Effect of forage-to-concentrate ratio in dairy cow diets on emission of methane, carbon dioxide, and ammonia, lactation performance, and manure excretion

Abstract

Holstein cows housed in a modified tie-stall barn were used to determine the effect of feeding diets with different forage-to-concentrate ratios (F:C) on performance and emission of CH4, CO2 and manure NH3-N. Eight multiparous cows (means ± standard deviation): 620±68kg of body weight; 52±34 d in milk and 8 primiparous cows (546±38kg of body weight; 93±39 d in milk) were randomly assigned to 1 of 4 air-flow controlled chambers, constructed to fit 4 cows each. Chambers were assigned to dietary treatment sequences in a single 4×4 Latin square design. Dietary treatments, fed as 16.2% crude protein total mixed rations included the following F:C ratio: 47:53, 54:46, 61:39, and 68:32 [diet dry matter (DM) basis]. Forage consisted of alfalfa silage and corn silage in a 1:1 ratio. Cow performance and emission data were measured on the last 7 d and the last 4 d, respectively of each 21-d period. Air samples entering and exiting each chamber were analyzed with a photo-acoustic field gas monitor. In a companion study, fermentation pattern was studied in 8 rumen-cannulated cows. Increasing F:C ratio in the diet had no effect on DM intake (21.1±1.5kg/d), energy-corrected milk (ECM, 37.4±2.2kg/d), ECM/DM intake (1.81±0.18), yield of milk fat, and manure excretion and composition; however, it increased milk fat content linearly by 7% and decreased linearly true protein, lactose, and solids-not-fat content (by 4, 1, and 2%, respectively) and yield (by 10, 6, and 6%, respectively), and milk N-to-N intake ratio. On average 93% of the N consumed by the cows in the chambers was accounted for as milk N, manure N, or emitted NH3-N. Increasing the F:C ratio also increased ruminal pH linearly and affected concentrations of butyrate and isovalerate quadratically. Increasing the F:C ratio from 47:53 to 68:32 increased CH4 emission from 538 to 648g/cow per day, but had no effect on manure NH3-N emission (14.1±3.9g/cow per day) and CO2 emission (18,325±2,241g/cow per day). In this trial, CH4 emission remained constant per unit of neutral detergent fiber intake (1g of CH4 was emitted for every 10.3g of neutral detergent fiber consumed by the cow), but increased from 14.4 to 18.0g/kg of ECM when the percentage of forage in the diet increased from 47 to 68%. Although the pattern of emission within a day was distinct for each gas, emissions were higher between morning feeding (0930h) and afternoon milking (1600h) than later in the day. Altering the level of forage within a practical range and rebalancing dietary crude protein with common feeds of the Midwest of the United States had no effects on manure NH3-N emission but altered CH4 emission. © 2011 American Dairy Science Association.