The sample used for analysis should be representative of the whole batch of material, therefore the most appropriate method for taking samples should be used. For good results, the following procedure is recommended:
In bulk lots of under 10 tons, take two samples per ton.
In bulk lots of over 10 tons, take one sample per ton.
For packaged materials, for 1 to 10 sacks take a sample from each one; for more than 10 bags, sample 10% of the total at random.
Take the samples from different places so that the material is representative of the total lot; mix thoroughly and divide into batches weighing 1–2 kg. Place in airtight containers and store as appropriate until required for analysis.
A record should be kept of each material identifying the type of process it has undergone before (industrial by-products), origin (vegetable, animal, mineral, medicaments) and the part used as feed (especially if it has been subjected to a process that prevents its identification). This data is particularly important when agricultural products are being used, because composition can vary, depending on the variety, growing conditions, or time of harvesting. They may also contain pesticide residue or be contaminated by moulds or, in the case of animal by-products, by antibiotics and hormones (Frazer, 1967; Harris, 1980).
For material to be used in laboratory analysis it must be prepared in a proper way so that the results obtained will represent the total and can be used confidently for either formulating the feed or evaluating it. Therefore, the following recommendations are made:
The quantity of material should be sufficient for all analyses necessary; it should be homogeneous and representative.
The sample must be handled carefully to avoid any change or contamination.
The sample should be ground finely, sifted and thoroughly mixed. This should all be done quickly, with minimum exposure to the atmosphere. Overheating during grinding must be avoided, therefore heat-sensitive materials should be ground by hand. Make sure the grinder is perfectly clean before using it.
If the sample has a high moisture content and cannot be prepared without changing it significantly, determine the moisture content both before and after preparation.
Macroscopic and microscopic physical examination is recommended to detect the presence of contaminants.
Mix the sample well and divide into two equal parts; if necessary carry out a preliminary grinding to facilitate this stage. Store one part in a clean, dry, airtight bottle; the other part will be used in the analyses and should be sufficient for all the tests required.
Unless the method of analysis indicates otherwise, materials should be ground at once and passed through a 1 mm2 screen; mix the sifted sample thoroughly and store in an airtight container. Before taking material for each analysis, mix again.
Unless there are instructions to the contrary, wet samples must be dried before grinding and sifting, following the instructions above.
Liquid and semi-liquid samples must be stored in sealed bottles and mixed well before being analyzed.
Materials should be stored under refrigeration or at temperatures that avoid alterations in their composition. Samples to be analyzed for vitamins or other light-sensitive substances should be stored in amber coloured glass containers.