[M_Grey]

Well the seaweed itself would take up some CO2 during growth... so you have that bonus too. I'd guess the major costs would come during transport.

[mrob]

Seaweed dries and rehydrates well, and like all vegetables it's mostly water, so if it's grown somewhere where it can be sun-dried then total energy cost for transport could be low.

[jstsch]

Also, seaweed is a good source of protein, which for cows tends to come from (imported) soy or corn.