Influence of soil fertility management on striga reproduction and grain yield of sorghum in semiarid areas of Tanzania

Influence of soil fertility management on Striga reproduction and sorghum grain yield in semiarid areas of Tanzania. PhD thesis, Faculty of Agriculture, University of Nairobi. Witchweed (Striga), a root parasite, has become a major biological constraint to sorghum production with severe yield losses of about 85% reported in semi-arid areas of sub-Saharan Africa. In Tanzania, Striga asiatica is the most dominant Striga species and covers over three quarters of the country, from the northern parts to the southernmost regions. The success of this weed is due to its ability to produce large numbers of tiny dust-like seeds that cause a rapid increase in parasite population density after the first infestation. A long-term control program must focus on reducing Striga seed inputs into the soil sufficiently so that a reasonable crop yield can be obtained. Therefore a study was carried out in two regions of Tanzania with the following specific objectives: (a) to determine the agronomic practices adopted by farmers in Striga control and the relationship between soil chemical characteristics and Striga reproduction; (b) to determine the influence of organic and inorganic fertilizers on Striga asiatica reproduction and sorghum grain yield; and ( c) to evaluate the effect of combined farmyard manure and urea on Striga asiatica reproduction and sorghum grain yield. Surveys were carried out in 20 farmers' fields in 2010/2011 cropping season in semi arid areas of Dodoma and Morogoro regions for the first objective. Data collected included the agronomic practices used by farmers in Striga control, soil chemical characteristics, Striga plant counts and number of capsules/Striga plant. A semi-structured questionnaire was used to generate information on the cropping practices, farmers' knowledge and perceptions of the Striga problem and agronomic practices adopted by farmers for Striga control. Soils were sampled at 0 - 20 em depth from different farmers' fields and analyzed for soil moisture content, pH, organic carbon, nitrogen, phosphorus and potassium. The trials for the second objective were conducted in two consecutive years (2009/2010 and 2010/2011) at Hombolo Research Station in Dodoma region. Treatments consisted of farmyard manure (5.9 tons/ha), chicken manure (2.5 tonslha), urea (50 kg Nlha) and triple superphosphate (TSP) (40 kg P/ha) which were evaluated on sorghum (variety Macia) in a field naturally infested with Striga asiatica (L.) Kuntze). The 5.9 tonslha farmyard manure and 2.5 tonslha chicken manure tested were each equivalent to 50 kg Nlha. A randomized complete block design with four replications was used. The trials for the third objective were carried out in 2010/2011 at Hombolo and Melela locations in Dodoma and Morogoro regions respectively. A factorial experiment in a randomized complete block design was set up in fields naturally infested with Striga asiatica. The treatments which comprised farmyard manure at rates equivalent to 0, 25, 50, 60 kg N/ha and urea at rates of 0, 25, 50 and 60 kg Nlha were replicated three times. Results from the surveys showed that the recommended agronomic practices for the control of Striga have not yet found large scale application on farmers' fields. The majority of farmers indicated that Striga infestation in the area was increasing and they were not able to control in their fields. Two species of Striga (Striga siatica and Striga forbesii) were observed in the farmers' fields, although Striga asiatica was the dominant species. Potassium was highly positively related to the number of capsules/Striga plant. This study also showed that fertilizer applications indeed affected Striga reproduction negatively. Plants supplied with chicken manure, urea and TSP had significantly fewer number of capsules per plant than farmyard manure treated and control (without fertilizer application) plants. Combination of urea at 50 kg Nlha or 60 kg Nlha with all rates of farmyard manure significantly reduced the number of capsules/Striga plant. Generally, fertilizer applications improved sorghum grain yield with chicken manure giving significantly the highest grain yield. Combined application of urea and farmyard manure increased sorghum grain yield compared to the application of either urea or farmyard manure alone. Economic analyses showed that the marginal rates of return from applying urea alone at 25,50 and 60 kg Nlha were 31, 54 and 48% respectively. Combination of urea at 60 kg Nlha and FYM at 60 kg Nlha equivalent had the highest marginal rate of return (70%). Farmyard manure alone at 25, 50 and 60 kg Nlha equivalent rates had lower rates of returns than the non-treated control. Chicken manure and combined farmyard manure with urea can be used to improve sorghum production in semiarid arid areas of Tanzania. Low quantities of FYM and high soil K may promote Striga infestation as they are associated with increased Striga reproduction. Even though fertilizers play a significant role in Striga reproduction, calculations of seed production per capsule indicated that considerable amounts of seeds were still added to the soil. Therefore, the use of fertilizers alone is not enough to reduce the seed inputs and seed bank. This calls for an integrated approach in which cultural, chemical and crop resistance strategies are deployed.