Nutrient Density/Digestibility and Structure Value of a Feedstuff

Nutrient density (digestibility) and structure value of a food are both related with CF (cell- wall) content. The higher the cell-wall content, the lower the nutrient density and the higher the structure value of a food. Nutrient density of a food is defined as its energy content per kg DM. Digestibility of a food is closely related to nutrient density (and CF content). The digestibility values can be used as a guideline for the nutrient density .

The nutrient density values are usually 5-10% below digestibility values (D% or DC). The nutrient density is important. If it is too low (<50% digestibility in the DM) its use in feeding dairy is limited. Therefore, low quality/density feedstuffs (roughage) must be balanced with feedstuffs of high density (concentrates). A cow producing 10 kg of milk requires at least 65% digestibility in the DM. On the other hand, to assure good rumen functioning and to avoid that rumen mass may become too much compacted, the ration must contain sufficient "structure-materials" (fibre), indicated by structure value. Structure value is expressed on a scale from 0 - 1.2 (on DM base). Long, dry roughage have a high structure value (1 or more), while concentrates have little or no structure value (< 0,2). A practical recommendation is, that at least 1/3 of the total DM of a ration is "structure value". In Poland, roughage has generally a rather high structure value (1 or more). The general guideline is that at least 30% of the total DM of a ration should be roughage. To preserve the structure value of a roughage, it is necessary to have a chopping length of over 1 cm. Feedstuff Structure value % DM Straw 1.2 90 Good quality grass hay 1.0 85 Wilted grass silage 0.7-0.8 40 Maize silage (0.8-1.0 cm long) 0.6 25 Pasture grass 0.5-0.6 15 Concentrates 0.0 90 The digestibility of a food is most accurately defined as the DM proportion not excreted in the faeces, and therefore to be assumed absorbed by the animal. The digestibility of a food is commonly expressed as a coefficient or % DM. Example A cow consuming 9 kg of hay with a 90% DM content has a DMI of 8 kg. In the faeces 3 kg of DM is recovered (DMO). In formula: DC = DMI – DMO/DMI = 8 - 3/8 = 0.625 D% = DMI – DMO/DMI * 100% = 62.5% with DMI Dry Matter Intake (DM eaten) DMO Dry Matter Output (DM in faeces) 3.4.1 The Influence of Digestibility The Digestibility of a ration has an influence on heat-increment and DMI. 3.4.1.1 Influence of Digestibility on Heat-increment There is quite a variation of heat-increment between different feedstuffs, 40-80% from ME into NE. This difference depends for a big part on digestibility. Poor digestibility (poor quality roughage) leads to high heat-increment. An aspect especially to be considered for warmer climates/seasons, and feeding during hot parts of the day. For Poland, this will be exceptional, and only applicable during a hot summer. In this case, some of the consequences are: - to avoid heat-increment peaks by offering roughage ad lib - to offer at least roughage during the night - to consider aspects of housing (roof, ventilation) - to provide shade in daytime in the yards, especially above feeding areas and drinking troughs - to offer good quality roughage, which is essential for a high intake of food as to reach high production levels - to distribute concentrates evenly during feeding (minimum 3-4 times/day) 3.4.1.2 Influence of Digestibility on Dry Matter Intake (DMI) If a food is not digested easily, it will stay longer in the rumen. The rumen will remain rather full and the cow does not develop a physical feeling of hunger. Therefore the cow will eat less (lower DMI) and consequently produce less milk! Importance of a low digestibility food intake is indicated by following example: Example With a certain grass, with two different D%, it was found that: D% = 60%, and DMI = 80 gr DM per kg bodyweight D% = 40%, and DMI = 50 gr DM per kg bodyweight A decrease of 33% digestibility (from 60% to 40% = 20/60 * 100% = 33%). The intake of digestible DM decreased from 48 gr/kg bodyweight (0.60 * 80) to 20 gr/kg bodyweight (0.40 * 50): a decrease of almost 60% (28/48 * 100%)! This indicates a very important principle in cattle feeding. The higher the digestibility of a food, the higher the DMI. This results in a proportionally increase of total nutrient intake and, naturally, vice versa! Conclusion If a food is of a good quality, an animal will eat more. If quality is lower, than an animal will eat less with all consequences in performance. One should notice that a high digestibility of a food indicates a low CF contents and consequently a high nutrient contents. 3.4.2 Factors Affecting Digestibility 1. Various factors are affecting the level of digestibility: 2. Food composition Ration composition 3. Preparation / treatment of food Feed additives 4. Level of feeding Animal Factors 3.4.2.1 Food Composition The CF fraction (lignin) and ash fraction (silicium, soil) are important factors in the digestibility of a food. More mature roughage will have a lower digestibility. 3.4.2.2 Ration Composition In a ration, the total CP (or DCP) contents and available energy are important. There are associated effects (balance in quality) amongst different feedstuffs. These associated effects can be positive or negative. 3.4.2.3 Preparation / Treatment of Food  Milling, grinding, and crushing - Essential for cereal grains and pulses  Boiling - No real effect for ruminants  Chopping - No real effect on digestibility - Reduces selectivity and therefore requires better quality supplement - May reduce losses when chopped < 15-20 cm (long hay, stover, straw) - Below 1 cm loss of structure value consider labour and machinery input  Fine chopping/grinding of roughage - Nett effect not positive - 20% less fibre digested as food passes quicker through rumen - Loss of structure - Change of VFA's (less acetate) 3.4.2.4 Feed Additives As for treatment of straw for low production systems (extensive production), Urea is a possible additive. NPN as an additive is not advised in high yielding dairy production systems. 3.4.2.5 Level of Feeding A higher level of feeding may reduce the digestibility as food passes quicker through the rumen, but less degradation of protein in the rumen is possible. The nett effect is not clear. Reduction on digestibility due to increased passage rate (rumen turn-over rates) are greatest for the slowly digested components (cell-walls/fibre). The greatest reduction of D% with increased feedings level are found with ground and pelleted roughage and some fibrous by- products (straw, stover, chaff). Digestibility may be reduced by as much as 20%. 3.4.2.6 Animal Factors Differences occur between breeds and individuals. This last aspect offers some scope for selection (records!). 3.5 Minerals and Vitamins See for details Chapter 1. 3.6 Special Aspects Special aspects to be considered for feed evaluation are - Constant availability of quality volumes - Constancy/reliability of supply, transport, handling & storage requirements - Influence on milk production (cabbage and brewers grain usually have a positive effect) - Influence on milk quality (smell, taste, colour, quality of butterfat) - Certain toxic or other substances like aflatoxin in groundnut products, gossypol in cottonseed cake, goitrogenic substances in Brassica family, oestrogenic substances and mimosine in legumes - Possible contamination or adulteration (soil, sand, dirt, chaff, sawdust etc.) - Palatability, usually closely related to digestibility - Factors affecting digestibility like tannin 3.7 Physical Judgement of the Feedstuff If possible, feedstuffs could be physically examined to assist in the evaluation of quality. Judge the overall quality in relation to the average product, using all senses (feel, look, taste, smell) and your experience. One should have the same approach to all other kind of products (vegetables, molasses etc.). 3.7.1 Roughage -Estimate maturity, indicating fibre contents and digestibility (coarseness) -Look at the ratio leaf : stem -Determine species and possible varieties, as well as length Hay colour: - yellow/green ------- good - yellow/green -------- medium - brown/black ----------- poor Mould, dust, smell to be checked Presence of weeds, thorns etc. Silage - smell (butyric acid) - wetness, structure - colour - soil contamination