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Beef Breeder MANUAL
3.2.3. Digestibility
Presence of a nutrient in a feedstuf is one consideration, but the availability of that nutrient to the animal is another.
Digestibility, the proportion of a dietary component available for animal metabolism, is estimated using various
techniques including animal studies and laboratory analyses. It may be expressed as the digestibility of the
feedstuf as a whole, or of one or more components of the feedstuf. In general, as digestibility declines so does intake.
Perhaps the most important single factor inluencing the digestibility of a feed is the level of lignin in the
feed. Lignin is a compound laid down by the plant to give strength to the plant ibres. Unfortunately, lignin is
indigestible, and as plants grow older, the level of lignin in their tissues increases. The longer the period from one
cutting or grazing of a forage to the next, the lower will be the digestibility of the plant leaves and stems. This in
turn leads to reduced forage intake, and poorer animal performance. The balance zone between maximising
leaf and minimising stem is shown in Fig. 3.3.
3.2.4. Fibre
Dietary ibre, or sometimes called roughage is the indigestible portion of plants. The most common
deinition of plant ibre comprises the three polysaccharides cellulose, hemicellulose and pectin, and
lignin. Some proteins and waxes are also included in the deinition of Plant Fibre.
From an analytical approach to ruminant nutrition, ibre is classiied into three categories;
• Crude Fibre - is a traditional measure of ibre content in feeds. Neutral detergent ibre NDF and acid
detergent ibre ADF are more useful measures of feeding value, and should be used to evaluate
forages and formulate rations • Neutral Detergent Fibre - Structural components of
the plant; speciically cell walls. NDF is a predictor of voluntary intake because it provides bulk or ill. In
general, low NDF values are desired because NDF increases as forages mature
Figure 3.3. Optimum time to cut or graze grass forage is between young leaf and early lowering stages. Source: FAO http: www.fao.orgdocrep005af298eaf298E09.htm.
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Cold Winter Climates
• Acid Detergent Fibre - The least digestible plant components, including cellulose and lignin. ADF
values are inversely related to digestibility, so forages with low ADF concentrations are usually higher in
energy.
Whilst dietary ibre has the ability to depress productivity, particularly plant material high in ADF, ibre is necessary
to enhance body function. Adequate ibre level of suitable particle length assures normal chewing activity,
saliva production and ruminal function.
Efective ibre is a measure of the feed’s ability to stimulate chewing and includes ibre content and texture
particle size. Efective ibre particles are between 10mm and 40mm long, and have a scratchy feel. Symptoms of
insuicient efective ibre include:
• Decreased rumination
• Decreased growth rates
• Reduced appetiteintake
• Low body condition score.
The proportion of stem relative to that of leaf increases with age and the onset of lowering in plants generally.
Further, the level of lignin in the stems of grasses may be double that of the level in the leaves. The pattern is similar
in legumes, but diferences are usually less pronounced. In work with the grasses, cocksfoot, perennial ryegrass,
timothy and tall fescue, and the legumes, lucerne and sainfoin, in vitro digestibility levels of leaf and stem are
uniformly high at early stages of growth, but decline as the plants mature.
3.2.5. Palatability of Feedstuffs
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