The influence of oscillating dietary crude protein concentrations on milk production and nitrogen utilization in lactating dairy cows

................................................................................................................................ ii ACKNOWLEDGEMENTS ........................................................................................................ iv TABLE OF CONTENTS ............................................................................................................. v LIST OF TABLES ..................................................................................................................... vii LIST OF ABBREVIATIONS ................................................................................................... viii 1. GENERAL INTRODUCTION ............................................................................................ 1 2. LITERATURE REVIEW .................................................................................................... 3 2.1 Environmental Impacts of Excess N ................................................................................ 3 2.2 Rumen nitrogen metabolism ............................................................................................ 4 2.2.1 Protein Requirement ................................................................................................. 4 2.2.2 Ruminal Protein Degradation and Microbial Protein Synthesis ............................... 5 2.2.3 Ammonia Absorption, Ureagenesis and Urea-Recycling ......................................... 7 2.3 Factors That Regulate Urea Recycling............................................................................. 9 2.3.1. Dietary N Supply ...................................................................................................... 9 2.3.2. Supply of Ruminally Carbohydrate Fermentation .................................................. 11 2.3.3. Efficiency of Nitrogen Utilization in Dairy Cows .................................................. 13 2.4 Strategies to Increase the Efficiency of N Utilization in Ruminants ............................. 14 2.4.1. Dietary N Supply .................................................................................................... 14 2.4.2. Nitrogen Source ...................................................................................................... 16 2.4.3. Fermentable Carbohydrate Supply ......................................................................... 18 2.4.4. Frequency of N supply – Oscillating Dietary Crude Protein .................................. 19 2.5 Summary ........................................................................................................................ 23 2.6 Hypothesis ...................................................................................................................... 24 2.7 Objectives ....................................................................................................................... 24 3. MATERIALS AND METHODS ....................................................................................... 25 3.1 Animals, Experimental Design, and Dietary Treatments ............................................... 25 3.2 Data and Sample Collection ........................................................................................... 25 3.3 Sample Analyses ............................................................................................................ 30 3.4 Calculations .................................................................................................................... 32