A field study was initiated to generate information on dry matter yield and some biometric parameter effects on yield of seven sweet potato genotypes in weathered ultisols in the humid tropics. The experiment, which was laid out in a randomize complete block design with four replications was conducted at National Root Crops Research Institute, Umudike, Abia State, Nigeria (05°29'N, 07°32'E,122 m asl). Umuspo 1 (31.2 t haG 1), closely followed by TIS 87/0087 (27.45 t haG 1) significantly (p<0.01) gave the highest storage root yield compared to the other genotypes and also exhibited highest Bulking Rate (BR) (7.78 g dayG 1), Total Dry Matter yield (TDM) (71.99 g plantG 1), Crop Growth Rate (CGR), Relative Growth Rate (RGR) and Absolute Growth Rate (AGR). High yield of Umuspo 1 was partly due to greater partitioning of assimilates to the storage roots relative to the other genotypes. The mean sequence of TDM content and BR at 56 Days after Planting (DAP) in the genotypes was in the order: Umuspo 1>Solomon 4 > Ex-Igbariam>TIS 87/0087>TIS 8164>Umuspo 3>Centinneland Umuspo 1 >TIS 87/0087>TIS 8164>Solomon 4>Ex-Igbariam>Umuspo 3>Centinnel, respectively. Storage root yield had significant (p#0.01) and positive correlation with number of storage roots/plant with correlation coefficients (r) of 0.53, bulking rate (r = 0.99), crop growth rate (r = 0.59), absolute growth rate (r = 0.59) and total dry matter at 2, 4, 6 and 8 weeks after planting. Umuspo 1 may be adjudged to be better endowed genetically because of its high dry matter content, early bulking ability and high yield potential recorded in the study.

This review presents an overview of the importance, constraints, and prospects on different aspects of cassava (Manihot esculenta crantz) breeding for enhanced micronutrient level, including carotenoids (precursors for provitamin-A) and especially beta-carotene. Early cassava-breeding efforts concentrated on crop yield, dry matter, and disease resistance, which are farmer-preferred traits. However, unacceptably high levels of preventable human diseases caused by malnutrition prompted breeders and nutritionists to screen wild relatives and unimproved germplasms (landraces) to increase micronutrient density in staple crops. The ultimate objective is to reduce diseases caused by micronutrient deficiencies. Nigeria, with 140 million people and the largest producer and consumer of cassava in the world, is characterized by rampant malnutrition and high incidence of nutrient deficiency-related diseases. The tuberous root of cassava is low in micronutrients. It is also well known that vitamin A deficiency is primarily caused by dietary inadequacy that results in progressive eye damage and eventually leads to blindness, especially in children. In addition, affected children suffer from a weakened immune system. Present interventions to eliminate this Breeding for Enhanced β-Carotene Content in Cassava 561 deficiency rely on supplementation and food fortification programs, do not reach all those affected and do not get to the root of the problem, which is an inadequate diet. The development of high micronutrient-content cassava varieties (especially, higher β-carotene and other carotenoids) will contribute to a more sustainable solution of the problem of vitamin A deficiency. A current thrust of research (HarvestPlus initiative) is to determine the genetic potential for increasing the concentrations of bioavailable Fe, Zn, and provitamin A carotenoids in the edible portions of several staple food crops including cassava, rice, wheat, maize, and beans. Currently, the International Institute of Tropical Agriculture (IITA), International Center for Tropical Agriculture (CIAT), and National Root Crops Research Institute (NRCRI), Umudike, are working in collaboration to develop an elite cassava gene pool and to develop varieties that will be released to farmers soon in hope of addressing part of micronutrient malnutrition.

The effects of five stake weights (50, 63, 75, 88 and 100 g) of three cassava cultivars (TMS 30572, TMS 91934 and NR 8083) on the growth and yield of cassava were studied for 2 years (1992\93 and 1993\94) in Nigeria. The leaf area index (LAI), crop growth rate (CGR), net assimilation rate (NAR), leaf area duration (LAD), harvest yield and harvest index (HI) were dependent on the weight of stake planted and were greatest at the stake weight of 88 g (875 kg\ha) irrespective of cassava cultivar. Variations in LAI, LAD and HI of stake weights between 75 and 100 g were non-significant. The profusely branching cassava cultivars, TMS 30572 and NR 8083, achieved greater LAI, CGR, NAR and LAD relative to the sparsely branching cultivar, TMS 91934. The significant growth and yield differences among the stake weights with the optimum at a stake weight of 88 g (875 kg\ha) indicates the need to adopt this stake weight for sustainable and improved growth and yield in cassava production.

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