Hybrid rice seeds are genetically packed to produce high yields due to heterosis. Seeds are also the most effective and inexpensive medium to transfer hybrid rice technology to farmers. Isolated evaluation reports on hybrid rice were biased and conclusions were based on insufficient data13,14. We have used the largest data generated on hybrid rice by AICRIP in METs5 for 32 years (Additional file 1: Table S1). Although the irrigated ecosystem is made homogenous by regular applications of water, environmental and yearly variations may influence the yields recorded at test locations. Therefore, yield data sets were statistically corrected suitably to neutralize the year effect as well as the location effect before any analysis. Further, the three floating checks namely T3HM, EXPM and ILCVM were used to remove pitfalls in estimating absolute yield performance. When similar trend is observed in the floating checks, the actual genetic gain or loss over years is obtained in the re-analysis by deducting year-wise experimental or check mean yield from the mean yield of top-three genotypes (Fig 1-2).
Grain yields of hybrid genotypes
The top-three hybrid genotypes in IHRT trials produced significantly higher grain yields than that of the experimental mean or the inbred check variety mean yields over the years and across locations evaluated (Table 2). Over 32 years (1988-2019), the grain yields of T3HM increased by 0.6 to 0.8 t/ha than that of the EXPM and by 0.9 to 1.4 t/ha than that of ILCVM. The EXPM of hybrid genotypes assessed was also higher by 0.1 to 0.6 t/ha than that of the inbred LCVM (Table 2). The contention on yield advantage due to heterosis in hybrids was not apparently present in most of these hybrid F1 genotypes. The linear regression models on the three floating checks over the years showed significant annual increases of 5-29 kg/ha in T3HM and 11-34 kg/ha in EXPM in early, mid-early and medium maturing hybrids. However, the mean grain yield of ILCV decreased in early maturing hybrids and those with medium slender grains by 41-46 kg/ha but increased in mid-early and medium maturing hybrids by 16-24 kg/ha. The consumers in India are well-known to prefer and accept short bold or long slender grains. The stress on market preferred grain type rather than on the level of heterosis of hybrid F1 genotypes was the evident cause of such declining yields of floating checks in IHRT-MS trial over the years. The classical stability parameters often do not consider the absolute performance of genotype cohorts evaluated in experiments. A few statistical parameters were developed to combine both stability and performance6,15,16. The floating checks of Jensen6 however, combine both stability and performance17. The trend observed was more or less similar across the three floating checks studied using AICRIP experimental data. When a linear model showed a significant increase or decrease in T3HM, the environmental effects were eliminated in that data year-wise by deducting EXPM or ILCVM from T3HM. The linear models developed after removing environmental effects became statistically non-significant. Evidence thus obtained clearly revealed lack of genetic gain or loss in hybrid genotypes developed over the 32 years. Our analyses have established that grain yields of 7.0 to 7.9 t/ha were harvested in hybrid F1 genotypes with early, mid-early and medium maturity, and in those with medium slender grains at 20 locations and in 362 experiments performed during 1988 to 2019 (Tables 4-7).
Disarray in the assessment of hybrids
The assessment of hybrid genotype performance has generally been defining rather than experimenting with them due to following reasons. During 1988-2019, 102 locations were used to test 2070 hybrid F1 genotypes in 2376 experiments across rice growing irrigated areas in India. Due to ambiguity in nomenclature of some locations in the initial years, data from 159 experiments were removed before analyses. Grain yield assessments were done on hybrid genotypes for only 1-4 years in 270 experiments performed and were omitted as inadequate for analysis12. Boyles et al18 had also excluded data from locations that were not represented for five or more years from the long-term analysis of red wheat experiments. The remaining assessments were done for 5-24 years. In 962 experiments, hybrid F1 genotypes had produced yields lower than that of inbred LCV in one or more trials. Thus, nearly two-thirds of all experiments performed had demonstrated the lack of hybrid vigor in hybrid F1 genotypes evaluated. This is a colossal loss of efforts at 102 locations on field evaluation, time and money used to conduct these experiments made with hybrid F1 genotypes from 1988-2019 (Additional file 1, Table S1). Initiation of an additional singular trial for hybrids with medium slender grains from the year 2006 further demonstrated the speculative organization of testing in METs of AICRIP as early, mid-early and medium maturing hybrid F1 genotypes with this grain type were already available in adequate quantities (Additional file 4: Table S2). The limited use of released 10 commercial hybrids from 2005 as checks (Table 8) further confirmed the difficulty of producing and supplying hybrid seed in sufficient quantity for tests. However, 54 inbred checks have been used with frequent and rather irrational replacements. Astoundingly, while high yields (~9 t/ha) were recorded in ILCV in some experiments, at most locations (Additional file 6, Table S4) but the ILCVM remained low (~6 t/ha). Apparently there was no attempt to select an appropriate inbred variety for use as check. Our analysis further adds to doubts raised by Muralidharan et al7,9 that either the yield of inbred local check is underestimated or the yield of hybrid F1 genotype is over estimated. The lack of required repetitions of experiments has made the bulk of resource and efforts on assessment of grain yields of expensive hybrid F1 genotypes irrelevant. There was evidently no critical assessment of the data on hybrid rice experiments in AICRIP from time to time.
