Methane emissions of geese (Anser anser) and turkeys (Meleagris gallopavo) fed pelleted lucerne

Highlights

• We document CH₄ emissions in live geese and turkeys on a lucerne diet.

• CH₄ emissions are in the range of similar-sized mammals of low emission status.

• CH₄ production by poultry should not be dismissed.

• Poultry CH₄ production may be linked to dietary protein via uric acid fermentation.

Abstract

In contrast to mammalian herbivores, birds are generally perceived to produce little methane (CH₄) during digestion, and accounting for poultry in greenhouse gas inventories is considered unnecessary. We measured CH₄ emissions in six domestic geese (Anser anser, 5.0 ± 0.9 kg) and six domestic turkeys (Meleagris gallopavo, 6.3 ± 0.6 kg) kept on a diet of lucerne pellets only, using open-circuit chamber respirometry. Measurements of oxygen consumption were similar to previously published values in these species. Absolute CH₄ emissions per day were lower in geese (0.58 ± 0.10 L) than in turkeys (1.48 ± 0.16 L) and represented 0.4 ± 0.2 and 0.6 ± 0.1% of gross energy intake, respectively. These results confirm previous findings on the presence of methanogenes in the digestive tract of poultry species, and in vitro measurements performed on poultry caecal contents. In relation to mammalian herbivores in terms of absolute CH₄ emissions, CH₄ yield per dry matter or gross energy intake, or the CH₄:CO₂ ratio, the lucerne-fed geese and turkeys had comparatively low values. The emission of CH₄ in spite of the very short digesta retention times and low fibre digestibility, as measured in the same animals, gives rise to the hypothesis that that in some birds, caecal fermentation and the associated CH₄ production may be related to the microbial digestion of uric acid. The hypothesis that CH₄ emissions in poultry may depend not only on dietary fibre but also on dietary digestible protein (that is excreted as uric acid in urine and retrogradely transported from the cloaca into the caeca) remains to be tested.

Introduction

Herbivorous birds produce methane (CH₄). This is known from in vitro experiments in ratites, Anseriformes and Galliformes using faeces (Hackstein and Van Alen, 1996; Saengkerdsub et al., 2007) or caecal content (Shrimpton, 1966; Marounek and Rada, 1998; Fievez et al., 2001; Saengkerdsub et al., 2006; Chen et al., 2014). Methane production was also demonstrated by spot-sampling air from rock ptarmigans (Lagopus mutus), ducks (Cairina moschata) and geese (Anser anser) kept in closed-circuit respiration chambers (Gasaway, 1976; Chen et al., 2003; Chen et al., 2014). By contrast, continuous measurements in open-circuit respiration systems are rare. The few measurements allowing extrapolations for daily CH₄ emissions in birds seemed to indicate extremely low levels of CH₄ (reviewed in Fritz et al., 2012). The to our knowledge only open-circuit respiration measurements of CH₄ in chickens (Gallus gallus) (Hadorn and Wenk, 1996a, Hadorn and Wenk, 1996b) support this impression. However, recent open-circuit measurements suggest that at least ratites (ostriches Struthio camelus, emus Dromaius novaehollandiae, and rheas Rhea americana) produce CH₄ in magnitudes expected of similar-sized nonruminant mammalian herbivores (Frei et al., 2015a; Frei et al., 2015b).

It is generally assumed that CH₄ production is mainly related to microbial fibre digestion, and a substantial fibre degradation is in turn dependent on prolonged digesta retention (Stevens and Hume, 1998). Therefore, it may appear astonishing that avian species such as geese or emus, with poor fibre digestion and very short digesta retention times (Frei et al., 2015c; Frei et al., 2017), are nevertheless among the CH₄-producing birds (Hackstein and Van Alen, 1996; Frei et al., 2015b). By comparing caecotomized and intact geese, Chen et al. (2003) demonstrated that the caeca are the main origin of CH₄ in this and, given the in vitro studies mentioned above, probably also other avian species. Methanogenic archaea have been detected in the digestive tracts of chicken, geese and turkeys (Meleagris gallopavo) (Miller and Wolin, 1986). Nevertheless, the most recent version of the guidelines for national greenhouse gas inventories (IPCC, 2006) still does not take domestic poultry into account, most likely because of the very low levels of dietary fibre in commercial poultry diets, and the literature reports on generally low avian CH₄ emissions mentioned above.

In order to increase available in vivo CH₄ measurements, and to further explore potential differences between mammals and birds in relation to CH₄ emissions, we performed open-circuit respiration measurements in geese and turkeys as part of a study that measured intake, digestion and digesta retention on a diet of pelleted lucerne in these species (Frei et al., 2017). For the two investigated species, we hypothesised that turkeys would produce more CH₄ than geese because of their more voluminous caeca and longer digesta retention times (Frei et al., 2017).