| Summary: | The present study investigated the effects of different dietary forage to concentrate ratios on animal performance, and enteric and manure greenhouse gas emissions in growing calves. Fifteen male Murrah calves (153.5 ± 18.17 kg; 6 to 12 months) were randomly assigned to 3 dietary treatments and fed corn fodder, wheat straw and concentrate in 3 different proportions: 20:60:20 (C20); 20:40:40 (C40) and 10:30:60 (C60), for a period of 120 days. Increasing dietary concentrate proportion had no significant (P > 0.05) effect on dry matter intake (DMI) but increased crude protein (CP) and total digestible nutrient intake (P < 0.05). Average daily gain and feed conversion efficiency were significantly higher (P < 0.05) for C60 compared with C20 and for C40, these did not differ with C20 and C60 (P > 0.05). The apparent digestibility of dry matter, organic matter and CP were higher (P < 0.05), but acid detergent fiber digestibility was lower (P < 0.05) for C60 compared with C20 whereas, ether extract and neutral detergent fiber digestibilities were not affected (P > 0.05). Daily methane (CH4) emission (g/d), CH4 energy loss (MJ/d) and CH4 yield (CH4 g/kg organic matter intake [OMI], CH4 g/kg digestible OMI, and CH4 % of metabolizable energy intake) were significantly higher for C20 compared with C60 (P < 0.05). Methane yield as g/kg DMI although lower for C60 compared with C20 but the difference was not significant (P > 0.05). Manure CH4 (g/kg DMI) and nitrous oxide (N2O mg/kg nitrogen) emissions were not affected (P > 0.05), but N2O emission on mg/kg DM basis was significantly higher (P < 0.05) from the manure of calves fed C60 than that for C20. Thus, increasing dietary concentrate proportion improved animal performance, and reduced enteric CH4 emission (g/day) without any significant effect on manure N2O (mg/kg nitrogen) and CH4 emissions. Keywords: Concentrate proportion, Growth, Digestibility, Manure, Methane, Nitrous oxide
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