Variability of Bypass Protein in Forages
by Patrick C. Hoffman and Nancy M. Brehm
Department of Dairy Science
University of Wisconsin-Madison
Until recently there have been no commercially viable tests to evaluate bypass protein content of forages. As a result, nutrition consultants and producers have estimated the bypass protein content of forages to formulate rations. The principal source of information on which these estimates are based is the 1989 Nutrient Requirements of Dairy Cattle1. Using the 1989 Nutrient Requirements of Dairy Cattle to estimate the bypass protein content of forages is especially troublesome because, while the publication offers mean bypass protein values, it makes little reference to possible variations. Recent research efforts by University of Wisconsin dairy scientists have demonstrated that large variations in bypass protein contents of forages exist. In addition, this research has lead to the development of an unbiased, commercially available, low-cost bypass protein evaluation system for forages. This research suggests estimating bypass protein values for forages using book values or empirical evaluation systems has the potential for gross errors. In view of the development and use of unbiased bypass protein evaluation systems for forages, the practice of using book values or empirical evaluation schemes should be de-emphasized.
Factors Affecting Bypass Protein Variation in Forages
There are many factors that can influence the bypass protein content of a forage. Listed below are common factors most often associated with creating variation in forage bypass protein content.
Numerous research projects have demonstrated that immature forage legumes and grasses contain more degradable and less undegradable protein than mature forages. Immature forages contain more non-protein nitrogen primarily composed of ammonia, nitrate, amines, amides, and free amino acids which are rapidly degradable in the rumen. With advancing maturity, true plant protein synthesis advances and the cell wall matrix becomes more complex, rendering forage protein less accessible to rumen bacteria and less degradable. These factors ultimately reduce degradation potential of forage proteins. The effect of maturity on bypass protein content of eight perennial forage species is presented in Table 1. This data2 suggest that maturity has a profound and large influence on bypass protein content of forages.
Species is also known to affect bypass protein content of forages. The affect of species on bypass protein is also presented in Table 1. In general, legume protein is more degradable than grass protein. This is due in part to grasses containing more neutral detergent fiber which reduces rates of nutrient digestion. Within grasses, bromegrass and quackgrass appear to have greater levels of bypass protein, while perennial ryegrass protein appears to be quite degradable. In addition, germplasms within a forage species have been demonstrated3 to vary in bypass protein content.
Grasses assimilate soil nitrate (NO3) and ammonium (NH4) into non-protein nitrogen and true protein fractions. Increasing soil N supply increases forage N (crude protein). The increase in forage N (crude protein) is, however, disproportionate with the non-protein nitrogen pool increased to a greater extent than the true protein pool. Because non-protein nitrogen is readily degradable in the rumen, nitrogen fertilization generally reduces the amount of bypass protein. This concept is demonstrated in Figure 1. Fertilizing grasses with 0 or 120 lb./acre of N resulted in increasing crude protein 1.5% units and increasing protein degradability4.
When forages are ensiled, bacteria ferments the forage and breaks forage protein down into smaller fractions which are more degradable by rumen bacteria. This process is called proteolysis. Some researchers5 estimated that only 9% of forage macro protein molecules remain after fermentation. The effect of proteolysis can have a dramatic effect on the bypass protein content of forages. The concept of proteolysis can be demonstrated from a recent study6 from our laboratory (Figure 2). Alfalfa silage was made at three different maturities and wilted for 0, 10, 24, 32, 48, and 54 hours before ensiling. Ruminal degradability of ensiled forages was compared to a non-ensiled forage (NE). In all cases, the percent crude protein remaining (bypass) was less for the ensiled forages as compared to the non-ensiled forages. The effects of reduced bypass protein content were also more pronounced as wilting time decreased (increased moisture content). Red clover silage is a notable exception to the conceptual effects of proteolysis on ruminal protein degradation. Red clover contains polyphenol oxidases which have been demonstrated7 to inhibit or reduce proteolysis during fermentation. Therefore, red clover silages will generally have a higher bypass protein content than alfalfa or grasses silages within a similar maturity and/or nutritional level.
When forages are ensiled too dry and/or elimination of oxygen from the silage mass is not satisfactory, significant levels of heat can be produced during the fermentation process. Significant levels of heating can also occur when legume or grass hays are made too wet. In these situations when excessive heating occurs, forage protein may become bound (maillard reaction) to forage carbohydrate fractions, rendering the protein fraction less degradable. There are few studies that have quantified these effects under field conditions. Controlled research, however, has clearly demonstrated the effects of heated forages on ruminal protein degradability. Canadian researchers8 heated silages at 200°F for just 2 min and observed a 6.0 percentage unit increase (24.5 vs. 18.9% of CP) in the amount of bypass protein in alfalfa silage (Figure 3).
