At this time it's not known when Robert joined the Fiftieth Tennessee but his name shows up several times as either the surgeon or assistant surgeon throughout the War. His name first appears in November, 1862 as being Dr. R.G. Rothrock, then of Nashville, Surgeon of the consolidated regiment and battalion.
During the battles of Chickamauga and Missionary Ridge (November 25, 1863) the Fiftieth had lost all its field officers, many of its company officers, and more than half it's men. The Fiftieth Tennessee, the First Tennessee Battalion and the Fourth Confederate Tennessee were then consolodated.
To fully understand the grief these boys went through, you'll have to read "Military Annals of Tennessee", Regimental Histories and Memorial Rolls" pages 558 - 566.
Below is a brief passage from the book:
"Ragged and weary and heart broken, when the men of the old Fiftieth fell into line for the last time, and stacked arms in the presence of the enemy on that dreary April morning, only 37 men along with Colonel, S.H. Colms; Lieutenant-colonel, George W. Pease; Adjutant, John W. Childress; Surgeon, J.B. Sugg; Assistant Surgeon, R.G. Rothrock and Chaplain, J.G. Bolton were left to answer at roll-call."