Alternate Bearing Information

The tendency of fruit trees to bear fruit in two-year cycles, consisting of large crops followed by little or no crop, is termed alternate or biennial bearing. Alternate bearing occurs in almost all tree-fruit crops, but is especially severe in pecan. This severity is linked to three traits inherent to pecan:

1. the time of fruit maturity,
2. the nature of fruit growth, and
3. the chemical composition of the kernel (Sparks, 1974).

In contrast with many fruit crops, pecan fruit matures late in the growing season leaving little time prior to leaf fall for carbohydrate storage to support the next season's developing flowers and fruit. In addition, most of the dry matter of the nut accumulates at the end of the season. Finally, kernels contain about 70% lipids, which require considerable energy to produce. These traits, when combined with a heavy fruit-set, can leave trees depleted of carbohydrate reserves, especially in the roots, at the end of the season and contribute to poor fruit-set the following year.

The alternate bearing index (I) is a measure of a cultivar's tendency to produce alternating high and low yields. Ranges are from 0 to 1, with 0=no alternation and 1=complete alternation. I values in our cultivar tests have ranged from a low of 0.19 to a high of 0.90. Generally, the lower the value the better, because this means the cultivar tends to produce moderate yields yearly and quality and production is often superior in these cultivars. We have found that cultivars with I values above 0.65 are much less likely to be accepted by the industry. This is especially true for cultivars with large nuts because it is difficult to fill large nuts when the crop is excessive. However, if methods are used to reduce crop lode in years with an excessive crop, the usefulness of some cultivars with high I values may be increased.

Formula used to calculate I

I=1/(n-1) X {|(a₂– a₁)|/(a₂ + a₁) + |(a₃– a₂)|/(a₃ + a₂) … + |(a_n)– 
a(_n-1))|/(a(_n) + a(_n-1))}

where n = number of years, and a₁, a₂, …, a(_n-1), an = yield of corresponding years

Alternate bearing index of some popular cultivars.