Cucurbit Genetics Cooperative Report 5:18-20 (article 9) 1982
Todd C. Wehner
North Carolina State University, Raleigh, NC 27650
A selection index (SI) can be of great help in guiding the decisions of plant breeders by summarizing several variables into just one number. With the wide availability of computers for data analysis, it is now fairly simple to calculate an index for each entry in a trial (or progeny row in a population) and print them in rank order.
A number of selection indices have been developed and evaluated for efficiency (1), some requiring complex calculations, but it is easier (and perhaps just as useful for the breeder) to develop an index based on the importance the plant breeder feels each trait should have. The index should not be relied upon too heavily, but should be used as a guide along with the other traits in making decisions.
I have presented 3 indices that I use: one for progeny rows in a segregating population, and one each for pickling and for fresh-market cucumber variety trials. The index is constructed as follows: 1) list the traits to be included in the index; 2) write the approximate range expected for each trait; 3) scale the trait so it has a value from 1 to 10 with 10 being best and 1 being worst; 4) weight each trait according to its importance by multiplying it by a fraction of one (all weights should add up to one). The sum of all the weights multiplied by the scaled values is the index, which will be a number from 1 to 10.
The index for pickling cucumber variety trials (calculated from Table 1) is: SI = .00064Y + .00128E + .14SH + .05C + .08SC + .05PO + .0167PR + .14(10-D) + .01(10-A).
Table 1. Selection index for choosing the best lines in a pickling cucumber trial.
Trait |
Index abbreviation |
Approximate range |
Scaled 1-10 |
Weighted by importance |
| Yield ($/ha) | Y | 500-5000 | .002 x Y | .32 |
| Earliness z ($/ha) | E | 125-1250 | .008 x E | .16 |
| Shape y | SH | 1-9 | 1 x SH | .14 |
| Color y | C | 1-9 | 1 x C | .05 |
| Seedcell y | SC | 1-9 | 1 x SC | .08 |
| Potential x | PO | 1-9 | 1 x PO | .05 |
| Pressure test w | PR | 3-30 | .333 x PR | .05 |
| Disease v | D | 9-0 | 10 – D | .14 |
| Anthracnose | A | 9-0 | 10 – A | .01 |
| 1.00z |
Value for the first harvest out of 5.
y Quality scores are subjective (9=best, 5=average, 1=worst).
x Potential is a score given for the overall impression of the line.
w Pressure test in lbs. using Magness-Taylor tester with 5/16 diameter tip.
v Average score for all diseases rated (0=no disease, 1=trace, 9=plant dead).
The index for fresh market cucumber variety trials (calculated from Table 2) is: SI = .0028M + .013F + .0178E + .14SH + .05C + .08SC + .05PO + .14(10-D) + .01(10-A).
Table 2. Selection index for choosing the best lines in a fresh market cucumber trial.
Trait |
Index abbreviation |
Approximate range |
Scaled 1-10 |
Weighted by importance |
| Marketable yield (q/ha) | M | 70-700 | .014 x M | .20 |
| Fancy yield (q/ha) | F | 13-130 | .0769 x F | .17 |
| Earliness z ($/ha) | E | 9-90 | .111 x E | .16 |
| Shape y | SH | 1-9 | 1 x SH | .14 |
| Color y | C | 1-9 | 1 x C | .05 |
| Seedcell y | SC | 1-9 | 1 x SC | .08 |
| Potential y | PO | 1-9 | 1 x PO | .05 |
| Disease y | D | 9-0 | 10 – D | .14 |
| Anthracnose | A | 9-0 | 10 – A | .01 |
| 1.00 |
z Weight for the first harvest out of 5.
y Quality scores are subjective (9=best, 5=average, 1=worst).
x Potential is a score given for the overall impression of the line.
w Average score for all diseases rated (0=no disease, 1=trace, 9=plant dead).
The index for selecting progeny rows from a segregating population of pickling or fresh market cucumbers (calculated from Table 3) is: SI = .04Y + .18SH + .06C + .10SC + .06PO + .2(10-D).
Table 3. Selection index for selecting the best progeny rows in a population improvement program.
Trait |
Index abbreviation |
Approximate range |
Scaled 1-10 |
Weighted by importance |
| Yield z | Y | 10-100 | .1 x Y | .40 |
| Shape y | SH | 1-9 | 1 x SH | .18 |
| Color y | C | 1-9 | 1 x C | .06 |
| Seedcell y | SC | 1-9 | 1 x SC | .10 |
| Potential x | PO | 1-9 | 1 x PO | .06 |
| Disease w | D | 9-0 | 10 – D | .20 |
| 1.00 |
z Yield (no. fruit/3m row) can be measured by harvesting all plots at the earliest possible time to favor early maturing types.
y Quality scores are subjective (9=best, 5=average, 1=worst).
x Potential is a score given for the overall impression of the row.
w Average score for all diseases rated (0=no disease, 1=trace, 9=plant dead).
The value for disease score has to be reversed by subtracting it from 10 so that “no disease” (0) will be given the largest value possible (10). Some traits may be included more than once in the index. For example, anthracnose score is included by itself, and as part of the average disease score in the indexes for the variety trials. The selection index should be printed out in the data summary table along with all of the other traits evaluated. That will permit the breeder to check for any lines or progeny rows that are defective for one or more traits. The selection index alone will not be sufficient for decision-making because superior performance in several traits can hide a single defective trait.
Literature Cited
- Crosbie, T. M., J. J. Mock, and O. S. Smith. 1980. Comparison of gains predicted by several selection methods for cold tolerance traits of two maize populations. Crop Sci. 20:649-655.