Feeding cows seaweed as just 2% of their diet can decrease their methane emissions (aka burps and farts) by 99%! Methane is a potent greenhouse gas, 21X more potent than CO2 by certain measures, so any strategy for decreasing emissions is a big deal. Globally, methane from ruminants (like cows) makes up 4-5% of annual greenhouse gas emissions. Obviously, eating less meat could accomplish drastic emissions reductions also.
I recently saw this concept getting some press and remembered that the book Drawdown I helped research covered the idea as well. Drawdown mentions that seaweed has been used as livestock feed for thousands of years, so this isn't some new untested feed.
Seaweed (as part of Marine Permaculture) is being looked at as a climate solution in many ways since growing it removes CO2 from oceans, provides habitat for fish and other things people eat (people also eat seaweed), and apparently it can decrease emissions when incorporated into the diet of livestock. Seaweed is pretty nutritious, so it could have other benefits for cows as well.
http://www.drawdown.org/solutions/coming-attractions/cow-walks-beach