This article is not about a livestock feed, it's about creating a pasture polyculture that reduces the need for nitrogen fertilizer application.
The 60% claim is "in comparison with pastures without the use of nitrogen fertilization." And of course only applies to the region and cattle breed they studied, not to mention only cattle raised on pasture in the first place.
It's not claiming to be a magic growth tonic for all cattle. Though I agree the title is a bit exaggerated and invites misinterpretation.
I'm curious if the nitrogen fixation is done by symbiotic bacteria.
If so, can both organisms survive in all the climates where cattle is raised?
Yes, legumes are fairly unique in that their root nodes host symbiotic bacteria capable of fixing nitrogen for the atmosphere. That's why they are so important in crop rotation or polyculture.
Every region could have different species that work best. The finding of this study, as I understand it, is identifying this foreign species that happens to work well for the location and agricultural needs.
Cows graze.
https://en.wikipedia.org/wiki/Feedlot