Mycotoxins can be present on crops despite negative analytical results for two main reasons. Firstly, they are not homogeneously distributed, but tend to accumulate in certain areas known as “hot-spots” (Figure 1) and therefore might stay undetected. Secondly, masked mycotoxins could occur in feed and raw materials.
Proper sampling and sample preparation are the foundations of quality mycotoxin testing. The main problem of assessing the presence of mycotoxins in feed and raw materials is their uneven distribution in the commodity, especially in whole kernels. Different parts of the lot may contain different concentrations of mycotoxins (Figure 1). Effective sampling leads to a representative sample of feed commodities. Sampling is usually the largest source of error in the mycotoxin analysis and can result in up to 76% of the total uncertainty.
Every individual item in the lot should have an equal chance of being selected: this method is called random sampling. It is essential to select equipment that is adequate for sampling. For example, probes should be able to sample large particles and reach every location in the lot. If the lot has been blended thoroughly during handling then it is assumed that all particles are distributed uniformly and representative samples may be collected. However, when particles are not distributed uniformly, the aggregate sample should be an accumulation of several small incremental samples taken from many different locations throughout the lot (Figure 2).
Depending on the type of product that will be sampled, a minimum number of incremental samples should be collected in order to obtain a representative sample for analysis. Figure 2 illustrates the basic steps for a successful sampling procedure. Minor differences exist for solid feeds, when sampled during transfer or during storage, and for roughages.
When sampling during transfer collect incremental samples of products (100 g) at periodic intervals while product is being transferred from one place to another (Figure 3).
When transferring with a loading bucket, collect samples of grain in each bucket being loaded (Figure 4).
For roughages, the most practical way to take a sample is by hand. If possible, collect samples while the material is in motion. Be sure that all layers are represented equally. For sampling after ensiling, wait until the fermentation is complete.
In the case of fresh roughages, leaves and stems are usually distributed at the edges of the truck. Collect incremental samples by hand as the truck is unloading.
In the case of bag and bunker silos, collect incremental samples by puncturing the plastic cover using a sharp coneshaped sampling device. Holes should be evenly distributed across the entire surface of the silo (Figure 5). Carefully refill each hole immediately after the sample has been taken and cover using a strong tape to prevent possible contamination.