Red Rice. Weed that dramatically affects cereal cultivation in Cuba . It is generally a superior competitor to commercial cultivars, due to special production conditions presented by this weed, such as its early vigor, higher tillering, larger plant size and self-shelling capacity. The seeds can have dormancy and maintain their viability for many years, which facilitates their persistence as weeds in the subsequent crop cycles.
Summary
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- 1 Generalities
- 2 Origin
- 3 Description
- 4 Biology
- 1 Latency and longevity of seeds
- 2 Emergency
- 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
- 9 Source:
Generalities
The term red rice or weed-rice generally includes all the species of the Oryza genus that behave like rice and grow as weeds of that crop . Red rice populations have been reported in many of the direct sowing 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. The weedy rice grains frequently present the red pericarp and for this reason the term red rice is commonly adopted in 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 color red may be absent in various forms of weedy rice.
Control of “red rice” is made difficult due to 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 difficult.
Panicles of commercial rice very different morphologically from red rice
Weed-rice infestations have been reported in 40-75% of the rice 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 season to the next, depending on the agrotechnical measures that are put in place during cultivation .
Panicles with florets of red rice or weedy rice
Origin
The phylogenic origin of the weed-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 worldwide. O. glaberrima is known as African rice and is mostly grown in West Africa .
Weedy or red rice grains, shelled
The Oryza genus 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 types of genomes have been identified within the Oryza genus . Wild species such as Oryza perennis , O. nivara , O. rufipogon, and O. longistaminata share the same genomium and can be easily crossed with cultivated species of O. sativa . The wild species O. barthii (= O. breviligulata ) is considered the progenitor of the African rice mutation.O. glumaepatula is a wild species endemic to Central and South America that is conventionally considered to be a subtype of O. rufipogon ; however, recent genetic analysis has determined that it is closer to forms of African rice.
Experimental field of red rice or weedy rice
In addition to the mentioned species, they also cause problems such as weeds, Oryza latifolia , O. punctata , O. officinalis (perennial) and Zizianopsis miliaceae (perennial); These species can appear in rice crops , on the edge of ponds or in channels, preventing 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.
Description
Weedy rice plants have a wide variety of anatomical, biological and physiological characteristics. A study carried out on 26 samples of weedy rice from Uruguay revealed two main groups. One group included plants with black glumes, purple apex and long edge, showing evident wild characters, while the other group had straw, glumes and apex and had no edges, simulating cultivated varieties.
Commercial rice field invaded by red rice (inflorescences stand out)
In the seedling stage, weedy rice plants are difficult to distinguish from cultivated plants, while weed-rice identification is possible after tillage thanks to multiple evident morphological differences with cultivated varieties: longer stems, numerous and fine, often hispid leaves on both surfaces, tall plants, pigmentation in various parts of the plant, seeds that are easily dispersed 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 catecholic tannins.
Red pigmentation is a dominant character and is controlled by more than one gene. The red layer of the rice-weed grains could be removed with additional work in the mill but this operation caused the breakage of the grains and a decrease in the commercial value.
biology
Latency and longevity of seeds
Unlike the cultivated varieties, weedy rice presents a variable degree of latency. The duration of dormancy varies according to the biotype and storage conditions of the seeds after shelling. Latency duration has been investigated in various countries under natural conditions. In Swaziland, Oryza punctata was dormant for over a year and up to five years in East Africa . Viable weed 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 dormancy duration. The weed rice-biotype seeds with straw of the same color of the pericarp, kept at -15 ° C showed a variable duration of dormancy in relation to the moisture content of the seeds after maturation. The dormancy break was faster when the seed moisture was between 6-14% and very low at moisture content of less than 5% or greater than 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 populations of weedy rice remained 90% viable after two years and 20% after seven years of being buried. The longevity of the weed-rice seeds can be up to 12 years.
Emergency
The emergence of red rice is strongly influenced by the texture of the soil, the presence of water in the field and the depth to which the seed is buried, which is directly related to the type of tillage that has been adopted for the preparation of the seed bed. The seedlings that emerge before planting the rice are mostly those born from the seed bank of 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 degrees / day of growth. The minimum temperature for germination of weed-rice is considered to be around 10 ° C, the same as for cultivated varieties.
Seed age , depth of ground, 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 wet soil and in 18 days in flooded soil. Seeds buried at a depth of 4-5 cm showed a delay of 15 days in germination, compared to seeds that were placed close to the soil surface. This behavior could be one of the reasons for the continuous emergence of these seeds in the rice fields.
Flowering
In cultivated and wild varieties, flowering begins at the upper panicle florets and continues toward 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 self-fertilized, cross-pollination is higher in weedy rice than in cultivated varieties. The likelihood of crossbreeding between red pericarp weed-rice plants with cultivated varieties has been studied by several researchers.
Flowering is induced by the length of the day (short photoperiods favor flowering), by the age of the plant (it is greater for younger plants) and the biotype (it is greater in biotypes of higher latitudes).
Glumes and seed pigmentation begin to develop in the terminal spikelet a few days after anthesis and continue as the spikelet matures.
Shelled
Early shelling is a specific characteristic of weedy rice. This behavior is controlled by the Sh gene that exhibits the character of shelled under dominant conditions of homozygosity (ShSh) or heterozygosity (Shsh). The seed falls as a result of the formation of an abscission layer formed by three layers of cells located between the spikelet and the pedicel. This cell layer is not fully formed in cultivated varieties and some bands of lignified tissue maintain the attachment of the spikelets to the pedicel.
