I'm not sure how you are adding your rods, but my suggestion in any event is to place an additional keyframe at the frame just before you are making your change. So, if at frame 5 you are going to make a rod appear, then make sure to put in a keyframe at 4 that completes your original scene. Then, your frame at 5 .... then one at 9 that shows both rods again, then at 10 put in your third rod .... and on like that.

Could be worse, could be raining.