Pruning, trimming, and training pecan trees
Pruning your pecan tree(s) is a necessary and on-going chore that begins with the initial planting of your tree.  Pecan trees are 'lazy' trees and will grow into gigantic 'pecan bushes' if annual pruning is not performed.  The goals you seek to achieve in pruning are to encourage upward growth, lateral growth that will allow your tree full advantage to sunlight, and improved tree management.  You also want your trees to well...look nice?
Suckers (small branches) growing along the trunk) will rapidly grow into unmanageable limbs, while growth in the upper areas of the tree will stall.  It take less energy, water, and nutrients to promote growth lower on the trunk, thus if lower branches are allowed to remain, the pecan tree will be content just being an unsightly and unmanageable 'bush.'
You can't hardly 'mess up' when it comes to pecan tree pruning, unless you (a) don't prune at all, or (b) you prune too heavily.  Since you are reading this page, I assume you intend to prune so we'll move on to (b) pruning too heavily.  Unless your tree has reached near death and severe pruning is your only hope to revive it, you should never remove more than one third of its branches in a season.  If you follow a yearly pruning schedule, you should  never have to prune even that severely.
We bought our pecan orchard when the trees were five to six feet tall.  They had been neglected for several years and had branches growing on the lower trunk.  With the hope (and the notion that those lower branches produced pecans, too)  that we could turn a profit from our pecan crop, or at least break even, we were cautious with our pruning for the first several years.  We 'worked our way up' the tree for the first three years, removing the branches from the ground to a height of three feet the first year and removing another two feet of growth each of the following two years.  This prevented throwing the trees into shock and allowed top growth to offset the lower branch removal.  We were successful in producing a crop each year by following this method.
Pruning is in fact training your tree to grow the way you want it to grow.  Pecan trees must have ample sunlight to produce a crop.  With this in mind, you should prune to allow the upper branches to spread and to take in as much sunlight as possible.  Many pecan experts say a single vertical trunk is essential.  We pruned our trees to achieve this for the most part.  We pruned one tree close to our house into a 'vase' shape because we designated it the 'climbing tree.'  Approximately two feet from the ground, it splits into three 'trunks' forming the vase shape.  It is a beautiful and well proportioned tree, and our kids have always claimed it as their tree.  So, when it comes to training a young tree, you can decide whether to prune to make it become 'just a tree' or make it an eye catching part of your landscape.  Just be sure you start out with a plan and carry it out throughout the years.
While mature trees will require annual pruning to an extent,  your major pruning comes during the first ten to fifteen years of the pecan trees life.  Keep in mind during those years that you need access to the orchard floor beneath your tree.  You will spend considerable time underneath your tree, watering, fertilizing, controlling weed growth, and oh yeah...picking up pecans.  When you are able to prune your tree's lowest limbs to a height just above your head, your major pruning chores are almost finished.  There should be no need to prune the lower branches any higher than your head.  You might keep in mind that someday you may be bringing in the heavy equipment (harvesting equipment) and be sure the shape of your trees will accommodate these machines.  Other than that, your biggest pruning chores are complete.
Mature pecan trees will enevitably require annual attention.  Small branches will grow downward in search of sunlight, and limbs bearing a heavy pecan crop will droop and continue a downward growth.  The removal of these branches should be the only pruning you will need to perform on mature trees.  Again, your goal is to be able to walk beneath your tree without a branch scraping your face!
Occassionally an upper branch will die and need to be removed.  I suggest you purchase a telescoping tree saw/lopper to handle these problems.  The cause of these higher branches dying range from high wind damage, freeze damage, disease, and improper care of your tree.  I highly suggest you remove any dead limbs from your pecan tree as soon as possible to avoid the possibility of disease spreading throughout your tree.

When to prune your tree
It seems that there is a mis-conception that pecan trees should be pruned in the dead middle of winter.  I personally think this is one of the biggest mistakes you could make.  Pruning during the cold winter months leaves a wound that is exposed to the harsh winter elements.  Those who practice this method say there is less chance for insect damage, however, that gaping wound you inflicted on your tree is still present when the bugs do come out in early spring.  I recommend you do not prune until early spring, prior to bud break.  We actually plan our pruning for late April, after the leaves have sprouted.  This enables the tree to immediately begin it's healing process.  Pecan trees can be pruned at any time without major setbacks.  I discourage summer pruning unless it consists only of small branch removal. 
We try to coincide our prunings with the expected crop-set.  Pecan trees tend to produce heavily every other year, followed by an 'off year.'  We will do our heaviest pruning on our 'on ' years, and prune lightly on our 'off' years.  This aids in balancing out our crops and to an extent, prevents the tree from stressing from a heavy crop, which would lead to a light crop the next year.

Pruning Tools
If you practice annual pruning faithfully, a pair of hand held pruning shears should be all you ever need.  They fit in your back pocket and you can continually lop off those small low hanging branches as you tend to your orchard throughout the spring.  You may need a larger 'two handed' lopper if you allow a branch to grow out of control.  If you really neglect to prune regularly, you may find a bow saw handy, however, I warn you, pecan wood is an extremely hard wood and will require some muscle to saw a large limb.  Somewhere along the way, you will probably decide you need a chainsaw to handle those large limbs.  I do recommend you purchase a telescoping tree saw/lopper to reach the highest branches of your tree.
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Tools for pruning pecan trees