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Forty Years of Forage Quality Research: Accomplishments and Impact from an Animal Nutrition Perspective.

Five publications stand out as authoritative references on the topic of forage quality as related to nutrition of ruminant animals. The first of these is the "Proceedings of the National Conference on Forage Quality Evaluation and Utilization". This publication was the result of a meeting on the same topic held at Lincoln, NE, on 3 to 4 Sept. 1969. One decade earlier, a group of scientists met informally at Purdue University to discuss problems inherent with the development of an in vitro technique for evaluating forages (Barnes, 1969). The need to discuss problems and coordinate research efforts was recognized at this meeting and was the impetus for the 1969 symposium. Johnson (1969) noted that until the early 1960s, "the gulf of understanding or agreement between the plant breeder, the forage management specialist and the animal nutritionist ... was deplorable and spoke for itself as justification for concerted effort in this area" (i.e., forage science). Another key reference was published this same year and is a "must read" for all students of forage science (Raymond, 1969).

The third key reference in this area is "Forage Evaluation: Concepts and Techniques" (Wheeler and Mochrie, 1981). This book resulted from a 5-d discussion of the techniques and underlying research philosophies used in the evaluation of forage and methods of its utilization. The meeting was held in Armidale, Australia, on 26 to 31 Oct. 1980. Participants reviewed methods of evaluating forage in laboratories and with confined or freely grazing animals, discussed the concepts of both forage evaluation and the means of utilizing pastures, appraised recent developments in methodology, and formulated agreed-upon procedures for evaluating pastures and pasture treatments.

On 7 to 10 Oct. 1991, the International Symposium on Forage Cell Wall Structure and Digestibility was held in Madison, WI. Resulting from this was the text "Forage Cell Wall Structure and Digestibility" (Jung et al., 1993). The objective of the symposium was to assemble a multidisciplinary group of expert scientists to present state-of-the-art information on forage cell wall structure and digestibility, and to identify limits to our knowledge where more research is required. A benchmark reference publication was desired that described current knowledge regarding the chemical structure and digestibility of forage cell walls.

Finally, on 13 to 15 April 1994, the National Conference on Forage Quality, Evaluation, and Utilization was held at the University of Nebraska, Lincoln, and a book of the same title was published (Fahey et al., 1994). This was the twenty-fifth anniversary of the first conference mentioned above. The book provided a multidisciplinary look at the field of forage science. Twenty-five years of progress in forage quality, evaluation, and utilization were detailed along with cutting edge research in this field. New directions for future research and development were explored as well.

If we are to take the general topic of this symposium quite literally, the 40-yr period under study would be 1957 to 1997 and the statement by Johnson cited above very bluntly sums up the state of affairs at the very beginning of that period. However, considerable progress began to be made in the early 1960s and this is the information that was summarized at the time of the 1969 conference.

Quality as Defined from a Nutritional Perspective

From an animal nutrition perspective, forage quality may be defined as some product of digestibility and intake of the forage, with intake being the more important of the two components (Mott and Moore, 1969). This definition has stood the test of time such that many present-day research efforts still center around studies of these components.

What Constitutes Accomplishment and Impact?

As noted by Reid (1994), advances in research often occur in small and incremental steps, sometimes with obscure or unknown origins, but generally as variations on a preceding theme. For our purposes, an accomplishment in a given area will have led to a significant advance in understanding and (or) application. Impact, by definition, is a strong influence or a powerful effect. For our purposes, we will review research that not only has had impact at the animal producer level but also that which has changed the thinking of the scientific community, even if that information has not made it to the application stage as of yet.

State-of-the-Art Prior to the 1969 National Conference

As noted by Reid (1994), the major milestone in definition of the nutritional quality of forages for ruminants, namely, the significance of the intake factor, had been established well before the 1969 Conference by researchers such as Crampton, Blaxter, and J. T. Reid. The common forages fed to ruminants had been evaluated extensively for energy content, digestibility, and utilization but not for intake (Waldo, 1969). Lack of tabulated intake data was thought to be a result of the complex interactions among feed, animal, and the environment which cause great variation in the intake measurement and great difficulty in making a unifying concept of forage intake by ruminants. It is known that of the variation in digestible dry matter or digestible energy intake among animals and feeds, 60 to 90% is related to differences in intake whereas only 10 to 40% is related to differences in digestibility (Mertens, 1994a).

