Garlic oil and its principal component diallyl disulfide fail to mitigate methane, but improve digestibility in sheep

Abstract

One focus of current ruminant research is identification of feed supplementation strategies for mitigating enteric CH₄ production that do not impair rumen fermentation. Previous in vitro studies have indicated a potential anti-methanogenic activity of garlic (Allium sativum), garlic oil (GO), and its main compound diallyl disulfide (DADS). In our study, effects of supplementation of a standard hay and concentrate diet with 5 g GO or 2 g DADS/kg dietary dry matter (DM) were tested in sheep in a duplicated 3 × 3 Latin square design experiment with three 23 d periods. Respiratory measurements were on d 17 and 18. Dietary supplementation with GO or DADS had no influence on the amount of CH₄ produced (27 g/d). When rates of CH₄ production/kg OM digested were compared, DADS, but not GO, tended (P=0.09) to decrease CH₄ production compared to control. DM intake was not affected by GO or DADS, although concentrate intake was slightly decreased (P=0.10) with GO compared to control. DADS increased (P=0.02) digestibility of OM and aNDFom (P=0.03), as well as energy utilization (P=0.03) compared to control. Low palatability and lack of effect on either CH₄ mitigation or energy use efficiency indicate that a GO supplemented concentrate is of little relevance in practice. As DADS supplementation only tended to decrease CH₄ formation relative to OM digested, it too is limited use as a means of CH₄ mitigation. However, DADS supplementation improved digestibility and energy use efficiency.

This article is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.

Introduction

Enteric CH₄ emissions from ruminant livestock are an anthropogenic source of CH₄ (Lassey, 2008). For this reason, the search for methods to mitigate enteric CH₄ emissions is ongoing, with feeding management approaches being the most developed (Martin et al., 2010). A few feed additives, such as ionophores, are known to mitigate ruminal CH₄ formation (Beauchemin et al., 2008), but their use is controversial and prohibited in some countries. Therefore, alternative strategies to decrease ruminal CH₄ formation need to be identified which do not impair ruminal fermentation (Newbold, 2007, Patra and Saxena, 2009), as well as energy utilization.

Extensive in vitro screening has been conducted in a search for the plants which are most effective at inhibiting ruminal CH₄ formation (Bodas et al., 2008, Garcia-Gonzalez et al., 2008, Patra et al., 2010). Special emphasis has been placed on plant secondary compounds, including essential oils (Calsamiglia et al., 2007, Hart et al., 2008, Kamra et al., 2008). Of these, garlic oil (GO), and its principal organosulfur compound DADS (C₆H₁₀S₂; Harris et al., 2001), have been shown in vitro to have a CH₄ mitigating effect (Soliva et al., 2011), possibly through inhibition of the methanogenic Archaea (Busquet et al., 2005). Effects of organosulfur compounds such as DADS might involve the specific inhibition of SH-containing enzymes of the methanogens (Busquet et al., 2006). Besides the CH₄ mitigating potential of GO and its organosulfur components, beneficial effects such as improved ruminal energy and N use have been reported from a short term in vitro incubation study (Busquet et al., 2005).

Although DADS has shown CH₄ mitigating effects in vitro (Busquet et al., 2005), it has not been shown to have these properties in vivo when supplemented in dietary proportions of 0.2 g/kg to cows (van Zijderveld et al., 2009). In addition, doses that are effective at mitigating CH₄ in vitro are often very high and may result in palatability and digestion problems in vivo. To date, very few animal experiments have tested garlic extract (Yang et al., 2007, Chaves et al., 2008a) or its organosulfur compounds (McSweeney and Denman, 2007, McSweeney et al., 2009) as feed supplements. To our knowledge, effects on CH₄ formation have only been measured in one in vivo study (van Zijderveld et al., 2009).

The hypotheses were that GO is effective in mitigating CH₄ and improving nutrient and energy utilization in ruminants, and that this effect is mediated by its principal organosulfur compound, DADS.