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Crop any plant product grown, harvested or collected for human use. Over 80 crops are grown worldwide, providing people with the major part of their food and supplying fibers, construction materials, pharmaceuticals, beverage, rubber, dyes and other materials. Crops grown for export are cash crops, while a cash crop is one grown in the interval between two main crops.

Crops are usually recognized under four major groups. The food crops provide the bulk of people's food worldwide; the major types being cereals, tubers, roots, pulses, vegetables, oil crops, fruits, tree nuts, sugarcane, and spices. Cereals make the largest contribution to human nutrition. In Bangladesh the major cereals are rice and wheat; the minor cereals include barley/jab, joar, bazra, cheena and kaon; of the oil seeds, rape and mustard are most important.

Forage crops are grown to feed livestock. Grasses dominate this group although cultivation in not widely practiced in Bangladesh. Bhadoi and Rabi fodder are cultivated in many districts. mulberry is mostly grown in Rajshahi and Nawabganj districts.

Fiber crops include jute, cotton, hemp and kenaf. Jute dominates among fiber crops having about 7,37,000 m tons annual production. Miscellaneous crops of Bangladesh include tea, tobacco, rubber, ornamental flowers, and plants that produce perfumes, pharmaceuticals, and dyes. Tea constitutes the second cash crop of Bangladesh. Its cultivation is however confined to Sylhet and Chittagong regions. [SM Humayun Kabir]

See also pulse; rice; wheat.

Major crop Crops that are grown on one percent or more of the gross-cropped area (GCA) of a country. In Bangladesh, only nine crops- rice (73.94%), wheat (4.45%), jute (3.91%), rape and mustard (3.08%), lentil (1.54%), chicklling vetch (1.25%), potato (1.13%), sugarcane (1.12%), and chilli (1.05%) are grown on 1 percent or more of the crop acreage (14.61 million ha) and may be considered as major crops.

Rice dominates the cropping pattern throughout Bangladesh. It has been broadly divided into three classes viz, aman (transplanted and broadcast varieties), boro, and aush according to the season in which they are harvested, namely, in December-January, March-May and July-August respectively. Again, of these varieties transplanted aman is the most important and covers about 46.30% of the paddy area, followed by boro (26.85%), aus (17.59%) and broadcast aman (9.26%). Transplanted aman is grown almost everywhere in Bangladesh, while broadcast aman is mostly grown in the low-lying areas of the south and northeast. Boro is grown to a certain extent in every district, especially in the irrigated part, while aus is a well scattered crop.

Next to rice, wheat is the most important crop in Bangladesh. It is grown mainly in the drier parts of the north and is cultivated only as a winter crop. Jute leads the country's list of export crops but is ranked third in terms of area cultivated. It is confined mainly to the low-lying areas of the Brahmaputra-Jamuna and Padma floodplains.

Mustard (including rape) is the fourth important crop. It is also grown mainly in the low-lying areas of Brahmaputra-Jamuna and Meghna floodplains.

Masur (lentil) and khesari (chickling vetch) are the two important varieties of pulses produced in Bangladesh. Masur is mainly grown in the Gangetic delta while khesari is a well-scattered crop in the islands and chars (any accretion in the riverbed, usually sandy islands).

Potato is the most important of the winter vegetables and is widely grown. At present potato is a most important crop to Munshiganj, Comilla, Rajshahi, Rangpur, Dinajpur, Bogra, Joypurhat and Nilphamari districts. Sugarcane is the eighth important crop the country. It grows best in Rajshahi, Natore, and Chuadanga districts.

Although chilli is produced to a certain extent in every district of the country, the cultivation is mainly concentrated in the southern districts including Bogra, Sirajganj and Pabna.

See also jute; mustard; pulse; rice; sugarcane; wheat.

