The dried white urine can be a problem. It's excess calcium in the urine. Whether it is or not depends on a lot of things.
If the pig is young (under a year), we don't usually worry too much about it. They need the excess calcium for strong teeth and bones, and pigs don't usually develop urinary tract stone problems at that age. Powdery spots mean that the pig has a lot of urinary calcium, and that's definitely implicated in the development of stones. There's a controversy over whether that's a problem or not, and you'll find a lot of people saying that powdery spots are ok. Gritty spots are a step in the development of stones, and definitely MUST be addressed.
For about two years, I did an informal survey of pigs who had developed bladder stones -- probably 150-200 pigs. EVERY pig except one had excreted calcium in its urine, some gritty, some powdery. The one pig that did not had only been with its owner about three weeks when it showed stone symptoms, and she had her other pigs on a VERY low calcium diet, so it's possible that pig had developed the stone before she got him, but didn't excrete excess calcium in her care because she fed so little calcium.
To me, that says that ANY excess calcium is something to be concerned about. I don't think you can completely eliminate it, but I know from my own experience that you can greatly reduce the size and number of the spots by carefully controlling calcium in the diet. My own pig is now 18 months post-surgery for stone removal, and is doing fine. I recently had her x-rayed again to see how she was doing, and there's no sign of any stone recurrence.
Additionally, there's the genetic factor, and no one really knows how that figures in. One person I know fed her pigs a very low calcium diet, but had one pig that repeatedly developed stones. After three surgeries, and the last stones developing within three weeks after the last surgery, she reluctantly had to euthanize the pig.
In short, you may or may not need to worry about the powdery spots. How old is your pig? What exactly do you feed her (hay, pellets, vegetables) and how much of each?
That's really interesting. I'm not trying to hijack this thread, but that was really helpful information. Our piggies are about 6 months old, and they've had powdery urine since we got them. I'm really happy that I learned this now.
We've been feeding our girls lettuce and carrots for breakfast (mainly Romaine or Green/Red Leaf mixture, depends on what's available) and for their evening treat, we'll give them a different treat everynight (Green Bell Pepper, Snap Peas, Apples or Cilantro) We started with Alfalfa hay, but switched to Timothy after 1-2 months, and they have unlimited amounts daily. For pellets, we have been using Oxbow Essentials Young Guinea Pig Food. We usually give them a 1/2 cup every other day. I don't think it needs to be said, but they also have fresh water every day.