The present invention relates to codon-optimized xylanase coding sequences and the expression of xylanases in microbes and yeast. The invention further relates to using multiple copies of the xylanase expression construct for high levels of protein expression. The invention also relates to the use of xylanases as feed or food additives. The invention also relates to methods of expression of enzymes to increase thermotolerance by expressing them in organisms that glycosylate proteins compared to expression that the same enzyme without the glycosylation. Further, the invention relates to methods of preparing feed, enzyme feed additives, and methods of reducing the feed conversion ration or increasing weight gain of animals.