June 11, 2020

1. Introduction to the problem

Crop by region

CIMMYT EA Maize

Problem specification

Genotypes are currently evaluated in three stages of testcross yield trials before recycling to ensure high accuracy of selections.

Breeding strategy component tackled

Evaluation

Breeders’ equation terms tackled

L, r

Hypothesis

Reducing cycle time by recycling earlier will increase the rate genetic gain.

2. Materials and methods

Treatments

Treatment Description
Stage 3 Current scheme, recycling after three testcross stages. Crossing block consists of 48 lines, the 16 best lines from each of the last three stage 3 evaluations.
Stage 2 Recycling one year earlier. Crossing block consists of 48 lines, the 16 best lines from the last stage 2 evaluation and the 16 best lines from each of the last two stage 3 evaluations.
Stage 1 Recycling two years earlier. Crossing block consists of 48 lines, the 16 best lines from the last stage 1, stage 2 and stage 3 evaluations.

 

Simulation procedure

A 20 year burn-in period was modeled using the current DH breeding scheme. The burn-in was followed by a 40 year evaluation period to measure rates of genetic gain for all treatments. Genetic gain was measured by assessing changes in grain yield for hybrids evaluated in the on-farm testing stage and for hypothetical hybrids created from all possible crosses between newly created DH lines. Genotype-by-year interaction variance was assumed to be equivalent to genetic variance (based on average correlation between locations being equal to 0.5). All evaluations were conducted using 40 replications.

3.1 Results (On-farm hybrids)

By year 40, all reduced cycle time treatments generated more genetic gain than the baseline Stage 3 treatment. The Stage 2 and Stage 1 treatments generated 1.09 (95% CI: 1.06,1.13) and 1.17 (95% CI: 1.14,1.2) times more gain, respectively.

3.2 Results (Hybrids between DH lines)

By year 40, all reduced cycle time treatments generated more genetic gain than the baseline Stage 3 treatment. The Stage 2 and Stage 1 treatments generated 1.08 (95% CI: 1.06,1.11) and 1.19 (95% CI: 1.17,1.22) times more gain, respectively.

4. Conclusion

Reducing cycle time by choosing parents earlier consistently improved the rate of genetic gain. We recommend developing an improvement plan for selecting some parents earlier.

Only grain yield was examined, but we expect similar results when using a selection index.

We recommend a follow-up study to examine the sparse testing scenarios not addressed in the present study. The follow-up study should also re-evaluate the present scenarios using refined simulation assumptions.