Critical appraisal of test locations for evaluation of hybrid rice
For commercial release of a hybrid genotype, the criterion set is the ability to produce at least 10% higher grain yield than the inbred check due to heterosis3. During 1996-2019, rice hybrid F1 genotypes were tested at 102 test locations in the irrigated ecosystem in four trials across India. The mean grain yields produced by hybrid F1 genotypes were higher than the yields harvested in ILCV at 30 locations (13 AICRIP, 11 private and 6 voluntary) (Tables 4-7). The coefficient of variation (CV) is a standardized, dimensionless measure of dispersion relative to a data set's average. It enables the comparison of several data sets on genotypes19. It is also used as a measure to compare the robustness of different biological traits20. At locations where heterosis in yield was detected in hybrid F1 genotypes, the CV remained low (10 to 11%) indicating robustness of analysis. Only 23 locations, hybrid F1 genotypes recorded ≥10 per cent higher grain yields than that of ILCV. These locations revealing such heterosis in hybrids are 8 public funded AICRIP locations at Coimbatore (Tamil Nadu), Karnal and Kaul (Haryana), Kapurthala and, Ludhiana (Punjab), Mandya and Mugad (Karnataka) and Warangal (Telangana); 11 private funded locations at Advanta, Bayer, Bio Seeds, HR International, Kaveri Seeds, Mahyco Seeds, Nuziveedu Seeds, Pan Seeds, Parry Monsanto, Syngenta India and VNR Seeds; and 4 voluntary locations at Allahabad (Uttar Pradesh), Gaddipally (Telangana State), Jabalpur (Madhya Pradesh) and Karaikal (Puducherry) in India. Although hybrid F1 genotypes have been tested and shown to produce higher yields at locations in the states of Andhra Pradesh, Karnataka, Haryana, Puducherry, Punjab, Tamil Nadu and Telangana State, commercial hybrids are grown only in Uttar Pradesh. The yields of hybrids were much less than those of inbreds at all the test locations in Bihar, Chhattisgarh and Jharkhand. The locations for testing hybrid F1 genotypes were evidently not selected based on the merit of execution of experiments. Periodical and critical examinations of testing programs are necessary as they can save cost incurred by human resource and infrastructure, and can free scientists for deployment in other activities. The irrational choice of locations makes hybrid rice experiments a wasted exercise and precludes any diffusion of technology developed due to exclusion of bulk of the potential target area.
Hybrid rice cultivation has been estimated to cover ~1.5-2.0 million ha mainly in the Indian states of Bihar, Chhattisgarh, Jharkhand and Uttar Pradesh21. Monetary incentives are provided by the Government of India for the training, production and distribution of hybrid seeds by public organizations and private seed business firms. Jharkhand is rainfed and has a limited irrigated area (<5%). By contrast, 8.51 million ha irrigated area in Bihar (2.13 m ha), Chhattisgarh (1.34 m ha) and Uttar Pradesh (5.04 m ha) are available for growing hybrid rice. Yield assessment of hybrid genotypes may be performed in future at select locations in these target areas to avoid wastage of resources, improve efficiency and overcome the difficulty of short supply of hybrid seeds.
