1a. Objectives (from AD-416)
The objectives of this research are to: 1) Improve accuracy, reproducibility, and ease of measuring/estimating feed digestibility for fiber and protein for use in developing feeding strategies for improving animal performance; 2) Develop rapid methods for measuring feed qualities to improve on-farm precision of feeding; and 3) Establish methods to measure feed characteristics of nutritional relevance in dairy cattle diets.
1b. Approach (from AD-416)
The approaches for the diverse projects in this research will also be diverse. They will include evaluation of feed evaluation equipment and methods used on the farm and in the laboratory to measure specific feed components or their digestibility. We will compare the results of the experimental approaches to values for the feed components measured with standard measurement systems, or to digestibility data from studies with dairy cattle.
3. Progress Report
Progress was made on all three objectives of this project, all of which fall under National Program 101 – Food Animal Production, Component 2: Enhancing Animal Adaptation, Well-Being and Efficiency in Diverse Production Systems, Problem Statement 2C: Improving Efficiency of Nutrient Utilization and Conversion to Animal Products. This research focuses on providing improved methods of analyses to dairy nutritionists, analytical labs, and researchers. In Objective 1 (addressing improvement of in vitro digestibility methods), the results of the ring test with commercial and research laboratories on fiber digestibility showed that the assay is inherently variable, but laboratories could reliably rank samples based on digestibility. Samples that compared forage processing were evaluated using in vitro methods. Discovery of existing data sets comparing in vitro to in vivo fiber digestibility removed the need for comparison of in vitro and in situ results (all Objective 1.A). For Objective 1.B (addressing ruminal protein degradability), comparisons of commercial protease-based vs. rumen inoculum-based in vitro assays indicated that commercial proteases were insensitive to differences in degradability of feed proteins and would not be useful for feed evaluation. Accordingly, near infrared reflectance spectroscopy calibrations have not yet been attempted. New approaches being investigated include an assay to measure compounds formed when soluble or insoluble proteins are broken down by microbes in the rumen. Additionally, we assessed a rumen in vitro method that applied enzyme kinetic computations to determine rates of degradation of feed proteins by rumen microbes. This method was applied to typical protein supplements (different soybean meals) and tropical grasses grazed by Brazilian dairy cattle. The work on Objective 2.A: “Developing rapid methods for measuring feed qualities to improve precision of feeding” is being continued by a post-doctoral student and a graduate student after the retirement of a key collaborating scientist. Measurement of feed dry matter composition on as-received whole plant materials is nearly complete and is giving good precision of measurement. However, initial attempts to transfer dry matter analysis calibrations among instruments have not been successful, and other approaches are still being evaluated. Research on methods to measure nutritionally relevant fractions in feeds (Objective 3) continues with the dietary starch method. The dietary starch method is under review by the Association of Official Analytical Chemists International to make it an official method. This method has been disseminated and is already in use by major feed analysis laboratories. With the sugar analyses, results of an additional experiment revealed a failure of the reducing sugar assay to measure all monosaccharides equivalently. This largely explains the inconsistencies among “sugar” assays in evaluation of feeds and specific carbohydrates. A manuscript will be revised to address this point.
1. Reassessment of in vitro neutral detergent fiber digestibility assays leads to more accurate evaluation and better utilization of feedstuffs. Tens of thousands of fiber digestibility analyses are purchased each year by nutritionists, farmers, and feed and seed companies to determine the nutritional value of forages and other feeds to cattle. Fiber digestibility analyses are commonly used to describe a feedstuff’s energy for diet formulation, and to determine the quality/value of forages for sale. Consumers raised concerns about the variability of the values generated by commercial laboratories. ARS scientists in Madison, Wisconsin evaluated the variability in fiber digestibility values that were used by commercial and research feed analysis laboratories. Results indicated that the fiber digestibility analysis is inherently variable, but the laboratories were far more reliable in ranking the forages in order from highest to lowest digestibility. Users of the digestibility values need to reassess how those values are used numerically in diet formulation or forage evaluation. Use of the values as relative rather than absolute values may be most appropriate, leading to more accurate formulation of diets to meet animal needs.
2. Variability and accuracy in measurement of sugars and water-soluble carbohydrates in animal feeds depends upon the methods one chooses. Measurements of sugars and other soluble carbohydrates in feeds are commonly used by nutritionists who formulate diets for dairy cattle and, increasingly, for horses. These formulations are done to meet the energy needs of the animals, take advantage of the benefits of inclusion of sugars, and avoid induction of health disorders specific to feeding certain carbohydrates. However, commercial laboratories, nutritionists, and animal owners have raised concerns about apparent inconsistencies among the analytical values they generate/receive and question how they should be interpreted. ARS scientists in Madison, Wisconsin evaluated reducing sugar and phenol-sulfuric acid assays used to detect soluble carbohydrates. They determined that sugars or extractable carbohydrate contents of feedstuffs may be over-, under-, or accurately estimated, depending on the assay applied. The methods typically differed in the values they measured for various carbohydrates, but gave similar values when used to measure sucrose under the conditions tested. This means that, except in samples where sucrose predominates, the results of the two detection methods will often not be comparable. These results also explain why the common practice of estimating fructan content of feeds as water-soluble minus ethanol-soluble carbohydrates (each measured with a different detection method) gives erroneous values. The study made clear the need for alternative soluble carbohydrate analyses to obtain accurate values for feeds in which sucrose is not the predominant soluble carbohydrate. Accuracy of sugar values is important to correctly formulate diets that meet animal needs and make efficient use of feedstuffs.
3. New laboratory procedures allow more accurate evaluation of dietary proteins fed to dairy cattle. The amount of protein escaping microbial breakdown in the rumen (commonly referred to by farmers and consultants as “bypass protein”) determines much of the value of feeds for dairy cows. Thus far, no laboratory test has proven reliable to measure bypass protein in dairy feeds. ARS researchers in Madison, Wisconsin developed an in vitro technique to estimate bypass protein in which digesta collected from the cow’s rumen were incubated with the feeds being tested. Chemicals were added to the incubation so that protein breakdown could be determined from appearance of protein breakdown products. This procedure did not account for all of the compounds formed when protein was broken down and, thus, it underestimated protein escape. This method was modified to measure all breakdown products including peptides, important protein fragments that were not accounted for earlier. This procedure was applied to a number of feed proteins commonly fed to dairy cows. The new technique gave more rapid rates of protein degradation and yielded values for bypass protein that were similar to measurements made in living dairy cows. The method was applied by ARS researchers plus university scientists to assess the value of protein concentrates (soybean meals, canola meals) and samples of tropical grasses collected in several grazing studies. This research also indicated that in situ tests (which are used throughout the world to measure bypass protein from the rumen) consistently overestimate protein escape. Better methods to accurately measure bypass protein allow improved evaluation of feeds and lead to the formulation of diets that are efficiently utilized by dairy cattle.
Colombini, S., Broderick, G.A., Clayton, M.K. 2011. Effect of quantifying peptide release on ruminal protein degradation determined using the inhibitor in vitro system. Journal of Dairy Science. 94:1967-1977.