fungal infections and pest damage.

Most grain is harvested and stored on farm or in silos for medium- to long-term storage, generally under conditions with poor aeration, poor moisture and temperature control, which are factors known to influence aflatoxin production during storage.

Prevention strategies for the reduction of aflatoxin contamination

• Control of stalk borers using chemicals such as Bulldock® (betacyfluthrin).
• Planting Bt maize hybrids that reduce stalk borer levels which in turn reduce aflatoxin contamination by indirectly reducing the dissemination of Aspergillus spores.
• Early harvesting and rapid drying of maize to moisture levels below 12% reduces aflatoxin production.
• Avoid mixing damp and dry grain during storage because an isolated pocket of damp grain supports fungal growth which can spread to the surrounding dry grain.
• Optimum conditions for storage are temperatures below 12°C, relative humidity below 85%, and grain moisture content below 12%.
• Storage facilities must be regularly monitored to ensure early detection and control of insect and fungal infestations. Old grain residue must not be mixed with new grain and storage areas must be sanitised before new grain is stored.
• Sorting of grain to remove discoloured grain before use for human consumption will assist in reducing aflatoxin levels.
• Contaminated grain should preferably be destroyed as aflatoxin has a way of finding its way back into the food chain via contaminated milk or meat.

Research on storage practices of subsistence farmers at the ARC-GCI

The researchers at the ARC-GCI are involved in a follow-up study to determine the effect of storage systems on contamination by aflatoxins and other fungal mycotoxins (fumonisins, deoxynivalenol, nivalenol and zearalenone) in maize produced by subsistence farmers in northern KwaZulu-Natal.

Since storage systems are important sources of contamination and aflatoxin accumulation in rural areas, grain is collected after harvest and after grain has been in storage for three to four months.

Results indicate that one sample each from Vryheid (1,23 ppb) and Mbazwana (0,09 ppb) were positive for aflatoxin while a sample from Jozini had aflatoxin levels of 44,9 ppb.

Since the maximum allowable limit for aflatoxin in human food in South Africa is 10 ppb, some farmers, such as the one in Jozini, are still exposed to dangerously high levels of aflatoxins. This indicates that the nature of contamination is sporadic, but potentially very dangerous and intervention systems need to be developed to reduce possible incidence of aflatoxicosis in animals and humans. Storage facilities presently used by farmers in northern KwaZulu-Natal (Photo 1 - Photo 6) show that there are no quality storage systems in place to reduce consumption of aflatoxin-contaminated grain in subsistence farming systems.

Contaminated grain is stored in plastic and metal drums, under poor ventilation which might form an environment conducive for further aflatoxin production during storage. Grain from subsistence farmers is often marketed locally or consumed within the household resulting in an increased exposure to aflatoxins. An improvement in storage facilities such as the availability of silos with temperature and air control systems as well as the implementation of practices such as drying maize grain prior to storage to moisture levels of 12% or less, cannot sustain the growth of fungi and could prevent further contamination by aflatoxins and reduce food wast-age during storage.

Development of affordable technologies to determine moisture content before storage, could also result in the control of aflatoxin production and fungal infection of grain during storage. For more information, contact the authors at 018 299 6100.