Agriculture is just one of many industries.The way I'm reading this is that say you have 50 farmers employing 500 migrants then if they didn't employ them and went bust,50 farmers would lose their livelihood(or just go and work for someone)and 500 migrants would go home.Sounds good to me.
In this situation 50 Brits lose jobs, 500 migrants lose jobs, and nobody gains. And this sounds good to you, just because it gets some foreigners out of the country?