The germinability of the fallen seeds was very low during the first 15 days after flowering, with a maximum of five percent. This behavior can probably be explained by the incomplete development of the seeds initially fallen due to climatic elements such as wind. The seeds that fell after 15 days from flowering contained full ripe and physiologically ripe kernels.
Competitive ability
Weedy rice can cause severe yield losses to cultivated rice in relation to density, type of weedy rice plants and cultivated varieties. Low varieties are generally more susceptible to competition from red rice than tall varieties. Several studies have been carried out to assess the effects of different weed densities. In studies carried out in Arkansas , United States of America, the yield of the semi-dwarf cultivar ‘Lemont’ was affected by a weed-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 weed-rice density and length of the competition period, a 50% yield reduction was observed when 24 plants / m2 of weed-rice competed with the crop during the first 40 days after emergence . At the same initial density, the performance loss reached 75% in the case of competition throughout the season. In an experiment in the greenhouse, significant effects on the growth of rice plants were recorded only when the competition period was greater than 70 days from emergence.
Control strategies
Red rice control methods that can be applied to rice cultivation are expensive, time consuming and generally do not lead to complete eradication of weed infestation. Incomplete weed control in a given year can lead to the nullification of the results of several years of good control. Weed rice leaks of 5% or less can produce enough seeds to restore the original seed bank population in the soil .
Control of weedy rice plants is much more difficult than 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 high spectrum of activity. 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 low varieties.
Control is also more complicated based on spaced germination over a long period of rice growth. The high elasticity of the germination process can favor the competitive activity of weedy rice plants that are capable of germinating before the cultivated plants or allow the escape of control treatments carried out in the rice pre-sowing.
Prevention
Prevention is the primary means of reducing weed infestation and can be done primarily by sowing rice seed free of weed-rice seed. This measure, however, is not so easy to apply since the seeds dressed in weedy rice are similar to the seeds of the cultivated varieties, without considering the color of the pericarp.
White and red pigmented seeds are difficult to recognize as the color of the pericarp is visible only after the glumes have been removed from the seed. The only possibility of obtaining rice seed free from weed-rice seed is to inspect the fields intended for seed production and remove the weed-rice plants, even manually, before starting the harvest of the seed 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 avoid the spread of the weed-rice to the non-infested fields.
Cultural methods
The best weed-rice control can be obtained through rotations, but this practice cannot be applied in certain particular environmental conditions such as saline or hydromorphic soils. Crops that normally go into rotation with rice in temperate climates include soybeans , corn , sunflowers , sorghum and other crops .
The introduction of mung bean cultivation in Viet Nam resulted in a huge decline in weedy rice plants and other species. Under these conditions many weeds can normally emerge but do not complete their cycle due to insufficient soil moisture during the mung bean growing season .
The emergence of weed-rice basically depends on the type of tillage of the soil that has been done for the preparation of the seed bed and on its water content during the germination of the weed.
The minimum tillage made at no more than 10 cm depth and with good soil moisture conditions creates the best conditions for the emergence of the weed-rice, while plowing and soil flooding severely affect the germination of the seeds of the weeds. Seedbed preparation with a plow can be considered as a useful agronomic medium when the infestation degree is low and chemical herbicide applications are not planned.
Control of weed-rice plants is sometimes done manually, but an expensive and time-consuming practice. Manual removal of weeds is practically impossible in the first 30-40 days after crop emergence since seedlings of cultivated rice varieties are difficult to distinguish at those early stages from those of weedy rice. This manual removal is then done when most of the damage caused by 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 case the former has not given satisfactory results and in order to avoid the dispersion of the grains. The manual control method is important in fields intended for seed production to obtain weed-free seed materials.
Mechanics
Various techniques using mechanical means can be applied to control weedy rice. Most of these can be applied before planting the crop, after the emergence of red rice, which can be stimulated by tillage for seed bed preparation.
Weed control obtained with this practice is satisfactory but takes more time and is generally less efficient than chemical treatment . Mechanical control can also favor the emergence of new weeds after their application due to the stimulation of germination of seeds brought to the soil surface by machinery.
Chemical
The close anatomical and physiological similarity of cultivated rice and weedy rice makes control with selective post-emergence herbicides extremely difficult. The most successful management techniques are based on the application of herbicides before planting the crop, before and after the emergence of weeds.
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 the conditions of planting rice 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 weed-rice problem can be faced 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 varieties of rice. Several glyphosate and sulphosate tolerant rice lines have been identified among more than 14,000 samples originating in Brazil, Colombia , the United States of America and India .
Conclusions
Several species of the Oryza genus behave like weeds even though they share many of the characteristics of cultivated rice varieties .
They are undesirable, firstly because the seeds can easily fall before the harvest of the crop and remain dormant in the soil for a long time . The varieties of weedy rices are similar to commercial varieties in terms of both plant morphology and herbicide tolerance. Due to their high competitive capacity, these weeds can notoriously affect rice yields.
Weed rice control cannot be based on a single method but must rest on a complex management program and based on an appropriate combination of preventive, cultural, mechanical, chemical and genetic means. Preventive practices include the use of weed-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 weed-rice infestations.
The introduction of herbicide- resistant varieties offers rice growers a good opportunity to manage weed-rice and other weeds, although their success depends on the extent to which cultivation strategies can prevent the transfer of resistance genes. to red rice.
