Adding 1.0% and 2.0% North Atlantic brown seaweed to pregnant replacement heifers cut methane emissions by about 8.2% and 8.7% without affecting growth performance

In live pregnant replacement heifers, 1.0% and 2.0% dry-matter inclusion of North Atlantic brown seaweed lowered methane from 11.89 g/h in controls to 10.93 and 10.86 g/h, reductions of about 8.2% and 8.7% (p < 0.05), whereas the 0.5% level reduced methane by only 1.16% and not significantly. The methane-mitigation potential of this product (Atlantic GRO®) had previously been assessed only in artificial rumen in vitro systems, where 1% supplementation showed up to a 16% methane reduction, but it had not been validated in live animals and the optimal dose remained unknown. Continuous measurement in headbox metabolic chambers, with each animal measured twice in two 24-h periods; repeated-measures mixed models (AR(1) covariance structure, Kenward–Roger degrees-of-freedom adjustment) with orthogonal polynomial contrasts for dose response; methane data were background-corrected and aligned across animals by cubic spline interpolation.

Methane reduction showed a significant linear dose response (p < 0.001) with a modest quadratic component (p = 0.046), and the difference between 1% and 2% was small, indicating a plateau at higher inclusion levels. This study provides the first in vivo dose–response curve for this brown seaweed product, indicating an effective threshold at 1% and above, whereas in vitro work (e.g., Ramos et al. 2025 with a blend of F. vesiculosus and L. longicruris) required a higher 5% inclusion rate. Graded design with four inclusion levels (n = 5 per group in the performance trial; in the gas phase the control group dropped to 3 animals after one heifer aborted, with 4 per remaining group), combined with linear and quadratic orthogonal contrasts.

The 1.0% and 2.0% groups had significantly lower carbon dioxide emissions (328.5 and 329.3 g/h) and oxygen consumption (242.6 and 242.9 g/h) than controls (342.2 g/h and 250.0 g/h, p < 0.001), while the 0.5% group did not differ significantly from controls in carbon dioxide. Prior work focused mainly on methane; this study also reports carbon dioxide and oxygen exchange, adding complementary information on how dietary strategies influence overall gaseous outputs. All three gases measured simultaneously with the same chamber system; the authors explicitly frame these as secondary gas-exchange observations under chamber conditions and note that rumen fermentation, archaeal abundance, and volatile fatty acid profiles were not measured.

No supplementation level adversely affected growth performance: average daily gain was 0.52–0.63 kg/day, feed conversion ratio 15.0–25.86, and dry matter intake did not differ significantly among treatments (p > 0.51), with only days in test significantly affecting dry matter intake (p < 0.001). This contrasts with some studies reporting performance improvements from seaweed, but agrees with several studies of red (A. taxiformis) and brown (A. nodosum) seaweeds; the authors note the lower daily gain reflects the controlled-growth goal for replacement heifers. 50-day performance trial with 20 heifers randomly allocated to four groups (n = 5 each), individual intake recorded by the Growsafe system, and body weights taken at the start, biweekly, and at the end; ADG and FCR analyzed as whole-period summary traits.

The study provides in vivo evidence for a regionally available brown seaweed product as a ruminant methane-mitigation strategy, applicable to pregnant crossbred replacement heifers on a silage-based diet in Atlantic Canada. For producers, both the 1.0% and 2.0% inclusion levels were associated with lower enteric methane emissions in this trial without negatively affecting production parameters, and the authors suggest the final inclusion level could be guided by product availability and economic considerations. For researchers, the results support further in vivo validation of other brown seaweed products and highlight that translating in vitro findings to live animals must account for palatability, rumen adaptation, and diet.

The authors state clearly that because only two seaweed samples were analyzed, the phenolic data should be treated as descriptive rather than direct evidence of mechanism; rumen fermentation, archaeal abundance, volatile fatty acid profiles, and specific phenolic fractions were not measured, so any mechanistic explanation for the methane response should be considered hypothetical. The gas-emission phase was a follow-up on a subset of animals from the performance trial under different management conditions, and the authors caution that Experiment II findings should be interpreted as follow-up observations under chamber conditions rather than as directly equivalent to Experiment I. In addition, one control heifer aborted after the habituation period and was removed from the study (the authors report the cause of death was unrelated to the study), reducing the control group to three animals in the gas phase. The carbon dioxide and oxygen exchange results are positioned by the authors as secondary observations whose relationship to fermentation dynamics or energy metabolism still requires further investigation.