"Fat supplements are often used in an attempt to increase energy intake, yields of milk and milk components, and body reserves of dairy cows. However, different fatty acids (FA) have different metabolic fates and therefore it is critical to understand how FA may affect energy partitioning and milk production. Importantly, physiological state (i.e. lactation stage, lactation number, production level) plays an important role in the efficiency of nutrient utilization and may interact with different nutrition strategies affecting metabolic and production responses. Our research examined the effects of varying the dietary ratio of FA under different physiological conditions on nutrient digestion, energy partitioning, and production responses of dairy cows. In the first research chapter, we evaluated the effects of varying the ratio of dietary palmitic (C16:0), stearic (C18:0), and oleic (cis-9 C18:1) acids on post-peak dairy cows. Among the combinations of C16:0, C18:0, and cis-9 C18:1 evaluated, FA supplements with more C16:0 increased energy output in milk, whereas FA supplements with more cis-9 C18:1 increased energy storage in body reserves. Increasing C18:0 in a FA supplement reduced FA digestibility and did not increase energy intake, which most likely explains its lower performance compared with the other FA treatments. In the second research chapter, we determined the long-term effects of C16:0 supplementation on primiparous and multiparous post-peak dairy cows. Our results demonstrated that supplementation with C16:0 consistently increased DMI, energy intake, milk yield, milk fat content and yield, energy-corrected milk (ECM), and NDF digestibility in both primiparous and multiparous cows. In addition, C16:0 supplementation increased body weight (BW) change in primiparous cows but not in multiparous cows. In the third and fourth research chapters, we determined the effects of timing of C16:0 supplementation on production and metabolic responses of early lactation dairy cows. Our results demonstrated that feeding a C16:0 supplement to early lactation cows consistently increased the yield of ECM compared with a non-fat control diet regardless of the timing of supplementation. C16:0 supplementation also increased NDF digestibility, energy intake, and milk energy output. When fed in the immediate postpartum period, C16:0 increased negative energy balance, plasma non-esterified FA, and BW and BCS loss, and decreased plasma insulin. In the fifth chapter, we evaluated responses of lactating dairy cows with different levels of milk production to alterations in the dietary ratio of C16:0 and cis-9 C18:1. Our results indicated that high producing dairy cows (averaging 60 kg/d) responded better to FA supplements containing more cis-9 C18:1, while lower producing cows (averaging 45 kg/d) responded better to FA supplements containing more C16:0. Regardless of production level, increasing cis-9 C18:1 increased total FA digestibility, BW and BCS change, with no effect on DMI. In our last research chapter, we determined the effects of altering the dietary ratio of C16:0 and cis-9 C18:1 on metabolic and production responses of early lactation dairy cows during the immediate postpartum period and the carryover effects of the treatment diets in early lactation. We observed that feeding FA supplements containing C16:0 and cis-9 C18:1 during the immediate postpartum period increased milk yield and ECM, tended to increase DMI and reduce BW loss compared with a non-fat control diet. Additionally, the yield of milk and milk components and ECM were higher during the carryover period for cows that received FA-supplemented diets compared with control during early postpartum period. Overall, our results indicate that different combinations of FA should be used according to production level and stage of lactation in dairy cows."--Pages ii-iii.
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