I'm assuming you meant subsistence farming? If so then yes, it would be a large problem because they would then not have enough food to feed themselves.
The correct answer is - industry.
The developing countries are most often occupied with making the economies heavily focused on the industry, so the countries engage into a big industrialization process.
The industries are usually based around manufacturing of the natural resources that the country has, and most often those are the metal ores, the agricultural products, creating materials...
The developing countries usually have a relatively big economic growth, and that is due to the bad economic basis that they had as a starting point.
Humans tend to congregate and form settlements around areas that are rich with natural resources such as forests and rivers, because these locations support life and the economy.
