additives may even be essential nutrients presently unrecognizedand unidentified. Molyb- 
denum is of current interest in poultry nutrition. There has been interest in the selenium 
toxicity problem for a number of years. Recently, selenium has become recognized as an 
essential element and certain areas of the country have been pointed to as producing crops 
so deficient in selenium as to affect livestock. In many cases specific substances are 
harmful in excess but are essential for animal health in lesser amounts. The hormone 
chemicals in many respects canbeclassedinthis category. It is not a safe rule, however, 
that if some is good, more is better. Besides a better understanding of basic functions, 
we still need to know the optimal and safe amounts to use of a lot of nutrients that go into 
animal diets. 
ei. 
rz. 
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