Dietary factors affecting fatty acid digestion and metabolism in lactating dairy cows

The five research chapters presented provide insight on the effects of different dietary factors on fatty acid (FA) digestion and metabolism in lactating dairy cattle. Together these experiments increase our understanding of factors that impact the amount of FA available for incorporation in milk fat and partitioning of energy into adipose tissue. In the first research chapter, digestibility estimates of individual FA from previous studies were analyzed using meta-analysis and meta-regression. Results from the meta-analysis indicated that minor differences in the digestibility of individual FA exist. However, when analyzing the data using meta-regression to generate best-fit equations, stearic acid (C18:0) digestibility was significantly reduced as the amount of stearic acid reaching the duodenum increased.;In the second research chapter, responses to diets differing in starch concentration were evaluated by lactating dairy cows across a wide range of milk production. The high-starch diet increased DMI, DM digestibility, yields of milk, fat and protein compared with the low-starch diet. Higher producing cows responded favorably for many production parameters to higher starch concentration in the diet whereas lower producing cows were able to maintain production on a diet containing only 12% starch.;The third research chapter investigated the effects of two diets similar in energy concentration yet differing in starch, NDF, and FA on milk production and energy partitioning. Intake was similar between treatments with the higher fiber and fat diet reducing milk yield, milk protein concentration, and milk protein yield. The higher fiber and fat diet increased milk fat yield and concentration and milk energy output. The higher fiber and fat diet partitioned fewer nutrients towards body reserves as evident by reduced body weight gain, body condition score, and subcutaneous fat thickness. Our results indicate that although diets can be similar in energy content, the composition of the diet impacts production and body composition changes.;Research chapter four investigated the effects of additional fat supplemented to the diet as either free FA or esterified FA on production parameters and the FA composition of milk. Supplemental fat reduced DMI and increased milk yield with no changes in milk fat or protein yield. Although there was a reduction in the proportion of both de novo and 16-carbon milk FA, there was compensation in the form of an increase in preformed FA. Fat supplementation changed the FA profile of milk, however, there was no significant difference between free FA and esterified FA for common FA associated with milk fat depression (MFD).;In the final research chapter increasing doses of stearic acid were supplemented to lactating dairy cows to determine the effects on production parameters, digestibility, and incorporation of FA into milk fat. Stearic acid was supplemented at 0.8, 1.5, and 2.3% of diet DM compared to a non-fat supplemented diet. Supplementation of stearic acid resulted in no effect on yields of milk or milk components. Stearic acid inclusion resulted in reduced FA digestibility with the largest reduction in digestibility being 18-carbon FA. Increasing stearic acid to the greatest inclusion rate resulted in no effect on preformed milk FA yield and only a small increase in stearic and oleic acid. Results from all trials provide insight on FA digestibility, ways to utilize FA to repartition energy, and incorporation of FA into milk fat; however, further work is required to understand mechanisms of action for differences in digestibility and partitioning of energy and to determine if results are consistent across stages of lactation.