Addition of fat supplements to dairy rations is becoming more common due to the increases in milk yield and milk fat yield that have been observed. This thesis contains two studies that evaluated the effects of palmitic (C16:0), stearic (C18:0), and oleic (C18:1) acids in the form of commercially available supplements (C16:0 and C18:0-enriched) or as custom blends (C16:0 and C18:1) on lactating dairy cows. The first experiment used two commercially-available products enriched in either C16:0 (PA) or C18:0 (SA) supplied at 1.5% diet dry matter (DM) and a control diet with no added fat. Fat supplementation increased milk yield, but decreased total FA digestibility when compared to control. PA increased digestibility of total, 16- and 18-carbon FA as well as NDF digestibility, energy corrected milk (ECM), and milk fat yield when compared to SA. In the second experiment, the effect of differing ratios of C16:0 and C18:1 (fed at 1.5% diet DM) was determined using blends that consisted of 80% C16:0 + 10% cis-9 C18:1 (80:10) or 60% C16:0 + 30% C18:1 (60:30) across a wide range in production level. Interactions between preliminary milk yield and treatment were observed for dry matter intake (DMI), and yields of ECM and 3.5% fat-corrected milk (3.5% FCM), indicating that higher producing cows responded better to the 60:30 and lower producing cows responded better to the 80:10. 60:30 increased digestibilities of total, 16- and 18- carbon FA compared with 80:10. Together, this work will provide information that can be used to guide feeding decisions to maximize performance and farm income while using commercial FA supplements.
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