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Determination of the wash value

Washing losses or a wash value of a sample are considered to be the amount of water soluble substances readily utilizable by the animal. This technique was first applied to ruminant studies concerning the digestibility of roughages.  It now has been suggested to be directly related to the cell contents (100 - NDF) of a sample, which in turn, could be of application in non ruminant nutrition studies too.

The method herein described it that proposed by Ly and Preston (1997) which studies some detail of the analytical technique, not taken into consideration previously. This technique requires a balance, an oven and a comercial semiautomatic washing machine. On the other hand, it is necessary to use nylon bags of 50 x 50 mm. This bag are made of nylon filter cloth with a pore size of 45 to 55 micrometers, and are available from some catalogue list.

Instructions

  1/ Weight a dry and clean bag

  2/ Add to the bag, very carefully, 1 g f air - dried sample or 5 g of fresh sample. Weight again   this operation is made at least in triplicate of the same sample. The mouth of the bag must   be tightly close, such as to avoid losses during washing. This procedure can be afford by a   rubber band.

  3/ Wash the bag in a comercial, semiautomatic washing macbine of 15 min - cycle, during 90   min, discarding the water every two cycle. Use tap water at room temperature. The ratio of   water per bag is of 3 litters.

  4/ After washing, let the bag to drain and dry it in an oven to constant weight

  5/ Determine the wash value taking into account the original dry matter of the sample.

  6/ The sample is followed for determining the wash value of nitrogen in the sample. In this   opportunity, the N content in the original and in the washed sample should be used for

     calculations.

  Calculations

                                                  OS - DR

                       % WL (DM) =  -------------- x 100

                                                       OS

  Where OS is the weight in g of the original sample, in dry basis, and DR is the weight in g of the dry residue.

  If it is desire to determine the wash value of N in the sample, then the calculation will be as follows

                                                  NS - NR

                         % WL (N) =  -------------- x 100

                                                      NS

Where NS and NR in the nitrogen content determined in the original sample and in the residue obtain after washing. The nitrogen content is usually expressed in percent in dry basis, in both cases.

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