I was walking through the oldest cultivar trial on the research station when I noticed obvious cultivar differences in over-all tree size. In the photo above, you can see that Major produced a much larger tree than Dooley. Both cultivars were grafted onto Giles seedling rootstock planted in the field back in 1981. The only difference between these two trees is the scion cultivar.
Looking around this 35-year-old trial, I found that trunk diameter was largely a function of cultivar. To create a visual record of cultivar differences in trunk diameter, I photographed the trunks of 6 pecan cultivars placing a one-foot-long ruler across the truck for scale (photo above ). Major and Peruque grew trees so large they seem to dwarf all others in comparision. Colby and Posey grew moderate sized trees while trees grafted to Giles or Dooley seemed small in comparison.
I would never consider tree size a very important tree characteristic in making cultivar decisions. However, tree growth rate becomes important when planning initial orchard spacing and making subsequent thinning decisions. Large growing trees, like Major or Peruque, should be established at a wider spacing--let say, 40 feet rather than the standard 35 feet. Tree growth rate will also influence tree thinning decisions. The larger a tree grows, the sooner you will need to thin out adjacent trees to make room for future growth.
It takes at least 30 years of cultivar testing to make accurate tree size determinations. In that time, many cultivars will prove unworthy of future propagation. Of the six cultivars pictured above, I would consider grafting only Major because of its scab resistance and regular nut production. But now, when I graft new Major trees, I'll also be sure to allow more space for these trees to grow.