Estimating crop load in edible chestnuts

As burs develop around Michigan, begin estimating crop load and planning for sales this fall.

Chestnut harvest in Michigan. Photo by Erin Lizotte, MSU Extension.

Chestnut harvest in Michigan. Photo by Erin Lizotte, MSU Extension.

Chestnut growers should accurately estimate their crop each year, as this is the start of the process of setting prices. Even though chestnut is a fall crop, market negotiations start as early as August, particularly in years with large crops. To estimate the yield in Michigan, consider tree age, type and weather events when we evaluate how large the yield might be. For example, if there was a frost in spring or a harsh winter, we would expect to see reduced yields. Droughts, excessive rain, pollination weather and excessive heat may also factor into the yield.

When considering tree types, we are primarily delineating based on whether the trees are seedlings or cultivars. Seedling trees are genetic individuals with high levels of variability, making the crop load of seedling trees much more difficult to estimate. Conversely, cultivars are genetically identical, making their performance more predictable and uniform.

That being said, each cultivar is different and requires we estimate them separately. For example, the cultivars Colossal and Labor Day would require separate crop load estimates and then be extrapolated proportionally based on their representation in a given orchard. For those of you with primarily Colossal and a couple other cultivars, the estimation is much simpler and should be more accurate. Experience and speed of crop load estimation improves quickly with practice.

Protocol

To estimate crop load, estimate the number of nuts per bur first and then estimate the number of burs per tree. These estimates should be performed for each cultivar and age group, as applicable.

To estimate the nuts, open five burs from five trees of each cultivar and age. Sample from all sides and reachable heights of the tree. Normally, there will be one or two nuts in a bur and sometimes three. Add up the total number of nuts you observed and divide it by the number of burs to estimate the number of nuts per bur.

For example, if you looked at five burs on five, eight-year old Colossal trees (25 burs total) and counted 45 nuts, you would divide 45 nuts by 25 burs = 1.8 nuts per bur on average. Perhaps your 12-year old Colossal trees had more nuts and averaged 1.9 nuts per bur.

To estimate burs, divide the tree into four quadrants (north, south, east and west) to help improve accuracy. Count the number of burs on the end of each branch within each quadrant on five trees. Again, trees of differing age should be measured separately. Add the number of burs on five trees in each age and cultivar class and divide by five to determine the average bur count per tree.

For example, if the five trees you evaluate contain 112, 126, 108, 100 and 145 burs, then the average burs per tree equals 591 burs divided by five trees = 118.2 burs per tree. Consider using a small, handheld click counter to assist you.

To extrapolate crop load from the nut and bur estimations, use the following formula:

(Nuts per bur x burs per tree x number of trees in class) ÷ nuts per pound* = crop load in pounds

*To complete this formula, you need to know how many nuts per pound to expect. Generally, Colossal can have about 20-30 nuts per pound, so for our purposes we could average that out to 25 nuts per pound. Nuts per pound is part of the estimation that has the potential to cause errors. If the nuts are smaller (30 nuts per pound), you will have fewer pounds; if larger (18 nuts per pound), you will have more pounds. If trees have been shaded in some areas of the orchard, there will be fewer burs, and if you have full sun, you might have more burs in some areas.

For the examples we have created above, let’s do the math:

(1.8 nuts per bur x 118 burs per tree x 50 trees) ÷ 25 nuts per pound = 424.8 pounds of nuts on the eight-year old Colossal trees in this example.

Again, this would have to be repeated for each cultivar and age class and then added together to get the farm total.

To get an accurate estimate on seedlings, estimate each tree separately. The pounds from a group of seedling trees may range from 0 to 60 pounds depending on the age, history and weather.

Timing

The earlier we can make an estimate, the better. However, it is often difficult to determine how many nuts are actually developing in the bur. In most cases, there will always be three nuts, but some only have fibers and some have a small amount of nut kernel or just a gelatinous embryo. The earlier burrs are opened, the harder it is to accurately count the number of nuts in a bur. Practice your estimate in mid-August and then recheck them for accuracy in early-mid September.

This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2015-09785. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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