Selection among genotypes in final stage sugarcane trials: effects of time of year

Low levels of commercial cane sugar (CCS) reduce relative economic value (REV) in sugarcane. In the Australian sugarcane industry, CCS is lower early (June) compared with the completion (November) of the harvest period. Performance of sugarcane genotypes in 2 Central region series and 1 Burdekin region series of final stage selection trials was examined to determine if independent selection programs are required to select elite genotypes for 2 target periods: (a) early (before July), and (b) mature (from July on). Across series, CCS (16.83 v. 12.02% fresh cane weight) and REV (AU$3937/ha v. $3123/ha) were significantly higher in the mature than in the early period, while genotypic variance for CCS (0.76 v. 0.33), and broad-sense heritability for CCS (0.96 v. 0.86) and REV (0.79 v. 0.69), were higher in the early than in the mature period. Genetic correlations between sample times less than 3 months apart were usually ≥0.9 for CCS, but generally declined to ≤0.6 for times greater than 3 months apart. Consequently, genotype × period (early compared with mature) interaction effects on CCS affected selection decisions, especially in the Central region, and genetic improvements for CCS would be expected via specific targeting of early and mature periods. However, genotype × period interaction effects were not important for cane yield or REV, such that selection for specific adaptation to early or mature periods would not improve gains in REV across the entire harvest period. Some final stage selection trials should be harvested early in the harvest period, when heritability and genotypic variance are highest, to capture high early CCS genotypes with acceptable cane yield for recycling in breeding activities. This protocol should enhance genetic gain for early CCS and simultaneously increase REV early in the harvesting period of the Australian sugar industry.