Integrated weed management systems in sorghum based cropping system in Nigeria

Sorghum (Sorghum bicolor (L) Moench) is the fifth most important staple food crop after wheat, rice, maize and barley. Sorghum is consumed by more than 70% of the population. Striga hermonthica is a serious biotic constraint to cereal production in the dry savannas of sub-Saharan Africa. Striga infestation in sorghum is reported to be higher in Nigeria than in other West African countries with about 80% (8.7 million ha) of land cropped to sorghum infested by this weed. Field trials were conducted in 2012 and 2013 to evaluate the effect of seed treatment, sowing date and trap crop in the management of S. hermonthica. During the investigation in two sorghum (resistance and susceptible) which involved intercropping with soyabean, seedtreatment with parkia biglobosa pulp and sowing at different dates (June and July), these were found to reduce the infestation of sorghum by S. hermonthica. The results showed that the effect of shading by soyabean, putative allelopathic mechanism effect of parkia material and high relative humidity due to established rainfall in July showed some benefits against S. hermonthica infestation. Growth of S. hermonthica was almost completely suppressed and yield increased with the resistant sorghum intercropped with soyabean, primed at 66 g/L parkia and planted in July.


INTRODUCTION
Producing enough food for an ever-growing population is the biggest problem facing the human race, worldwide. A large proportion of crops are lost to insects, diseases, weeds, and parasitic plants. In the developing world where farmers and governments struggle to feed hungry mouths, the cost of the damage caused by these pests on agriculture is of utmost concern. Sorghum is a very economic important cereal crop and represents major staple food crop for many developing countries (FAO, 2012). Sorghum was severely affected by weeds infestation during the 4-5 weeks after seeds emergence and seedling growth. As a consequence, severe uncontrolled weed infestations often cause poor crop establishment or complete crop failure (Pannacci et al., 2010). The origin of Striga hermonthica is unclear. It may have originated in Northeast Asia (Scholes and Press, 2008). It is the largest and most destructive of the Striga species and considered as one of the most serious weeds in Africa (Oswald, 2005). The Striga problem in Africa is intimately associated with human population growth. Traditional African cropping systems included prolonged fallow, rotations and intercropping, which were common practices that kept Striga species infestations at tolerable levels (IITA, 2004;Kanampiu et al., 2002). The use of Parkia pulp has been found to improve the soil's physicochemical properties and inhibits the germination of S. gesnerioides seeds in cowpea at Burkina Faso (Itta et al., 2014). Similarly, Magani et al. (2010) reported 29.1 and 38.8% less S. hermonthica emergence in field and greenhouse respectively, when Parkia based products were used in maize. Incidence and severity of S. hermonthica are exceptionally high on sorghum, pearl millet and maize, the main staple foods for over 300 million people in Sub-Saharan Africa (Scholes and Press, 2008). The impact of Striga damages depends on ecological conditions, cropping systems, local cultural practices and farmers' skills on the ecology (IITA, 2002). Intercropping of cereals with legumes, application of organic fertilizers and genetic resistance of host plants are three control methods with a high potential for adoption by farmers (Oswald, 2005). Allelopathy plays an important role in agricultural ecosystems and in a large scale, in the plant covers among the crop-crop, cropweed and tree-crop covers. These interactions are detrimental and occasionally, are useful and gave attention to allelopathy in natural and agricultural ecosystems. Today, allelopathy is recognized as appropriate potential technology to control weeds using chemicals released from decomposed plant parts of various species (Naseem et al., 2009). Bioherbicides represent solution to heavy use of synthetic herbicides which cause serious threats to the environment, consumers and increase costs of crop production (Asghari and Tewari, 2007). Moreover, continuous use of herbicides for weeds control causes herbicide resistant (Naseem et al., 2009). Many authors reported employing plants extracts for controlling weeds with variable success (Hussain et al., 2007;Iqbal et al., 2009;Naseem et al., 2009).

