By P.J. Huffstutter, Tom Polansek and Bianca Flowers CHICAGO (Reuters) – For nearly two decades, Abe Sandquist has used every marketing tool he can think of to sell the back end of a cow. Poop, after all, needs to go somewhere. The Midwestern entrepreneur has worked hard to woo farmers on its benefits for their crops. Now, facing a global shortage of commercial fertilizers made worse by Russia’s invasion of Ukraine, more U.S. growers are knocking on his door. Sandquist says they’re clamoring to get their hands on something Old MacDonald would swear by: old-fashioned animal manure. “I wish we h…