Red Rice.Weeds that drastically affect the cultivation of cereal in Cuba . It is, generally, a superior competitor to commercial cultivars, due to special production conditions that this weed presents, such as its early vigor, greater tillering, larger plant size and self-threshing capacity. The seeds can show dormancy and maintain their viability for many years, which facilitates their persistence as a weed in subsequent crop cycles.
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- 1 Generalities
- 3 Description
- 4 Biology
- 1 Seed dormancy and longevity
- 3 Flowering
- 4 Shelling
- 5 Competitive ability
- 5 Control Strategies
- 1 Prevention
- 2 Cultural methods
- 3 Mechanics
- 4 Chemical
- 5 Genetic and Biological
- 6 Conclusions
- 7 internal links
- 8 external links
The term red rice or weedy rice generally includes all species of the genus Oryza that behave like rice and grow as weeds of that crop . Red rice populations have been reported in many of the no-till areas for this crop; being a problem in the rice fields of Cuba . Although red rice belongs to different species and subspecies, they all share the ability to spread their grains before the rice is harvested.
Red rice can also adapt to a wide range of environmental conditions. Weedy rice grains frequently have a red pericarp and for this reason the term red rice is commonly adopted in the international literature to identify these wild plants. This term, however, is not entirely appropriate since red grains also exist in some cultivated varieties of rice and because, on the other hand, the red color may be absent in various forms of weedy rice.
The control of “red rice” is made difficult by its close taxonomic relationship with commercial rice and the complexity of applying measures that select differences between populations that are physically and physiologically very similar. This makes control with selective post-emergence herbicides very complicated.
Commercial rice panicles very different morphologically from red rice
Weedy rice infestations have been reported in 40-75% of the rice-growing area of European countries, 40% in Brazil , 55% in Senegal , 80% in Cuba , and 60% in Costa Rica ; although these values generally vary from one campaign to another, depending on the agrotechnical measures that are put into practice during cultivation .
Panicles with florets of red rice or weedy rice
The phylogenetic origin of weedy rice forms is closely related to that of cultivated rice. Many plants share most of the characteristics of the two cultivated species, Oryza sativa and O. glaberrima . O. sativa , which is also known as Asian rice, comprises the indica , japonica and javanica varietal groups and is cultivated throughout the world. O. glaberrima is known as African rice and is grown primarily in West Africa .
Weedy or red rice grains, without husk
The genus Oryza includes more than 20 wild species, most of which are diploid. Based on their morphological, physiological, and biochemical characteristics and their interrelationships in crosses, eight different genome types have been identified within the genus Oryza . Wild species such as Oryza perennis , O. nivara , O. rufipogon , and O. longistaminata share the same genome and can be easily crossed with cultivated species of O. sativa . The wild species O. barthii (= O. breviligulata ) is considered to be the parent of the African rice mutation.O. glumaepatula is a wild species endemic to Central and South America that is conventionally considered a subtype of O. rufipogon ; however, according to recent genetic analyses, it has been determined to be closer to African rice forms.
Experimental field of red rice or weedy rice
In addition to the species cited, they also cause problems such as weeds, Oryza latifolia , O. punctata , O. officinalis (perennial), and Zizianopsis miliaceae (perennial); these species can appear in rice fields , on the edge of ponds or in canals, impeding the flow of water. O.latifolia is a species of weedy rice widespread in Central America where it is known as “rice” or “duck rice”; it is up to 2 m tall and has seeds with a white pericarp.
Weedy rice plants present a great variety of anatomical, biological and physiological characteristics. A study carried out on 26 weedy rice samples from Uruguay revealed two main groups. One group included plants with black glumes, purple apex and long awn, showing obvious wild characters, while the other group had straw, glumes and apex and no awns, simulating cultivated varieties.
Commercial rice field invaded by red rice (inflorescences stand out)
At the seedling stage weedy rice plants are difficult to distinguish from crop plants while identification of weedy rice is possible after tillage thanks to multiple morphological differences evident with cultivated varieties: longer stems, numerous and fine, leaves often hispid on both surfaces, tall plants, pigmentation in various parts of the plant, seeds that disperse easily after formation.
The seeds of most biotypes of Oryza sativa and O. glaberrima present a pigmented pericarp caused by the presence of a variable content of different anthocyanins, catechins and catechol tannins.
Red pigmentation is a dominant character and is controlled by more than one gene. The red coating of the weedy rice grains could be removed with additional work in the mill but this operation results in breakage of the grains and a decrease in commercial value.
Seed dormancy and longevity
Unlike cultivated varieties, weedy rice has a variable degree of dormancy. The length of dormancy varies according to the biotype and the storage conditions of the seeds after shelling. The length of latency has been investigated in several countries under natural conditions. In Swaziland, Oryza punctata exhibited dormancy for more than one year and up to five years in East Africa . Viable weedy rice seeds with red pericarp remained dormant for up to two years in the United States of America and three years in Brazil .