Minor crop Crops that are grown on less than one percent of the gross cropped area (GCA) of a country. In Bangladesh gram (0.78%), millets and maize (0.60%), onion (0.58%), black gram (0.51%), sweet potato (0.45%), groundnut (0.40%), green pea (0.36%), sesame (0.33%), linseed (0.30%), garlic (0.20%), pea (0.12%), barley (0.10%), etc, are usually considered as minor crops. In addition, some crops, including vegetables, spices, etc, occupy a very insignificant proportion of the GCA (ie less than 0.10% to each crop), and they altogether account for 1.57 percent.

Tea, a minor crop in terms of area, comes second as an export crop. It is mainly grown in the hills of Maulvi Bazar district and small areas of Habiganj, Sylhet, Chittagong, and Cox's Bazar districts. Three other minor cash crops are tobacco, betel nut, and betel vine. Lalmonirhat, Nilphamari, Rangpur and Kushtia have the maximum share in tobacco cultivation. Betel nut cultivation is generally concentrated in the seaward districts, while betel vine is an important crop in certain areas of Barisal, Cox's Bazar, Rajshahi, Maulvi Bazar, and Satkhira.

Among minor cereals maize, barley and different varieties of millets are grown in certain localities. Groundnut, sesame and linseed are grown to some extent in most districts. Gram, black gram, green pea, and pea are also produced to a certain extent in every district. Nawabganj has the largest share in maskalai (black gram) cultivation. Among vegetables and spices, onion, garlic and brinjal are mostly grown as kitchen gardens in every district while Patuakhali and Bhola are generally leading in sweet potato cultivation.

Among fruits, banana, mango, coconut, and jackfruit are generally grown around homesteads almost throughout Bangladesh, while pineapple is the most common around Maulvi Bazar, Rangamati, Tangail, Khagrachhari and Bandarban districts. [Md Zahidul Hassan]

See also black gram; garlic; grass pea; groundnut; linseed; maize; onion; spice;tobacco; vegetable.

Kharif crop Crops grown during the season that starts from April and extends up to November, when the moisture supply from rainfall plus soil storage is enough to support rainfed crops. In other words, Kharif crops are grown in the spring or summer season and harvested in late summer or in early winter. The season is conveniently' divided into Kharif I and Kharif II. Kharif I, often called Pre-kharif, actually starts from the last' week of March and ends in May. The Kharif season is characterised by high temperature, rainfall and humidity.

Kharif crop: Newly transplanted rice field

The principal crops grown in the country during this season are as follows: (i) cereals- broadcast and transplant aus, transplant aman, prosomillet, foxtail millet, and sorghum; (ii) tuber and root crops- panikachu, mukhikachu, olkachu, mankachu, and pancha mukhikachu; (iii) oilseeds- sesame, groundnut, and soybean; (iv) pulses- black gram, mungbean, and pigeon pea; (v) summer vegetables- lady's finger, red amaranths, amaranths, Indian Spinach, sweet gourd, ash gourd, bitter gourd, squash, palwal, snake gourd, teasle gourd, yardlong bean, brinjal, and summer tomato; (vi) spices- green chillies, ginger, and turmeric; (vii) fibre crops- jute, kenaf, mesta, and cotton; (viii) sugar crops- sugarcane; (ix) stimulant- tea, and (x) fruit plants- banana, pineapple, papaya, and melon. Most Kharif crops are subject to drought and floods in areas where there are no irrigation systems and flood control measures.

Rabi crop Crops grown in one of the two agricultural seasons called Rabi that begins at the end of the humid period when the Southeast monsoon starts ceasing in November and extends up to the end of March.

Rabi crop: Mustard

The season is characterised by dry sunny weather and warm at the beginning and end, but cool in December-February. The average length of the Rabi growing period ranges from 100-120 days in the extreme west to 140-150 days in the Northeast part of Bangladesh.

