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To swab a vagina for semen, you must first have a... Muscle is made of protein, as is the body part in question. The process of anarobic bacterial post-mortem putrafication is the breakdown of proteins and everything else by other bacteria, mold and enzymes. Bacteria and mold will degrade DNA, as it is made of sugars and amino acids; all stuff that bacteria can digest. After 5ish weeks in the Iowa summer heat, in the shade of the corn plants, how much soft tissue will be left to be swabed on the skeletonizing body, in a pool of soupy liquification? Sure, you can get the body's DNA from a tooth if necessary, but what are the chances of finding markers for that small amount of male DNA left with the body, in that mess. And I have not even discussed insect and animal activity. The chances of finding CR's DNA on the body, if present, are likely none to maybe. But the FBI forensic labs are the best, and if any one can do it, they can!