A great deal of research was done prior to 1969 on factors affecting forage intake. These are reviewed by Minson and Wilson (1994) and include (i) negative effect on forage intake of low levels of P, Co, N, Mg, and Se; (ii) particle size effects on voluntary intake of forages, with grinding and pelleting of forage having a positive effect on intake; (iii) the longer retention time of silage in the rumen when compared with its dried counterpart, caused by the difficulty animals have in regurgitating the partially digested silage which is probably swallowed before it is thoroughly chewed; (iv) the more often observed high correlation of voluntary intake with rate of digestion; (v) the positive correlation between voluntary intake and digestibility (today, it is recognized that intake and digestibility are related but independent); (vi) the low intake potential of forages with [is greater than] 800 g [kg.sup.-1] moisture caused by this forage being swallowed before it has been properly chewed and, hence, before maximal particle breakdown has occurred; (vii) the negative correlation between voluntary intake and the lignin content of both temperate and tropical grasses; and (viii) the negative correlation between voluntary intake of forage and forage fiber concentration.

Relatively little research was conducted on the influence of environment on forage quality except for the work of Reid et al. (1959) which clearly showed the marked effect of time of cutting, latitude, and altitude on the digestibility and intake of forage crops grown in temperate regions. The effect of climate in depressing the digestibility of tropical vs. temperate forages was reported by Minson and McLeod (1970). On average, temperate grasses had digestible dry matter coefficients approximately 13 percentage units higher than tropical grasses at similar stages of maturity, with digestibility decreasing by 1 percentage unit for each 1 [degree] C rise in growing temperature. (However, their temperate database was largely perennial ryegrass. Other temperate grasses are not that much more digestible than tropical grasses. Further, at a given digestibility, tropical grasses may have a higher intake than temperate ones.)

On the methodological front, Van Soest (1969) defined the general theory of the detergent fiber system of analysis and its potential application to forage evaluation. The two-stage in vitro method of Tilley and Terry (1963) was considered superior to other laboratory methods in the prediction of in vivo digestibility, but was difficult to standardize. Nylon bag digestion measurements were useful in examining rate phenomena but no technique adequately estimated rate of passage. Much of the theory and many of the methods currently used in determining digestive kinetics were in place at the time of the 1969 National Conference.

As regards early work in the pasture area, determination of the energy, protein, and phosphorus requirements of beef cattle and sheep on range was emphasized, as was measurement of range plant nutrients, and testing methods for increasing nutrient utilization to meet livestock needs (Richards, 1969).

Other significant effort in the forage quality area during the early years of this 40-yr period includes (i) the impact of N fertilization on forage yield and quality; (ii) quantification of effectors of forage quality such as date of harvest, species of forage, harvest method, and fertility; and (iii) comparisons of hay vs silage, grasses vs legumes, and legumes vs corn silage as to their nutritive value.

Selected Accomplishments, Impact of the Findings, and Recommendations for Future Research

Methodology

The milestone analytical event of the 1970s was the application of near infrared reflectance spectroscopy (NIRS) to forage evaluation (Norris et al., 1976). They showed that forages could be analyzed by NIRS for quality constituents, including digestion and intake measurements. Several contributions in this area include use of a scanning monochromator (Shenk and Hoover, 1976) which provided additional accuracy of NIRS in predicting forage quality, use of tilting filters (Barton and Burdick, 1978) which made it possible to analyze the major quality constituents in forages, and establishment of the USDA NIRS Forage Network (seven laboratories) and their significant efforts to coordinate research findings and to transfer the technology to the private sector (Shenk and Westerhaus, 1994). These latter authors summarized the progress in the first decade (1976-1986) to include improvement of software and instrument design, calibrations for new applications, and verification of accuracy and utility by forage extension specialists and instrument manufacturers. Progress in the second decade (1987-1997), however, included instrumentation and calibration techniques, standardization of instruments and monitoring their performance, and the understanding of the spectra. From a ruminant nutrition perspective, the application of NIRS has offered a rapid and low cost analysis of nutrient composition of forages. Therefore, beef or dairy producers could formulate diets that are more accurately able to meet the nutrient requirements of their animals. The future advancements in the application of NIRS were proposed (Shenk and Westerhaus, 1994) to include improving calibration techniques, lowering instrument cost without sacrificing accuracy or precision, and increasing the sophistication of the software without requiring more expertise from the user.

