Spinach grows fast and is rich in Selenium. I grow Sprouts for my birds, chop up Spinach, Rocket Lettuce, Swiss Chard, Cucumbers for my birds and grow these in my garden for them. They once got loose in my garden and seemed to love devouring my Bell Pepper and Jalapeno plants. I have one Silkie that loves the fresh seeds out of my garden Bell Peppers. I tried giving her seeds out of store-bought Peppers and she ignored them. She wanted the fresh seeds out of the garden! My girls also love Cantaloupe but it takes too much room to grow the vines so we purchase Cantaloupe fruit at the store. Blueberries are rich in Selenium too. I once thought of planting these bushes all along the fence for the chickens to forage but I don't have the right climate zone.