Journal of Agriculture and Rural Development in the Tropics and Subtropics Vol. 112 No. 1 (2011) 19–28 urn:nbn:de:hebis:34-2011101139331 ISSN: 1612-9830 – journal online: www.jarts.info Nitrogen in combination with Desmodium intortum effectively suppress Striga asiatica in a sorghum-Desmodium intercropping system Carl Frederick Reinhardt ∗, Niguse Tesfamichael Department of Plant Production and Soil Science, University of Pretoria, Pretoria 0002, South Africa Abstract It is well known that the parasitic weed Striga asiatica (L.) Kuntze can be suppressed by Striga-tolerant sorghum (Sorghum bicolor L. Moench) cultivars, Desmodium intortum (Mill.) Urb. (greanleaf desmodium), and by fertilization with nitrogen. The study objective was the assessment of Striga control provided by integration of Desmodium density, timing of sorghum-Desmodium intercrop establishment, and nitrogen fertilization. Growth responses and yield of three sorghum cultivars were measured in three pot experiments. A soil naturally infested with Striga was used, and that part of the soil which served as uninfested control was chemically sterilised. Striga numbers and growth were affected significantly by sorghum cultivars, sorghum-Desmodium intercrop ratios, timing of the sorghum-Desmodium associ- ation, as well as by their interactions. Desmodium caused 100% suppression of Striga emergence when Desmodium was established in the 1:3 sorghum-Desmodium ratio at seeding of sorghum. Total control of Striga was also achieved with the 1:1 sorghum-Desmodium ratio when Desmodium was transplanted 30 days before sorghum seeding. How- ever, these two treatments also caused significant reductions in sorghum yield. In contrast, 100% Striga control and a dramatic increase in sorghum yield were achieved with 100 kg N ha −1 in the 1:1 sorghum-Desmodium intercrop. Com- patibility of sorghum and Desmodium was evident at the 1:1 sorghum-Desmodium intercrop established at sorghum seeding. Overall, the Ethiopian cultivars Meko and Abshir showed better agronomic performance and higher tolerance to Striga than the South African cultivar PAN 8564. It is recommended that the N × Desmodium × sorghum interaction be investigated under field conditions. Keywords: allelopathy, competition, Desmodium, intercropping, plant parasite, sorghum, Striga 1 Introduction acterized by a short or non-existent fallow period, de- pleted soil fertility and low inputs of fertilizers, pesti- Striga asiatica (L.) Kuntze causes substantial yield cides, improved seeds and other modern management losses throughout the sorghum growing areas of the practices (Oswald & Ransom, 2001). Striga infesta- tropics and sub-tropics (Parker & Riches, 1993). Small- tion is increasing in area and intensity in cereal mono- scale farmers with limited resources and knowledge to cultures which are associated with aggravated depletion control the parasite are the main sorghum producers in of nutrients from the soil. Crop yield losses attributed these areas (Haussmann et al., 2000). Consequently, to Striga infestation are estimated to range from 70 to Striga is considered as one of the biggest biological 100% (Sauerborn et al., 2007). constraints on sorghum production when compared with Because of the unique ecological adaptation of Striga diseases, insect pests and birds (Ejeta, 2007). The par- in terms of establishment, growth and development, asite is mostly problematic in agricultural systems char- 80% of its host-limiting effect on sorghum occurs at sub- terranean level, from when haustoria attach to the host ∗ Corresponding author roots until the parasite emerges above-ground (Stew- E-mail: [email protected] art & Press, 1990). In addition to the requirements 20 C. F. Reinhardt & Niguse Tesfamichael / J. Agr. Rural Develop. Trop. Subtrop. 112 - 1 (2011) 19–28 that other weeds have for germination, Striga spp. re- jectives of the study were: (1) to determine the op- quires a period of suitable environmental conditions for timum Desmodium intortum population in a sorghum- pre-conditioning and an exogenous chemical signal de- Desmodium intercrop and the role of timing in the inter- rived from the host plant to initiate germination, hausto- crop’s establishment; (2) to evaluate the integrated effi- ria development, attachment and penetration of sorghum ciency of N application, Desmodium intercropping and roots. These specific requirements to start its life cycle Striga-tolerant sorghum cultivars for suppressing Striga entail that implementation of control strategies should growth and its effect on this crop. best be directed at these physiological stages (Stewart & Press, 1990). Cereal-legume (grain or forage) intercropping repre- 2 Materials and Methods sents dual or multiple cropping systems that are often 2.1 Experimental site, soil collection and preparation used by subsistence farmers for higher productivity per unit land area as a result of gains in fertility and the con- Pot experiments were conducted in a