MATERIALS AND METHODS
Field experiment was conducted in 2012 and 2013 rainy season, at the Federal University of Technology Minna, (09° 39' N and 06° 28' E) in the Southern Guinea Savannah ecological zone of Nigeria with mean annual rainfall of 1300 mm. The experiment was carried out on sandy clay loamy in a field with a history of high S. hermonthica infestation. Three concentrations of Parkia biglobosa pulp at 0, 66 and 100 g/L was used to prime two sorghum cultivars (resistance ICSV 1002 and susceptible local variety), and two sowing dates (15 June and 21 July). The trap crop (soyabean variety TGX 1448-2E) was planted three days before the sorghum while planting distance was 75 cm between rows and 30 cm between plants. Seed were socked for 16 h and sown two to three seeds of sorghum per hill on the chosen dates and plant stands with excess seedling were thinned to two plants per hill at two weeks after sowing. The treatment design was a randomized complete block (RCBD) with three replicates. Hand pulling of weeds Mamudu et al. 21 other than S. hermonthca seedling was done at week 4 and second weeding was carried out at week 8 after sowing. Sorghum panicles was harvested at weeks 22 and 23 after sowing for June and July dates respectively, panicles were dried threshed and grain yield determined. Data collected include days to first Striga emergence, Striga count per stand and per plot, severity score of maize using a scale of 1-5, where 1 indicate no Striga damage and 5 indicating a very high severely level, plant height from tagged stand using tape rule and measuring from the soil surface to neck of last leaf, grain yield per plot using weighing balance. The data were subjected to analysis of variance (ANOVA) using the computer software Genstat (2010), and differences between variables means were compared using least significant difference (P < 0.05). The 2013 experiment was repeated on the same land where the 2012 experiment was conducted.

