Methanogenic potential of forages consumed throughout the year by cattle in a Sahelian pastoral area

Methane emission from ruminants is scarcely evaluated in African pastoral systems. Intake and proportions of the different plants composing the diet under such systems vary highly along wet and dry seasons, which may affect methane production. This study took place in a semi-arid Sahelian location of Senegal (27.8°C average temperature and 393 mm annual rainfall). In each of two different sites, three Gobra zebus were tracked throughout one year. A representative sample of their global diet was obtained one day per month by the simulated bites technique. Diet was mainly made of grasses, herbaceous legumes, tree and shrub foliage and pods, and “straw” defined as dried standing forage and plant residues. Methane production and volatile fatty acids (VFA) concentrations were determined using an in vitro batch fermentation system. Neutral detergent fibre was measured by near infrared spectrophotometry. Methane production varied between 24.2 and 35.2 mL/g forage dry matter (DM) according to months, was minimal in August (rainy season) and maximal in February (dry season). The acetate:propionate ration varied as methane (3.2 and 4.8 in August and February, respectively); VFA production was minimal in March and maximal in October (64.3 and 81.2 mmol/L, respectively). Methane production was closely related to dietary NDF content (r=0.88) and acetate:propionate ratio (r=0.96). Variation between animals was partly explained by dietary NDF content, and was reduced when methane was expressed in mL/g NDF. For six successive periods (February to July) and for the same animal, plant categories constituting the diet were incubated separately. Reconstituting the production of methane and VFA in the diet based on the proportion of plant components gave similar values to those of the global diet (33.9 and 33.7 mL methane /g DM and 74.0 and 77.7 mmol VFA/L, respectively). This suggests the absence of interaction on rumen fermentation between plant categories. (Texte intégral