Which of the following is NOT a component of volume in breeding heifers?

The shape of rib is indeed a fundamental characteristic when evaluating the volume of breeding heifers. Volume refers to the overall capacity of the animal, which is often assessed through measurements that provide insight into its ability to carry weight and efficiently produce milk.

Chest width is critical as it allows for a greater lung capacity and overall body frame, which contributes to better health and productivity. Depth of body relates to the heifer's ability to carry and support a larger gut capacity, which is essential for digestion and nutrient absorption. Meanwhile, the roundness of the rump, while it can indicate certain qualities of conformation, does not directly contribute to the overall measurement of volume in the way that the other components do.

Thus, when assessing volume in breeding heifers, the roundness of the rump is not considered a key component, making it the correct response in this scenario.