In recent years, other techniques have been applied to forage evaluation and appear to be powerful new tools for the examination of cell wall structures at the cellular level and for their differential breakdown by ruminal bacteria, protozoa, and fungi (Reid, 1994). These techniques include nuclear magnetic resonance (NMR) scanning and microspectrophotometry for cell wall analysis (Barton, 1985, 1991; Hartley et al., 1990), ultraviolet fluorescence microscopy for analysis of phenolics in cell walls (Akin et al., 1990), and light microscopy, scanning electron microscopy, and transmission electron microscopy analyses alone or in combination with immunocytochemical procedures (Grenet, 1991) that may increase our understanding of the plant/microbial interaction through the digestion process. "Summative equations" have been very useful in understanding and predicting digestibility. The Lucas Nutritive Entity concept was the basis of the Van Soest summative equation. This has been expanded by researchers H. R. Conrad and W. P. Weiss since 1969. Indeed, the Weiss equation probably will be adopted widely in the near future.

Intake

Probably no single factor related to forage quality has received more study than has forage intake. (Interestingly, most forage improvement programs give no attention to intake. Rather, considerable effort is being made to understand and model forage digestion. This effort is warranted because digestibility influences both the intake of metabolizable energy and the efficiency of metabolizable energy utilization. In tropical grasses, digestibility may be more variable than is intake.) Selected milestones related to this aspect of forage quality are outlined below.

1. Quantification of the depression in energy digestibility with increasing levels of intake (Moe and Tyrrell, 1975: Tyrrell and Moe, 1975). This finding had major impact as regards forage use by lactating dairy cattle in particular.

2. Recognition that physical distension of the reticulorumen is a major factor limiting intake of many forage and high fiber diets (Baile and Forbes, 1974). There is still much debate as to the importance of physical distension relative to other intake-limiting criteria.

3. Development of the NDF-Energy Intake System as reported by Mertens (1994a). The accepted static theories of intake regulation have practical utility in forage evaluation and ration formulation only when they can be related to specific feed and animal characteristics that are routinely measured. In addition, the theory can be tested only when the vague concepts of the feed's filling effect and available energy and the animal's intake constraint and energy requirements are quantitatively defined. Mertens (1994a) developed and refined the concept that NDF and net energy of lactation can serve as proxies for the filling effect and available energy in the accepted theories of intake regulation. The value of the NDF-Energy Intake System for predicting intake is that it not only provides a specific, quantitative system for estimating intake when formulating rations that maximize dry matter intake and forage use, but also it serves as a framework for quantitatively understanding intake regulation. However, it must be remembered that this system is applied to formulating diets for dairy cows, not to forages fed alone or supplemented for other classes of ruminants. This system is derived from pioneering studies of Conrad et al. (1964) and Montgomery and Baumgardt (1965). However, those studies were based on a range of forage/concentrate ratios, not on forages fed alone. Thus, associative effects were involved in determining intake of digestible energy.

4. Quantification of those factors that have usefulness in prediction of the voluntary intake of forages (e.g., leaf proportion, bulk density, grinding energy, resistance to chewing, near-infrared reflectance spectroscopy, in vitro and in situ digestion techniques, a range of chemical analyses, and models including all animal, plant, and management factors known to control voluntary intake).

5. Refinement of management practices that increase voluntary intake of forages (e.g., encouraging use of superior forage species; leaf/stem ratio; supplementing protein-deficient forages with either protein sources, non-protein nitrogen, or sulfur; recognizing that energy supplements depress the voluntary intake of forages but increase total intake, especially of mature forages).

