There are a number of political issues having to do with agriculture. Not all countries have to deal with all of these issues.
One issue that is common to many countries is how much (or whether) to subsidize or to protect agriculture. In developing countries, the agricultural sector is often unable to compete with farmers from rich countries. This means that the governments have to decide whether to allow free trade. In rich countries, there is often a feeling that farmers are socially or politically important and that the government should make sure that they can have relatively secure livelihoods. Thus, in places like Japan, there can be extremely high trade barriers placed on some farm products.
An issue with agriculture in the United States and in some other rich countries is how many immigrants to allow in to work in agriculture. In the US, there is a great deal of controversy over allowing unskilled immigrants to enter the country (or to stay in if they entered illegally). The agricultural industry wants them to be allowed in because it wants a large work force of people who are willing to work for very little pay. The desires of agriculture (and other labor-intensive sectors) then have to be balanced against those of people who worry that excessive immigration is socially and economically undesirable.