Generally I tend to stay away from a horn from Texas Longhorns, they like to split and are brittle. As for Bison, well they're a whole different beast. Holstein is a great source of your horns. I get mine now from Roland Cadle and I still have a bunch I got from Powder Horns & More when John owned the company. Locally I have a source from a meat processor and call every once in a while, but being in central NY the horns that are available will be small that work out great for small projects or a nice flat horn project.
Even horns from holsteins (just to name one) can differ from horn to horn, even on the same horn, you can come across a 'soft' or hard section. Just what one has to be prepared for when you go to engrave them. So no, they are not getting harder or brittle, just the nature of the hobby and that comes with experience of working many, many horns over the last 20+ years.
