cedar fence logs split- what do you fill the gap with?

My cedar logs have split as they dried. Some splits are two inches deep and .5 inch wide and extend for 7 ft. What is an economical way to seal them. I could fill with sand/sawdust and than seal over with something? They split on the side that faces the sun. I plan on rotating them so the good side faces out , hoping the good side does not also split when exposed to direct sun.
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