Pecan Crop Load Management

Charles J. Graham  |  11/2/2006 9:12:59 PM

Figure 1. Cross sections of pecan cultivars showing complete kernel expansion (Pawnee), three-quarters kernel expansion (Kiowa), and two-thirds kernel expansion (Sumner).

Most pecan growers managing trees over 10 years old will agree that one of their biggest problems is inconsistent production. While all pecan cultivars demonstrate some variation in annual yields, growers managing high-yielding, large-fruited cultivars generally suffer through the widest swings in yields and nut quality. Excess fruit load causes several problems including poorly developed kernels of little or no value, an increased susceptibility to cold damage, an increased incidence of shuck disorders, and tree limb breakage from excess weight.

Why Do My Pecan Trees Have a Large Crop Followed by a Small Crop?

The tendency of pecan trees to follow a high production year with one or more years of relatively low production is generally referred to as alternate bearing or biennial bearing. Alternate bearing has been known as one of the major problems of pecan since the sixteenth century. Pecan tree traits associated with alternate bearing include late season fruit maturation (trees defoliate only weeks after the nuts mature), late season dry matter accumulation (over 2/3 of the nut dry matter accumulates in the weeks prior to shucksplit), and the high lipid content of pecan kernels (lipids require more energy to produce than soluble carbohydrates and starch). These traits combined with a heavy crop will deplete the tree of carbohydrate and nutrient reserves at the end of the season and contribute to low fruit set the following year.

The exact cause of alternate bearing continues to elude researchers, but is generally attributed to the high energy requirement to produce high quality pecan kernels. Flower initiation and development in pecan is controlled by an interaction of several factors including carbohydrate reserves and a balance of plant hormones. Flowers that produce next year’s crop are initiated during the time the current season’s crop is maturing. Therefore, stress during flower initiation affects next year’s crop. Several popular cultivars suffer from this type of over-production including Cape Fear, Creek, and Pawnee. Pecan cultivars vary in their tendency toward alternate bearing, but all demonstrate a cyclic increase and decrease in their production as they increase in age. Alternate bearing is a strongly expressed genetic trait of pecan trees, but a grower can minimize this cyclic behavior by implementing intensive management techniques including a pesticide spray program, herbicide strips, irrigation, and fertilization.

Why Do I Mechanically Thin My Pecan Crop?

Thinning an excessive crop load will improve fruit size and quality, and this practice is routinely performed in many fruit crops such as apple and peach. Fruit thinning of prolific pecan cultivars will relieve stress on the tree by equalizing the balance between leaves and nuts. Problems with over production are related to a decrease in the number of leaves per fruit, especially in heavy crop years. Young trees have a higher number of leaves per fruit than older trees and usually have a more consistent annual production.

The process of kernel deposition requires a large amount of readily available carbohydrates. During this process, each nut competes with other nuts for the carbohydrates manufactured by surrounding leaves or held in the wood of supporting branches. Removing a portion of the crop (nut thinning) before nuts begin to actively compete for a fixed amount of carbohydrates provides each remaining nut with access to a greater supply of carbohydrates. It also provides the tree with enough reserves to support a flower crop for the following season, thus reducing alternate bearing.

Mechanical thinning research has been conducted in numerous states with positive results reported on many pecan cultivars. Research at the LSU Agricultural Center has also demonstrated the benefits of fruit thinning in pecan. Thinning fruit from overloaded Kiowa and Sumner pecan trees increased nut size and percent kernel. Return bloom (flower production for next year’s crop) was increased in both cultivars. Fruit thinning decreases total yield per tree for the current year, but in some cases increases marketable yield.

When Do I Thin My Pecan Crop?

First, you must determine the cultivars in your orchard which have an excessive crop. The initial evaluation can generally be done by the third or fourth week in June, when most of the natural nut drop will be finished and the real crop potential can be seen. Nutlet abortion from low vigor, lack of pollination, self-pollination, casebearer damage, and drought has usually ceased by then, and the potential crop carried by the trees can be more accurately evaluated. Crop evaluation is accessed by observing 50-100 terminal growing points at random and developing a percentage of nut-bearing terminals for each cultivar that you have. A light crop is a percentage less than 30%; medium to good crop is 50-70%; and a heavy or overloaded crop is 75-100%.

