Most households in Southern Africa depend on maize as their main source of food and energy, given the high volumes and ease with which it is produced. Alternative food crops that are consumed as substitutes include rice, wheat, sorghum, millet, and tubers such as cassava and potatoes. Consumption of these substitutes occurs mainly when maize is not available or among those households in areas where such substitutes are more easily available (for example, cassava in northern Mozambique). The majority of rural households do grow the other cereals — especially sorghum and millet, which are more drought resilient — in relatively small quantities as a buffer in bad production years for maize. Furthermore, wealthier households (especially in urban areas) with access to a variety of costlier cereals (such as rice and wheat) do consume them to diversify their diets. While wheat is widely consumed in the form of bread, it is produced in relatively small quantities in the region. South Africa is the only country that produces substantial amounts, but still in quantities insufficient to meet domestic requirements. South Africa is also the region's major producer of maize and acts as a major supplier and exporter. In years of relative maize surplus, sizable amounts of both formal and informal cross border trade occurs between neighboring countries.