Here is some information from UGA Pecan Horticulturalist Dr. Lenny Wells on pollination considerations at planting:
Historically, pollination has been overlooked when planting pecan orchards. The increased production from having the right pollinators in the right place is not always something we notice unless we have something to compare. The effects of poor pollination are obvious in some years more than others when spring weather conditions disrupt pollination.
I often hear someone say, “There are plenty of seedlings scattered around near my orchard to pollinate my trees.” Just because other pecan trees are around does not mean they are pollinating the trees in a particular orchard. In order to get effective pollination, the female flowers in an orchard must be receptive to pollen from another tree at the time other trees are releasing pollen. Pecan trees will self pollinate; however, a large percentage of these nuts fail to develop, abort, drop off, or fill poorly.
Pollinators must also to be in the right place. Research has shown that pecan trees need a pollinator within 150 feet. This is why we recommend placing a pollinator at every 5th tree on every 5th row if you prefer planting a solid block of one cultivar. On the other hand, if you block multiple cultivars in an orchard, change cultivars about every 4 rows. Pecan trees usually release pollen for 5-6 day,s and female flowers may only be receptive for as little as 4 days. Just a few days of the wrong weather conditions can wreak havoc if we have just a couple of cultivars in the orchard. This is why we recommend planting at least 3 cultivars in an orchard when possible. The greater the diversity of cultivars, the more we spread the risk.
Click on the pollination chart here to see which cultivars serve as pollinators for each other.