Historically, dairy cattle diets have been balanced to maintain cow health and achieve efficient milk production. More recently, scientists have also been studying how diets affect atmospheric emission of methane, ammonia and carbon dioxide. Our objective in this study was to determine the effect of feeding lactating dairy cows with diets differing in forage to concentrate ratios on milk production and the undesirable atmospheric emission of methane, ammonia and carbon dioxide. Increasing the proportion of forage in the diet from 47 to 68%, while maintaining dietary crude protein did not impact milk production or emissions of ammonia or carbon dioxide, but increased methane emission per unit of energy-corrected milk by 25%. This information shows that a proper balance between the forage and concentrate components of dairy rations is needed to maintain high levels of milk production and minimize emission of methane from dairy barns.

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 methane (CH4), carbon dioxide (CO2) and manure ammonia-nitrogen (NH3-N). Eight multiparous cows (means ± standard deviation (SD)): 620 ± 68 kg of body weight (BW); 52 ± 34 days in milk (DIM) and 8 primiparous cows (546 ± 38 kg of BW; 93 ± 39 DIM) were randomly assigned to one of four air-flow controlled chambers, constructed to fit 4 cows each. Chambers were assigned to dietary treatments sequences in a single 4 x 4 Latin squares design. Dietary treatments, fed as 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 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 (Innova Model 1412). 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 ± 1.5 kg/d), energy corrected milk (ECM, 37.4 ± 2.2 kg/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 solid-non-fat content (by 4, 1, and 2%, respectively) and yield (by 10, 6 and 6%, respectively), and milk nitrogen (N) to N intake ratio. Increasing F:C ratio also increased ruminal pH linearly and affected concentration of butyrate and isovalerate quadratically. Increasing the F:C ratio from 47:53 to 68:32 increased CH4 emission from 538 to 648 g/cow/d, but had no impact on manure NH3-N emission (14.1 ± 3.9 g/cow/d). In this trial, CH4 emission remained constant per unit of neutral detergent fiber intake (1 g of CH4 was emitted for every 10.3 g of neutral detergent fiber consumed by the cow), but increased from 14.4 to 18.0 g/kg of ECM when the percentage of forage in the diet increased from 47 to 68%. Within a day, CH4 emission was highest during the first 6 hours after morning feeding and declined thereafter until the next morning. Altering the level of forage within a practical range and rebalancing dietary crude protein with feed commonly found in the Midwest of the United States had no effects on manure NH3-N emission but altered CH4 emission substantially.