you will rarely see (low level) mycotoxin damage... all feed has the potential in the right environmental conditions for the spores to become active and cause problems (immunesuppression, malabsorption etc )
http://www.fao.org/docrep/S4314E/s4314e08.htm
Good storage is essential because the value of the feed that you present to your animals depends on it. Feed spoils during storage -whether it deteriorates quickly or slowly depends partly on its quality when you receive it but very largely on how you store it on your farm.
Fungal growth itself encourages local rises in temperature and moisture content. Many fungi are killed during the processing of ingredients but their
spores are resistant and remain present to re-infect the material later if the environmental conditions become favourable for their development.
There are various types of chemical reactions which cause rancidity to develop. The result, as far as feed quality is concerned, is similar. Rancid fats reduce the palatability of the feed and contain toxic chemicals which may depress growth (Appendix XV). Chemicals may also be produced which reduce the availability of amino acids in the feed proteins. .........
Vitamin potency decreases during storage (and processing), particularly in premixes which also contain minerals. Naturally occuring vitamins in feed-stuffs also deteriorate on storage. Vitamin C is particularly susceptible, as is thiamine (vitamin B1)........
As shown in section 7.1., many problems can occur during feed storage. Some deterioration is inevitable. Thus ingredients should be stored for as short a period as possible and compounded feeds used quickly....."