Most of the time, all the soldiers in the trenches had to eat was:

The bulk of their diet in the trenches was bully beef (caned corned beef), bread and biscuits. By the winter of 1916 flour was in such short supply that bread was being made with dried ground turnips.

Germany and Austria-Hungary - with food supplies hit by the Allied naval blockade - made immense efforts to keep their soldiers fed; even if this increased the hunger being felt by their citizens at home. But starvation eventually played a key role in the collapse of the latter's army in 1918.