Legitimacy of hybrid trials
Hybrid F1 genotypes, inbred checks and commercial hybrid varieties have been organized into four trials with IHRT-E (early 110-120 days), IHRT-ME (mid-early 121-130 days), IHRT-MED (medium maturity 131-140 days), and IHRT-MS (medium slender grains maturing in 130±5 days). Our analyses have shown the presence of medium slender grain types in the hybrid genotypes included in these trials in nearly equal numbers. Therefore, the IHRT-MS is redundant. The productivity per day was comparatively higher in early and mid-early maturing than medium maturing commercial hybrids or ILCV (Tables 4-6).
China has so far cultivated only commercially released hybrids that have a long maturity duration (>140 days).Yuan and Sun22 had reported on the development of an early maturing hybrid Weiyou 64. The compatibility of early maturity and high yield in rice has been a subject of debate23. Using the early-maturing restorer line Ce64 derived from IR9761-19-1, Minghui7724, Zaohui8925 and other early maturing hybrids were developed in China. Mapping of a dominant earliness gene, Early flowering-completely dominant (Ef-cd) encoding a long noncoding RNA (lncRNA)26 and its subsequent cloning27 have revealed the genetics behind the possibility of exploring this trait to combine early maturity and high yield for enhancing productivity per day. High-yielding rice hybrids in China usually take 160 to even 180 days from sowing to harvest. The newly developed early maturing (125 days) hybrid variety G3-1S/P19 tested at two sites produced 1.57 kg/m2 in central China's Hunan Province28. Several high yielding early maturing inbred commercial varieties have been developed in India29 and elsewhere, unconscious of the earliness gene (Ef-cd), based on simple phenotype selection30. We have demonstrated that early (110-120 days) and mid-early maturing (121-130 days) hybrid FI genotypes produce high yields (7 to 8 t/ha) with increased productivity per day (61-63 kg/ha). As the maturity duration is increased, water requirement, the cost of cultivation and the associated risks also increased. A scrutiny of AICRIP data indicated that most hybrid F1 genotypes matured between 115 and 135 days. Therefore, efforts may be more focused to breed early and mid-early maturing hybrids with high productivity per day.
Hybrid vigour and its exploitation
Hybrid genotypes had produced higher yields by 728-2588 kg/ha than the yields of inbred LCV (4922-5648 kg/ha) at only 30 locations tested in 5-24 experiments (Table 4-7). None of the hybrids evaluated for 32 years had shown a mean yield of ≥10 t/ha that has been long-established repeatedly with inbred LCV in METs of AICRIP7,9,11,31. Yet, the grain yields of 7.0 to 7.9 t/ha in hybrid genotypes attained in many experiments reveal the achievable higher yield owing to unhindered and better crop husbandry. This inference is confirmed by the lack of genetic gain for grain yields of hybrids in any of the trials examined in this study. The claim of heterosis can deceptively come from an under-estimation of inbred LCV, especially when inbred yields are as low as 5-6 t/ha as compared with their easily attainable level of ≥10 t/ha under efficient crop management. Heterosis is considered as an important trait to increase biomass and yield in hybrid F1 genotypes than in the specific parents used, but not over the yield of commercially cultivated high yielding inbred varieties. Rice-breeding using abundant and variable germplasm accessions for many decades has effected countless - and often unknown - changes in the genetic composition of inbred varieties. Breeding activity however, is solely dependent on the meager availability of male sterile lines to develop hybrid rice varieties. The genetic basis of heterosis is less understood32. The recent advancements in cloning the heterotic QTL GW3p6 and development of a near-isogenic rice line indicate the possibility of realizing high yield in inbred rice lines without even needing to develop hybrid rice33.
