Cow Performance. Throughout the study cows were milked twice daily, between 0600 and 0800 h, and between 1600 and 1800 h, with individual milk yields recorded automatically at each milking. During each CH4 measurement period (described below), milk fat, protein, and lactose concentrations were determined at each milking using a Milko Scan milk analyzer (model FT 120; Foss UK Ltd., Warrington UK). Cow BW was recorded after every milking, with a mean BW calculated for each week. Condition score was assessed weekly using a 5-point scale as described by Edmonson et al. (1989; 1 = emaciated; 5 = extremely fat).