I would try the niacin if you have any on hand. It certainly wont hurt. Have you ruled out other poisons and toxins - even bites - spider/snake etc.
I had several geese do this all on the same day fairly recently (I ended up losing one because I didn't spot it in time). I added brewers yeast to the water for several days and locked them up - the rest all came good. I put it down to them eating more kikuya than normal due to them spending more time out - I was at home sick so they got a heap of extra time out of their pen.
I hope she recovers for you.
Yillyita, how did you discover that Kikuya grass is no good for geese, re. niacin? here i have some lawn grass that´s very similar in looks, but it´s from E Asia, called Zoysia Japonica. the geese hardly touch it, they much prefer the grass in the marsh. I wondered Iif it may be a similar type of grass re. the niacin? How did you find out? Because I can´t find a site that gives this information.