Starting in mid-July, nuts should be periodically monitored for kernel and shell development so that the earliest time to thin the crop can be identified. After fertilization, the ovule (the tissue that becomes the kernel) begins to expand and lengthen from the tip of the nut toward the stem. As the ovule expands, the space is filled with fluid until the ovule extends to the stem end of the nut (water stage). The shell begins to harden from the tip shortly after the nut reaches full size. After expansion is complete, kernel deposition begins (dough stage). Research has shown that nuts should be removed when the ovule is 50% to 100% expanded (Figure 1), but before the kernel enters the dough stage. The calendar time varies with cultivar and orchard location. Thinning the nuts earlier, while they are too small, requires force that can damage the tree. Thinning too late, (after the nuts enter the dough stage) reduces thinning benefits on kernel quality, return bloom, and cold hardiness. A simple long-ways slice through the shuck from tip to base with a pocket knife will tell you how far toward the base of the shuck the water-filled "kernel sacks" have extended (wear gloves or you will have stained hands for a few weeks).

How Do I Thin My Crop?

Fruit thinning can be accomplished with a conventional tree shaker equipped with donut or cylinder pads. Donut pads are preferred, especially on Savage-style or three-point hitch shakers, because they a give better grip and result in less crushing or tearing of the trunk bark. It is important to have a firm grip on the trunk and it is generally beneficial to have someone on the ground helping position the clamps on the trunk when using that type of shaker. Mono-boom, self-powered shakers (OMC, Coe, Orchard-Rite) with cylinder pads filled with walnut hulls or glass beads do a good job holding the trunk; however, over-tightening of the clamp can cause compression damage to the trunk. The shaker manufacturer should be consulted about pressure settings, and pressure gauges should be checked for accuracy. Some growers will remove the packing and only have air in the cylinder pads to minimize compression damage. Always use silicon gel or grease lubrication between rubber flaps that cover the pads as an additional precaution. This allows movement between the flap and pad, preventing movement of the bark during shaking. Position the shaker as level as possible on the ground and hold the tree in the center of the shaker head for shaking.

Preventing bark damage is imperative in pecan nut thinning. Monitor the trunks for signs of shaker damage. Under conditions of abundant rainfall or if the trees are well irrigated, the cambium layer under the bark will slip more readily, making it easier to split or slip the bark off the tree trunk. If rainfall has been plentiful and trees appear to be flushing growth, it is advisable to shake a small number of trees and observe them for 1-2 days afterwards for signs of compression injury (splits and cracks in the bark in the area held by the pads).

How Much Do I Thin?

Proper management of crop load requires the grower to be thoroughly familiar with the orchard. Not all cultivars or trees within a cultivar will require nut thinning in a single year. But, as the percentage of fruiting shoots increases above the optimum nut load, the nut weight, return bloom, kernel percentage, and grade will decrease. Judging crop load is instrumental in making a determination about whether or not a crop needs thinning and is a practice which will improve with experience. Generally, a grower will underthin the crop more often than he will overthin a large crop. Optimum crop load varies with cultivar and may range from 50-60% for large nuts such as Kiowa or Cape Fear to 60-70% fruiting shoots for smaller nuts such as Elliott or Curtis. Trees with almost 100% of the shoots fruiting and a cluster size greater than three are overloaded and will definitely benefit from thinning. Trees should be shaken 2 to 3 seconds at a time, evaluated, and shaken again if needed. This process should be repeated 2 to 3 times when necessary until thinning is completed. The force required to remove nuts is not closely related to nut size. For example, relatively small fruited cultivars such as Curtis require about the same shaking force to remove nuts as some larger fruited cultivars such as Kiowa. It is advisable to have one person on the ground watching what is happening as the trees are shaken. This person can communicate to the shaker operator to stop or repeat the shaking process. It is important to shake briefly, stop, evaluate how many nuts are still in the tree, and then shake again if necessary. Growers should test nut thinning in a portion of their orchard and personally evaluate the results before using nut thinning as a regular practice.

If your counts show that most trees of a particular cultivar and age class are uniformly overloaded, then it is easy to determine which trees need to be shaken. However, if some trees are overloaded and some are not, then only the overloaded trees should be marked or flagged for shaking. Mechanical thinning can not only provide stabilization of year to year orchard production, but can also improve uniformity of tree-to-tree production.

Summary

Mechanical thinning of pecan crops has been practiced for over a decade, and both research trials and grower experience has confirmed that it is a sound management tool. Thinning the pecan nut load in heavy crop years generally increases nut quality as indicated by increased percentage of kernel, kernel grade, and nut weight. Nut thinning improves return bloom of some cultivars, thus reducing crop variation from year to year, and has been shown to reduce the tree’s susceptibility to cold injury. Growers are encouraged to test nut thinning in a portion of their orchards and personally evaluate the results.

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