Factors affecting the Total Digestible Nutrients(TDN) and ME value of the feed


Factors affecting the Total Digestible Nutrients (TDN) value of feed

  • Percentage of dry matter : The more the water present in a feed, the less is the other nutrients and resulting in a lower total digestible nutrient value (TDN) .
  • Digestibility of dry matter : Unless the dry matter of a feed is digestible, it has no TDN value. eg. Mineral oil has a high gross energy value, but it cannot be digested and so has no digestible energy or TDN value.
  • Amount of mineral matter in the dry matter : The more mineral matter a feed contains, the lower will be the organic matter and its TDN value.
  • Digestibility of fat in the dry matter: The more digestible fat a feed contains, the greater will be the TDN value.



Weakness of the TDN system

  • It is based on proximate analysis of the feed,which does not partition the feed into well defined chemical constitutents. Almost all proximate principles are composed of more than one chemical compound.
  • The highly digestible nitrogen free extract (NFE) contain part of hemicellulose and lignin, while crude fibre residue contains all the original cellulose, variable proportions of the hemicellulose, and small and variable proportion of lignin. That is why the assumption about high digestibility of NFE and low digestibility of crude fibre is always not true and for some feeds crude fibre is as digestible as NFE.
  • The factor, 2.25 used in case of fat to equalise its high energy content with that of carbohydrate and protein is not always a constant.
  • It is also based on human and dog experimental data. The ether extract of various feeds differ in the true fat content.
  • It does not measure energy in energy units.
  • It attempts to measure what feed 'contains' rather than what they accomplish or produce.
  • It over estimates the energy value of forages in relation to concentrates.
  • The term TDN implies that digestion losses only are taken into account. But actually this is not the case. To put protein on an equivalent carbohydrate basis, as was done for fat, digestible protein should have been multiplied by a factor namely,1.3 (5.2/4=1.3). But this is not being done. That is how calculation of TDN took account of urine as well as digestion losses. Actually, as calculated, it is a measure similar to ME for those species having no gaseous losses. Thus TDN doe not mean what it implies.

Factors affecting the metabolizable energy value of feeds

  • Main factors that affect the metabolizable energy value of a feed are those which influences its digestibility.
  • ME value of a feed will obviously vary according to the species of animal to which it is given, or more specifically, to the type of digestion to which it is subjected. Fermentative digestion incurs losses of energy as methane.
  • A disadvantage of the intervention of microorganisms in digestion is an increase in the losses of energy in either urine (as the breakdown products of the nucleic acids of bacteria that have been digested and absorbed) or faeces (as microorganisms grown in the hindgut are not digested). In general, losses of energy in methane and in urine are greater for ruminants than for non-ruminants. So feeds such as concentrates, that are digested to the same extent in  ruminants and non-ruminants, will have higher  ME value for non-ruminants.
  • The ME value of a feed will vary according to whether the amino acids it supplies are retained by the animal for protein synthesis or are deaminated and their nitrogen excreted in the urine as urea/uric acid. For this reason, ME values are sometimes corrected to zero nitrogen balance.
  • Preparation of feed: For ruminats the grinding and pelleting of roughages leads to an increase in faecal losses of energy, but this may be partly offset by a reduction in methane production. But for poultry the grinding of cereals has no consistent effect on ME values.
  • Increase in the level of feeding of ruminants may cause an appreciable reduction in the digestibility of their feed and hence in its, ME value. For finely ground roughages and for mixed roughages and concentrate diets, ME value is reduced by increase in the level of feeding.
  • The end products of rumen fermentation influence the value of ME. High concentrate, low roughage rations increase propionic and butyric acids and lowers the acetic acid in the rumen contents; methane production is also reduced.
Last modified: Wednesday, 18 April 2012, 6:06 AM