Optimizing Sugarcane Cultivar Selection For Frost-Prone Environments In South Africa.

Frost in low-lying areas of the midlands region of South Africa limits sugarcane (Saccharum spp.) yields by preventing the attainment of full crop maturity (12 vs. 24-month crop age). Quick maturing, high sucrose cultivars selected for warmer regions may achieve acceptable yields in such frost pockets. The objectives of this study were to i) test the performance of cultivars developed from other breeding programmes in frost pockets in the midlands, ii) investigate the genotype x environment (G x E) interactions and identify candidate control cultivars to optimise future pre-release selection strategies, iii) evaluate cultivar differences in post-frost deterioration, and minimum temperatures observed in crop canopies. A series of cultivar trials established between 2004 and 2009 at four frost sites were analysed.  Cultivar x crop interactions were significant (p<0.05) for cane yield (TCANE), estimated recoverable crystal (ERC) and ERC yields (TERC) in most trials. As a group, test cultivars produced between 2.0 and 9.5% lower TCANE compared to standard cultivars. In contrast, ERC and TERC of test cultivars were between 15 and 29, and 6 and 27% higher than the standards, respectively.  The TERC of USA cultivars HOCP96-540 and LCP85-384 were not significantly different from the top performing test cultivar N36, showing their possible adaptability to South African conditions. . Genotype + genotype x environment (GGE) biplot analysis for TERC showed that frost pockets on humic and sandy soils are necessary when developing a breeding strategy, and that cultivars N36, N41 and N48 will be suitable standard cultivars in future breeding trials. Rates of post-frost deterioration in three crops were 0.7, 1.3, and 1.7 g ERC/stalk/week for test cultivars compared to 1.5, 1.7 and 2.1 g ERC/stalk/week in the standard groups, respectively. The dense-canopying cultivar N36 showed significantly higher daily minimum temperatures at the apical meristem compared to other cultivars when temperatures dropped below -2°C on three separate occasions.