6. Establishment of techniques where intake of forage by grazing ruminants can be determined for short or long periods of time (e.g., change in body weight, grazing behavior, cutting methods, fecal techniques, and regressions that predict intake from liveweight and production).

7. Recognition of the importance of plant anatomical factors on forage intake (e.g., ratio of thin- to thick-walled cell types; degree of lignification; variation in structural organization of organs and tissues that influence the size of fiber structures and their resistance to breakdown to particles sufficiently small to pass rapidly from the rumen). As regards areas of future research on this topic, prediction of feed intake under a variety of management situations is the ideal to strive for and any research focus on this topic would be appropriate and ultimately beneficial to the livestock producer.

Optimization of Microbial Growth and Efficiency

Microbial efficiency is defined as the proportion of substrate energy fixed into cells and is, therefore, inversely related to production of fermentation end products (e.g., volatile fatty acids, lactic acid, and gases). Extensive research using the newer techniques (ruminally and duodenally cannulated animals and continuous cultures of ruminal contents) and analytical methods (e.g., microbial markers) has indicated that microbial efficiency varies with the quality and quantity of forages and depends on the mix of microbial species in the rumen (Nolan, 1993). This microbial mix is influenced by specific components of the forage and ruminal conditions (e.g., partial pressure of [H.sub.2], pH, and turnover time). Preservation of the forage appears to influence microbial protein synthesis in the rumen. Siddons et al. (1985) reported higher efficiency of microbial protein synthesis (26 vs. 21 g of N/kg of organic matter apparently digested in the rumen) for hay than for silage from the same forage. Properly supplemented forage-based diets have the potential to increase microbial yields and improve efficiency (Rooke et al., 1987). Progress in this area will be dependent upon improvement in techniques to estimate microbial protein synthesis in the rumen.

Environmental Factors Affecting Forage Utilization by Ruminants

As noted by Buxton and Fales (1994), "no single factor impacts forage quality more than plant maturity, but plant environment modifies the impact of plant maturity". Forage growth rate, developmental rate, yield, and herbage quality are factors affected by environment. Major accomplishments in this area include definition of the effects of temperature on forage quality, with decreases in in vitro digestibility of both leaf and stem tissue occurring with a temperature increase. Fales (1986) found that high temperature had no effect on the rate of digestion of potentially digestible fiber, but reduced total fiber digestibility mainly by increasing the amount of indigestible fiber. Ford et al. (1979) found a strong negative correlation between digestibility and lignin concentration for warm-season forages but not for cool-season forages. Systematic identification of the effects of environment, to include plant stressors, on forage quality is also important (Table 1). These stressors include water deficit, insects, shading, and plant nutrient deficiencies. Information on this topic has had impact in both the scientific community and at the producer level. However, more information is needed about interactions among environmental factors and how this impacts overall forage quality.

Table 1. The average effect of a unit increase in several environmental factors on leafiness and digestibility of grasses (Deinum and Dirven, 1974).
                          Stage of
Factor                    development       Leafiness

Temperature (C)           Vegetative             -2.0
                          Reproductive           -0.7
Light supply (MJ
 [m.sup.-2] [d.sup.-1])                           0
Light duration
 (h [d.sup.-1])
N fertilization
 (100 kg N [ha.sup.-1])                         +20.0
Other nutrients
Moisture supply
Age of herbage (d)        Vegetative             -2.0
                          Reproductive
                           [C.sub.3] spp.        -9.0
                          Reproductive
                           [C.sub.4] spp.        -8.0

                              Digestibility
Factor                     Leaf    Stem    Total

                              g [kg.sup.-1] DM
Temperature (C)            -6.5    -10.0     -8.5
                           -0.8    -15.0    -13.0
Light supply (MJ
 [m.sup.-2] [d.sup.-1])    +0.5     +0.5     +0.6
Light duration
 (h [d.sup.-1])             No direct effect found
N fertilization
 (100 kg N [ha.sup.-1])    none    none      none
Other nutrients             generally no effect
Moisture supply            generally small increases
Age of herbage (d)         -1.4     -1.8     -1.5