Breeding hybrids has demonstrated improved yield potential in many cross-pollinated crops such as sorghum, maize and cotton34,35,36. Hybrid breeding takes advantage of heterosis (hybrid vigour). It is a phenomenon where F1 hybrids derived from crosses between genetically distinct inbred varieties exhibit superior phenotypic performance over their parents. Superior performance may specify an improved function of any biological process in the hybrid offspring37. Rice is a self-pollinated crop and commercial hybrid breeding mostly depends on a few cytoplasmic male sterile systems. It requires a lot of financial resource to evaluate field performance of all possible crosses among a large number of inbred lines38. In general, only a small proportion of crosses can be evaluated in the field and many potential superior crosses may not get tested. The evidence generated in our study has confirmed the production of the stipulated 10% more grains by some hybrids than what was produced by inbreds in many experiments. The grain yields recorded in these experiments however, were well below the attainable yield levels of inbreds established ever since 1968. The genuineness of the claim of a yield advantage in any hybrid over inbred variety is untenable. The optimized canopy, architecture, dark green leaves, erect flag leaf, narrow leaf, dwarf plants with a plant height of ~one meter, panicle length of 25 cm, 180 grains per panicle, grain filling rate of 81%, 1000-grains weight of 26 g, and adaptability to a wide range of growing environments determine yield level of rice (data collected from 2013 ~ 2015 by China Rice Data Center, http://www.ricedata.cn/variety/varis/604222.htm)39. The architectural and physiological features associated with high yield were recently studied in two elite historical hybrid rice cultivars, i.e., YLY1 and LYP939. The canopy photosynthesis was found to increase the proportion of biomass allocation to above ground tissues (1.5%), productive tillers (25%), photosynthate reserve in leaf sheath (5-11%) before grain filling and photosynthate translocation to grains. Yet, the yields of hybrids have remained below or at best on par with the yield records of inbred varieties (~10 t/ha).
A longer root, a larger number of tips, a better developed aerenchyma, a higher capacity for N uptake, and reduced NH4+ efflux from roots are associated with higher N-use efficiency and growth performance in hybrid super rice Yongyou 12 and Jiayou 640. Lin et al41 used a linkage map consisting of high-density SNPs, to identify heterosis-associated five genes, which contributed to the high yield, with repeated occurrence of qSS7 and qHD8 in both hybrid populations. The two super hybrids Yongyou12 and Jiayou 6, however, produced grain yields of 11.0-11.8 t/ha compared to 9.1 t/ha in common (inbred) variety Xiushui 134 at 200 kg N/ha40.
The expression high yield of any variety is to be used in relation to the one that produces lower yield. However, whenever high yields of hybrids are reported, the yields of inbreds have been less than their demonstrated attainable yield. The question therefore, arises of whether the extra yield in the hybrid is due to heterosis? Is it only an assumption that the heterosis of hybrids developed leads to increased yields over inbreds? There is little evidence to indicate that heterosis of hybrids bred is due to induction of better traits in F1 plants leading to enhanced yields. It is essential to prove that hybrids have certain improved traits compared with those of inbreds to produce more grain yields.
Opportunities to achieve desirable heterosis for yield
Dominance42 and overdominance43,44 hypotheses were coined long ago to explain heterosis. The molecular mechanism of heterosis still remains mysterious despite research on heterosis for more than a century. Unfolding research findings indicate the potential to improve heterosis by exploiting inter-specific heterosis between African rice and Asian rice45, differential expression of genes46, divergent selection and genetic introgression47, genomic hybrid breeding using whole genome markers48, epistasis interactions between select parents49, chromosome-segment substitution lines47, heterotic loci from wild Oryza longistaminata for yield heterosis50, wide compatible neo-tetraploid rice line with long panicles51 and genome wide transcriptome profiling of O. rufipogon52.
The hybrid performance in self-pollinated crops may depend on the initial strategy to form heterotic pools. Simulation studies of the future hybrid performance indicated an advantage in using heterotic pools only when there was high allelic diversity (number of quantitative trait nucleotides (QTN) ≥ 2000) and/or high dominance at loci (≥0.4) for a quantitative trait53. In 97 parental lines used with CMS line to produce Indian hybrids, Sruthi et al54 have reported limited diversity among and within the maintainer and restorer groups. Novel approaches of base-broadening before forming heterotic pool of QTNs and employing reciprocal recurrent genomic selection are expected to improve yield performance of hybrid breeding in self-pollinated crops like rice in future53.