Major Rabi crops grown in the country include: (i) cereals- wheat, maize, barley, and boro rice; (ii) tuber and roots crops- potato and sweet potato; (iii) oilseeds- mustard, sesame, groundnut, niger, sunflower, linseed, and safflower; (iv) pulses- chickpea, lentil, grass pea, and cowpea; (v) winter vegetables- cabbage, cauliflower, brinjal, tomato, carrot, turnip, radish, spinach, lettuce, bottle gourd, country bean, and garden pea; (vi) spices- chilli, onion, garlic, coriander, sweet cumin, black cumin, and fenugreek; (vii) fibre crops- sunhemp; (viii) sugar crop- sugarcane; (ix) stimulant- tobacco, and (x) fruit plants- watermelon. Rabi crops can use residual moisture stored down to 125 cm in soils. [MS Islam]

High yielding crop varieties Bangladesh has a climate favourable for growing a large variety of crops. The major crops grown in the country are rice, wheat, jute, sugarcane, oilseeds, pulses, potato, sweet potato, tobacco, different vegetables, and fruits. Local or indigenous varieties of most of the crops are poor yielders. Much advances have been made during the past few decades by different research institutes including bangladesh agricultural research institute, bangladesh rice research institute, bangladesh institute of nuclear agriculture, bangladesh jute research institute and bangladesh sugarcane research institute in developing high yielding varieties of crops as well as in introducing improved varieties from different international crop research centres. [Md Shahidul Islam]

Rain-fed food crop Cultivating food crops under natural rain-fed conditions; in other words, rain-based agriculture that utilizes little or no irrigated water. Due to agroecological and socio-economical specialities, the farmers of Bangladesh deviced a sustainable, low-input, risk-aversion type of mixed farming to attain a minimum food security in the face of natural hazards. Due to its geographical location, Bangladesh has a unimodal monsoon climate bracketed by a hot-humid summer and a dry mild winter. The total rain-fed food grain cropped area in 1994-95 was 10.6 million ha which was about 76% of the total cropped area.

The rain-fed rice crop consists of aus and aman groups, high yielding varieties, pajam, local aus, broadcast aman, and local transplant-aman. The total and non-irrigated areas are given in a table. The extent of the rain-fed area is about 93-94% of the total area under aus and aman crops.

In the Kharif season apart from rice, sesame (til), cowpea, sorghum (jowar) and some millets (cheena, kaon, bajra, etc) are grown. However, in the Rabi season boro rice, wheat, maize, cheena, kaon, oilseeds and pulse crops are grown extensively. About 56% of the wheat crop is rain-fed. Only 10% of the boro crop is rain-fed which mostly covers local boro crops. The rain-fed area of pulse and oil seeds exceeds more than 95% of the total area.

Pulse crops are essential as these supply cheaper protein and enrich the soil with symbiotically fixed nitrogen through the rhizobium, the common nodule bacteria. According to the area covered, the first five places are occupied by khesari, Lathyrus sativus (248,365 ha), lentil, Lens culinaris (209,049 ha), chickpea, Cicer arietinum (85,641 ha), black gram Vigna phaseolus (68,492 ha) and mung, Vigna radiata (54,445 ha). These cover about 93% of the total pulse crop area. Pulse crops are grown with residual soil moisture with the occasional light rain of winter months.

The total pulse production in 1994-95 was 533,590 metric tons. This could provide only 18% of the recommended daily need of 58 gm/head/day. The deficit is partly covered by imports. About 95% of the edible oil-producing crops are grown under residual soil moisture. Mustard and sesame respectively cover 73 and 19 percent of the total area under edible oil seeds. Oil seed production in 1994-95 was 307,602 metric tons, from which 179,524 metric tons of oil could be produced (after deduction of seeds and storage loss). From this only 27% of the daily per head oil need could be met, forcing the government to import a huge quantity from abroad.

