Weeds are a major biotic constraint to increased rice production   worldwide.   Its   occurrence   is   a   constant component of the ecosystem in comparison with the epidemic  nature  of   other  pests  which  makes  farmers unaware of the significant losses they incur from their infestation (Ismaila et al., 2015). Farmers can spend over US $400 ha⁻¹, or 20% of their production costs to control weeds during the growing cycle (Islam et al., 2005). Weeds can cause serious yield reduction in rice production worldwide. Losses caused by weeds vary from one country to another, depending on the predominant weed flora and the control methods practised by the farmers (Mishra et al., 2016). The extent of loss varies depending upon cultural methods, rice cultivars, weed species and the density and duration of competition. For example, uncontrolled weed growth is reported to have caused 28 to 74% yield losses in transplanted lowland rice, 28 to 89% in direct-seeded lowland rice, and 48 to 100% in upland ecosystems (Rodenburg and Johnson, 2009). The potential yield loss from weed is less in transplanted rice than in dry-seeded rice (Joshi et al., 2013).

However, improved weed control has been estimated to increase rice yields by 15 to 23%, depending on production system in the ecosystem (Rodenburg and Johnson, 2009). It is rare, however, for farmers not to undertake some form of weed control and therefore losses on farmer’ fields are likely to be less depending on the control measures adopted. Common weed management practices in rice-based cropping systems include soil tillage, clearance by fire, hand- or hoe-weeding, herbicides, flooding, fallow and crop rotations, and these are often used in combinations (Rodenburg and Johnson, 2009). Good cultural practices cannot be underestimated in their importance to weed management. Most, if not all of these cultural methods should be a necessary part of crop management procedures in controlling weeds (Gbanguba et al., 2011).

The management of weeds requires integrated strategies to be successful. The combination of direct weed control methods, such as herbicides or hand weeding, with indirect methods such as land preparation, flooding and competitive crops, can suppress weed growth. The optimum combination of weed control methods will depend on the farming system, economic conditions, and farmers’ resource and knowledge base (Johnson, 2009). During the off-season, rain fed rice lands are typically fallowed (Gbanguba et al., 2014). The straw and fallow weed vegetation are subjected to grazing by livestock. In a minor fraction of the area with conducive residual soil water-holding capacity, and/or a high ground water table, upland crops, including legumes, are grown in the post-rice season. This practice is most common in well-drained rice lands. In this case, upland crops are grown prior to rice during the dry-to-wet season transition period. Very short duration crops are advantageous to permit maturity before the soil becomes waterlogged. Rotating crops with different planting dates and growth periods, contrasting competitive characteristics and dissimilar management practices can be used to disrupt the regeneration niche of different weed species  in  rice  field.  For  example,  Liebman  and Davis (2000) reported that Bromus tectorum (L.) density remained relatively stable when winter wheat (Triticum aestivum L.) was rotated with oilseed rape (Brassica napus L.), whereas the density of the weed increased rapidly when wheat was grown continuously. Also, Liebman and Davis (2000) mentioned that Setaria faberi Herrm. seedling density tended to be greatest in continuous maize, intermediate in a two year maize/soybean rotation and lowest in a three year maize/soybean/winter wheat rotation. A well-planned crop rotation system can help producers avoid many of the problems associated with weeds, particularly perennial weeds (Mohler, 2012). Crop rotation is an effective practice for controlling weeds because it affects weed growth and reproduction negatively and in turn reduces weed density (Sims et al., 2018). In a previous study, Filizadeh et al. (2007) found that rice yields in rotation with soybean were higher by 17 to 21% compared with continuous rice. Anders et al. (2004) reported higher yield of rice in rice grown after soybean than in rice wheat rotation. Toomsan et al. (2000) recorded 50% higher rice yields in rice followed cover crop green mixtures than rice in bare fallow rotation. The grain yield of rice preceded by a legume fallow had been reported to be on average 0.2 kg ha^-1 or 30% greater than that preceded by a natural weedy fallow control (Gbanguba et al., 2014).

 In Nigeria, cassava/legume intercropping preceding rice production is a common practice among farmers in the Southern Guinea agro-ecology. But it is not known, how this practice under varying weed control methods affects weed growth, growth and yield of rice in this region. Thus, this study was aimed at evaluating the effect of one year rotation of pre-rice cropping with cassava/legume intercrops and weed management practices on weed suppression, growth and yield of low land rice.

