The concept of improving animal performance by going beyond simply meeting requirements and synchronizing ruminal availability of protein and energy has been with us for at least 3 decades. Although theoretically appealing, research and field results have not supported this approach to diet formulation. Why? Essential to successful ruminal synchrony is the ability to predict available amounts and fates of diverse substrates. The substrates come from varied sources; their efficiencies of use and yields of products are affected by inherent properties, interactions, transformations, and passage. However, substrate quality and availability in the rumen are affected only in part by diet. For example, NPN, true protein, and peptides are contributed by diet and intraruminal recycling, with additional endogenous NPN contributions by the cow. Changes in factors that alter the rate or extent of substrate fermentation, such as the rate of passage or ruminal pH, can alter nutrient yield from the rumen and must be accounted for in order for synchrony to work. Our ability to estimate ruminally available substrate is also challenged by normal variation in feed composition and imprecision in component and digestibility analyses. Current in vitro assays may not be adequate to accurately describe the digestibility of feed components in vivo in mixed diets. There are some indications that the amount or pattern of supply of fermentable carbohydrate has a greater impact on microbial production and efficiency than does the pattern of protein supply. Animal responses to modifications in the supply of true protein from the rumen may be masked if additional protein is oxidized by tissues or if AA from endogenous sources cover deficiencies. Animal factors, such as response to immune challenge and sustained damage to tissues, will also affect partitioning of nutrients for production and may alter an animal's response to changes in nutrient supply. With the array of factors internal and external to the diet that must be considered, "synchrony" implies a greater deliberate precision in diet manipulation than may be currently possible to effect. Perhaps we should consider balance. Within the rumen and cow, can we generate conditions so that needed substrates or nutrients are available from the diet or accessible from endogenous resources to meet requirements and enhance productivity and efficiency? This approach involves the whole animal, rather than only the rumen and feed we offer to the cow.