Pulse and oil seeds are grown with low inputs. As a result production is lower, causing severe protein and oil deficiencies. Barley, maize and millets production can be increased at least by 50% along with pulse and oil seed crops to provide a better balanced diet. [SM Hasanuzzaman]

Crop combination The pattern of cultivating two or more crops in a cropping season. This practice provides farmers with opportunities for harvesting diverse crops from the same land increasing total land productivity, and maintaining or improving soil fertility through use of legumes.

The collective pattern of crops (crop combination) has been analysed with the help of a combinational technique. The technique consists of comparing the actual percentage of cropped areas occupied by different crops with their theoretical distribution in an enumeration unit. For example, theoretical distribution of monocultute of any unit will be 100,0,0... percent. Similarly, for 2-crop combination it will be 50,50,0,0... percent and so on for more crop combination. In this process the theoretical combination having the lowest sum of the square deviation has been taken as the established crop combination.

According to this combinational technique, the collective pattern of crops in Bangladesh during 1994-95 as a whole represents 3-crop (three varieties of paddy) combination ie, aman-boro-aus. On a detailed analysis, monoculture to 9-crop combination have also been identified. Monoculture of aman is found in Khulna, Bagerhat and Patuakhali districts. Three forms of 2-crop combination have been identified in 13 districts. Aman-boro combination is found in Gaibandha, Joypurhat, Bogra, Naogaon, Jhenaidaha, Satkhira, Gazipur, Chittagong and Cox's Bazar. Aman-aus combination is found in Panchagarh, Jhalokati and Bhola, while Sunamganj represents the boro-aman combination.

Four types of 3-crop combination are found in 21 districts. Aman-boro-aus combination occurs in Rangpur, Narail, Netrokona, Mymensingh, Sherpur, Kishoreganj, Gopalganj, Sylhet, Habiganj, Comilla, Chandpur, Lakshmipur, and Feni. Aman-boro-aus combination occurs in Nilphamari, Lalmonirhat, Kurigram, Pirojpur, Barisal and Noakhali. Aman-boro-jute combination is found in Narsingdi while Barguna represents the aman-aus-khesari combination.

Seven categories of 4-crop combination have been identified as boro-aman-jute-mustard, boro-aman-mustrad-aus, aman-aus-boro-tea, aman-boro-jute-wheat, aman-wheat-boro-aus, aman-boro-wheat-aus, and aman-aus-maskalai-boro; and these are found in the districts of Narayanganj, Brahmanbaria, Maulvi Bazar, Jamalpur, Thakurgaon, Dinajpur and Nawabganj respectively.

Ten forms of 5-crop combination have been identified as aman-aus-boro-banana-mustard, aman-aus-banana-boro-mustard, boro-aman-mustard-aus-jute, aman-boro-potato-jute-aus, aman-aus-boro-jute-mustard, aman-aus-wheat-masur-jute, aman-boro-aus-jute-masur, aman-wheat-aus-boro-sugarcane, aman-boro-aus-wheat-sugarcane, and aman-boro-wheat-jute-mustard; and these are respectively found in Khagrachhari, Bandarban, Dhaka, Munshiganj, Madaripur, Kushtia, Jessore, Rajshahi, Natore and Sirajganj districts.

Five types of 6-crops combination have been identified as aman-aus-wheat-jute-masur-boro, aman-aus-jute-boro-masur-gram, aman-boro-aus-mustard-wheat-jute, aman-aus-boro-mustard-wheat-jute, and aman-banana-boro-aus-mustard-pineapple. These combinations are practiced in the districts of Meherpur, Magura, Tangail, Manikganj and Rangamati respectively.

Three forms of 7-crop combination have also been identified as aman-aus-wheat-masur-boro-gram-jute, aman-aus-boro-wheat-jute-masur-gram, and aman-aus-boro-jute-wheat-gram-mustard; and these are found in Chuadanga, Faridpur and Rajbari respectively.

