The Reduction of Methane Production in the In Vitro Ruminal Fermentation of Different Substrates is Linked with the Chemical Composition of the Essential Oil

SIMPLE SUMMARY: There is growing concern about how animal-derived foods are produced. Methane production in ruminants has received much attention in relation to its contribution to greenhouse gases and its effect on global warming. Another aspect of livestock production that is questioned by consumers is related to in-feed antibiotics added to improve feed efficiency, and due to health safety issues, their use has been banned or under revision in some parts of the world. Hence, there is the need to find new solutions to mitigate methane production in the rumen in a way that is considered safe and environmental-friendly by consumers and feasible, and without a negative impact on the farmers. Among the alternatives, the use of essential oils to modify rumen fermentation has attracted attention. This paper explores the effectiveness of essential oils obtained from two plants, Lippia turbinata and Tagetes minuta, to reduce methane production during the in vitro fermentation of substrates that are representative of different livestock production systems. The main conclusion to which we arrived is that the extent of the reduction in methane production depends on the interaction between the fermentation conditions that are generated by different substrates and the chemical profile of the essential oil, especially regarding its proportion of oxygenated compounds. ABSTRACT: There is interest in identifying natural products capable of manipulating rumen microbial activity to develop new feed additives for ruminant nutrition as a strategy to reduce methane. Two trials were performed using the in vitro gas production technique to evaluate the interaction of substrate (n = 5) and additive (n = 6, increasing doses: 0, 0.3, 3, 30, and 300 µL/L of essential oils—EO—of Lippia turbinata or Tagetes minuta, and monensin at 1.87 mg/L). The two EO utilized were selected because they differ markedly in their chemical composition, especially in the proportion of oxygenated compounds. For both EO, the interaction between the substrate and additive was significant for all variables; however, the interaction behaved differently for the two EO. Within each substrate, the response was dose-dependent, without effects at a low level of EO and a negative outcome at the highest dose. The intermediate dose (30 µL/L) inhibited methane with a slight reduction on substrate digestibility, with L. turbinata being more effective than T. minuta. It is concluded that the effectiveness of the EO to reduce methane production depends on interactions between the substrate that is fermented and the additive dose that generates different characteristics within the incubation medium (e.g., pH); and thus, the chemical nature of the compounds of the EO modulates the magnitude of this response.