Experimental site

The experiment was conducted in 2011 to 2013 growing seasons at the lowland experimental field of the National Cereals Research Institute, Badeggi (latitude 09° 45′N, longitude 06° 70′E, elevation 70.57 m above sea level). The area is located in the Southern Guinea Savanna zone of Nigeria with mean annual rainfall of 2066.3, 1163.6 and 899.7 mm distributed between April and October in 2011, 2012 and 2013, respectively. The average maximum and minimum air temperature was 30 to 38°C and 14 to 26°C, respectively. The soil texture of the experimental site was sandy clay. All the soils were moderately acidic with a pH (H₂0) around 5.24.

Treatments and experimental design

The treatments were a factorial combination of Cassava (IIT 427) intercrop with Mucuna or Velvet bean [Mucuna pruriens (L.) DC.], Cowpea  [Vigna  unguiculata (L.) Walp.], Soybean [Glycine max (L.) Merr.], Hyacinth bean [Lablab purpureus (L.) Sweet.] and Porcupine Jointvetch (Aeschynomene histrix Poir.) legumes, sole cassava and natural fallow. The weed management practices were (i) application of propanil at 1.44 kg a.i. ha^-1 plus 2, 4 D at 0.80 kg a.i ha^-1 (Orizo plus^R)) at the rate of 2.24 kg a.i ha^-1 at 3 weeks after transplanting (WAT) followed by hand weeding at 6 WAT, (ii) two hand weedings at 3 and 6 WAT, (iii) one hand weeding at 3 WAT, and (iv) weedy check. Cassava/Legume intercrop was assigned to the main plot, while weed management practices were assigned to the sub plot. The trial was laid out in a split plot arranged in a randomized complete block with three replications. Main plot size was 12.5 × 5 m and sub plot size was 3 × 5 m replicated three times.

Cultural practices

The experiment was initiated by intercropping of cassava with the legumes in January in 2011, 2012 and 2013 on manually prepared raised beds (2.5 m × 0.5 m × 0.75 m) using residual moisture. Beds were spaced 0.5 m. Cassava (IIT 427) was planted on the top side of the bed in two rows at intra-row spacing of 0.5 m (ten stands per bed) and legumes were planted by the side of the beds at intra-row spacing of 0.25 m, except for soybean (TGX 1019EN) which was drilled at 5 cm intra row immediately the beds were constructed. Cassava cutting and soybean varieties were sourced from the International Institute of Tropical Agriculture, Ibadan. Among the other legumes, Aeschynomene histrix was sourced from the International Institute of Tropical Agriculture, Kubwa-Abuja, and cowpea (IAR 48) and Mucuna seeds were obtained from the Institute for Agricultural Research, Samaru-Zaria. The cassava/ legume cropping lasted till August when cassava was harvested. Superimposition of rice (Faro 52) variety obtained from seed unit of National Cereals Research Institute, Badeggi, on the plots previously cropped with cassava/legume intercrops were levelled for rice cultivation on 6, 5 and 9th August of 2011, 2012 and 2013, respectively. Rice was transplanted at rate of two seedlings per hill at a spacing of 20 cm × 20 cm. Weeding was carried out as per the treatments. Orizo Plus^® was applied with a CP3 knapsack sprayer using a spray volume of 250 L ha^-1 at 206 kPa. Fertilizer application was done by broadcasting in split application of NPK 40: 60: 60 as basal, while N 40 kg ha^-1 was applied as top dressing. Harvesting was achieved by using sickle. Drying, threshing and winnowing were manually carried out.

Data collection

Before the superimposition of the rice seedling,  soil  samples  were taken from three randomly selected spots from each cassava/ legume intercrop plot, bulked and used to determine organic carbon, total nitrogen, and available phosphorous. The method of Walkley and Black (Anderson and Ingram, 1993) was used to determine the organic carbon. Total nitrogen was determined by the macro Kjeldahl method (Jackson, 1962). Available phosphorous was determined by the Olsen method (Okalebo et al., 2002).

