Improving grain yield and lodging resistance are two important targets for improving rice production. The aim of this study was to link grain yield and lodging resistance with growth patterns in rice. A nitrogen (N) fertilization experiment was conducted in 2020 and 2021, which consisted of two N rates (150 and 225 kg ha–1) and three N split-application ratios among the basal, early tillering, and panicle initiation stages (6:3:1, 5:3:2, and 4:3:3). The results demonstrated that increasing the N rate from 150 to 225 kg ha–1 did not affect grain yield but increased the plant lodging index by 16%. This increase in the plant lodging index was attributable to a higher plant fresh weight resulting from improved post-heading growth and a lower plant lodging load, which was partially due to a higher leaf area index. Altering the N split-application ratio from 6:3:1 to 4:3:3 increased grain yield by 4% but did not affect the plant lodging index in rice. The increase in grain yield was due to improved post-heading growth mainly resulting from increased radiation use efficiency during the post-heading period, which was partially attributable to increased specific leaf weight. An improvement in pre-heading growth in 2021 did not affect grain yield but increased the plant lodging load and decreased the plant lodging index by 36% compared to 2020. The improvement in pre-heading growth was due to a higher intercepted solar radiation resulting from higher leaf area index and incident solar radiation during the pre-heading period. The results of this study indicate that improving pre-heading growth can maintain grain yield and increase lodging resistance, while improving post-heading growth by increasing canopy radiation use efficiency (but not the leaf area index) can increase grain yield and maintain lodging resistance.

The occurrence of chalkiness has decreased in new hybrid rice cultivars in China. As both chalkiness occurrence and starch granule size are associated with the biosynthesis of starch, we hypothesized that there may be a correlation between chalkiness occurrence and starch granule size, and this may partially explain the decreased chalkiness occurrence in the new hybrid rice cultivars. To test this hypothesis, a field experiment was conducted over eight environments (two years × four sowing dates) with two hybrid rice cultivars: one recently developed with low chalkiness occurrence, Jingliangyou 1468 (JLY1468) and a relatively older cultivar with high chalkiness occurrence, Liangyoupeijiu (LYPJ). Results showed that JLY1468 had a higher cumulative distribution of large-diameter (7.51–19.50 μm) starch granules and a lower grain weight of milled rice compared to LYPJ. As a consequence, mean and relative starch granule diameters were 6% and 21% higher in JLY1468 than in LYPJ, respectively. Although both the chalky grain rate and chalkiness degree were negatively correlated with mean and relative starch granule diameter, they were more closely correlated with the relative granule diameter. These results support our hypothesis regarding the relationship between chalkiness occurrence and starch granule size and suggest that the relative starch granule diameter is a relevant parameter in understanding the occurrence of chalkiness in hybrid rice.

Weeds are ubiquitous and economically damaging in southern U.S. rice systems. Barnyardgrass has consistently been one of the most prevalent and troublesome of these. Although most rice cultivars do not suppress weeds dramatically, certain Indica cultivars and commercial hybrids are known to suppress barnyardgrass aggressively in conventional, drill-seeded rice systems in the southern United States. A field study was conducted to determine the degree to which either reducing or increasing standard seeding rates would affect natural suppression of weeds by conventional inbred and weed-suppressive cultivars. Five cultivars were evaluated at three seeding rates (160 [low], 320 [medium; conventional recommendation for inbred cultivars], and 480 [high] seeds m−2) and two weed levels (weed-free and weedy). Cultivars included a conventional, non–weed suppressive long-grain, ‘Wells’; high-tillering weed-suppressive cultivars ‘PI312777,’ ‘Rondo,’ and ‘4612’ from Asia; and the commercial hybrid ‘XL723.’ Overall, PI 312777 produced the most tillers, whereas XL 723 exhibited the greatest midseason shoot biomass and the greatest weed suppression. Yields of PI 312777 and 4612, both of which are Indica cultivars considered to be good weed suppressors, changed minimally across all seeding rates when compared with the other cultivars and thus tolerated weeds at the low rate nearly as well as at the high rate. Such a tolerance to weeds might be useful in the maintenance of weed suppression at reduced rice-seeding rates and suggests that reduced seeding rates of PI 312777 and 4612 would be less risky for yield loss when compared with the other cultivars tested. Visual suppression ratings were positively correlated with rice yield within weed-infested plots, suggesting that yield performance under weed pressure might be a good indicator of weed-suppression ability of cultivars in these systems. In contrast with PI 312777 and 4612, yields of the conventional inbred cultivar and commercial hybrid appeared to benefit from the high seeding rate. Overall, moderate to high seeding rates are likely to be needed for consistent weed suppression for all of the cultivar types evaluated in this study.

