Your underestimating the amount of food cows need to eat a day and likely overestimating global seaweed production in relation to even supply only 1% of cattle feed.
89 millions cows in the US alone eating eating 20+ pounds of food a day is 890,000 tons or nearly 325 million tons per year. That’s per day. Global production of seaweed was 358,200 tons in 2019. Only about 11% of what would be needed to be included in 1% of feed of every cow in America. And that wouldn’t leave any seaweed to be used for any of its other uses or the millions of other cows around the world.
https://worldanimalfoundation.org/advocate/how-many-cows-in-...
https://beef.unl.edu/cattleproduction/forageconsumed-day
https://fppn.biomedcentral.com/articles/10.1186/s43014-022-0...
https://documents1.worldbank.org/curated/en/0990814231045205...