Expanded North American ethanol production has pushed up corn and wheat prices and increased the availability of by products, like corn and wheat dried distillers grains. University of Alberta feed industry research chair Dr. Ruurd Zijlstra says there are several concerns related to higher inclusion of these by products in rations to keep in mind.

High content of vomitoxin
One of them would be mycotoxin content particularly in parts of North America where we use grains that have a high content of vomitoxin. This vomitoxin would not be destroyed by the production of ethanol during the fermentation process.
A second concern would be, especially for corn dried distillers grain, is very high in fat and all of this fat ends up in the carcass and corn oil is actually a very soft fat so, if you have a very high inclusion rate of dried distillers grain, it actual results in carcasses of a less than desirable quality and the third one would simply be just as for any alternative feed stuff what is the impact on performance.
If you see reports on formulating diets on digestible energy and total amino acids that would be part of the reasons why you sometimes see a reduction in performance when you start feeding higher levels of byproducts, where as if you formulate your diets properly there would be less of a concern.

Dramatic variation
Zijlstra notes the quality of these by products can vary dramatically from processing plant to processing plant.
He recommends partnering with the by products producer to identify ways, whether through contracting or specific sourcing, to reduce the variability of these ingredients.