Seaweed Feed Additives and Methane: What the Angus-Cross Trials Actually Show

Of all the ideas for reducing methane from cattle, red seaweed from the genus Asparagopsis has drawn the most attention. Several trials have used Angus or Angus-cross cattle, and the headline numbers vary a good deal, from roughly a third to nearly four-fifths depending on the study. This article walks through what was actually measured and what a producer should take from it.

Why the numbers differ

Methane is produced by microbes in the rumen as cattle digest forage. The active compound in Asparagopsis is bromoform, which interferes with the final step of methane formation. How much methane falls depends on how much bromoform the animal actually eats, what the diet is, how long the trial runs and how emissions are measured. Comparisons across studies are tricky for that reason.

A grazing trial in Montana

A study from the University of California, Davis, published in the Proceedings of the National Academy of Sciences, tested a pelleted seaweed additive on steers grazing irrigated pasture, according to the open-access version on PubMed Central. It involved 24 Wagyu-Angus cross steers over 70 days. Enteric methane fell by an average of 37.7 percent during the optimal and decreasing phases, and weight gain showed no significant difference between treated and control groups. Pasture made up more than 95 percent of total feed intake, so it is one of the closer matches to a grazing operation.

The authors also listed limits. The sample was small, intake of the dry matter was predicted rather than measured and bromoform intake declined later in the study, likely because of palatability, meaning the effect over longer periods is unclear. Hydrogen emissions rose, and the lead author disclosed an advisory role with the company that supplied the product.

A feedlot-style trial

An earlier UC Davis study on 21 Angus-Hereford steers, summarized by Beef Central, tested seaweed at 0.25 and 0.5 percent of feed. It reported methane reductions of up to 80 percent in the high-dose group on a low-forage diet, with overall reductions of about 45 percent and 68 percent for the low and high dose across diets. The report noted that the study was small, partly supported by a company selling the product and based on a preprint at the time. It also said diet type affected the result, which helps explain the difference between this study and the grazing trial.

Cows on extensive pasture in Australia

A University of Adelaide trial, described in its news release, used 80 pregnant and lactating Angus cows in an extensive grazing system for eight weeks and tracked calf growth to 150 days. It used a bromoform extract oil from Asparagopsis, and reported methane reductions between 49 and 77 percent. The release said more work is needed to optimize dosage. The information I reviewed did not give carcass or calf performance outcomes, so look at the full publication before drawing conclusions.

What the range tells you

The spread is the lesson. Nobody should promise a producer a fixed percentage. The reduction in any given herd will depend on dose, delivery method, forage quality and how consistently cattle eat the additive. In grazing systems, getting cattle to consume a consistent amount is the central practical problem. Supplement delivered in a feed bunk is easy to dose. Supplement delivered to cattle on large pastures is not, which is why the Montana trial used pellets dispensed through monitoring units that also measured emissions.

Questions a producer should ask

• What does it cost per head per day? Without a premium or payment for reduced emissions, the additive is a cost with no direct return.
• How is it delivered? Many pasture operations lack the infrastructure to feed it consistently.
• Is the product stable and verified? Bromoform content can vary between products and can degrade in storage.
• What are the animal and food safety findings? Researchers continue to look at bromoform residues and animal health in longer trials.
• Who will pay? A buyer program or carbon market payment may eventually justify the cost, but terms vary.

Our piece on beef climate goals explains the market pressure behind this research, and our look at grass-fed Angus covers the forage side of the picture.

The bottom line for now

Seaweed additives are a real, studied tool, and the grazing results are encouraging, but the evidence is early, trials are small and short and commercial use at scale is still developing. For most Angus producers the sensible step is to follow the research, ask your extension office what is available locally and keep improving the basics that also lower emissions per pound: fertility, growth and forage quality. For general background on feeding programs, see farm.fm's page on feed formulation for Angus beef cattle, and use agriculture.directory to find nutritionists and extension offices near you.

Research findings and product availability change, so check the original studies and current regulatory status before making decisions. This site is independent and not affiliated with the American Angus Association or Certified Angus Beef.