**3. Rice ecosystems**

Based on IRRI, rice farming is categorized into four ecosystems depending on source and water supply. This are irrigated, flood prone, rainfed lowland and rainfed upland and [14].

Irrigated ecosystems are the most widely utilized rice farming ecosystems accounting for over 75% percent of total yield. The ecosystem includes lager parts of Europe, Australia, America, Asia and Africa. Irrigation ecosystem is again grouped into irrigated wet season and irrigated dry season. Irrigated wet season involves cultivation of rice during the wet season and irrigation water supplements rainfall. Irrigated dry season involves planting rice when rainfall is usually low and water is majorly supplied by irrigation in places that usually experience high solar radiation and evapotranspiration. In irrigated ecosystem the fields are bunded and leveled, water level maintained at between 2.5 cm to 1.5 cm determined by availability of water. Rice seeding is by either transplanting or direct seeding. In Kenya irrigation farming is done in irrigation schemes under the management of the National Irrigation Board (NIB). Major irrigation schemes include Ahero, Bunyala, West Kano, Perkera, Hola, Bura and Mwea. Small holder irrigation farming is practiced along river valleys namely; Kore, Alungo Nyachoda, Wanjare, Anyiko and Gem Rae in Western Kenya and Kipini, Malindi, Shimoni and Vanga at the coastal region [11]. Dry irrigation entails continuous flooding and it is practiced in Mwea, Ahero, Bunyala and Western Kano irrigation farming. Dry irrigation must have continuous water supply and soils must have high water retention capacity. During drought water is rationed hence reducing productivity though currently, System Rice Intensification (SRI) has been introduced.

Flood prone ecosystems involves paddy-fields being subjected to unbounded flooding for a duration that is about 5 months and water depth might range up to a maximum of about 5.0 M during plant growth. In this deep-water condition rice plants, mostly floating rice varieties outstretch their stems to get to the water surface. Flood prone ecosystem is largely practiced in Africa and Asia and accounts for 7% of the world land under rice cultivation. The cultivation is mostly located in river deltas for instance the Ganges in India, Brahmaptura in Bangladeshi,

the Mekong in Vietnam and Cambodia, Niger delta in Niger and Chao Phraya of Thailand. Deep water rice system is also extensively practiced in coastal areas of India, West Africa, Vietnam and Bangladeshi based on daily tidal inundation. Key constraint in this environment is soil and water salinity and flash floods. This ecosystem extremely variable due to unpredictable flooding and drought. Farmers in this ecosystem records about 1.5 t ha−1 average yield with the main stress being environmental making most applicable farming inputs ineffective [14].

Rainfed lowland ecosystem involves slightly bunded and leveled field where water supply is mainly by rainfall and the water depth and the duration depends on the rain season. The water level cannot be controlled and rice plants are severely exposed to drought, deep floods, and alterations between anaerobic and aerobic environments [14]. In Kenya Rain-fed lowland rice cultivation is practiced in Kwale, Kilifi and Tana River counties at the Coast region.

Upland ecosystems involve rice fields in straighten valley bottoms to hilly mountainous lands with slopes ranging from 40% to about 0% descend. In Upland ecosystem rice cultivation done by preparing fields that are seeded when dry. These ecosystems form about 13% of harvested rice areas worldwide but accounts for only 4% of the world's total production. Upland rice is largely for a subsistence crop with yields approximated at 1 t ha−1 in areas with little inputs to 3–4 t ha−1 in situations where fertilizer application and supplementary irrigation is practiced. An estimated population of 100 million people are believed to depend on upland rice as their staple food. Upland rice is mostly grown in Asia (Bangladeshi and India), Africa and Latin America. These ecosystems have many constraints, mostly attributed to insufficient soil fertility, weed invasion and disease infection. Worldwide, rain fed ecosystem accounts for approximately 54 million ha of rice, mostly found in Africa and Asia [14]. In Kenya rain fed upland is grown in Kisumu, Busia counties in western Kenya and Kilifi, Kwale and Tana river counties of Coastal Kenya [15].

#### **4. Rice varieties under rainfed ecology in Kenya**

Rice varieties under rainfed conditions are categorized as traditional, introduced improved and hybrid rice. Traditional varieties are characterized by late maturity, low yields, lodging. However, they are adapted and are able to tolerate stresses such as pests and diseases, drought, weeds, salinity and even bird's attack. Some traditional lines possess farmers preferred traits like aroma and good gelatinization temperature.

#### **4.1 Rainfed lowland ecology varieties**

Under rainfed lowland ecology traditional lines include Madevu, Kitumbo, Kichana chawa, macho ya wanda, kijego, Matako Nyeusi, Moshi and Mtumbatu. Introduced improved lines show improved yield, earliness and less lodging. Rainfed lowland introduced improved lines include Komboka and MWIR 2. Komboka was introduced by Kenya Agricultural and Livestock Research Organization (KALRO) in co-operation with International Rice Research Institute (IRRI) in 2013. It is high yielding, good grain quality, semi aromatic and has high tillering ability. Supaa, a local landrace that is aromatic and late maturing is also grown particularly at the Kenyan coast. Highbred low land ecosystem rice lines are Arize Tej Gold and Arize 6444 Gold from Bayer East Africa that were evaluated and found promising by National Irrigation Board (NIB) however, there adoption remain low.
