Cows grazed perennial ryegrass (Lolium perenne)- based swards throughout this experiment, with the 40 experimental cows part of a larger group of 88 cows. This group commenced grazing on April 13, 2011, and were grazing full time by April 19, with all cows offered 8.0 kg of concentrate/cow per day during this transition period. Over the following 14-d period, concentrate feed levels for all cows within each group were gradually adjusted to the designated treatment levels (2.0, 4.0, 6.0, or 8.0 kg/cow per day). Cows were split into their 4 experimental groups on April 27 (22 cows/group), with the experiment beginning on May 4. The grazing period continued until September 30, 2011