glasshouse on trol of weeds, diseases and insect pests; such systems the experimental farm of the University of Pretoria. also reduce the risk of crop failure attributed to drought, The experimental farm is located at 28016’E longitude, thus giving better returns for precious resources used, 25045’S latitude, at an altitude of 1372 m above sea such as family labour (Willey, 1990). The role of inter- level. The soil used as growth medium was collected cropping in sustaining farm productivity and diversity from a farm field in the Settlers district about 110 km of farm products, either as feed or food, justifies the northeast of Pretoria. The field had been heavily in- practice (Willey, 1990; Ransom, 2000; Hess & Dodo, fested by Striga asiatica for several years prior to the 2004). study, and sorghum was grown there in monoculture for In most of the sub-Saharan African countries, farmers at least five seasons. The soil consisted 73.2% sand, do not apply recommended rates of fertilizer in Striga 7.4% silt and 19.4% clay, 0.36% organic matter (%C) prone areas, possibly due to the high cost of fertiliz- and had a pH(H2O) of 6.3. Soil levels of P, K, Ca and −1 ers and low returns from the added input costs. For in- Mg at sampling were: 46, 183, 347, and 202 mg kg , stance, N rates between 120 and 280 kg ha −1 have been respectively. Soil from the field was thoroughly mixed reported to be effective in reducing Striga damage and on a concrete floor in order to obtain as homogenous as emergence (Kim et al., 1997). In contrast, farmers in possible distribution of the Striga seed contained in the most sub-Saharan countries apply 20 to 50 kg N ha −1 soil. The soil was subsequently separated into two parts; for sorghum and 20 to 90 kg N ha −1 for maize (Kim & one part, representing the non-infested treatment, was Adetimirin, 1997), which is apparently too low to con- heaped as a seed bed (6 × 2 × 0.3 m) and covered by a trol the parasite and improve crop productivity. How- gas-proof tarpaulin for fumigation for 24 h with methyl −3 ever, the need for relatively high N rates may be at least bromide at the rate of 40 g m . The other part of the soil partially be obviated by combining Striga-tolerant culti- was not fumigated, thus serving as both growth medium vars with sub-optimal N in an intercropping system that and the source of Striga seed. includes a legume. Khan et al. (2001) reported that intercropping of 2.2 General practices and treatments Desmodium species with sorghum or maize enhanced Two Ethiopian sorghum cultivars, Abshir and Meko, ff soil fertility and increased the e ectiveness of applied and one South African cultivar PAN 8564 were grown N for suppressing the parasite. Desmodium intortum in pots in all three experiments. Each pot contained (greenleaf desmodium) could be one of the forage crops 10 kg of either Striga infested or non-infested soil de- that have the potential for intercropping with sorghum pending on the treatment involved. Plants were grown cultivars for the trapping of Striga through stimulation in a glasshouse at a mean daily temperature regime of of suicidal seed germination (Khan et al., 2001). A 18/33°C (min/max). The soil was not fertilized in case legume such as Desmodium is likely to supplement ap- it would have influenced the crop and parasite responses plied N, and it has been reported that N inhibits Striga to the treatments. Watering was done every second day seed germination (Farina et al., 1985). Striga-tolerant to replenish water lost by applying the same amount of cultivars will further contribute to exclusion of parasite water to each pot. The volume required was estimated attachment and root penetration (Ejeta, 2007). by considering the combined mass of pot + soil + wa- The hypothesis tested in the present study was ter at the inception of each experiment. Soil moisture whether the tolerance of sorghum cultivars towards at field capacity was gravimetrically determined to be Striga could be promoted by intercropping with 15% for the particular soil, i.e. field capacity = 1.5 L Desmodium intortum and N application. Specific ob- H2O per 10 kg soil. Once a week for the duration of the C. F. Reinhardt & Niguse Tesfamichael / J. Agr. Rural Develop. Trop. Subtrop. 112 - 1 (2011) 19–28 21 trials all pots were weighed to adjust soil moisture lev- chlorosis, blotching, scorching, and stunting, where 0 els to as close as possible to the field capacity. Weekly indicates no symptom development and 9 indicates the adjustments in water supply were made on a pot by pot highest intensity of symptoms (Table 2). basis for sorghum and Desmodium in order to compen- sate for biomass increases over time. Calculations for Table 1: Striga asiatica vigour ratings on parasitized sorghum sorghum were based on data for the relationship plant plants (adapted from Sinebo & Drennan (2001) height/biomass, and on the leaf number/biomass rela- tionship for Desmodium, which were determined in pre- liminary experiments.