Livestock production, particularly enteric methane production, contributes to greenhouse gas emissions globally. Various mitigation strategies developed to reduce enteric emissions have limited success. Although in vitro studies have shown a considerable reduction in methane emissions using Asparagopsis spp., no studies have been conducted to investigate the effect of any species of Asparagopsis in dairy cattle. Our objective was to evaluate quantitatively the response of cows consuming Asparagopsis armata on methane production (g/kg), yield (g/kg feed intake) and intensity (g/kg milk yield). Twelve post-peak lactating Holstein cows were randomly assigned to three treatments (control, 0.5% and 1% inclusion levels of A. armata on organic matter basis) in a 3
۳ Latin square design with three 21-day periods. Enteric methane emissions were measured using the GreenFeed system. Methane production by cows decreased significantly by 26.4% at the low (0.5%) level of A. armata inclusion and 67.2% at the high (1%) level of inclusion. Feed intake was reduced by 10.8 and 38.0%, in cows fed the low and high level of macroalgae inclusion, respectively. Methane yield decreased significantly by 20.3 and 42.7% in cows fed diet including 0.5% and 1% A. armata inclusion levels, respectively (P ¼ <0.0001). Methane intensity significantly decreased by 26.8% from cows fed at 0.5% level and 60% at the 1.0% A. armata inclusion level. Bromoform concentrations in milk were not significantly different between treatments. Our in vivo results showed that A. armata has potential to be used as a feed additive to reduce enteric methane emissions.

Introduction

The livestock sector contributes 14.5% of global GHG emissions (Gerber et al., 2013), with global methane emissions contributing to about 2.1 Gt CO2 equivalent in 2010 (Smith et al., 2014). There are considerable differences in contribution of enteric methane in different regions and countries of the world. The main source of anthropogenic methane emissions in the United States is generated by enteric fermentation of livestock (25%; NASEM, 2018). US EPA (2019) estimated the total methane emissions from enteric fermentation in the United States to be 6.46 Tg in 2017, which is equivalent to 27% of the nation’s anthropogenic methane emissions. Enteric methane is a natural by-product of microbial fermentation of nutrients in the digestive tract of animals. Enteric methane emissions represent up to 11% of dietary gross energy consumed by ruminants and in North American dairy cattle it is estimated to be about 5.7% of gross energy intake (Moraes et al., 2014). Hristov et al. (2013) reported that mitigation options including nitrates, ionophores, tannins, direct-fed microbials and vaccines may offer opportunities to reduce enteric methane emissions; however, the results have been inconsistent. Knapp et al. (2014) estimated that nutrition and feeding approaches may contribute to reducing methane emission intensity (i.e., emissions per milk yield) by 2.5e15%, whereas rumen modifiers had little success in sustained methane emissions without compromising milk production. Methane inhibitors may be a more successful approach in reducing emissions from enteric fermentation. For example, 3- nitrooxypropanol (3NOP) has been reported to substantially decrease methane emissions from ruminants (Duin et al., 2016).