Environmental conditions during seed formation and temperature and humidity during storage are considered the main factors that can affect the length of dormancy. Weedy rice seeds of the straw biotype of the same pericarp color, kept at -15 °C, showed a variable duration of dormancy in relation to the moisture content of the seeds after their maturation. Breaking dormancy was faster when seed moisture was between 6-14% and very low at moisture content below 5% or above 18%.
Seed longevity has been investigated in several studies that have shown contrasting results. In a study carried out in the United States of America, seeds from different weedy rice populations remained 90% viable after two years and 20% after seven years of being buried. The longevity of weedy rice seeds can be up to 12 years.
The emergence of red rice is strongly influenced by the texture of the soil, the presence of water in the field and the depth at which the seed is buried, which is directly related to the type of tillage that has been adopted for the preparation of the crop. seed bed. The seedlings that emerge before rice planting are mostly those that emerge from the seed bank in the soil. Almost all plants that grow freely in untilled soil are able to emerge from mid-April to mid-March after having accumulated 200 degree/day of growth. The minimum temperature for weedy rice germination is considered to be around 10 °C, the same as for cultivated varieties.
Seed age , burial depth, flood conditions and heavy soils have a negative influence on weed germination and emergence .
The emergence of seedlings located in the 0-1 cm layer was completed in 14 days in the moist soil and in 18 days in the flooded soil. Seeds buried at a depth of 4-5 cm showed a 15-day delay in germination, compared to seeds that were placed near the soil surface. This behavior could be one of the reasons for the continuous emergence of these seeds in the rice fields.
In cultivated varieties and wild varieties, flowering begins on the upper florets of the panicle and continues down to the lower florets. In red rice, the florets begin to open between 08:00 and 09:00 in the morning and continue for at least an hour longer than in cultivated varieties. For this reason, although all plants of all rice species are selfing, cross-pollination is higher in weedy rice than in cultivated varieties. The probability of crosses occurring between red pericarp weedy rice plants and cultivated varieties has been studied by several researchers.
Flowering is induced by day length (shorter photoperiods favor flowering), plant age (greater for younger plants) and biotype (greater for biotypes from higher latitudes).
Pigmentation of the glumes and seed begins to develop in the terminal spikelet a few days after anthesis and continues as the spikelet matures.
Early shelling is a specific characteristic of weedy rice. This behavior is controlled by the Sh gene, which presents the character of shelling in dominant conditions of homozygosis (ShSh) or heterozygosis (Shsh). The seed falls as a result of the formation of an abscission layer made up of three layers of cells located between the spikelet and the pedicel. This layer of cells is not completely formed in cultivated varieties and some bands of lignified tissue maintain the attachment of the spikelets to the pedicel.
The germinability of fallen seeds was very low during the first 15 days after flowering, with a maximum of 5%. This behavior can probably be explained by the incomplete development of the initially fallen seeds due to climatic elements such as the wind. The seeds that fell after 15 days of flowering contained full and physiologically mature grains.
Weedy rice can cause severe yield losses to cultivated rice in relation to density, type of weedy rice plants and cultivated varieties. Short varieties are generally more susceptible to competition from red rices than tall varieties. Several studies have been carried out to assess the effects of different weed densities. In studies carried out in Arkansas , USA , the yield of the semidwarf cultivar ‘Lemont’ was affected by weedy-rice density as low as two plants/m2. Five and 20 plants/m2 of weedy rice caused a yield loss of 40 and 60 %, respectively, in the cultivar ‘Oryzica 1’.
Combining the effects of weedy rice density and the duration of the competition period, a 50% reduction in yield has been observed when 24 plants/m2 of weedy rice competed with the crop during the first 40 days after emergence. . With the same initial density, the yield loss reached 75% in the case of competition throughout the season. In a greenhouse experiment, significant effects on rice plant growth were recorded only when the competition period was greater than 70 days from emergence.
Red rice control methods that can be applied in rice cultivation are costly, time consuming and usually do not lead to complete eradication of the weed infestation. Incomplete weed control in a given year can lead to nullifying the results of several years of good control. Weedy-rice escapements of 5% or less can produce enough seed to restore the original population of the seed bank in the soil .
The control of weedy rice plants is much more difficult than the control of other weeds due to their great morphological variability, the particular behavior of their growth and the great biological affinity with the cultivated varieties.
Chemical control of weeds in rice with selective herbicides is generally not as effective in weedy rice, with the sole exception of transgenic varieties that have been appropriately transformed to tolerate selective herbicides for rice grown with a large activity spectrum. For this reason, chemical weed control cannot be applied to cultivated rice during its growth except when herbicides are applied with the impregnated glove or rope system in combination with short varieties.
Control is also more complicated due to germination spaced over a long period of rice growth. The great elasticity of the germination process can favor the competitive activity of weedy rice plants that are able to germinate before cultivated plants or allow the escape of control treatments carried out in the pre-planting of rice.
