We evaluated the effects of altering the dietary ratio of C16:0 and cis-9 C18:1 on production responses and energy partitioning of lactating dairy cows. Cows were blocked by milk yield and assigned to 3 groups (12 cows per group) in a main plot. Production groups were: a) low (45.2 ± 1.7 kg/d); b) medium (53.0 ± 1.6 kg/d); and c) high (60.0 ± 1.9 kg/d). Within each production group, a truncated Latin square arrangement of FA treatments was used in 2 consecutive 35 d periods. The FA treatments supplemented at 1.5% diet DM were: 1) 80:10 (80% C16:0 + 10% cis-9 C18:1); 2) 73:17 (73% C16:0 + 17% cis-9 C18:1); 3) 66:24 (66% C16:0 + 24% cis-9 C18:1); and 4) 60:30 (60% C16:0 + 30% cis-9 C18:1). The statistical model included the random effect of cow within production group, and the fixed effect of treatment, production group, period, and their interactions. Treatment by production group interactions were observed for milk yield (P = 0.09), FCM (P = 0.05), ECM (P = 0.05), milk fat yield (P = 0.02), milk protein yield (P = 0.06), milk lactose yield (P = 0.08), and energy partitioned to milk (P = 0.02). Increasing cis-9 C18:1 in FA treatments reduced FCM, ECM, and milk energy output in low producing cows (linear, P < 0.05), but increased these in high producing cows (linear, P < 0.01). Increasing cis-9 C18:1 in FA treatments tended to reduce milk fat yield in low producing cows (linear, P < 0.10), but increased it in high producing cows (linear, P < 0.01). Increasing cis-9 C18:1 in FA treatments did not impact milk yield, milk protein yield, and milk lactose yield in low and medium producing cows, but increased these in high producing cows (linear, P < 0.01). Regardless of production level, increasing cis-9 C18:1 in FA treatments increased BW change (quadratic, P = 0.02) and BCS change (linear, P < 0.01); however, there was no effect of treatments on DMI (P = 0.98). Our results demonstrate that high producing dairy cows respond better to fat supplements containing more cis-9 C18:1, while lower producing cows respond better to supplements containing more C16:0.