Weed samples were collected from 1 m^-2 quadrant randomly placed in each plot in each year at 9 WAT for determination of weed density, dry weight and control efficiency. All the weed species in each quadrat were counted and clipped above the soil surface; oven dried at 70°C to a constant weight for weed dry weight determination. Rice plant height, tiller and panicle numbers were determined by collecting rice samples from the 1 m² quadrat used for weed sampling at 9 WAT. Rice grain yield was obtained from tillers harvested from 2 × 4 m net plot. Tillers were harvested at physiological maturity and manually threshed and winnowed. Grains obtained from the tillers were measured and converted to kg ha^-1. Data collected were subjected to analysis of variance (ANOVA) and differences between means were separated using Duncan Multiple Range Test (DMRT) at P≤ 0.05 using M-Stat-C software (M-Stat-C Version 1.3).

Some soil nutrient status after cassava/legume intercrop

A significant effect of cassava/legume intercrop was observed on organic carbon, total nitrogen and available phosphorous in this study (Table 1 ). Cassava/Cowpea, cassava/soybean and cassava/Aeschynomene intercrops in each year and cassava/Mucuna intercrop in 2013 added more organic carbon to the soil than all other treatments. Total nitrogen was the highest and comparable in cassava/Mucuna, cassava/cowpea and cassava/ Aeschynomene intercrops in 2012 and 2013, and cassava/soybean or Lablab intercrops in 2013 than all other treatments. In terms of available phosphorous, cassava/cowpea intercrops significantly contributed more of this nutrient, which was comparable to cassava/ Mucuna, cassava/soybean and cassava/Aeschynomene intercrops in 2011 and 2012, and cassava/Lablab intercrop in 2011 only.

Weed density

Rice grown after cassava/Mucuna intercrop produced the least weed density compared with other intercrops in each year of the study (Table 2). Furthermore, weed density produced in rice after cassava/legume intercrops and natural fallow ranged from 29.5 to 47.2%.  Rice grown after natural fallow consistently had the highest weed density followed by sole cassava.

On weed management practices, application of Orizo Plus fb hoeing at 6 WAT and two hoeing at 3 and 6 WAT produced similar lower weed densities in 2011, 2012 and 2013 rainy seasons (Table 2). Weedy check accounted for the highest weed density in both years of the study which ranged between 30.0 and 38.0% over Orizo Plus fb hoeing at 6 WAT, and two hoeing at 3 and 6 WAT.

A significant (p≤0.05) interaction between cassava/ legume intercrops and weed management practices was recorded for weed density in each year in this study (Table 3). In 2011, weed density was significantly (p≤0.05) lowest in cassava/Mucuna intercrop in combination with each of the weed management options. Furthermore, in 2012 and 2013, cassava/Mucuna in combination with Orizo Plus fb hoeing at 6 WAT and two hoes weeding at 3 and 6 WAT caused similar significant decrease in weed  density.  Similarly,  reduction  of  weed density by cassava/Mucuna intercrop in combination with one hoe weeding at 3 WAT was comparable to that recorded in cassava/cowpea intercrop in 2012 and 2013. Irrespective of the weed management practice, the highest weed density was observed in natural fallow which were similar for each weed management practice in the three years of the study.

Weed dry matter

Table 4 shows that the lowest weed dry matter was obtained in rice grown after cassava/Mucuna intercrop compared with others throughout the period of study. Weed dry matter production in rice grown after sole cassava and natural fallow was found to be 16.7 to 56.9% and 31.2 to 64.5% more than that of the intercrops, respectively.

Considering weed management practices effects, application of Orizo Plus fb hoe weeding at 6 WAT and hoe weeding at 3 and 6 WAT had similar lowest weed dry matter, than the other treatments in the  study period (Table 4).  

Irrespective of the weed management practice, maximum weed dry matter was recorded in combination natural fallow with each of the  weed  management option in each year in this study. The lowest weed dry matter was obtained from cassava/Mucuna intercrop irrespective of the weed management practice in each year of study (Table 5). Contrarily, optimum weed dry matter was produced in natural fallow.

Rice plant height

Table 6 shows that rice plant height differed between cassava/legume intercrops, such that cassava/Mucuna, cassava/cowpea and cassava/Aeschynomene in 2011 and 2012 produced similarly taller plants. The natural fallow plots had the shortest plants.