Hybrid rice development, a major advance in rice breeding, has demonstrated its immense potential in enhancing the rice productivity. In the three line system of hybrid rice, the restoration of fertility of the widely used wild-abortive type of cytoplasmic male sterility is controlled by Rf3 and Rf4, two fertility restoration genes. Exploration for restorers in diverse genetic backgrounds is a critical step in hybrid rice breeding. As genetic resources constitute the major source of new genes/alleles, an attempt was made to assess the distribution of these two restorer genes in the native populations collected from the geographic regions known to be the primary and secondary centres of origin of rice. In addition, assays were performed on the populations of Oryza rufipogon and Oryza nivara, the wild progenitors of rice. The results suggest that the Rf genes are well distributed in the wild forms at a higher frequency, more in O. rufipogon, while in cultivated forms, their frequency is low, particularly for the Rf3 gene. The genes and the alleles identified in the genetically divergent, low-performing land races and wild relatives can be successfully employed for the development of new hybrid rice cultivars.

Cultivated rice yield losses due to red rice infestation vary by cultivar, red rice density, and duration of interference. The competition effects of red rice could be influenced further by emergence characteristics, red rice biotype, and planting time of cultivated rice. We aimed to characterize the emergence of red rice biotypes at different planting dates and evaluate the effect of red rice biotype, rice cultivar, and planting date on cultivated rice yield loss. Field experiments were conducted at the Southeast Research and Extension Center, Rohwer, AR, and at the Arkansas Rice Research and Extension Center, Stuttgart, AR, in the summer of 2005 and 2006. The experimental design was a split-split plot with three or four replications. Planting time, ClearfieldTM (CL) rice cultivar, and red rice biotype were the main plot, subplot, and sub-subplot factors, respectively. There were three planting times from mid-April to mid-May at 2-wk intervals. CL rice cultivars, CL161 and hybrid CLXL8, and 12 red rice biotypes were planted. The emergence rate and coefficient of uniformity of germination differed among some red rice biotypes within a planting time. Planting date affected the emergence characteristics of red rice biotypes. The mean emergence rate of red rice was 0.043 d−1 in the mid-April planting and 0.058 d−1 in the late April planting. For the mid-April planting, 50% of red rice biotypes emerged in 20 ± 2 d compared with 15 ± 2 d for CL rice cultivars. Yield losses due to red rice biotypes generally increased in later planting dates, up to 49%. Yield losses due to interference from red rice biotypes ranged from 14 to 45% and 6 to 35% in CL161 and CLXL8, respectively. Cultivated rice became less competitive with red rice in later plantings, resulting in higher yield losses.

The response of new, high-yielding rice cultivars to varying barnyardgrass control levels and rice plant densities is not well understood. Studies were conducted in 2002 through 2004 at the Rice Research and Extension Center near Stuttgart, AR, to evaluate three new rice cultivars at plant densities ranging from 79 to 392 plants/m2 grown in competition with barnyardgrass. Representatives from each of the three classes of long-grain rice produced in the United States were selected. ‘Wells’ represented conventional, long-grain rice, ‘CL161’ represented semidwarf, imidazolinone-tolerant, long-grain rice, and ‘XL8’ represented hybrid, long-grain rice. Rice density and barnyardgrass control affected rice total aboveground biomass production, panicle weight, and harvest index. As rice density decreased or barnyardgrass control increased, total aboveground biomass production and harvest index increased. Rice density had no effect on panicle density or yield. Panicle density increased 14 panicles/m2 for every 10% increase in barnyardgrass control. Cultivar yields were affected similarly by barnyardgrass control, and increased 750 kg/ha for each 10% increase in barnyardgrass control. XL8 and Wells produced the highest average yields, with CL161 producing the lowest over the three-year experiment. XL8 yield components fluctuated more than CL161 or Wells over the range of rice densities and barnyardgrass control levels, indicating that XL8 is highly sensitive to intra- and interspecific competition at high plant populations.

Research was conducted in 2012 and 2013 to evaluate the response of the rice cultivars ‘Cheniere', ‘CL151′, ‘Caffey', ‘CL261′, and ‘CLXL745′ response to POST applications of saflufenacil. Treatments included a nontreated control, saflufenacil at 50 g ai ha−1, and carfentrazone at 35 g ai ha−1 applied mid-POST (MPOST) to rice in the three- to four-leaf stage. Pooled across cultivars, injury was greatest at 3 and 7 d after treatment (DAT), 21 and 17%, respectively. Rice injury was only 5% at 14 DAT and 1% at 28 DAT. Hybrid long-grain cultivar CLXL745 was injured more than inbred long-grain cultivars CL151 and Cheniere. Cheniere was more tolerant than inbred medium-grain cultivars CL261 and Caffey. All cultivars exhibited tolerance to saflufenacil as evidenced by similar normalized difference vegetative index (NDVI), maturity, mature plant height, and rice yield (rough, whole, and total milled rice).

