SELECTION-BASED APPROACH TO EARLY BULKINESS FOR IMPROVED PROVITAMIN A CAROTENOID CONTENTS IN CROSSES OF YELLOW CASSAVA (Manihot esculenta Crantz) GENOTYPES IN NIGERIA

Abstract

The problem of poor yields and the need for lands encourages piecemeal harvesting whereby farmers harvest from five months after planting and mostly at the stage of reasonable yield but of a poor marketable size and value. This study therefore aims to provide farmers with biofortified cassava genotypes that bulks early so that farmers can harvest their cassava early enough with reasonable yield. This study was conducted under two experiments in different agroecologies. The first experiment was a bulking rate experiment conducted under a rainforest agroecology in two cropping seasons where 42 F1 progenies obtained from crossing block involving 39 parental genotypes from different crossing combinations were evaluated at different harvesting periods of 6, 9 and 12 months after planting (MAP) in a 42x3 factorial experiment in a randomized complete block design in Ibadan. And the second experiment, a bulking rate experiment was conducted in a rainforest and southern guinea savanna zone where ten (10) cassava genotypes were evaluated at different harvesting periods of 3,6,9 and 12 months after planting at Ubiaja and Mokwa locations in a randomized complete block design. Data were collected on plant height, height at first branching, number harvested, root number, shoot weight, root weight, harvest index, root size, storage root diameter, pulp colour, inner skin colour, dry matter content, fresh storage root yield, total carotenoid, beta carotenoid content. In both experiments, fresh root yield of genotype progressively increased from earlier months up until 12 MAP while some shows discontinuous patterns of growth. Discontinuity in yield, that is a genotype/accessions showing retrogressive pattern in their root yield across months after planting had no effect on overall root yield performance. At 9 MAP, most cassava genotypes and accessions had lower dry matter (DM) with low root yield while in some others, their root yield increased as DM reduces. In the experiment at Ibadan, the path analysis shows that root weight had direct effect on fresh root yield while seasons significantly and negatively contributed to total carotenoids (TC). The cropping seasons (2019/2020) significantly and positively contributed to fresh root yield while MAP contributed negatively to DM. In both experiments, a negative correlation was observed between TC (total carotenoids) and DM (dry matter) as well as between TC and fresh storage root yield. However, the cassava progenies in Ibadan exhibited greater variability, with some cassava accessions demonstrating both high root yield and high total carotenoids content. In both experiments, early bulking of cassava demonstrated similarity, with both Experiment I and II recording early bulking rates exceeding 60%. In the experiment at Mokwa/Ubiaja, genotypes falling into the early bulking category, specifically IBA141092, exhibited notably higher beta carotenoid levels (10 µg/g). Conversely, in experiment at Ibadan, most accessions in the late bulking category demonstrated higher total carotenoid content compared to other bulking categories. In Ibadan, among the progeny, accession IBA180058 displayed the highest best linear unbiased prediction (BLUP) values for total carotenoids (4.85 µg/g), while IBA180146 emerged as the top-performing accession in terms of root yield (5.04 t/ha). To enhance the progeny's root yield and carotenoid content, cross-breeding with accessions boasting the highest BLUP values for root yield (IBA180146) and total carotenoids content (IBA180058) is recommended