Nevertheless, CH4 production, CH4/DMI, CH4/GEI (P< 0.001), and CH4/MEI (P = 0.012) tended to decreasewith period, whereas CH4/milk yield (P = 0.49) and CH4/ECM yield (P = 0.54) did not change with period. However, no period × concentrate level interactions existed for any of the CH4 data examined (P >0.1); the main effects of concentrate level are presented in Table 2.