                           -2.5     -4.5     -4.0

                           -4.0     -5.0     -5.0


Physical Structure of Forages in Relation to Their Subsequent Utilization

Van Soest (1982) emphasized the importance of physical properties (i.e., nature of cell wall structure) of forages in determining their susceptibility to microbial breakdown and rate of passage through the digestive tract. The significance of such' physical properties in plants in relation to their digestive utilization also was emphasized by development of scanning and transmission electron microscopic techniques (Akin, 1979). Perhaps the greatest variable in feed utilization by ruminants is the plant cell wall. Cell wall width and rigidity in connecting cells and tissues and other physical characteristics of cell walls influence digestibility and physical disruption by ruminants. For example, cool season grasses are more digestible than warm season grasses because the proportions and arrangement of tissues differ as a result of differences in photosynthetic pathways and optimal growing temperatures (Akin, 1989). Continued investigations with techniques to study the chemistry and structure of various layers within specific plant cell walls should provide useful information on degradation of the cells and tissues.

Synchronization of Available Energy and N in Forages with Use of Supplements

The economic impact of protein and (or) energy supplementation of forage-based diets has been recognized during the last 40 yr. Such supplementation improved animal productivity by enhancing forage intake and (or) digestibility. The improved performance was a result of the tremendous research efforts that were summarized by Paterson et al. (1994). This research was conducted under different grazing systems, with different types of forages, and during different stages of the production cycle. Accomplishments in this area include explanation of how the level and source of protein and (or) energy supplements influence forage intake (Paterson et al., 1994), comparative evaluation of supplemental non-protein N vs true protein and their roles in enhancing utilization of grazed or harvested forages (Petersen, 1987), and recognition of the influence of the source and form of energy supplement on forage intake. For example, replacing starch with readily fermentable fiber sources (e.g., soybean [Glycine max (L.) Merr.] hulls (Martin and Hibberd, 1990) or wheat (Triticum aestivum L.) middlings (Sunvold et al., 1991) minimized the reductions in forage intake that were associated with feeding starch. Despite the body of information available in the supplementation area, there are still many challenges. The strategies that include meeting the nutrient requirements of the ruminal bacteria in general and the cellulolytic ones in particular when selecting the supplement need to be elucidated. This can be accomplished by focusing future research on in vivo measurement of bacterial protein synthesis and fiber digestion in the rumen using duodenally-cannulated sheep or cattle under different grazing systems and different supplementation strategies.

Supplements enhance performance by increasing total energy intake, but the increased performance may be greater than or less than expected depending upon the effects on forage intake and digestibility. More attention should be given to these "associative effects" in practice.

Digestive Kinetics and Its Application to Forage Evaluation

The digestion of forages and other dietary ingredients is a complex process involving dynamic interactions among dietary, microbial, and animal factors. Therefore, separating the process into distinct components provides a conceptual framework that not only clarifies our understanding of the process, but also provides mathematical description (i.e., models; Mertens, 1993). Such mathematical models can provide information that helps in discovering the factors limiting the digestion process. We refer the reader to the most recent reviews of mathematical models on digestive and (or) passage kinetics of forages (Faichney, 1993; Mertens, 1993; Murphy and Kennedy, 1993; Nolan, 1993; Ellis et al., 1994; Illius and Allen, 1994).

From the application of the concept of ruminal protein degradability to feed evaluation arose the need for laboratory methods (e.g., in situ incubations) to estimate solubility and (or) degradability of protein in forages and other dietary ingredients (Orskov and McDonald, 1979). In their exponential model [degradation = a + b (1 - [e.sup.-ct])], they were able to predict the ruminal degradation of forage protein by measuring the soluble fraction (a) and insoluble but degradable fraction (b) and calculating the rate of degradation of b. This accomplishment has significant impact on N evaluation in forages. With the emphasis on the ruminally undegraded feed protein contribution to dairy (NRC, 1989) or beef (NRC, 1996) diets, standardization of the in situ technique and other laboratory (solubility and enzymatic degradation) methods (Aufrere et al., 1991; Broderick, 1994) will continue to be a major challenge.