Pabna represents 8-crop combination (aman-aus-wheat-jute-khesari-mustard-onion-masur), while Shariatpur represents 9-crop combination (aman-aus-boro-masur-chilli-mustard-jute-khesari-wheat). [Md Zahidul Hassan]

Crop diversification Culture of agroecologically different crop taxons useful to provide for a balanced economy, daily diet, crop rotation, and conservation of the soil environment. In an agroecological area, organic evolution, usually forced and fostered by the farming and social community, provides the basis of crop diversification (CD).

About 50% of Bangladesh's land area remains inundated from mid-June to October. Crops tolerant to flood waterlogging can only be grown in these areas. In the non-flooded areas various crops are grown both under rainfed and irrigation conditions. During the dry-cooler period non-rice crops are grown under residual moisture regime. Other crops and boro rice are grown extensively with irrigation increasing the CD. On the other hand, in the coastal saline areas of 833,000 ha, cropping intensity (CI) remains limited to 62-159%. Due to socio-economic and agroecological stresses, farmers have no choice but to grow rice in almost 74% of the total cropped area. In the remaining 26%, they grow major crops like wheat, legumes, oil seeds, sugarcane, spices, jute, cotton, etc. Within such limitations the farmers grow crops belonging to 23 major families, 70 genera, about 80 species, and hundreds of varieties, exhibiting a unique spectrum of biodiversity. The ratio of total cropped area to the single cropped area provides an index of crop-diversification (CDI). Higher CI is also correlated with CDI. Rangpur, Bogra, Jamalpur and Mymensingh (all lie in favourable agroecological zones) have higher CDI and CI (CDI from 10.4 to 12.0 and CI from 196-202). A CDI value less than 4 is considered as a lower degree of CD.

The World Bank provides the following formula to measure CD:

S = 1 - S (Ai / A)2,

Where A = total annual cropped area, Ai is the area under a crop in a given season and S indicates a measure of CD termed as Simpson's Index which varies from 0 to 1. When S = 0.5 it means that two different crops were grown in consecutive seasons in one plot. S usually provides a better measure of crop biodiversity.

The World Bank gives another formula for measuring CD, which is

R= S (ar) /A),

Where ar is the area devoted to each rice crop. R varies between 0 and 1. However, here a higher value of R indicates a lower CD.

Stagnation of present crop productivity, unbalanced and less nutritious diet, and soil nutrient deficiency are all cumulative effects of lower CDI of Bangladesh. However, higher crop diversification may bring food security; extra profits by reducing production costs and eliminating edaphic, biotic, ecological and economic pressures; initiate commercial farming; favourable market price for wheat, maize cotton, jute, sugarcane, vegetables, and annual fruits by creating local and foreign markets; optimum investment in infrastructure, support service (including inputs), 'improved' processing facilities, along with specialization at local levels to device profitable single, mixed, and multiple cropping patterns; and organic matters to the soil including green manuring to improve soil health and environment. [SM Hasanuzzaman]

See also agroecological zone; irrigation.

Crop modelling A method of expressing agronomic knowledge in a quantitative way- in terms of predictive equations- and to combine these into an integrated description and understanding of the whole crop system. A crop model is a simple representation of a crop, used to study crop growth and to compute growth responses to the environment. Crop models in common use can be distinguished as descriptive and explanatory models. A descriptive model defines the behaviour of a system in a simple manner. The model reflects little or none of the mechanisms that are the causes of the behaviour. An explanatory model consists of a quantitative description of the mechanisms and processes that cause the behaviour of a system. A simulation model is a relatively simple representation of a system in the world around us. In other word, a simulation model is a module that represents the relevant processes of a system, usually in the form of a computer programme.

Comprehensive models represent a system in which essential elements are thoroughly understood and which incorporate much of this knowledge. Summary models are abstracts of comprehensive models. Nowadays, both summary and comprehensive models are used. These are dynamic and mechanistic and also called the crop simulation model. The crop modelling approach draws together knowledge from different areas: soil science, crop physiology, agroclimatology, phytophathology, agricultural economics, and so on. In the past 30-40 years, crop growth simulation models have also been developed.