Previous research has examined the extent to which red rice affects both yield and grain quality of cultivated rice. However, this research was conducted over 15 yr ago. Modern long-grain rice cultivars have the potential to produce yields above 10,000 kg ha−1; however, it is unknown whether modern rice cultivars sacrifice competitiveness to achieve higher yields, or if, in fact, they are more competitive. Field studies were conducted in 2002 and 2003 at the Southeast Research and Extension Center near Rohwer, AR, and at the University of Arkansas Pine Bluff Research Farm near Lonoke, AR, to investigate the effect of red rice density on interference between red rice and five rice cultivars (‘CL161’, ‘Cocodrie’, ‘LaGrue’, ‘Lemont’, and ‘XL8’). White rice yield reductions were between 100 and 755 kg ha−1 for every red rice plant m−2. The hybrid rice, XL8, had higher yields than the conventional cultivars. Red rice contamination in milling samples increased linearly as a function of red rice density at Lonoke and Rohwer in 2003. Dockage for each cultivar was calculated on the basis of the relationship between red rice density and red rice contamination. Semidwarf Lemont was the most contaminated and hybrid XL8 the least contaminated by the various densities of red rice.

To maximize heterosis, it is important to understand the genetic diversity of germplasm and associate useful phenotypic traits such as fertility restoration for hybrid rice breeding. The objectives of the present study were to characterize genetic diversity within a set of rice germplasm groups using coefficient of parentage (COP) values and 58 simple sequence repeat (SSR) markers for 124 genotypes having different attributes such as resistance/tolerance to various biotic and abiotic stresses. These lines were also used for identifying prospective restorers and maintainers for wild abortive-cytoplasmic male sterile (CMS) line. The mean COP value for all the lines was 0.11, indicating that the genotypes do not share common ancestry. The SSR analysis generated a total of 268 alleles with an average of 4.62 alleles per locus. The mean polymorphism information content value was 0.53, indicating that the markers selected were highly polymorphic. Grouping based on COP analysis revealed three major clusters pertaining to the indica, tropical japonica and japonica lines. A similar grouping pattern with some variation was also observed for the SSR markers. Fertility restoration phenotype based on the test cross of the 124 genotypes with a CMS line helped identify 23 maintainers, 58 restorers and 43 genotypes as either partial maintainers or partial restorers. This study demonstrates that COP analysis along with molecular marker analysis might encourage better organization of germplasm diversity and its use in hybrid rice breeding. Potential restorers identified in the study can be used for breeding high-yielding stress-tolerant medium-duration rice hybrids, while maintainers would prove useful for developing new rice CMS lines.

The characterization of genetic diversity and structure for improved cultivated varieties/elite lines is tremendously important to assist breeders in parental selection for inbred and hybrid breeding and heterotic group construction. In this study, a total of 737 improved indica varieties/lines developed recently and/or widely used by present indica breeding programmes worldwide were genotyped with a 384-single-nucleotide polymorphism assay. Model-based population structure analysis revealed the presence of two major groups with six subgroups (SGs), wherein no clear correlation was found between the groups/SGs and breeding programmes or geographical origin of the accessions. Over half of the accessions (51.8%) appeared to have less than 0.6 memberships assigned to any one of the six model-based groups, highlighting the wide range of gene flow within improved indica varieties/lines and the genetic integration of valuable alleles shared by ancestries among improved high-yielding varieties/lines through germplasm exchanges. Distance-based clustering revealed that Latin-American cultivated indica lines have tended to form their own ecological cline, which could serve as a potential heterotic ecotype for hybrid rice breeding, although they are still closely related to Asian indica lines. African cultivated indica lines, on the other hand, have not yet formed their own ecological cline. It was also observed that the most well-known hybrid rice parents, Zhenshan97B and Minghui63, were unexpectedly clustered in the same SG with a relatively narrow genetic distance, which suggests that a significant genetic distance between parents is not a prerequisite for all elite hybrid rice lines with high heterosis.

Investigation of genetic diversity and the relationships among varieties and breeding lines is of great importance to facilitate parental selection in the development of inbred and hybrid rice varieties and in the construction of heterotic groups. The technology of single nucleotide polymorphism (SNP) is being advanced for the assessment of population diversity and genetic structures. We characterized 215 widely cultivated indica rice varieties developed in southern China and at the International Rice Research Institute (IRRI) using IRRI-developed SNP oligonucleotide pooled assay (OPA) to provide grouping information of rice mega-varieties for further heterotic pool study. The results revealed that the Chinese varieties were more divergent than the IRRI varieties. Two major subpopulations were clustered for the varieties using a model-based grouping method. The IRRI varieties were closely grouped and separated clearly from the majority of the Chinese varieties. The Chinese varieties were subclustered into three subgroups, but there was no clear evidence to separate the Chinese varieties into subgroups geographically, indicating a great degree of genetic integration of alleles and shared ancestries among those high-yielding modern varieties.