Milk fatty acids (FA) are derived from two sources. Fatty acids from 4 to 14 carbons are synthesized de novo in the mammary gland using acetate and beta-hydroxybutyrate (BHB) produced during ruminal fermentation or liver ketogenesis. Those with 18 or more carbons are “preformed”, meaning they originate from the diet or adipose tissue. An interdependent relationship was proposed by Glasser et al. (2008) where de novo FA and preformed FA drive the synthesis of one another other to increase their incorporation into milk TG. The first experiment in this thesis evaluated the effect of supplemental palmitic acid (PA) on the molar yields of milk FA sources and individual FA. Molar yields were evaluated to adjust for differences in molecular weight of individual FA and standardize the incorporation of FA into TG. Supplementing diets with PA increased mixed FA yield and shifted FA in favor of increased yields of C4:0 and cis-9 C18:1 and decreased yields of FA from C8:0 to C14:0 and C18:0. Additionally, evaluating the relative concentration of FA using molar yields better reflects the contribution of de novo FA to milk fat compared to on a mass basis and highlights the important contribution of de novo FA to milk fat synthesis. The second experiment in this thesis evaluated the interactions between altering the dietary supply of de novo FA and preformed FA to maximize milk fat yield by supplementing sodium acetate (NaAc), PA, and whole cottonseed (WCS). Three-way interactions between NaAc, PA, and WCS were observed for the yields of milk fat, 3.5% fat-corrected milk (FCM), de novo FA, mixed FA, and preformed FA. Diets containing NaAc and PA independently increased milk fat yield whereas diets containing NaAc, PA, and WCS led to the greatest increase in milk fat yield. The results from our two studies emphasize the importance of measuring diet-induced changes in milk fat composition on a molar basis. Additionally, our results emphasize the importance of balancing the supply of FA from all sources to increase milk TG synthesis in the mammary gland.
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