Stagnating yield in inbred varieties in breeders’ experiments
Absence of any genetic gain for grain yield has been demonstrated in successively generated inbred commercially released varieties compared to the early release of Indian semi-dwarf inbred varieties in 1968 by analyzing yield data from AICRIP7,9 and from IRRI’s international rice testing program11,31. Yields of major crops have stabilized or even stagnated in many regions of the world31,55,56,57. Any genetic gain of wheat, barley, rice and other cereal crops after 1967 is rightly attributed to modification of plant architecture, especially reduction in plant height, and the consequent non-lodging at high levels of nutrient application7 and increased harvest index and grains per unit area58. Nevertheless, AICRIP’s progress in conventional breeding has added numerous traits of value in rice especially the ability to withstand abiotic and biotic stress conditions leading to enhanced stability in yield performance and overall production in the country. Improvements in the quality of grains of inbred varieties have enabled huge rice exports. Increases in national rice production have confirmed the continued improvement in crop production skills at the farm level along with effective use of irrigation, water-use-efficient varieties and other factors9. Inbred LCV had recorded overall mean grain yields ranging from 4.8 to 5.5 t/ha (Table 2) in 2376 experiments across the country executed for 32 years along with hybrid F1 genotypes. The mean grain yields produced by inbred LCV were low even at the best performing 20 locations (5.0 to 5.7 t/ha). Nonetheless, the inbred check had shown high yields ranging from 9.1 to 9.8 t/ha at several locations. Overwhelming evidence is available to indicate the stagnating grain yields of LCVs since the release of inbred variety Jaya that established 10 t/ha as the easily realizable yield in 1968. A decisive introspection is warranted concerning crop husbandry practices that lead to low grain yields in inbred varieties compared to earlier records of high yields reported in support of their release. Unless the attainable yields are reached in inbred checks with appropriate crop production practices in an experiment, it is futile to make any comparison with new genotypes to estimate a genetic gain for grain yields.
Dilemma in choosing inbred or hybrid variety
Many indica and japonica hybrids have been cultivated in addition to inbred varieties in China. SY63 is a rice hybrid derived from the female parent Zhenshan 97 A (ZS97A, a WA CMS line) and the male parent MH63 in China59. The indica/japonica hybrid has more productive tillers, larger sink size, increased accumulation of non-structural carbohydrate in the stems at heading time and its remobilization to grains, higher enzyme activity for sucrose-to-starch conversion process in the grains, and greater photosynthesis during the ripening period; the hybrid has greater root biomass, deeper soil distribution at heading time, and higher root oxidation activity during the ripening period. Due to enhanced agronomic and physiological traits, indica/japonica hybrid produces improved yields under low N input conditions60. Deng et al61 estimated potential yields of 8.6 to 10.8 t/ha, while current farm yields varied from 5.2 to 8.8 t/ha across climate zones and rice systems in China. To ensure high yields, N fertilizer overdosing relative to actual needs is practiced by most rice farmers, especially in China. For example, approximately 50-90% of the N fertilizer is applied as a basal dressing, and the total N fertilizer input is as high as 270-330 kg/ha in Jiangsu Province, China62. These reports indicate that the estimated requirement is 27-33 kg N/t grains. Averaged across 11 years (Hunan Province, China, from 2004 to 2014), grain yield and N requirements were 9.5 t/ha and 20.2 kg/t or 192 kg/ha, respectively63. Application of 165 kg N/ha along with a plant density of 24-27×104 hills/ha significantly increased the grain yield in a widely cultivated hybrid Liangyou 390564. Super hybrid rice varieties in China had achieved record high yields of 14 t/ha (Chaoyou1000) 65 to 15 t/ha (YLY900) under optimal management conditions66. However, these high yield records occurred only in specific areas with adequate solar radiation, fertile soils and large differences between day and night temperatures etc67. Nitrogen requirement was calculated at 20 kg N/t grains from a database covering a wide range of climatic conditions, soil types and field management across China68. At 270 kg/ha N, the mean yield of japonica/indica hybrids was 12 t/ha (at 22.5 kg N/t grains) and was higher than 9-10 t/ha yield of conventional rice69. Breeding green super rice varieties was pursued to obtain higher grain yield with less environmental impact. The average grain yield of green super rice varieties developed in China was more or less similar to that of super hybrid rice varieties70. Chang et al39 conducted experiments in three plots planted (150 hills at 0.2 x 0.2 m) with elite super hybrids YLY1 or LYP9. Fertilizer was supplied according to local standard agronomic practice for growing rice with 250 kg N/ha, 150 kg P2O5/ ha and 250 kg K2O/ha. Grain yields from 50 hills were measured and used to derive the theoretical yield as 10-10.6 t/ha. The N required to produce one tonne grains was 24-25 kg/ha.
