Upvote:2
The Confederates were better at keeping units up to full strength, both by replacements, and by "merging" units. (This was managed at a regimental, not company level.) At the battle of Gettysburg, for instance, there were seven Union Corps versus three Confederate Corps, but the ratio of Union to Confederate troops was 7 to 6, not 7 to 3, because the Union Corps were at half strength (or less), while the Confederate corps were close to full strength. Union troop shortfalls were legendary; the First Minnesota Regiment had an original strength of 1,000, went into Gettysburg with 264, and left the battle with 47, before it was finally merged with another unit. Josh Chamberlain's 20th Maine started with 1000, and went to Gettysburg with just over 200, before it was offered 120 deserters from other Maine regiments. (Chamberlain talked 117 of those men into joining his unit.)
One reason was that Confederate state governors were tasked with sending men to the central army, while Union state governors were tasked with sending regiments. So Confederate regiments could be "filled out" as long as state governments met their manpower "quotas." Union governors sent raw new regiments while old regiments went shorthanded. This changed only after 1863 when the Union instituted the draft and had a central "pool" of men. Then, they worked around the state quota problem by combining units from more than one state, and funneling draftees into these "mixed" units.
Upvote:8
The usual way of discussing troop replacement is by regiment, not company. This would be under control of the regimental officers.
Confederate regiments did get replacements periodically, although the system was not as formal as, say England's depot system where there was a permanent training force at home all the time. The adding of raw troops to experienced troops soon worked out the kinks in the raw troops. They would send people home to try and bring back recruits, even behind Union lines to occupied areas. They were trained with their own comrades at the front.
Even so, regiments that fell too far below the line in size were consolidated into other regiments to make one more to size. So at one point, you might have the 1st/27th Tenn. Consolidated. Companies were often consolidated earlier, so a small regiment might have 5 co. rather that 10, to cut down on the officers required.
The Union did not replace men in regiments until the draft, since most governors would rather appoint a new colonel in a regiment than beef up an old one. An exception was Wisconsin, which did replace like the Confederates and got a reputation as elite troops because of the improvement in size plus veteran skill over other state regiments.