They showed that in the movie. They would work those animals until they drop from exhaustion and then shoot em. Lot of mud and they would pull those heavy guns up hills with a team. Even tanks had to be pulled by team of horse out of the mud. Horrable war, just horrable. The amount of death was staggering, battles with thousands of men on both sides being killed in one day of battle.
Thus the term old maid coin after that war, many women in Germany and England from that generation never married because the shortage of young men. Many young women married much older men back then.
The machine guns were deadly but most of the carnage was from artillery fire, mortars. They used so much artillery that they could drop those shells in a pickle barrel. literally dead on ranging.