Predicting feed quality—chemical analysis and in vitro evaluation
Introduction
All feed evaluation techniques attempt to identify the degree to which individual feedstuffs contribute to the nutritional requirements of the animal. The ideal situation would be for each feed to be offered to the appropriate class of animal and observe the production response obtained. Obviously, this option is neither practical nor cost effective but does provide the most accurate ranking of feedstuffs in terms of their “nutritive quality”. At the other extreme “wet chemistry” provides an exact description of the chemical composition of a feed. However, as no animal–feed interactions are considered, effects such as palatability, the impact of diet composition on digestibility or the extent to which anti-nutritive factors influence feed intake, cannot be determined. As a result a range of feed evaluation techniques have been developed at various hierarchical levels, between these two extremes, depending on the degree of animal–feed interaction. Each of these balances some degree of compromise with the practical situation against data generation. However, due to the influence of intake and utilisation over and above that of feed quality, the target animal remains the ultimate arbitrator of nutritional value.
While many feed evaluation studies with respect to health, reproduction and production (e.g. lactation, growth, draught power) are conducted at the applied level they are expensive in terms of the number of animals, quantity of feed, time, labour and facilities required. As a consequence they are generally undertaken to confirm results obtained from screening work conducted at lower hierarchical levels (Fig. 1), rather than as independent studies. On the next level down digestion, nutrient utilisation, calorimetric and intake estimations provide highly detailed information, however these tend, by necessity, to be conducted under highly controlled experimental, rather than practical conditions. Digestibility studies are traditionally conducted in sheep offered single feeds at maintenance. While this standardisation provides a basis of comparison, the results cannot readily be applied to a high producing dairy cow consuming a mixed ration at multiple maintenance levels. Minson (1981) indicated that offering feeds close to ad libitum would provide a closer approximation to the practical situation, although Blaxter et al. (1956) argued earlier that the possibility of refusals (selection) at near ad libitum levels would interfere with the steady-state conditions required for the study. While digestibility estimates provide a measure of the potential availability of the major nutrients, they omit energy losses from methane, urine and heat and offer no indication regarding either digestion dynamics, site of digestion or nutrient supply absorbed. It should also be noted that intake and digestibility are non-independent variables. Although intake is directly related to digestibility, digestive capacity of animals is inversely related to feeding level with the magnitude of this effect greater with concentrate feeds than poor quality, long or coarsely chopped forages. Further, where excess feed is available, intake is greatly influenced by selection with that consumed generally being of a higher quality that offered (Mayne and Thomas, 1986, Velasquez et al., 1999).
At the next level is the in sacco methodology (e.g. Ørskov et al., 1980) which estimates degradability simply by exploiting the existing rumen microbial activity. The only animal interaction considered is the estimate of outflow rate to model effective degradability. Similarly the mobile bag technique (Hvelplund, 1985) in which feeds, pre-incubated in the rumen, are inserted proximal to the duodenum and recovered in the faeces utilises bile salts and the endogenous enzyme activity of the lower intestinal tract to estimate digestibility of undegradable feed protein. At the second lowest hierarchical level are the in vitro methodologies. The majority of these simply examine rate and extent of feedstuff degradation either directly as substrate loss or indirectly as end-product generation, however, it is possible to model complete animal processes in vitro. For example the rumen simulation technique (Czerkawski and Breckenridge, 1977) permits long-term effects on rumen fermentative processes to be examined. As previously host animal input is minimal and essentially restricted to the microbial inoculum (rumen fluid or faecal material) used. At the lowest hierarchical level, no animal interactions occur, with feed composition values obtained via wet chemistry or NIRS calibrations.
Section snippets
Chemical analysis
In contrast to other estimates of feed quality, chemical analysis is the only one that provides absolute values (e.g. the exact quantity of nitrogen or ash present). All other methods merely provide a ranking in which one feed is identified as being superior to another. For example digestibility of a feed varies with the type and physiological state of the test animal, the level of intake and the combination of feeds that it is offered with. Neither does chemical analysis offer any estimate of
Near-infrared reflectance spectroscopy (NIRS)
Although this procedure will be discussed in its application will be briefly discussed. The method relies on the measurement of light absorption by a sample when scanned using wavelengths in the near-infrared region (1100–2500 nm). The resulting absorption spectrum depends on the chemical bonds within the components of the scanned sample and it is therefore possible to identify specific regions of the spectrum correlated with constituents such as starch, fibre or CP. Norris et al. (1976), using
In sacco technique
While the in situ technique procedure (also referred to as in sacco, nylon-, artificial- or Dacron-bag technique) is not an in vitro technique it is often discussed in parallel as it contains a number of similarities. The technique is not new (e.g. Quin et al., 1938, McAnally, 1942), however, it was Mehrez and Ørskov (1977) who first standardised the technique and Ørskov and McDonald (1979) and later McDonald (1981) who provided the interpretative mathematical models that allowed rumen protein
In vitro evaluation systems
While considerable advances, especially in the areas of fermentation end-product analysis and degradation dynamics, have been made to in vitro feed evaluation systems since the seminal (Tilley and Terry, 1963) paper was published, many researchers are of the opinion that little, if any useful or additional information can be obtained from the application of such techniques. The original Tilley and Terry methodology comprised two stages, broadly representing the rumen and lower digestive tract,
Alternative inoculum
While in vitro techniques are generally more rapid and require less substrate than in sacco studies, those based on a rumen fluid fermentation are still dependent on surgically modified animals to provide the microbial inoculum. This requirement introduces additional sources of error due to variation within animals (time effect) between animals, time of sampling relative to feeding, basal diet, etc. Alternative microbial inocula have been suggested, e.g. faeces (El Shear et al., 1987) or dried
Future prediction of feed quality
Chemical analysis of feedstuffs is, and will continue to be an indispensable part of feed evaluation, whether using traditional wet chemistry or calibrated NIRS methodologies. However, the lack of any animal interaction means that as studies at higher hierarchical levels are required, the use of in vitro feed evaluation systems as initial screens will continue to expand. To meet both this demand and to provide more accurate information, in vitro systems will become increasingly complex to
Conclusion
Those that utilise evaluation systems have to understand both the function and limitations of each methodology as well as being able to accurately interpret their findings in order to draw the appropriate conclusions. The application of say in vitro gas release alone to select for feed quality, the presentation of in sacco results as potential rather than effective degradability, or the unnecessary application of mathematical functions to model data which then reduces its inherent value all
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