The derivation of a model goes through several steps, such as, derivation, calibration, evaluation (testing and validation) and involving many other interactions. Crop models are only an extension to the normal research process of data collection and analysis. Data analysis inevitably involves analysis in some mental framework ('conceptual model') which is usually expressed as a 'mathematical model' (ie a set of one or more equations).

In Bangladesh, a very limited number of studies have been carried out on crop modelling, particularly as institutional mandate or work. An international collaborative project was initiated at the Bangladesh Rice Research Institute (BRRI), with the International Rice Research Institute (IRRI), Philippines, entitled 'Simulation and Systems Analysis for Rice Production (SARP)'. This project was a collaborative effort of IRRI, the Centre for Agrobiologic Research of the Agricultural Research Department, and the Department of Theoretical Production Ecology of the Wageningen Agricultural University (TPE-WAU), both located in Wageningen, the Netherlands. The overall goal of the project was to build research capacity in the field of crop simulation and systems analysis in the Asian rice research institutes/centres.

Four broad themes were identified to focus activities: agroecosystems, potential production, crop and soil management, and crop protection. Under this project, a series of MACROS and ORYZA crop models were developed, parameterized, validated and tested at IRRI, Philippines. Different teams, including the BRRI team, adapted some of these models. A few rice crop models were also developed at BRRI and some models were adapted under this project. In addition to these, some models were developed or adapted by several scientists of the institute. Recently, some projects have been undertaken using the DSSAT3 (Decision Support System for Agrotechnology Transfer) crop model using the IBSNAT approach at the Bangladesh Rice Research Institute. The purpose of the International Benchmark Sites Network for Agrotechnology Transfer (IBSNAT) Project, a programme of the United States Agency of International Development (USAID), implemented at the University of Hawaii, is to assemble and distribute decision support software that enables its users to match biological requirements of crops to the physical characteristics of land so that the objectives specified by the user may be attained. Some work on crop modelling can be seen at the bangladesh agricultural research council and bangladesh agricultural university.

Crop models can be used to identify weaknesses in design experiments; and based on climatic regimes, and year to year variation in weather, they can assist in taking management decisions regarding crop planting, sowing, variety etc., irrigation scheduling, design, test, and evaluate cropping systems, and so on. Actually, crop simulation models are now potentially very useful and can make real contributions to the research and development processes. Large number of potential agrotechnology packages can be prescreened by using crop models. Models and field experimentation can complement each other and traditional research, based on field experimentation, cannot always be relied upon when the problems are too immediate and far-reaching. [Md Serajul Islam]

Crop rotation Practice of regularly changing the crops grown on a piece of land. Crops are grown in a particular order to utilize the nutrients in the soil, and to prevent the build-up insects and fungal pests. Inclusion of a legume crop in the rotation helps build up nitrate in the soil because the roots contain bacteria capable of fixing atmospheric nitrogen.

In many countries systematic 3 or 4-year crop rotation is widely practiced, usually with an autumn-sown cereal crop followed by a root crop, then spring cereal, and ending with a leguminous crop, such as peas or beans.

In Bangladesh crop rotation is followed, although not in a planned way by farmers for better use of soil resources. Generally deep rooted crops (jute) are grown after shallow rooted crops (rice) followed by winter crops (rape and mustard, pulses, etc). [SM Humayun Kabir]

Cropping season Climatic type associated with a particular time of the year that determine the cropping pattern. Because of geographical location in the subtropical region, Bangladesh has favourable temperature range for crop cultivation throughout the year. On the basis of cultural methods, the whole of the crop-growing period is divided into two main seasons, namely Kharif and Rabi. Crops (such as rice, jute, maize, millets, etc) which are grown during the Kharif season are called Kharif crops and those (such as wheat, mustard, chickpea, lentil etc) grown during the Rabi season are called Rabi crops. The Kharif season extends from May through October, while the Rabi seasons starts from November and continues up to April. In addition to these two main seasons, another transition season called Pre-kharif has been identified. This season starts from March-April and ends in May-June. The major characteristics of the cropping seasons of Bangladesh are described below:

Pre-kharif season Is characterized by unreliable rainfall and varies in timing, frequency and intensity from year to year, and provides only an intermittent supply of moisture for such crops as jute, broadcast aman, aus, groundnut, amaranths, teasle gourd etc. During this transition period, soils intermittently become moist and dry. The relative lengths and frequency of such periods depend on the timing and intensity of pre-monsoon rainfall during this season in individual years.

On the basis of moisture status, the whole of Bangladesh has been divided into six zones. They range from zone P1 with a mean length of 10-20 days in Sylhet and north-east of Sunamganj districts to P 6 with a mean length of more than 60 days in the west of Jessore and Satkhira districts.

With the expansion of irrigation facilities, some of the Pre-kharif crops are now grown under irrigated conditions. These include sugarcane, maize, jute, amaranths, groundnut, banana, sesame, lady's finger, teasle gourd, sweet gourd, white gourd, bitter gourd, balsam apple, ribbed gourd, Indian spinach, ginger, turmeric etc.

The Kharif season Starts from May when the moisture supply from rainfall plus soil storage is enough to support rainfed or unirrigated Kharif crops. The season actually begins on the date from which precipitation continuously exceeds 0.5 Potential Evapotranspiration (PET) and ends on the date when the combination of precipitation plus an assumed 100 mm of soil moisture storage after the rainy season falls below 0.5 PET. During the greater part of this season, precipitation exceeds full PET and water can be held on the surface of impermeable soils by bunds. The period of excess precipitation is called the humid period.

On some kinds of land, soil moisture status needs to be considered in the light of prevailing situations. For example, water may remain in fields on highlands after precipitation falls below the full PET rate. The crops most extensively cultivated during the Kharif season are jute, aus, broadcast aman, transplant aman, sesame, different kinds of summer vegetables, ginger, turmeric, pepper, green chilli, different kinds of aroids, cotton, mungbean, black gram, etc. Most Kharif crops are subject to drought and flood in areas without water control.

The Rabi season Starts at the end of the humid period and extends up to the pre-kharif season. The mean length of the Rabi growing period ranges from 100-120 days in the extreme west to 140-150 days in the Northeast of the country. Rabi or winter crops can use moisture stored down to 1.25 m (where there is no ploughpan). The mean starting date of the Rabi season ranges from 1-10 October in the extreme west to 1-10 November in the Northeast, and in central and eastern coastal areas. The mean end dates range from 1-10 February in the following year in extreme west to 20-31 March in the Northeast. Most Rabi crops such as wheat, maize, mustard, groundnut, sesame, tobacco, potato, sweet potato, sugarcane, lentil, chickpea, and grass pea perform better when sown/planted after 1 November. They can utilize the residual soil moisture as well as benefit from growth during the cool winter period. [MS Islam]

On lowlands, very lowlands and bottomlands where flooding continues even after the end of rainy season, the Rabi season starts from the date when flooding ends.

Relay cropping Growing a crop in a field where a standing crop is either at the ripening/maturing stage in case of a seasonal crop, for example, it could be T aman in the T aman-grass pea sequence or at the establishing stage in case of an annual crop, it could be sugarcane in the sugarcane-mustard sequence. The main objectives of relay cropping are to make sue of the season as well as to increase cropping intensity. The relay crops can utilize the residual soil moisture and benefit themselves from the residual effects of applied nutrients in the preceding crop. The most common examples of relay cropping sequence in the country are as follows: T aman-black gram, T aman-chickpea, broadcast aus-aman, boro rice-green manuring (dhaincha), jute-red amaranths, sugarcane-chickpea, sugarcane-lentil, sugarcane-onion, sugarcane-garlic, sugarcane- potato, potato-garlic, potato-palwal, mustard-lentil, mustard-chickpea, chickpea-linseed, maize-groundnut, maize-sweet gourd, onion-watermelon, country bean-Indian spinach, cotton-black gram, sesame- broadcast aus, and chilli-radish. [Md Shahidul Islam]

