The big attack is supposedly occurring just after sunup, but most of the shadows are short, indicative of shooting the sequence mainly around the middle of the day.
In the opening scene, Peso fires three shots into a water barrel as a means of teaching Luis about firing a gun, with some of the water spilling onto the ground. But with water in the desert being such a precious commodity, Peso wouldn't waste the water of someone he liked just to prove a point.
When Gilbert stabs the Indian he'd just thrown to the ground, he obviously misses the stunt man's shirtless torso, hitting the dirt beside his ribs instead.