So the wiki sayeth: "If the animals are inside the radius of the trough, they will eat from the trough before feeding on the grassland. The quality of the fodder from the trough is the average seed quality placed inside the trough. For example, if you placed a quality 5, 12, and 16, the quality of the fodder will be 11. The trough will hold a maximum of 200 unit of fodder."
Except that my animals are within range of the trough (the pen is smaller than its radius) but they still seem to prefer the heath they're parked on. The trough quality is 40ish right now, never went below 30, but these animals have somehow eaten quality 12, quality 26, and quality 18 food. I'm guessing it's averaging the quality of the trough and the grass their grazing but I thought they wouldn't graze unless the trough went empty? (It's never been below 3/4 full).