I guess I over-simplified by calling geese "herbivores." That's not really accurate. What I should have said was that traditionally, geese do get the vast bulk of their food from pasture during the growing season. One assumes this would be particularly true of the older, thriftier, heritage breeds (eg. Pilgrim and especially Cotton Patch). I've never raise geese (yet), but I took this from the 10th edition of the Encyclopedia of Country Living (written and compiled by Carla Emery):
"They can thrive on no more than grass, can be a cash crop, and will lay eggs for an amazing number of years....
Provide green grass to goslings as soon as they will eat. Although you can feed goslings the same diet as ducklings, you can also offer them bread or cornbread moistened with milk. You can gradually reduce the number of feedings until the fourth week....
At 5 to 6 weeks your goslings can survive entirely on pasture if they can get enough of it, although it's helpful to their diet if you continue the supplemental grain and/or pellet feeding until they're completely feathered.... They can also eat grocery store culls of lettuce, cabbage, endive, and celery leaves... For best results, plan on 1 acre of prime pasture for each 20-40 geese.... You can compensate for poor pasture by feeding supplemental grain. In winter... they'll also need some good (not moldy!) alfalfa hay, corn fodder, grain... or whatever you're feeding your chickens. Geese enjoy warm cull potatoes and can thrive on them....
Geese love green stuff. It's their natural food. Grown geese can thrive on grass and weeds alone, if the plants are tender.... Feeding geese grass gives them cheap nourishment and keeps them healthy."