Bypass Protein Evaluation
Until now, bypass protein evaluation in forages involved empirical assessment. Typically, a benchmark bypass protein value for a forage was chosen from tabular values1, with the attending nutritional consultant making arbitrary judgements to lower or raise the value depending primarily on protein (maturity) and dry matter (proteolysis) content of the forage. Experiments in our laboratory suggest that at best this process can account for only 50% of the variance in bypass protein content in forages. This process is flawed because it does not take species, fertilization, heat damage, and other factors which are known to precipitate variation in bypass protein content into account. To remedy this problem, our laboratory conducted nine experiments designed to develop an unbiased system to estimate bypass protein content of forages. We will not attempt to define the details of the experiments6,9,10 in this paper, but in short, we developed a calibrated cow in situ technique to measure bypass protein and utilized NIRS technology to predict the results. The system accounts for 87.0% of variance in bypass protein content of legume-grass silages. The system has been extensively field tested and is available at a number of commercial forage testing laboratories. In addition, while the NIRS bypass protein prediction system was developed for legume and grass silages, unpublished data from our lab suggests the system works equally as well for corn silage, legume hay, and grass hay.
Variation of Bypass Protein in Forages
Because a new rapid unbiased system of evaluating bypass protein is now available, it is possible to make field determinations of bypass protein variability in forages. Recently we evaluated large populations of alfalfa/grass silages, corn silage, legume hay, and grass hays for bypass protein using the new NIRS evaluation system. These data are presented in Figures 4, 5, 6, and 7. The average bypass protein content of alfalfa/grass silage, corn silage, legume hay and grass hay was nearly identical to industry norms1 at 22.0, 31.0, 31.0, and 38.0% of CP, respectively. While the average bypass protein content fits industry norms, the range does not. Data suggest that alfalfa grass silages, corn silage, and legume hays can range up to 20 percentage units of bypass protein. Grass hays may range up to 30 percentage units of bypass protein.
There is great variation in bypass protein content of forages which is caused by many factors. Because so many factors influence forage bypass protein, an unbiased system of evaluation should be used. A system is now available at commercial forage testing laboratories and is an excellent alternative to empirical estimation of forage bypass protein.
1National Research Council. 1989. Nutrient Requirements of Dairy Cattle. 6th rev. ed. Natl. Acad. Sci., Washington D.C.
2Hoffman, P. C., S. J. Sievert, R. D. Shaver, D. A. Welch, and D. K. Combs. 1993. In situ dry matter, protein and fiber degradation of perennial forages. J. Dairy Sci. 76:2632.
3Griffin, T. S., K. A. Cassida, O. B. Hesterman, and S. R. Rust. 1994. Alfalfa maturity and cultivar effects on chemical and in situ estimates of protein degradability. Crop Sci. 34:1654.
4Cuomo, G. J., and B. E. Anderson. 1996. Nitrogen fertilization and burning effects on rumen protein degradation and nutritive value of mature grasses. Agron. J. 88:439.
5Messman, M. A., W. P. Weiss, and M. E. Koch. 1994. Changes in total and individual proteins during drying, ensiling, and ruminal fermentation of forages. J. Dairy Sci. 77:492.
6Hoffman, P. C., N. M. Brehm, D. K. Combs, L. M. Bauman, J. B. Peters, and D. J. Undersander. 1999. Predicting the effect of proteolysis on ruminal crude protein degradation of legume and grass silages using near infrared reflectance spectroscopy. J. Dairy Sci. (In press.)
7Jones, B. A., R. E. Muck, R. D. Hatfield, and D. R. Mertens. 1994. Nutrient detergent fiber nitrogen and proteolysis in alfalfa silages treated with red clover herbage and extracts. J. Dairy Sci. 77(Suppl. 1): 275(Abstr.).
8Chamley, E., and D. M. Veira. 1990. Inhibition of proteolysis in alfalfa silages using heat at harvest. J. Anim. Sci., 68:2042.
9Hoffman, P. C., N. M. Brehm, L. M. Bauman, J. B. Peters, and D. J. Undersander. 1999. Prediction of laboratory and in situ fractions in legumes and grass silages using near infrared reflectance spectroscopy. J. Dairy Sci. (In press).
10Hoffman, P. C., N. M. Brehm, J. J. Hasler, L. M. Bauman, J. B. Peters, D. K. Combs, R. D. Shaver, and D. J. Undersander. 1999. Development of a novel system to estimate protein degradability in legume and grass silages. J. Dairy Sci. (In press).