Steve,
another great portrait. Wilhelm Hornung was promoted to Oberleutnant on 27 January 1910 and left active military service on 18 October 1910 and transferred to the 1st levy of the Landwehr Infantry, initially in Sangerhausen and shortly thereafter I Berlin. He died on 17 March 1929 as a Major der Landwehr a.D. That being said I am assuming your photo was taken a couple of years or so before 1911.
Regards
Glenn