A retrospective analysis of maize systems tolerance to combined drought with heat stress and low soil nitrogen in Zimbabwe.

Abstract

Climate change and variability have resulted in emergence of new constraints to maize production putting at risk the livelihoods of millions of people in Zimbabwe. Drought and heat stresses have increased in occurrence, intensity and severity threatening food and nutrition security. Over the years, grain yield losses were mainly linked to drought and poor soil fertility especially low soil nitrogen, but in recent times, reports have also attributed it to heat stress, occurring simultaneously with drought. Unfortunately, most smallholder farmers are resource-poor and cannot rescue their crops from the vagaries of these stresses. Crop improvement is highly regarded as the most cost effective and sustainable approach to building climate change resilience. However, the potential of integrating improved varieties in farming systems threatened by combined drought and heat, and low nitrogen stress is still poorly quantified. Therefore, this study aimed to make a retrospective analysis of maize systems tolerance to combined drought with heat stress and low soil nitrogen in Zimbabwe. The specific objectives were i) to identify sub-tropical maize inbred lines with combined drought and heat stress adaptation attributes among maize populations developed within the CIMMYT maize breeding program and the Crop Breeding Institute of the government of Zimbabwe, ii) to identify pre-release maize hybrids developed within CIMMYT maize breeding program with combined drought and heat stress adaptation attributes, and iii) to do the retrospective analysis of maize performance under low nitrogen stress conditions related to combined drought and heat stress in sub-Saharan Africa. Managed low nitrogen experiments were conducted in Eastern and Southern Africa, and combined drought and heat stress experiments were conducted at Chiredzi research station in Zimbabwe. The combined drought and heat stress experiments were conducted in Zimbabwe during the dry, hot, and rain-free season to ensure simultaneous occurrence of drought and heat stress. Drought at reproduction phase was simulated by withdrawing irrigation three weeks before flowering. The results showed important traits, number of ears per plant, number of kernels per row and cob girth that could be prioritised for selecting germplasm and revealed maize inbred lines and hybrids with adaptive attributes under these conditions. These will be recommended for incorporation and advancement in the national maize breeding program in Zimbabwe. Genetic correlation analysis showed that the number of ears per plant, number of kernels per row, and cob girth were highly associated with grain yield (p<0.05) of hybrids under combined drought and heat stress. In the inbred lines experiment, anthesis to silking interval, plant height, ear height, husk cover, grain moisture content at harvest, number of leaves above the ear and internode length were the key traits associated with grain yield under combined drought and heat stress. The newly developed pre-release maize hybrids, DJ65-32, DJ65-1 and DJ65-28 predominantly and significantly outperformed the old and widely grown commercial varieties by 34.97% under both combined drought and heat, whereas OPVs performed well under low nitrogen conditions. These hybrids and their parents will be advanced in the breeding program DJ65-32, DJ65-1 and DJ65-28. The study demonstrated the potential benefits of integrating improved maize varieties in farming systems under threat from climate change-induced abiotic stresses. Overall findings from this study have implications for improving the maize breeding program to continuously churn out new varieties with enhanced adaptation to low nitrogen and combined drought and heat stress in Zimbabwe.