to Asia in the early 1980s in the form of an applied breeding program in close collaboration with national The Centro Internacional de Agricultura Tropical (CIAT, headprograms. The Department of Agriculture (Thailand) quartered in Colombia) established a cassava (Manihot esculenta and CIAT established a collaborative cassava breeding Crantz) breeding program in the beginning of the 1970s with the aim program that distributed the advanced breeding materiof extending the Green Revolution success to less privileged sectors als to many national programs in Asia. of the tropical populations. The initial decade was mainly dedicated to the collection of germplasm and generation of basic breeding mate-The goal was defined as the establishment of a cassava rials. The later decades were devoted to applied breeding in collaborabreeding program with a global perspective that would tion with international and national programs in Latin America, Asia, generate economic benefits targeted to the less priviand Africa. This paper focuses on the basic breeding at CIAT/Colombia leged in the rural sector. During the whole period of (CIAT/HQ), applied breeding at CIAT/Thailand (CIAT/Thai), and development of the program, the following operating distribution and selection of improved materials with many collaboraprinciples or processes were closely adhered to: (i) estors in Asia. Fresh root yield of populations was improved by Ͼ100% tablishment of breeding methodology; (ii) generation of and root dry matter content by Ͼ20%. The national program collaborauseful breeding materials; (iii) distribution of advanced tors used these populations to develop many improved cultivars in breeding materials to national programs; (iv) establishmany countries. The biological factors considered as critical for this ment of competent national cassava breeding programs; successful breeding effort were as follows: inclusion of a broad base (v) development of improved cultivars; and (vi) dissemiof genetic variability obtained in the center of crop origin and diversifination of cultivars. cation; evaluation of breeding materials under diverse environmental conditions including high stress environments; and a clear understand-Within these procedures there were three distinct ing of the different operational principles at different stages of breed-technical phases performed under specific institutional ing advancement, as illustrated by the emphasis on harvest index in arrangements, each of which was critical to accomplishselection within populations and on biomass in population building. ing our goals: first, germplasm collection and evaluation, The understanding of crop germplasm being a common heritage and which formed the most important part of the basic the determination of agricultural scientists to use this for the welfare breeding at CIAT headquarters in Colombia; second, of the neediest people were the social factors for the overall success.

Genetic and environmental variabilities of root dry matter content (RDMC) of cassava (Manihot esculenta Crantz) clones (accessions and breeding lines) were estimated at different harvest months (8 12 months), seasons (wet and dry), and locations (three altitudes). To assess the possibility of obtaining higher dry matter yield through selection for RDMC, genetic studies based on F1 variability and parents‐offspring regressions also were conducted. Age of plant, season, and location significantly affected root dry matter content. Temperature appeared to be an important factor related to the effect of location. The highest RDMC was obtained with 8 month‐old plants harvested at the beginning of the dry season in the high altitude location, while the lowest occurred with 12‐month‐old plants harvested at the beginning of the wet season at the low altitude location. The clone effect was highly significant and a broad‐sense heritability of 0.87 was obtained. Clone ✕ location interaction was also significant, but the magnitude of this effect was much smaller than that of the clone effect. Genetic analyses suggested that inheritance of RDMC was controlled mainly by polygenic additive factors. Narrow‐sense heritabilities ranging from 0.51 to 0.67 were obtained. There was no indication of negative response in root dry matter content after intensive mass selection for root fresh yield. The data indicate that RDMC can be increased by simple breeding techniques such as phenotypic mass selection. Selection of clones at one location was largely valid for other locations; however, final selection should be made at each specific location for maximum potential gain.

No abstract

Intraspecific‐intergenotypic competition, competition with weeds, and spacing response were studied in 25 rice (Oryza sativa L.) cultivars with different growth habits in Lambayeque, Peru. The actual yield of genetically mixed rice populations was always below the yield of the better component genotype in pure populations. The yield of the mixtures seldom exceeded the average yield of the pure populations of the component genotypes. Thus, yield reduction in one genotype as a result of competition was not compensated by the yield increase in another.Intraspecific competition, competition with weeds, and spacing response were highly intercorrelated with each other, suggesting that these were controlled largely by the same genetic factors through the same physiological process. Vegetative vigor, large leaf area, a high rate of N absorption in early growth stages, and plant height were the most significant characters related to competitive ability. Tillering itself was not related to either competitive ability or to spacing response. Tall, vigorous genotypes with a long growth duration were adapted to extremely primitive agronomic conditions. The evolution of cultivars has been accompanied by the evolution of cultural methods and the loss of competitive ability.

No abstract