Agriculture is an important sector in driving socio-economic development and enabling food and nutrition security. However, it is facing significant challenges which, if addressed, present opportunities for increasing sustainable productivity for global food security. The challenges include: rising/changing food demand; natural resources scarcity; food safety risks (aflatoxins, water/soil pollution and zoonoses); and climatic changes, which increase the threat of pests and diseases. Addressing such challenges may require difficult tradeoffs between raising productivity, building resilience to climate risks and other shocks, as well as reducing agriculture’s negative impacts on the environment.
While crop yield increases need to be achieved in some locations, in others, land reallocation for better conservation of wild relatives, carbon storage, flood protection etc. may be more important. This calls for diversified crop systems – which involve growing different crops and/or focusing on different crop varieties – and ecologically-sound practices that are location and context specific. One such ecological practice is the maintenance of wildlife areas for the conservation of wild relatives. Wild relatives are sources of genetic diversity as we get different varieties of crops through their domestication and evolution. Even without going through the domestication process, wild relatives are an important source of genes for breeding resistant crop varieties, and the necessity of their conservation cannot be ignored.
The benefits of agricultural diversity include provision of ecosystem (environmental) services, like the control of pests and diseases; soil fertility enhancement; soil erosion control etc.
Pest and disease resistance
The use of pesticides on pest-resistant crop varieties leads to short-lived productivity as pests and pathogens overcome resistant genes when improved varieties are grown over wide areas. In the long term, the loss of species diversity and their genetic variation can have high costs for society and farmers themselves. However, crop genetic diversity is effective in controlling pests and diseases on-farm and overcomes the risks to food safety which arise from using pesticides.
Different crop varieties have varying susceptibility and resistance levels to pests and diseases, which means that as the number of on-farm varieties increases, the average damage levels from diseases and pests decreases. The decrease in damage corresponds with how evenly different varieties are distributed. The method of how crops are grown, either in the form of monocultures or mixtures of varieties, is therefore important. Monocultures facilitate the spread, multiplication and evolution of pests and diseases throughout the field due to crop genetic uniformity. The Irish potato famine in the mid-1800s is an example of how genetic uniformity (monocultures) can result in devastating crop loss. Reducing the risk of disease epidemics demands the planting of several varieties in farmers’ fields.
Varietal mixtures, which are based on the principle of having resistant plants between susceptible ones to slow down the spread of pests and diseases, help to minimise yield losses due to epidemics. This method has been successfully used in disease and pest management in several crops, including wheat, common bean and rice, as it provides an effective buffering effect against damage, as well as increasing yields. The proportion of resistant varieties and their arrangement in a mixture determines the methods’ effectiveness.
The value of diversification
Some people think that crop variety diversification is outdated, traditional and only suitable for poor farmers in developing countries, which is simply not true. Close to 50% of wheat fields in Europe and paddy fields in China have been planted as mixtures of different varieties. There are arguments against relying on crop genetic diversity, such as: growing more than one variety in a single field may not be compatible with modern agricultural equipment; and varieties may differ in terms of maturity period, growing character or cooking time, which makes them incompatible in terms of harvesting, marketing, cooking or growing.
Nevertheless, in addition to overcoming prejudices, more research into the suitability of crop diversification techniques for different environments – in respect to compatibility of species and varieties, management practices and the most appropriate acreage for optimum yield – is necessary.