Genetic Progress in 45 Years of Irrigated Rice Breeding in Southern Brazil

Abstract

Irrigated rice (Oryza sativa L.) is a crop of extreme social and economic importance in Brazil, and the state of Rio Grande do Sul accounts for >70% of the national production. The Brazilian Agricultural Research Corporation (Embrapa) is focused on rice breeding, with the aim of developing cultivars with significantly increased grain yield and improved sustainability. The objective of this study was to estimate the genetic progress of 45 yr of the irrigated rice breeding program of Embrapa in Southern Brazil from 1972 to 2016 by conducting a comparative analysis of cultivars in the same environment and by meta‐analysis of line yield assays. The estimates were results from a meta‐analysis obtained by evaluating 455 genotypes in 145 trials of regional line yield and value for cultivation and use of 44 agricultural crop seasons, and by comparing cultivars obtained by evaluating 25 cultivars in 10 agricultural crop seasons. Genetic gains were evaluated for grain yield, plant height, and days to flowering. The cultivars released by the breeding program were also evaluated for important agronomic characters. The genetic estimates determined a genetic progress for grain yield via meta‐analysis and via comparison of cultivars of 0.62 (37.91 kg yr−1) and 0.73% (47.78 kg yr−1), respectively. It was also verified that during the period there was a reduction in plant height and days to flowering. Three distinct historical phases that defined changes in research focus and in genetic gains can be described: (i) 1972 to 1983, before the rice Green Revolution; (ii) 1983 to 2000, after the rice Green Revolution; and (iii) 2000 to 2016, selection intensification for industrial grain quality attributes. Other relevant genetic aspects, selection strategies, and phases of the breeding program were discussed.

Research Article Performance of a breeding program for irrigated rice in Southeast Brazil

“…Another explanation is that the best genotype in one location may not be the best in another, that is, differentiated behavior of genotypes versus environmental variations, because of this difference between environment, which expresses the interaction genotypes by environments (Colombari Filho et al, 2013 andKleinknecht et al, 2016). However, in addition to the productivity that the genotype-environment interaction is visible, it is worth noting that the attributes related to irrigated rice grain quality are highly related not only to genetic but also environmental factors (Cameron et al, 2008;Hakata et al, 2012;Lyman et al, 2013;Li et al, 2014;Xu et al, 2015;Streck et al, 2017;Streck et al, 2018). The physical attributes of the grains are very complex quantitative traits because they are controlled by maternal and cytoplasmic effects (Streck et al, 2018).…”

“…However, in addition to the productivity that the genotype-environment interaction is visible, it is worth noting that the attributes related to irrigated rice grain quality are highly related not only to genetic but also environmental factors (Cameron et al, 2008;Hakata et al, 2012;Lyman et al, 2013;Li et al, 2014;Xu et al, 2015;Streck et al, 2017;Streck et al, 2018). The physical attributes of the grains are very complex quantitative traits because they are controlled by maternal and cytoplasmic effects (Streck et al, 2018). Therefore, many genetic mechanisms and interactions with the environment are still obscure (Shi et al, 2002;Zhou et al, 2009).…”

“…An important caveat that must take into account the high maintenance rates that limit genetic gains to rice productivity, mainly due to the low exploitation of the genetic base for the crop available in germplasm banks, and also because this characteristic is quantitative (Streck et al, 2018). This restricts the potential for genotypic variability of elite materials to be explored.…”

“…In view of these results, the gain to productivity has not been satisfactory at the present time, and the elite lineages have not exceeded the cultivars already launched. This can be justified by the importance that the program has given to the selection of other traits than grain yield (Santos et al, 1999), for example, the one focused on releasing cultivars with high productivity potential, good quality and healthy grains, resistance to biotic and abiotic factors and good adaptation to growing conditions (Streck et al, 2018). Corroborating the previous discourse, it is necessary to point out the need to establish new breeding strategies aiming at obtaining cultivars superior to those already launched, since it is known that irrigated rice still has sufficient productive potential and variability to be exploited (Santos et al, 1999).…”

“…We can summarize the dynamics of a breeding program through an indicator that expresses genetic progress. This information, in addition to verifying the success of the breeding program, quantifies the impact of favorable allele transfer strategies during selection process, guides future research and re-evaluates the methods used to obtain new varieties (Soares et al, 2005;Menezes Júnior et al, 2008;Streck et al, 2018). Finally, global gain estimates are useful as indicators of the effectiveness of the choice and conduct of the methodology used, as well as the potential of the exploited germplasm (Breseghello et al, 2011).…”