What quantity of seed from a plant must be obtained per hectare in order to consider the seed suitable - in terms of quantity - for production of livestock feed.
To achieve higher agronomic yield, it depends on many factors such as soil type, cultivar potential and other climatic conditions. Plant generally express its full yield potential when these are favorable. For a production of animal feed (seed), the plant will express similarly as for grain production and yield.
The profit from selling the 'quantity' of seeds produced need to be greater than the money invested in producing them. Then, they will consider growing these varieties.
The interest of a raw material for feed formulation is a balance between price and quality. In a specific area, it also depends on the alternative uses of land and the value you can expect from their cultivation.
In the same context (e.g. Brasil), soybean can be valuable at 3t/ha and maize at 8t/ha - because they dont have the same agronomical properties, and cultivation costs.
Some resources with lower yield can be valued thanks to a specific nutritional advantage : for example, linseed which is a source of w3 fatty acids can be interesting even with yield such as 1.5t/ha.
I think the critical issue is the quality of the product after harvest and not necessarily the quantity per se. Several factors weigh into this among them storage quality, resistance to diseases during storage, acceptance to feed users and ethical issues (GMO vs non GMS) etc.
Beside the quality also the cost has to be considered (cost per energy unit/cost per protein unit) and also the potential losses (field losses, storage etc.).