 After trucks became popular in the <unk> , the Omaha Stockyards grew <unk> . <unk> , <unk> and sheep were shipped cheaper by truck than by trains . In 1919 27 % of livestock at the Stockyards was shipped by truck ; by 1940 's it rose to over 75 % . In 1955 the Stockyards became the biggest livestock distribution center in the United States , and almost all of the cattle was shipped by truck . 