RESULTS AND DISCUSSION
There were significant (P < 0.05) differences in the combined effect of sorghum varieties, Parkia concentration at 66 g/L and sowing date on days to first Striga shoot emergence. Sowing ICSV1002 sorghum in July delayed the Striga emergence of all Parkia combinations in 2012 as shown in Table 1. The delayed Striga emergence in priming of sorghum with 66 g/L Parkia concentration compared to 100 and 0 g/L in 2012 and 2013 might be due to allelochemical in the Parkia pulp which inhibited Striga development at that concentration or level. A similar observation was made by Kolo and Mamudu (2008) that dressing of maize seed with P. biglobosa pulp gave better maize development both vegetative and in grain yield especially with the resistant varieties as described in Table 1. Irrespective of the year of planting, the combined effects of sorghum varieties with Parkia treatments and sowing date on Striga count per stand of sorghum were not significantly (P < 0.05) different in all the sampling periods except at 10 WAS in 2013 as shown in Table 1. Generally, sorghum variety ICSV 1002, priming at 66 g/L Parkia concentration and planting in July had fewer Striga shoot count compared to other treatment combinations. There were no significant (P < 0.05) differences in interaction effect of sorghum varieties, Parkia concentration and sowing date on Striga count per plot at 10 and 18 WAS in 2012. The ICSV1002 variety primed at 100 g/L Parkia concentration and sown in July supported fewer Striga compared to other treatments at 14 WAS. In the local variety, priming at 66 g/L Parkia concentration and sowing in July supported fewer Striga count compared to other treatment combination in 2012. In 2013, ICSV1002 variety primed at 66 g/L Parkia concentration and sown in July supported fewer Striga count per plot compared to other treatment combinations at 10 and 14 WAS, but in the local sorghum variety, fewer Striga count was recorded at 66 g/L Parkia treatment and sowing in July for 10 WAS; and at 100 g/L Parkia concentration and sown in July for 14 WAS compared to other treatment combinations. Fewer Striga count in 66 g/L Parkia concentration in 2012 and 2013 compared to 100 and 0 g/l confirms the ability of Parkia concentration in controlling Striga; although the mobility of Parkia pulp phytochemical in sorghum has not been documented, it is likely that the Parkia pulp concentration has an indirect mechanism by which it reduced Striga level. This is similar to the findings of Marley et al. (2004) that all plant material like neem and Parkia extract significantly reduced Striga emergence as described in Table  1. Fewer Striga count observed in July sowing date compared to June might be due to the lower weed pressure in July because of cooler soil temperature, high relative humidity and regular rainfall which cause the Striga seeds to undergo wet dormancy and fail to germinate. Dugje et al. (2008) had also reported that sowing maize in mid-July reduced Striga infestation compared to sowing earlier in mid-May or mid-June in parts of the Northern and Southern Guinea Savanna of Nigeria (Gressel et al., 2004). The interaction effects of sorghum varieties, Parkia concentration and sowing date did not significantly affect severity score in 2012. However in 2013, ICSV1002 variety primed at 66 g/L Parkia concentration in both June and July suffered less Striga damage compared to 0 g/l Parkia concentration in July. In the local sorghum variety, priming at 66 g/L Parkia treatment and sowing in July suffered less Striga damage compared to other treatment combination. The significance of the lower Striga damage in 66 g/L compared to 100 and 0 g/L Parkia concentration could be due to lower Striga population in the former which decreased severity of attack on host. This is in agreement with the work of Ndungu (2009) that coating sorghum seed with herbicides reduced Striga infestation. Reduction in Striga infestation accounted for fewer Striga damages. The reduced Striga damage in planting in July compared to June could be attributed to less weed pressure and unfavorable environmental condition of low temperature and high humidity which inhibited Striga emergence and population and reduced attack on host. This is similar to observation by Odhiambo and Ariga (2011) that when planting is delayed, Striga seeds are unable to germinate and seedlings fail to attach to host root systems due to unfavorable low soil temperature during the middle of the rainy season. This translated into less Striga damages as shown in Table 2. Sorghum varieties with Parkia concentration and sowing date were not  Table 3. In all the sampling periods (10, 14 and 18 WAS), Striga count per stand of sorghum was not significantly affected by cropping system, Parkia concentration and sowing date in both 2012 and 2013. The interaction effect of cropping system Parkia concentration and sowing date on Striga count per plot showed similar trend with Striga count per stand. There were no significant (P < 0.05) difference in all the sampling periods and the planting years 2012 and 2013 as described in Table 3.
The combined effects of intercropping system with 66 g/L Parkia concentration and sowing in July significantly suffered less Striga damage compared to other treatment combinations in 2012. The less Striga damage in sorghum intercropped with soyabean compared to sole sorghum confirms the effectiveness of soyabean as trap crop to induce suicidal germination of Striga seed. As cover crop, soyabean interfered with the sun radiation and chemical environment of Striga seed, lowering the light and daily temperature and inhibiting emergence of Striga seed, as well as increasing soil fertility through nitrogen fixation. All these caused unfavorable 24 J. Agric. Biotech. Sustain. Dev. condition for Striga seed germination and resulted in less attack and damages. This is similar to observation by Carsky et al. (2000) and Schulz et al. (2003) that varieties of cowpea, groundnut and soyabean have potential to cause suicidal germination of S. hermonthica and improve soil fertility. The significance of the lower Striga damage in 66 g/L compared to 100 and 0 g/L Parkia concentration could be due to lower Striga population in the former which decreased severity of attack on host. This is in agreement with the work of Ndungu (2009) that coating sorghum seed with herbicides reduced Striga infestation. Reduction in Striga infestation accounted for fewer Striga damages. There were no significant differences in 2013 among the treatments; also, there were no significant (P < 0.05) difference in interaction effects of cropping system, Parkia concentration and sowing date on plant height in the two planting years of 2012 and 2013 in all the sampling periods as shown in Table 4. Intercropping with priming at 66 g/L Parkia concentration and sowing in July produced highest sorghum grain yield compared to other treatment combinations in 2012. The less Striga damage in sorghum intercropped with soyabean compared to sole sorghum confirms the effectiveness of soyabean as trap crop to induce suicidal germination of Striga seed. As cover crop, soyabean interfered with the sun radiation and chemical environment of Striga seed, lowering the light and daily temperature and inhibiting emergence of Striga seed, as well as increasing soil fertility through nitrogen fixation. All these caused unfavorable condition for Striga seed germination and resulted in less attack and damages. This is similar to observation by Carsky et al. (2000) and Schulz et al. (2003) that varieties of cowpea, groundnut and soyabean have potentials to cause suicidal germination of S. hermonthica and improve soil fertility.
The significance of the lower Striga damage in 66 g/L compared to 100 and 0 g/L Parkia concentration could be due to lower Striga population in the former which decreased severity of attack on host. This is in agreement with the work of Ndungu (2009) that coating sorghum seed with herbicides reduced Striga infestation. Reduction in Striga infestation accounted for fewer Striga damages as presented in Table 4.

CONCLUSION AND RECOMMENDATION
The results demonstrate resistant sorghum varieties to reduce the impact of Striga, the high potentiality of using Parkia based products for S. hermonthica control by seed soaking at high concentration and the intensifying cropping by integrating soyabean varieties and sorghum; this could provide a sustainable system than the sole sorghum cultivation. The relatively low of Striga count and high yield in ICSV1002 resistant sorghum variety at 66 g/l Parkia concentration and under intercropping system indicates a reduced potential for flowering and capsule production and consequently, a reduced capacity of increasing the Striga seed bank in the soil. Parkia pulp powder might be used in S. hermonthica control to reduce dependence on herbicides. However, further studies are needed to determine if the efficacy of Parkia could be enhanced, as well as to analyze the active allelochemicals in Parkia pulp powder. This would be a promising start in producing bioherbicide for S. hermonthica control.