The theoretical yields calculated for super hybrids are closer to the estimates of national potential yield (9.8-10.5 t/ha) determined using the bottom-up approach61 and those using the top-down framework of Global Agro-Ecological Zone (GAEZ) protocols (IIASA-FAO 2012)71. Potential yield is a location-specific attribute as it depends on the crop growth duration and local weather. However, the current farm yield in China varied from 5.2 to 8.8 t/ha61. Huanghuazhan is the most common inbred rice cultivar planted in central and south China and is widely grown in 4.5 million ha across seven major rice-producing provinces (http://www.ricedata.cn/variety/) with high and stable yield, good quality, and wide adaptability. Although Huanghuazhan is an inbred cultivar, the yield is comparable to that of hybrid cultivars61,72. Therefore, it is clear that to harvest grain yields beyond 10 t/ha in hybrid super rice even in a limited area at select locations, adoption of intensive crop management and application of high levels of fertilizers and other input will be required. Rice grain yields of 10 to 11.6 t/ha in inbred varieties were recorded in yield trials performed at many locations in India7,31 and in international trials across rice growing areas in many countries of the world11,31. Muralidharan et al9 further concluded that the potential yield is limited to 15-16 t/ha and attainable yield to 10-11 t/ha with choice inbred varieties in the best rice growing stress-free environment under intensive management. He et al73 estimated hybrid rice to yield 7.5 t/ha when compared with 6.5 t/ha yields of inbreds in Jiangsu Province, China. From 1976 to 1995, hybrid rice varieties covering 15.7 million ha in China have yielded on an average 6.6 t/ha compared with 5 t/ha for conventional varieties74. Shanyou 63 (SY63), the most widely cultivated indica hybrid rice, had recorded 7325 kg/ha in various regional, provincial and national yield trials from 1982 to 1985. In spite of such yield reports, the increased grain production in China from 1984 to 2012 due to SY63 was estimated at an average yield increase of 300 kg/ha over check varieties59.
Li et al75 had analyzed the data set on the performance of 7686 rice varieties generated by the government crop variety administration department of China from the 2-3 year-long regional trials performed from 1978 to 2017 at 10-20 test sites to study the relationship between yield and other agronomic traits. A scrutiny of (Supplementary Data Sheet 1 Trait values for the 7686 rice varieties studied, Table_1_Exploring the Relationships Between Yield and Yield-Related Traits for Rice Varieties Released in China From 1978 to 201775 indicated that mean yields were 9.2 t/ha (4.6 to 12.4 t/ha) from 1809 japonica hybrids (156 days) compared to 8.7 t/ha (3.3 to 13.9 t/ha) from 296 japonica inbreds (153 days)75. The late maturing japonica (glutinous with low amylose content of 10-15% in grains or sticky rice) are grown in areas exposed to long days, and where grain filling continues for a longer period. Li et al75 further reported that the mean yields were 8 t/ha (3.8 to 13.6 t/ha) from 4814 indica hybrids (132 days) compared to 6.7 t/ha (3.2 to 12.4 t/ha) from 767 indica inbreds (123 days). Our analysis of 2070 indica hybrid F1 genotypes in trials of AICRIP performed in India from 1988-2019 also showed at 42 locations similar higher grain yields of 7.0 to 7.9 t/ha in early (110-120 days), mid-early (121-130 days) and medium (131-140 days) maturing hybrid F1 genotypes, and 5.8 to 7.6 t/ha in those with medium slender grains (130±5 days) as compared to 5.0 to 5.7 t/ha in 583 indica inbreds (Tables 4-7). In all these yield trials of AICRIP, yields of inbred varieties were lower than their known attainable levels. None of the hybrids tested could match the mean grain yields of ≥10 t/ha recorded by indica inbred varieties such as Rasi, IR 36 and IR 50 (early 115-120 days), Jaya and IR 8 (medium 131-135 days), and Swarnadhan and Savitri (late maturing >140 days) in rice yield trials7,9,11,31. The inbred variety Pusa 44 is well known to produce grain yields of 7 to 8 t/ha29. It was released for commercial cultivation in 1994 and matures in 140-145 days. In 2017, a mid-early maturing green super rice (GSR) variety HHZ of China was released in Punjab as PR126. It matured in 123 days and yielded on par with the dominant variety Pusa 44. The