Have you ever cracked open a pecan and gotten frustrated but the shell's inner packing material trapped in the grooves of the kernel (the dorsal grooves). It seems that some cultivars are more prone to this problem and the reason may be as simple as the shape of the nut's shell. I'm not talking about the length of the nut or how pointed the nut appears. When it comes to narrow dorsal grooves and trapped packing material, the important shape to observe is the shape of the shell in cross section. Not all nuts are perfectly round in diameter. In the photo below, I have arranged the nuts of three cultivars so you can see the how nut diameter can differ if measured 90 degrees from the shell suture (nut on left) or on the suture (nut on right). Below each pair of nuts I've listed the diameter ratio (diameter 90 degrees from suture/ diameter on the suture). A nut that is practically round in cross-section, like Kanza, has a diameter ratio close to 1. Cultivars that produce "flattened" nuts, like Greenriver, have a diameter > 1. Cultivars with a diameter ratio < 1, such as Chetopa, often appear narrow when viewed suture side up.
So, what does all this have to do with packing material stuck in the dorsal grooves? Its all about how kernels are oriented inside the shell. When you look at the shell's suture, underneath is a full kernel half. In other words, the inner wall partition between the kernel halves is oriented 90 degrees from the suture line. Lets look at the kernels of these same three cultivars (photo below).
Cultivars, such as Chetopa, that have a diameter ratio < 1 typically produce long narrow kernels. The dorsal grooves on these narrow kernels are not only narrow themselves but they tend to flare outward into the kernel. The result of this kernel geometry is frequently trapped packing material.
In contrast, round nuts or flattened nuts have broad kernels with wide dorsal grooves. These grooves also penetrate straight down into the kernel. The result of this geometry are kernels that fall free of all inner shell packing material.