Shackelford, 25, split the 2014 season with Class A Brevard and Double-A Huntsville. After he had a 0.87 ERA in 12 games at Brevard, he was 2-4 with a 4.86 ERA in 40 relief appearances.
Once a catcher for Marshall University, Shackelford was a 21st round Draft pick of Milwaukee in 2010 and was promptly converted to a pitcher.
In 27 games -- including 18 starts -- for Class A Wisconsin, Astin was 8-7 with a 4.96 ERA and four saves. Over 121 2/3 innings, he allowed 132 hits, 36 walks and 12 home runs while striking out 81 batters.
Astin, 22, was a third-round Draft pick in 2013 out of the University of Arkansas.
Neither Shackelford, nor Astin, were ranked among the Brewers' top 20 prospects by MLB.com.
Milwaukee traded to get late-inning bullpen help but has faded down the stretch. The Reds were seeking some payroll relief. Broxton is owed $9 million for the 2015 season and had a $9 million club option for 2016 with a $1 million buyout. A clause in his contract stipulated that his option switched to a mutual option once traded with an additional $1 million added to the buyout.