This video states that. And also, most interesting - it states that Alexander's the Great army was vegan.
So officially, would carry wheat and barley but they ate whatever they could find,
Armies back then were principally fed with wheat, the soldiers would have likely ate plain whole wheat bread loaves. They would supplement this with whatever they could forage, wild animals, fruits, vegetables etc. Answers.com
assessment that ancient soldiers required an ideal minimum of 3,000 calories per day to function properly in battle. Different breads, biscuits, and porridge provided the soldier with grains, which comprised for 2/3 of his caloric requirements. If meat was available, it took the form of pork, beef, or mutton and provided between 200-400 calories. Vegetables such as beans, lentils, herbs, and others, supplied the soldier with an adequate amount of vitamins while dairy products including milk and cheese offered 90-160 more calories. Finally, wine and beer, which were frequently the only source of liquids, supplied the remaining 350 calories Analysis of Ancient Soldier Stomach
So to answer your question, they ate whatever they could. If food was scarce they turned to wheat and barley but would forage for meat when they could. This idea that they were vegan/vegetarian by choice is false.
The concept that people got to choose what they ate is a relatively new concept especially when it involved Army's. Even during the civil war, the Confederacy had most of the cattle and pig farms and they still couldn't get meat to their soldiers.
Confederate soldiers were also faced with inadequate food rations, especially as the war progressed. There was plenty of meat in the Confederacy. The unsolvable problem was shipping it to the armies, especially when Lee's army in Virginia was at the end of a long, tenuous supply line. Union victory at Vicksburg in 1863 shut off supplies from Texas and the west.
-Vandiver, Frank E. (1944). "Texas and the Confederate Army's Meat Problem"
To feed an army with meat you need industrial farming, curing and canning techniques and transportation to bring the meat from home or recently conquered area to the front lines. Problems that weren't solved until modern times (WWI and WWII) Interestingly enough, look up why Spam is so popular in Hawaii.