With regard to the fiber fraction of forages, the incorporation of a lag phase (L; no disappearance occurs) in the model [digestion = ([e.sup.-kpL]) (a) (kd)/(kd + kp)] by Mertens (1977) was a major contribution. In this model, a is the potentially digestible fiber fraction, kd is the first-order rate constant for digestion, and kp is the first-order rate constant for passage. This model has been modified twice (Mertens and Ely, 1979; Allen and Mertens, 1988). Mertens (1987) concluded that fiber digestibility in forages is always determined by the fraction of the forage that is potentially digestible and the first-order rate constants for digestion and passage. Although, passage rates of digesta can be reasonably measured (France et al., 1991), the concerns related to markers (i.e., lag and mixing times and marker migration) raised by Owens and Goetsch (1986) still need to be addressed. Understanding the physiology of passage will remain a major scientific challenge (Ulyatt et al., 1986).

Forage Conservation and Treatment Procedures That Impact Ruminant Animal Performance

The major accomplishments on the conservation front during the past 40 yr occurred at the processing technology level (i.e., development of hay and silage packaging equipment) and in the testing of methods to prevent nutrient loss and(or) enhance quality (Reid, 1994). In addition, research efforts summarized by McDonald (1982), Minson (1990), and Reid (1994) have increased our understanding of the significant effects of the conservation process on the amount and the degradation characteristics of nitrogenous compounds in forages. With regard to hays and artificially dried forages, N solubility was found to decrease with increasing time and (or) temperature of drying (Manson et al., 1989). Ammoniation of heated forages also decreased N solubility by increasing the acid detergent insoluble N (ADIN) fraction (Perdok and Leng, 1987). The insoluble N fraction in dried hays or artificially dried forages may benefit the ruminant animal if it is digestible in the small intestine. However, ADIN is considered a waste of N because it leaves the digestive tract intact.

The major accomplishments in hay making in the 1980s and the 1990s include the development of equipment (i.e., mechanical conditioning and mechanized bale handling systems (Jarrige et al., 1982), the use of chemical conditioning (e.g., potassium carbonate and formic acid) for dry forages (Harris and Tulberg, 1980; Jones, 1991), and the use of preservatives (e.g., propionic acid, ammonia, and urea) with forages of high moisture content at the time of baling to inhibit microbial activity, reduce heating, and improve quality (Reid, 1994).

With regard to silage, the advances in its making and feeding include development of the flail, double-chop, and precision-chop harvesters, development of additive applicators, improved polythene coverings, and improved methods of mechanical cutting and feeding of silage (Jarrige et al., 1982). More recently, new methods of bagging and wrapping the silage have been introduced and demonstrated ability to decrease spoilage (Fenlon et al., 1989) and increase non-protein N content of the silage (Nicholson et al., 1991). Silage additives (e.g., bacterial inoculants, enzymes, acids, nutrient sources) have a significant role in enhancing quality. The benefits of these additives include stimulation of lactic acid fermentation, inhibition of microbial growth (e.g., mineral acids, formic acid, formaldehyde, and [SO.sub.2]), inhibition of aerobic fermentation (e.g., [NH.sub.3] and propionic acid), and provision of nutrients (e.g., nitrogenous compounds and minerals). Wilting of forages before ensiling has been shown to reduce protein degradation in the silo through inhibition of unfavorable bacterial species (Thomas and Thomas, 1985). The nutritional impact of such conservation of silage protein is improved feed intake and, as a result, improved performance (Waldo, 1985). The effects of the ensiling process on both the amount and form of N in forages were previously summarized (McDonald, 1981; Woolford, 1984; Thomas and Thomas, 1985). The work of Tamminga et al. (1991) showed that the method of conservation influences the ruminally undegraded protein fraction of the forage. They reported lower ruminally undegraded CP in silage than in hay from the same forage. Their in situ data also showed that the ruminally undegraded fraction of silage protein was influenced positively by the dry matter content and the date of harvesting and negatively by the crude protein content of the forage. Therefore, increasing dry matter content of the forage before ensiling and (or) using additives may enhance the undegraded protein fraction in silage and improve N retention in growing steers (Waldo, 1985).

