This is all just a scam run by Big Cow to get some tasty Asian flavors in the trough.
Granted, any reduction is good, but talking like seaweed is the pariah that will make cattle farming environmentally sound wont happen.
The TL;DR was: in order to supply 1.5 billion cows with the seaweed we might need up to 291.000 metric tons of algae per day. ~ 106 Million metric tons per year. In 2014 the world wide Aquatic Plants (which include ALL Macro-Algae) production was around 27 Million tons [0].
[0]: http://www.fao.org/3/a-i5555e.pdf, page 24, table 7