average productivity per day of PR126 was 61 kg/ha76. In AICRIP experiments, a higher level of productivity per day (62 to 63 kg/ha) was recorded with the early maturing and mid-early maturing hybrid F1 genotypes at several locations and in many experiments (Tables 4-7). The N requirement to produce 8 t/ha of hybrid rice has been estimated at a minimum of 160 kg N/ha or 20 kg N/t in China68,69 compared to 120 kg N/ha or 15 kg N/t used in AICRIP experiments with hybrid genotypes across India. Elevating the expression or activity of NGR5 (nitrogen-mediated tiller growth response) in rice can possibly reduce nitrogen fertilizer use while increasing grain yields further77. Application of an optimum level of nitrogenous fertilizer can additionally reduce emission of nitrous oxide, a greenhouse gas that is 300 times more potent than carbon dioxide in affecting climate change78. Nitrogen use is inefficiently distributed spatially across global food systems79. India has a large population and a large yield gap in rice production (~ 6 t/ha)9. The funds saved by reducing hybrid rice trials and test locations may be diverted to support sustainable intensification of rice production and research on precision farming and on increasing nitrogen use efficiency to reduce pollution.
Incidence of diseases and pests were also documented on hybrid genotypes tested in AICRIP trials80. Genetic gains in terms of resistance to diseases and pests have been remarkable. Within a decade after the release of Jaya in 1968, rapid strides were made in the development of stable and widely adaptable varieties that were insulated with resistance against biotic and abiotic stresses7,29. The Indian varieties that produced stable high yields in the international tests, or those which possessed resistance to stresses or good quality traits found wide acceptance and claimed release in several countries around the world31,81. Genetic gain for resistance was amply proved when several AICRIP improved genotypes were shown to possess resistance7,81,82. Besides yield attributes, it is a simple task to incorporate genes possessing resistance to pathogens and insect pests in susceptible inbreds to achieve definite genetic gains. Although restorer lines with resistance genes are used to generate hybrids, little information is available on their full expression in hybrids. It is apparently easier to manage resistance and prevent yield losses from diseases and pests in inbreds than in hybrids. The emerging molecular understanding appears to indicate a metabolic conflict between heterosis and defense mechanisms. Down-regulation of defense response genes in hybrid has been reported to lead to heterosis83,84. Yields in hybrid seed production of the three-line system are still in the range of 1.5 to 2.0 t/ha in Asia85. Hybrid rice seed production requires adoption of appropriate but non-farmer friendly cultural practices and skilled labor at premium wages to ensure a good seed set, which ultimately increases the cost of F1 seeds. Focus on increasing hybrid seed yield is needed. Apomixis, the asexual formation of seeds can be used by breeders to fix heterosis in hybrid seeds and rapidly generate doubled haploid crop lines to produce more hybrid seeds at a low cost. Using CRISPR/Cas9 gene editing technology, synthetic apomixis was established, but it reduced the numbers of clonal seeds produced86. This uneconomical and scanty hybrid seed production forces the Government of India to provide a massive financial support for training, production and distribution of hybrids seeds by public organizations and private seed business firms87. Yet, due to easy availability and much less price of seeds, farmers would undoubtedly prefer to grow high yielding inbred commercially released rice varieties to reduce the production cost. We have demonstrated that at the same level of cultural and agronomic management from 1988-2019, hybrid F1 genotypes with different maturity periods produced yields that are comparable or less than inbred LCV in 1391 experiments; in 985 experiments hybrid F1 genotypes produced yields that are ≥10% higher than yield of inbred varieties. Further, records on high yields of 10-13 t/ha in hybrid F1 genotypes and 9-10 t/ha in inbred varieties at many experiments beyond doubt proves the need to use efficient agronomic management practices in rice production to acquire higher yield benefits.