Nutrient composition, rate of fermentation and in vitro rumen methane output from tropical feedstuffs

In vitro rumen methane output (IRMO) of over 200 feed/feed mix samples representing approximately 74 feed types was investigated in a series of completely randomized experiments. The samples comprised dry fodder, grass, tree leaves, cultivated grasses, cereal by-products, cereal grains, oilseed/meals, compound feeds and total mixed rations (TMRs) from the tropical regions. These samples were subjected to three in vitro gas production tests at 39 °C in 100 ml Heberle syringes. The first incubation was conducted with 200 mg dry matter (DM) substrate for 96 h to determine half-time gas production (t ¹/², h) value of each sample. The second and third incubations were carried out simultaneously. The second incubation was done with 200 mg DM substrate until t ¹/² time to determine IRMO and third with 500 mg DM to estimate in vitro dry matter digestibility (IVDMD) of each samples, respectively. The IRMO was expressed as ml/100 mg digestible substrate. Crude protein content (g/kg DM) was lowest in dry fodder samples and highest in oilseed meals, whereas it was similar in local grass and tree leaves. The IVDMD values ranged from 0·48 to 0·87; the lowest digestibility was recorded in tree leaves. The potential gas production (PGP, ml/200 mg DM) ranged from 9·76 to 61·3. The PGP from grasses and compound feeds was similar, whereas it was lowest in tree leaves. The rate constant (mg/h) was maximum in compound feed followed by oilseed meal. The rate constant was similar among other group of feedstuffs. The t ¹/² time ranged from 9·8 to 19·4 h. The highest t ¹/² time was recorded in local grass samples followed by dry fodder and cultivated grasses. However, they were similar among tree leaves, cereal grains, by-products and compound feeds. The methane % in the total gas varied from 9·79 (tree leaves) to 20·2 (local grasses). Among straw, IRMO varied from 3·88 (Zea mays fodder) to 12·0 (Sorghum vulgare) and it was lower in fruit tree leaves than cultivated grasses. Among protein and energy sources, IRMO was higher in cereal by-products as compared with cereal grains, oil meals and compound feed. The IRMO was similar among TMR, irrespective of the composition of the concentrate mixture. Nevertheless, it varied with the amount of concentrate in the TMR. This is the first exhaustive data on IRMO from the tropical region. Because of the substantial amount of dietary gross energy lost in methane, knowledge of the methane output from these feed ingredients will help in formulating low methane emitting diets for ruminants. Incorporation of tropical tree leaves in the diets and feeding TMR are potential strategies to reduce enteric methane emission in ruminants.