I recommend using LaVern Manilla seedlings as rootstocks because they are easily found and relatively uniform in regards to their performance around SoCal. We also have the option of using random seedlings as well and the benefit here is that you save a lot of money but you risk getting a sloweror faster growing tree compared to Lavern Manilla. The huge trees in Leo Manuels yard is proof that random seedlings do perform well here but some varieties may be more or less prone to diseases.
Here are some pictures of some seedlings that were direct seeded into the ground. Unfortunately, I was not even considering the ramifications of grafting such young seedlings with mature scionwood. This Venus grafted onto Kent seedling is in full bloom and is wasting its energy on this senseless flowering.
Here is a CAC/COC grafted onto another Kent seedling but this variety is currently not blooming even though it is only several inches away from the Venus and gets the same fertilizer, water and experiences the same weather. There are definitely scion/rootstock interactions at work here and probably the stage and maturity of the scion( before it was removed) has some influence on it vegging out.
For those planting random mango seeds into the ground, do not graft the tree with mature scions until it is fully established and at the size where it can actually bear fruit. These trees that I grafted too early will be stunted with annual blooms until it reaches the size that I should have grafted them at. I just want others to learn from my mistakes.
The blooms on my Venus have been on the tree for several months now, mango blooms on young trees take many months to form and removing them early will only trigger another bloom as I explained above. Even though the CAC/COC is not blooming now, it will very likely bloom this Winter and every year there after.
Because the bloom cycle takes so long, young seedlings will likely only get one or two growth flushes and because it expended much energy on blooms, the vegetative flushes will not be as strong.
A seedling that was not grafted will simply stall and swell buds in Winter and have a strong vegetative flush as soon as the weather warms. A seedling that was not grafted will likely have 2-3 vegetative flushes or more if fertilized properly.
With each set of new leaves a seedling veges out, it will increase the total surface area for photosynthesis. This extra amount of surface area for gathering energy needs to be multiplied by each day it receives sunlight and hopefully you can see that soon there will be a drastic difference in growth rate between the grafted and nongrafted seedling.
In essence, the grafted seedling will be growing at a linear rate where as the non grafted seedling should grow at more of a logorithmic rate.