Subsistence farmers are force to migrate when

Subsistence farming, also called subsistence agriculture, is a form of farming in which the family works to produce only enough goods to support the family unit, without excess intended for sale and trade. Subsistence farmers grow a variety of crops and keep several types of animals in order to cover a large range of dietary needs.

Subsistence farmers are forced to migrate when they go in search of fodder for their animals.

Explanation:

Subsistence farming is a kind of life where communities focus in survival, the kinds of crops that they plant are determined according to the kind of soil and whether they have besides what they will need during the following year, a big part of their food necessities are covered by the animal product, that's the reason why they have to take care of what their animals eat and have to provide them with the correct food.