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The influence of percent of net energy in the herd's ration derived from concentrates on age-season effects was studied in New York Holstein Dairy Herd Improvement records. Ten feeding groups were defined by herd-feeding information. There was some increase in milk yield with an increasing proportion of concentrates in the ration. For six of these groups, the two highest, two middle, and two lowest in percent net energy from concentrates, age-season effects were calculated. The trends in age effects and seasonal effects were similar for all groups with the groups having a higher fraction of net energy from concentrates showing relatively higher mature milk yield. The multiplicative age-season factors calculated from the age-season solutions were similar for all groups, indicating that the proportion of concentrates over the current range does not have an important influence on age-season factors.