Yep. I remember doing the math when this info first came out years ago and seaweed production would have to scale astronomically to add an appropriate amount to reduce all cattle feed.
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].
Granted, any reduction is good, but talking like seaweed is the pariah that will make cattle farming environmentally sound wont happen.