Enteric methane emissions from zebu cattle are influenced by seasonal variations in rangeland fodder quality and intake

Rangeland fodder resources used to feed ruminants in the Sahel decline considerably in both quantity and quality from the wet to dry seasons. While it is widely assumed worldwide that this seasonality of fodder supply impacts intake levels and therefore enteric methane (eCH₄) emissions, there are very few references based on in vivo measurements of eCH₄ in this region. The purpose of this study was to test the assumption that the seasonality of fodder supply impacts intake levels and consequently eCH₄ in ruminants. Thus, eCH₄ was measured in vivo in Sudanese Fulani zebu cattle during three main seasons of the year (wet season: WS, cold dry season: CDS, and hot dry season: HDS). The experiment was carried out on 10 steers aged 32 months with an average (± SD) initial BW of 138 ± 5.8 kg (i.e. 0.55 Tropical Livestock Unit - TLU) and kept in individual pens. Animals were fed with natural rangeland fodder harvested each season following herders' practices, i.e. green fodder in the WS and dry fodder hay in the CDS and HDS. Different levels of fodder were offered to the animals to reproduce the gradient of fodder availability on rangelands over the year (six trials): 3.3% BW during the CDS; 3.3, 2.3, and 1.4% BW successively during the HDS; and 2.3% in two sequential studies in the WS. Each trial lasted 3 weeks, split into 2 weeks of fodder adaptation and 1 week of data collection. The BW, quantity of voluntary DM intake, digestibility of DM digestibility and of OM digestibility, and eCH₄ (GreenFeed® system) were measured for each animal. Fodder composition varied considerably between seasons (P < 0.05). The DM intake (g/kg BW per day) varied from 23.9 in CDS to 15.7 in HDS and 22.3 in WS (P < 0.001). The DM digestibility varied from 0.50 in CDS to 0.46 in HDS and 0.57 in WS. The eCH₄ yields (g/kg DM intake per day) varied significantly from 25.2 in the CDS to 31.8 in the HDS and 20.5 in the WS. When extrapolated over a full year and irrespective of season, eCH₄ emissions for steers amounted to 68.1 g/d (24.6 g/kg DM intake per day, 46.7 ± 3.34 kg of eCH₄/TLU per year). Variations in the various parameters recorded in different areas and during the main seasons must therefore be accounted for in national inventories to refine eCH₄ data for ruminants in Sub-Saharan Africa.