Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows

Asparagopsis taxiformis (AT) is a source of multiple halogenated compounds and, in a limited number of studies, has been shown to decrease enteric CH4 production in vitro and in vivo. Similarly, oregano has also been suggested as a potential CH4 mitigating agent. This study consisted of 2 in vitro and 2 in vivo experiments. Experiment 1 (Exp. 1) was aimed at establishing the effect of AT on total gas production and CH4 emission in vitro. Two experiments [Exps. 2 (Supplemental Material) and 3] with lactating dairy cows were conducted to determine the antimethanogenic effect of AT and oregano (Exp. 3) in vivo. Another experiment (Exp. 4) was designed to investigate bromoform stability in AT over time. In Exp. 3, 20 Holstein cows were used in a replicated 4 × 4 Latin square design with 4, 28-d periods. Treatments were basal diet (control) or basal diet supplemented with (DM basis): 0.25% AT (LowAT), 0.50% AT (HighAT), or 1.77% oregano (Origanum vulgare L.) leaves (OL). Enteric gas emissions were measured using the GreenFeed system and rumen samples were collected for fermentation analysis using the ororuminal technique. In Exp.1 (in vitro), relative to the control, AT (at 1%, DM basis, inclusion rate) decreased CH4 yield by 98%. In Exp. 3, HighAT decreased average daily CH4 emission and CH4 yield by 65% and 55%, respectively, in experimental periods 1 and 2, but had no effect in periods 3 and 4. The differential response to AT among experimental periods was likely a result of a decrease in bromoform concentration in AT over time, as observed in Exp. 4 (up to 84% decrease in 4 mo of storage). In Exp. 3, H2 emission was increased by AT and, as expected, the proportion of acetate in the total volatile fatty acids in the rumen was decreased and those of propionate and butyrate were increased by HighAT, compared with the control. Compared with control, HighAT decreased DMI, milk yield, and energy-corrected milk yield in Exp. 3. Milk composition was not affected by treatment, except lactose percentage and yield were decreased by HighAT. Concentrations of iodine and bromide in milk were increased by HighAT, compared with the control. Milk bromoform concentration and its

organoleptic characteristics were not different between control and HighAT. Oregano had no effect on CH4 emission or lactational performance of the cows in Exp. 3. Overall, AT included at 0.50% in the ration of dairy cows can have a large mitigation effect on enteric CH4 emission, but DMI and milk production may also decrease. There was a marked decrease in the CH4 mitigation potential of AT in the 2nd half of Exp. 3, likely resulting from bromoform decay over time.