8.4.1.5 Livestock management
Livestock, predominantly ruminants such as cattle and sheep, are important sources of CH4, accounting for about one-third of global anthropogenic emissions of this gas (US-EPA, 2006a). The methane is produced primarily by enteric fermentation and voided by eructation (Crutzen, 1995; Murray et al., 1976; Kennedy and Milligan, 1978). All livestock generate N2O emissions from manure as a result of excretion of N in urine and faeces. Practices for reducing CH4 and N2O emissions from this source fall into three general categories: improved feeding practices, use of specific agents or dietary additives; and longer-term management changes and animal breeding (Soliva et al., 2006; Monteny et al., 2006).
a. Improved feeding practices: Methane emissions can be reduced by feeding more concentrates, normally replacing forages (Blaxter and Claperton, 1965; Johnson and Johnson, 1995; Lovett et al., 2003; Beauchemin and McGinn, 2005). Although concentrates may increase daily methane emissions per animal, emissions per kg-feed intake and per kg-product are almost invariably reduced. The magnitude of this reduction per kg-product decreases as production increases. The net benefit of concentrates, however, depends on reduced animal numbers or younger age at slaughter for beef animals, and on how the practice affects land use, the N content of manure and emissions from producing and transporting the concentrates (Phetteplace et al., 2001; Lovett et al., 2006). Other practices that can reduce CH4 emissions include: adding certain oils or oilseeds to the diet (e.g., Machmüller et al., 2000; Jordan et al., 2006c); improving pasture quality, especially in less developed regions, because this improves animal productivity, and reduces the proportion of energy lost as CH4 (Leng, 1991; McCrabb et al., 1998; Alcock and Hegarty, 2006); and optimizing protein intake to reduce N excretion and N2O emissions (Clark et al., 2005).
b. Specific agents and dietary additives: A wide range of specific agents, mostly aimed at suppressing methanogenesis, has been proposed as dietary additives to reduce CH4 emissions:
Ionophores are antibiotics that can reduce methane emissions (Benz and Johnson, 1982; Van Nevel and Demeyer, 1996; McGinn et al., 2004), but their effect may be transitory (Rumpler et al., 1986); and they have been banned in the EU.
Halogenated compounds inhibit methanogenic bacteria (Wolin et al., 1964; Van Nevel and Demeyer, 1995) but their effects, too, are often transitory and they can have side-effects such as reduced intake.
Novel plant compounds such as condensed tannins (Pinares-Patiño et al., 2003; Hess et al., 2006), saponins (Lila et al., 2003) or essential oils (Patra et al., 2006; Kamra et al., 2006) may have merit in reducing methane emissions, but these responses may often be obtained through reduced digestibility of the diet.
Probiotics, such as yeast culture, have shown only small, insignificant effects (McGinn et al., 2004), but selecting strains specifically for methane-reducing ability could improve results (Newbold and Rode, 2006).
Propionate precursors such as fumarate or malate reduce methane formation by acting as alternative hydrogen acceptors (Newbold et al., 2002). But as response is elicited only at high doses, propionate precursors are, therefore, expensive (Newbold et al., 2005).
Vaccines against methanogenic bacteria are being developed but are not yet available commercially (Wright et al., 2004).
Bovine somatotropin (bST) and hormonal growth implants do not specifically suppress CH4 formation, but by improving animal performance (Bauman, 1992; Schmidely, 1993), they can reduce emissions per-kg of animal product (Johnson et al., 1991; McCrabb, 2001).
c. Longer-term management changes and animal breeding: Increasing productivity through breeding and better management practices, such as a reduction in the number of replacement heifers, often reduces methane output per unit of animal product (Boadi et al., 2004). Although selecting cattle directly for reduced methane production has been proposed (Kebreab et al., 2006), it is still impractical due to difficulties in accurately measuring methane emissions at a magnitude suitable for breeding programmes. With improved efficiency, meat-producing animals reach slaughter weight at a younger age, with reduced lifetime emissions (Lovett and O’Mara, 2002). However, the whole-system effects of such practices may not always lead to reduced emissions. For example in dairy cattle, intensive selection for higher yield may reduce fertility, requiring more replacement heifers in the herd (Lovett et al., 2006).