Crop water requirement The quantity of water required to produce each unit of grain or dry matter, exclusive of roots. It is dependent on soils, crops, climatic factors, local conditions, and farming practices. Water is essential for plant growth, for seeds to germinate, seedlings to emerge, and for many other growth functions. It prevents the dehydration of plants and provides the transport mechanism for plant nutrients and the products of photosynthesis.

Water constitutes about 80-90% of most plant tissues and cells in which there is active metabolism. Most of the water entering plants is lost in transpiration. Failure to replace this water loss results in the loss of turgidity, cessation of growth, and eventual death of the plants from dehydration. Crop yield (grain or dry matter) is directly related to crop evapotranspiration (ET) requirement. ET includes evaporation of liquid water from soil and plant surface plus transpiration of liquid water through plant tissues. Evapotranspiration is determined by direct field measurements (eg by lysimeters tanks, evaporation pans, etc) or from estimates based on climatological and crop data. In the field, total water requirement for irrigation purposes is the sum of water required for land preparation (LP), meeting evapotranspiration demand of crop (ETc), and percolation loss (P).

This total water is termed as field water requirement (FWR). The difference between FWR and the sum of effective rainfall (Re) and water available from (residual) soil moisture (ASM) is the Field Irrigation Requirement (FIR), which is to be supplied by irrigation. In Bangladesh irrigation schemes are never 100 percent efficient. Therefore, additional water is needed to be pumped to make up for the different losses in the system. Thus, the gross irrigation water requirement (GIWR) will be higher than the FIR. However, both water stress as well as excess water are harmful to crops. [M Harun-ur-Rashid]

Crop pest control Selection and use of pest control measures that ensure favourable economic, ecological and sociological consequences. Those include the monitoring of pest increase, selection of pest control method, the judicious use of a pesticide, or the effective communication that no action is necessary. Available techniques for controlling individual insect pests are almost inexhaustible and involve a very wide range of applied science and technology. They are conveniently categorized in increasing order of complexity as cultural, mechanical, physical, biological, chemical, genetic, and regulatory methods. In Bangladesh about 85 percent of the population is directly or indirectly engaged in agriculture and about 64 percent of the land is under crop cultivation. It has been found that about 700 insect pests damage agricultural crops either through direct feeding or through transmission of crop diseases. From the available statistics it has been found that about 10 to 15% crops are destroyed in the field each year due to insect pests.

It has been estimated that the amount of crops damaged is nearly double the amount of food grain imported in this country each year. Since most farmers are poor and illiterate, they do not know the modern technologies and procedures of crop protection and are not able to buy fertilisers and appropriate pesticides even if these are available. Besides, most farmers have not been trained in the use of modern materials and are not convinced that the new techniques are better than the old, because of the lack of demonstration farms or plots. Lack of agricultural advisory personnel is another serious weakness. Since pesticides have quick remedial properties, use of pesticides is still the most popular technique for controlling crop pests in the country, which has resulted in consumption of 14,340 m tons of pesticides annually. Recently, the Food and Agriculture Organisation (FAO) and the Department of Agricultural Extension (DAE) jointly initiated Integrated Pest Management Programmes in 72 upazilas of 63 districts of Bangladesh. Under this programme a huge number of farmers are getting training and acquiring preliminary knowledge of modern technologies for crop pest control. This will certainly minimize the use of pesticides and enhance the activity of natural enemies. [Masum Ahmad and Monawar Ahmad]

See also crop pests and diseases; pest.