Methane emissions and productivity of defaunated and refaunated sheep while grazing

Highlights

• Defaunated sheep displayed a decreased proportion of acetate and increased proportion of butyrate in ruminal VFA.

• Methane yield tended to be lower in defaunated than refaunated sheep while grazing.

• Liveweight gain and wool production were not different between defaunated and refaunated sheep.

Abstract

Rumen protozoa produce hydrogen, which can be utilised by methanogens to produce enteric methane (CH₄) that is a loss of digested energy and has an adverse environmental impact as a greenhouse gas. The aim of this study was to examine the effect of the absence of rumen protozoa on pasture intake, ruminal fermentation and enteric CH₄ production and performance of grazing sheep. An incomplete crossover experiment was conducted with eleven crossbred ewes (6 without [defaunated] and 5 with protozoa [refaunated]) on 2 × 2 ha pastures with daily CH₄ production (DMP) being measured by GreenFeed Emission Monitoring (GEM) units. Grazing defaunated sheep exhibited a lower concentration of rumen ammonia (P = 0.01), but similar concentrations of total rumen volatile fatty acids compared to refaunated sheep (P > 0.05). The molar proportion of acetate was decreased and butyrate proportion was increased by defaunation, while the proportion of propionate was unchanged. Estimated pasture intake was not different between defaunated and refaunated sheep (P > 0.05). Defaunated sheep tended to have a higher total dry matter intake (tDMI; P = 0.06), being the sum of pasture intake and pellet supplement intake. There was a tendency towards a lower CH₄ yield (g CH₄/kg tDMI; P = 0.07) in defaunated sheep, but no differences in average daily gain or wool growth occurred due to defaunation.

Introduction

The requirement to produce at least 70% more food in order to feed 9 billion people by 2050 (World Bank, 2008; Bijl et al., 2017) is a major challenge to animal production. The livestock population has surged in many developing countries in response to this rapid growing demand for livestock products and is forecast to rise further (FAO, 2006; Herrero et al., 2014). In association with an increased global human demand for food, an increased livestock products result in increased greenhouse gas (GHG) emissions which are expected to increase in coming years (van Beek et al., 2010). Recent meta-analysis confirms that removal of ciliate protozoa from the rumen of ruminants can increase livestock average daily gain (ADG) by 9% and reduce enteric methane (CH₄) emissions by 11% (Newbold et al., 2015). The positive effect of defaunation on animal growth is often seen with poor quality roughage diets that are low in nitrogen content and provide insufficient rumen degradable protein for the growth of rumen microbes (Bird and Leng 1978; William and Coleman, 1992). This may be advantageous in the tropics where forages are often deficient in protein and have higher fibre content than do temperate grasses (Minson, 1990). Since higher fibre content is associated with a great CH₄ yield (g CH₄/kg DMI; Margan et al., 1988; Pelchen and Peters 1998), a greater CH₄ yield as well as reduced animal performance can be expected to coincide if dietary fibre content increases. This suggests that elimination of rumen protozoa can improve growth and reduce CH₄ emissions of ruminants grazing on tropical or other low quality forages. However, there is little data from grazing animals available (Bird and Leng, 1984, Hegarty et al., 2000) and no grazing CH₄ production data. This study was conducted to quantify whether effects of defaunation on volatile fatty acid, intake, wool growth and CH₄ production observed in controlled feeding studies are also evident in grazing environments.