Grazing Systems

In the USA, there has been an increasing interest in pasture utilization research in the last 20 yr. This is due to changing economics, shifts in consumer attitudes towards the quality of the food, potential environmental problems, and the development of highly sophisticated electronic and remote sensing equipment for the measurement of animal behavior and plant resources. Reid (1994) summarized the accomplishments in this area to include the development of improved pasture systems for the production of forage-fed beef in the southern states, the expansion of intensified grazing methods for dairy and beef cattle and sheep in the northeast and north-central states, and the success of the two Grazing Livestock Nutrition Conferences in the western states (Judkins et al., 1987; McCollum and Judkins, 1991). With regard to techniques, the accomplishments include the development of a dual-marker system [which uses the long chain N-alkanes for the estimation of forage intake (Mayes et al., 1986, 1988)] with its high accuracy and precision when applied (Vulich and Hanrahan, 1992), the use of controlled release [Cr.sub.2][O.sub.3] capsules for determining fecal output (Laby et al., 1984), and the advances in design of equipment for infusing markers and sampling digesta in grazing animals to allow for more accurate prediction of nutrient flow and microbial protein synthesis. The challenges in this area are still the lack of appropriate designs for grazing trials (Marten, 1989) and the confusion as regards grazing terminology (Allen, 1991).

Modeling

So much effort has occurred in this area in the last 20 yr that it is difficult to discuss the topic when brevity is required. Those models dealing with the forage-ruminant animal relationship that have had an impact on the thought processes of scientists include the model of Waldo et al. (1972) for estimation of ruminal cellulose digestion and passage. In this model, cellulose is divided into two fractions, an indigestible component that disappears from the rumen by passage and a potentially digestible fraction that disappears as a result of digestion and passage. From this model, one is able to derive both digestion coefficients and filling effects of the diet as functions of digestion and passage rates. Digestion and passage are competing processes for any potentially digestible molecule. Other models based on the Waldo concept (e.g., Mertens and Ely, 1979; Illius and Gordon, 1991) assume that digestion is a first-order process in which the digestion rate is an intrinsic Characteristic of the feed and varies among feed sources. Other models such as those of Baldwin et al. (1977), France et al. (1982), and Hyer et al. (1991) assume that digestion is a second-order process that depends on the concentration of both ruminal microbes and substrate. Mechanistic models exist regarding the effects of lignin. These include the incrustation hypothesis of Crampton (1939) and two surface models (Conrad et al., 1984; Fisher et al., 1989; Weiss et al., 1992). Others involve factors such as chemical linkages, cell wall structure, and microbial interactions (Jung and Deetz, 1994; Chesson, 1994; Pell and Schofield, 1994). Lag time and microbial attachment have been considered in other models (Allen and Mertens, 1988). Other models deal with the problem of rate (Fadel, 1984; Fisher et al., 1989).

In future years, it will be important to relate purely mathematical models to structural models that will be more useful if they can be formulated mathematically (Van Soest, 1994). Simulations of the various integrated models of digestion and passage offer insight as to the sensitivity of intake and digestibility by ruminants to changes in intrinsic and extrinsic factors affecting cell wall digestion (Mertens, 1994b). As noted by Van Soest (1994), "the future of modeling on the nutritive quality of plant cell walls needs integration and the sorting out of colliding ideas".

Conclusion

A great deal of progress has been made in the area of forage quality as related to ruminant animal nutrition in the past 40 yr. Many advances have helped the producer while others have helped scientists better understand particular processes, which ultimately might reach the application stage. Of concern in the future is lack of funding to conduct forage quality research. Few public or private organizations currently have this topic on their list of priorities. And without the financial support to fund forage quality research, advancements will most certainly be slow in coming.

Abbreviations: ADIN, acid detergent insoluble nitrogen; CP, crude protein; NDF, neutral detergent fiber; NIRS, near infrared reflectance spectroscopy; NMR, nuclear magnetic resonance.

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G. C. Fahey, Jr.(*) and H. S. Hussein

G.C. Fahey, Jr., Dep. of Animal Sciences, Univ. of Illinois, Urbana, IL 61801; H.S. Hussein, School of Veterinary Medicine, Univ. of Nevada, Reno, NV 89557. Received 18 March 1998. (*)Corresponding author (g-fahey@uiuc.edu).
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