The Math Behind Light Crop Sets In Almonds

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The inclement weather during the 2010 almond bloom period has resulted in some very light Nonpareil crop sets. While areas in the far South and North regions of California have reported good sets, many orchards in the Northern San Joaquin Valley area are looking at a down year for Nonpareil. Though increases in kernel size will slightly offset this lower production, there are factors to be aware of in light crop years.

Most are familiar with the associated “higher reject” percentages that usually accompany a short crop. Let’s look at some of the mathematical reasons for this occurrence. Take a typical orchard with a 22-foot by 18-foot spacing with 110 trees per acre. In a year where the yield per acre is 3,000 pounds, each tree is producing about 27 pounds of meats. Using an average nut size of 28 per ounce, that would mean that each tree produced 12,096 nuts. As an example, if 100 navel orangeworm eggs hatched per tree and infested a nut, this would result in a grade sheet with 0.8% insect damage (not counting pickup machine and huller air-leg blowouts).

Take this same orchard with a 1,200 pound per acre yield. This would equate to 10.9 pounds of meats per tree. Since the kernel size will be larger, let’s use 22 per ounce. This means that each tree produced 3,837 nuts. If we take those same 100 navel orangeworm eggs hatching and infesting a nut, this would result in a grade sheet with 2.6% insect damage. This is more than a 3-fold difference in the percentage due to yield differences. These calculations are based on a percentage calculated from damaged kernels and not on a weight basis, however the results will be very similar.

What this example does illustrate is the need for highly scrutinized pest management in years with light crops. If you look at this from a weight based perspective, each addition single nut from the 1,200 pound orchard that a worm damages will cause a 21% greater change in the reject percentage when compared to the 3,000 pound orchard. This is because a size 22 nut weighs 21% more than a size 28 nut. So for every nut a worm damages in the light cropped orchard, the incremental change is 21% greater than in the heavy cropped block. This changes the previously calculated reject percentage of 2.6% from the 1,200 pound orchard to 3.1%. Thus we are looking at a difference of 0.4% versus 3.1% using the same number of damaged kernels per tree.

Mummy Issues

Another factor contributing to the potential reject problem is the high number of overwintering mummy nuts that remain in some orchards. Shaking was difficult in 2009, and some orchards were riddled with mummies going into the bloom period. If this is coupled with a block that is light set in 2010, these are the makings of a problematic reject situation. Also keep in mind that in light set years with large kernel sizes, the shell seal on soft-shelled varieties is generally poorer, allowing easier access for worms. This really reinforces the importance of winter mummy nut sanitation, a prompt harvest as soon as nuts are ready, and proper handling (shelling immediately or if stockpiling, fumigation).

Once nuts are on the ground, the same math comes into play with ants. Monitoring ant populations and treating accordingly will be very critical for those with light set soft-shelled varieties.

Wes Asai is a former University of California farm adviser and is currently owner of Wes Asai Pomology Consulting in Turlock, CA.

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