Two hoe weeded plots consistently produced taller plants (Table 6). Similar taller plants were observed in plots with application of Orizo Plus fb  hoeing  at  6  WAT, and one hoeing at 3 WAT in 2011 only.

There was significant interaction between cassava/ legume intercrop and weed management practices on rice plant height in 2012 and 2013 (Table 7). The use of cassava/cowpea intercrop in combination with the weed management practices generally had taller rice plants in 2012. Similarly, in 2013, irrespective of the weed management practice, rice plant height was tallest under cassava/cowpea, and cassava/Aeschynomene.

Cassava/Aeschynomene intercrop gave greater number of rice tillers per stand than all the other intercrops, but compared with cassava/Mucuna intercrop. This intercrop practice was able to provide season long weed control, which in turn provided favourable condition for enhanced crop growth and production of yield attributes of rice. This finding is in agreement with the work of Anders et al. (2004) who observed higher rice yield in rice grown after soybean than in rice-wheat rotation.

The practice of two hoeing at 3 and 6 WAT gave greater number of rice tillers, suggesting that this treatment gave efficient weed control, which provided good crop yield attributes. Kolo and Umaru (2012) and Hakim et al. (2013) also observed the production of more rice tillers in weed free plots that received two or three hoe weedings.

The significant interaction between cassava/legume intercrop on number of rice tillers per stand affirmed that the combined use of cassava/Mucuna, cassava/ Aeschynomene intercrops with two hoeing at 3 and 6 WAT probably gave rise to better weed control and soil nutrient availability and utilization.

The greater number of rice panicles per plant produced by two hoeing at 3 and 6 WAT than the other weed management treatments, though comparable to application of Orizo Plus fb hoeing at 6 WAT was due to efficient weed control. This  provided conditions for better crop yield. The present result is consistent with previous studies in which plots weeded twice at 15 and 30 DAS, increased number of rice panicles, and comparable to plots given bispyribac-sodium or ethoxysulfuron fb manual weeding at 30 DAS  (Ihsan et al., 2014).

The interaction between cassava/legume intercrop and weed management practice on number of rice panicle revealed that cassava/Mucuna, cassava/cowpea, cassava/Aeschynome intercrops with two hoeing at 3 and 6 WAT produced similar highest number of rice panicles per  unit  area.  This  could  be  attributed  to  the  greater number of tillers produced per stand, which might have produced more panicles per unit area (Maite et al., 2015).

The greater number of rice panicles per plant from cassava/cowpea and cassava/Aeschynomene intercrops compared with cassava/Mucuna intercrops could be attributed to effective weed growth reduction, which translated into enhanced crop yield attributes. Mobasser et al. (2007) also observed that greater number of panicles m^-2 gave higher grain yield.

In this study, cassava/cowpea intercrop gave the highest paddy  yield  than  the  other  intercrop  practices, though comparable to cassava/Aeschynomene. These intercrop practices gave taller rice plants and more rice panicles, thereby provided conditions for enhanced rice growth and yield.

Expectedly, paddy yield of rice was more in plots given two hoeing at 3 and 6 WAT, because it gave the best weed control (reduced weed density and biomass), produced taller plants, greater number of tillers and panicles per stand which translated into enhanced paddy yield of rice. Conversely, the highest paddy yield recorded with two hoeing at 3 and 6 WAT was comparable with application of Orizo Plus fb hoeing at 6 WAT. This may be attributed to efficient weed control which suggests reduced nutrients depletion by the weeds, which in turn enhanced rice growth, yield and yield attributes. Similar results have been reported in a previous study in Pakistan in which paddy yield of rice was increased with two hoeing at 15 and 30 DAS, which was comparable to application of bispyribac sodium or ethoxysulfuron ethyl followed by one manual weeding at 30 DAS (Ihsan et al., 2014).

Based on the results of the present investigation, it can be concluded that in terms of weed management, the best treatments were cassava/Mucuna intercrop under two hoeing at 3 and 6 WAT, or Orizo Plus fb hoeing at 6 WAT. Increased growth and yield of rice can be realized in this agro ecology with cassava/cowpea, cassava/ Aeschynomene intercrop in combination with two hoeing at 3 and 6 WAT or with application of Orizo Plus fb hoeing at 6 WAT.

The authors have not declared any conflict of interests.