Prevention is the fundamental means to reduce weed infestation and can be done mainly by sowing rice seed free of weedy rice seeds. This measure, however, is not so easy to apply since the dressed seeds of weedy rice are similar to the seeds of the cultivated varieties, regardless of the color of the pericarp.
The white and red-pigmented seeds are difficult to recognize as the color of the pericarp is visible only after the seed has had its glumes removed. The only possibility of obtaining rice seed free of weedy rice seed is to inspect the fields destined for seed production and remove the weedy rice plants, even manually, before starting the harvest of the seeds of the varieties. of rice.
Another important element of prevention is to ensure perfect cleaning of the equipment used during the rice harvest in order to prevent the spread of weedy rice to non-infested fields.
The best control of weedy rice can be obtained by means of rotations, but this practice cannot be applied in certain particular environmental conditions such as saline or hydromorphic soils. Crops that normally enter the rotation with rice in temperate climates include soybeans , corn , sunflowers , sorghum , and other crops .
The introduction of mung bean cultivation in Viet Nam led to a huge decline in weedy rice plants and other species. Under these conditions many weeds can normally emerge but do not complete their cycle because of insufficient soil moisture during the mung bean growing season .
The emergence of weedy rice basically depends on the type of soil tillage that has been done for the preparation of the seed bed and its water content during weed germination.
Minimum tillage done no more than 10 cm deep and with good soil moisture conditions create the best conditions for weedy rice emergence, while plowing and soil flooding severely affect weed seed germination. weeds. Seedbed preparation with a plowshare can be considered a useful agronomic means when the degree of infestation is low and chemical herbicide applications are not planned.
Control of weedy rice plants is sometimes done manually, but it is a costly and time-consuming practice. Manual removal of weeds is practically impossible in the first 30-40 days after crop emergence as it is difficult to distinguish seedlings of cultivated rice varieties from those of weedy rice at these early stages. This manual removal is then done when most of the damage caused by the competition has already occurred.
Manual removal of weedy rice plants is sometimes carried out in the case of light infestations and is carried out together with other means of control such as chemical control, in the event that the first has not given satisfactory results and in order to to avoid grain dispersion. The manual control method is important in fields intended for seed production to obtain materials free of weed seeds.
Various techniques using mechanical means can be applied to control weedy rice. Most of these can be applied before crop planting, after red rice emergence, which can be stimulated by tillage done for seed bed preparation.
Weed control obtained with this practice is satisfactory but is more time consuming and generally less efficient than chemical treatment . Mechanical control can also favor the emergence of new weeds after application due to the stimulation of germination of seeds brought to the soil surface by machinery.
The close anatomical and physiological similarity of cultivated and weedy rice makes control with selective postemergence herbicides extremely difficult. The most successful management techniques are based on the application of herbicides before planting the crop, before and after weed emergence.
Some antigerminative herbicides such as chloroacetamides, thiocarbamates and dinitroanilines, applied alone or in mixtures with other herbicides, were effective in controlling weedy rice before its emergence.
Good control of these weeds -often greater than 75%- can be obtained under rice growing conditions in Europe with pretilachlor and dimethenamid, used alone or in combination, at a rate of 1.5 kg ai/ha and 0. 48 kg ai/ha, respectively. To avoid any risk of phytotoxicity, both herbicides must be applied at least 25 days before planting the rice.
Genetic and Biological
The genetic and biotechnological approach is being consistently adopted to address abiotic and biotic problems in rice cultivation such as water scarcity, high and low temperatures, pests and diseases, and weed control .
The weedy rice problem can be tackled with the introduction of herbicide -tolerant varieties that allow selective post-emergence control of this plant.
Many traditional plant breeding works , and especially biotechnological research, have been aimed at obtaining varieties resistant to glyphosate, glufosinate ammonium, imidazolinones and broad-spectrum herbicides that are not selective for traditional rice varieties. Several glyphosate- and sulfosate – tolerant rice lines have been identified among more than 14,000 samples originating from Brazil, Colombia , the United States of America, and India .
Several species of the genus Oryza behave as weeds even though they share many of the characteristics of cultivated rice varieties.
They are undesirable, first of all, because their seeds can easily fall before the crop is harvested and remain dormant in the soil for a long time . Weedy rice varieties are similar to commercial varieties in both plant morphology and herbicide tolerance. Due to their high competitive capacity, these weeds can notoriously affect rice yields.
Weedy rice control cannot be based on a single method but must rest on a complex management program based on an appropriate combination of preventive, cultural, mechanical, chemical and genetic means. Preventive practices include the use of weedy rice seed-free rice seed and clean work equipment and machinery and are the starting point for successful application of other means of control. Among cultural practices, rotation is often the best way to reduce severe weedy rice infestations.
The introduction of herbicide -resistant varieties offers rice farmers a good opportunity to manage weedy rice and other weeds, although their success depends on the extent to which breeding strategies can prevent the transfer of resistance genes. to red rice.