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The proportion of net energy in the herd's ration derived from concentrates as reported to Dairy Herd Improvement was the basis for defining five groups of milk production records from New York and New England. The relative daily milk production by season and stage of lactation was estimated for each group at two ages by a generalized least squares procedure. These solutions showed little variation due to the grouping by proportion of concentrates, but season of freshening and age were important. The mean yield did increase with an increasing proportion of concentrates, and, consequently, extension factors would differ by group due to these different means.