It would also not supplant grain, which is really what causes cows to emmit so many greenhouses gases. If you ate the grain that cows eat you would have terrible gas too. Grain is intended as a supplement to grass as it is loaded with different nutrients, salt, etc., but it also makes cows grow faster and produce more milk and so it is a competitive advantage for a farmer to feed more than is really required for the cows to be healthy.

Seaweed may be better than grass in regards to gas emissions, but probably not 99 percent better. However, grass or seaweed alone won’t let cows get fat enough fast enough. Time is money and it is a commodity market unless you specifically target the organic, non-GMO, grass-fed crowd, which, while growing, is still a niche market that adds a bunch of complications.

On some farms, grain only supplements the grass diet, but on many beef cattle farms a larger portion of their diet is grain so they grow and fatten up more quickly. Dairy cows often fair a bit better in this regard, although the calorically rich diet of grain is also good for increasing milk production, so they are fed a good bit of grain also.

The grain does serve a health purpose for cattle, it is just often probably given in excess of what would be required for optimal cow health.