Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Animal and Veterinary Sciences

Committee Member

Dr. Gustavo Lascano, Committee Chair

Committee Member

Dr. Thomas Jenkins

Committee Member

Dr. Susan Duckett

Committee Member

Dr. William Bridges


Diet-induced milk fat depression is a multifactorial disorder characterized by sustained reduction in milk fat. Feeding high amounts of starch and unsaturated fatty acids have been shown to reduce milk fat yield and concentration, and alter ruminal biohydrogenation. In addition, starch with high rates of degradability may lead to a decrease in ruminal pH and an increase in the outflow of ruminal biohydrogenation intermediates. It is possible that starch sources of high degradability could still be utilized in diets for the recovery of milk fat depression, provided the unsaturated fat level is reduced. Replacing a portion of the starch, especially sources with a high rate of degradability, with sugar or soluble fiber have the potential to improve biohydrogenation and rumen fermentation. There is a relationship between the rate of starch degradability, the level of starch in a diet, and the influence on biohydrogenation in the rumen. The objectives of the study presented in Chapter 2 was to compare animal performance and ruminal fermentation in animals recovering from milk fat depression when corn with low or high starch degradability is fed. Overall, results from this study indicates that animals receiving high degradable starch diets during recovery from milk fat depression had greater milk trans-10, cis-12 CLA than animals receiving low degradable starch diets. Rumen pH, biohydrogenation rates were similar with both treatments. However, the animals receiving the high degradable starch recovery diet lagged in recovery of milk fat to normal levels. These data indicate that corn sources with rapid rates of degradability can be utilized in the recovery process from milk fat depression but may take longer, provided unsaturated fatty acid levels in the diet are corrected. Recent evidence suggests that replacing a portion of the starch, especially starch with a high degradability, with sugar may improve biohydrogenation and production of trans fatty acids during milk fat depression. The objective of the study in Chapter 3 was to determine the effects of adding lactose or sucrose to high or low starch degradable diets on biohydrogenation, digestibility, and rumen fermentation in continuous culture fermenters with a basal level of soybean oil. The outflow of cis-9, trans-11 CLA was reduced and total CLA outflow tended to be reduced with overall sugar addition. These data provide evidence that when lactose or sucrose replace a portion of the starch in continuous cultures with a high unsaturated fatty acid level the biohydrogenation rates of C18:2 and C18:3 were similar to C for L + S. Furthermore, starch degradability also alters the proportions of CLA produced in the culture, and increases pH. Modifying other carbohydrate fractions, such as the soluble fiber fraction, is another potential approach to improving conditions within the rumen with respect to low pH, rumen biohydrogenation, and production of milk fat inhibiting isomers. There is limited information about how starch degradability and soluble fiber interact when fed to continuous cultures with a basal fat level. Chapter 4 presents a study that was aimed at the utilization of soluble fiber in continuous cultures. The objectives of the study were to determine the effects of replacing starch with beet pulp in high or low starch degradable diets with a basal soybean oil level on biohydrogenation, digestibility, and rumen fermentation in continuous culture fermenters. The findings in this experiment suggest that replacing starch with soluble fiber (SF) can modify pH and influence the outflow of fatty acids, particularly trans-10 C18:1, presenting a possible positive effect in situations with a high risk for milk fat depression. The relationships between soluble carbohydrates and rumen biohydrogenation are evident based on the previous chapters. The objectives of the study detailed in Chapter 5 were to evaluate the effects of feeding diets with or without soybean oil and the replacement of a portion of the starch for sources of high sugar and or high soluble fiber, on animal production and in vitro fermentation. Milk fat was reduced with all high fat treatments but neither sugar nor soluble fiber improved milk fat production. Sugar and soluble fiber also influenced changes in rumen VFA. The results of this study imply that when a portion of the starch is replaced with sugar or soluble fiber, rumen fermentation profiles and animal production is altered.



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