Experimental site

The field trials were conducted during  the 2018 and 2019 rainy seasons at the Teaching and research farm of the University, Port Harcourt, Rivers State, (latitude 04° 54 538’N, and longitude 006° 55 329’E; 17 m above sea level), Nigeria. The site had an average rainfall between 2500 – 4000 mm and a mean temperature of 27°C, relative humidity of 78% and Nwankwo and Ehirim, 2010).  The area has two seasons (wet and dry). The wet season has double rainfall peaks with two cropping seasons in the area: early from March to July and late from August to December. The experimental site had been planted to mixed crops of maize, pepper and watermelon before commencement of the experiment. The dominant weed species found in the experimental site was identified with a weed handbook (Akobundu et al., 2016). These weeds were: Ageratum conyzoides Linn Aspilia africana (Pers.) C.D. Adams. Chromoleana odorata (L.) R.M. King & Robinson, Cleome rutidosperma DC.  Cyperus esculentus Linn.. Mariscus alternifolius Vahl., Mitracapus villosus (Sw.) DC., Oldenlandia corymbosa Linn. and Panicum maximum Jacq.

Soil analysis

Soil samples were collected before planting operations at a depth of 0-15 cm deep using an auger of 10 cm in diameter at ten different points from the experimental site. The samples collected was air- dried at ambient temperature for two weeks and pulverized to facilitates laboratory analysis and for the removal of plant debris. The dry pulverized samples was assessed through a 2 mm mesh sieve and analyzed for physicochemical properties using standard methods (IITA, 1982).

Rice variety used

The rice variety used was (UPIA 2) and UPIA is an acronym for University of Port Harcourt, International Rice Research Institute and AGRA.  It has an outstanding characteristic of high yield and tolerance to iron toxicity and African rice gall midge, matures between 110 - 120 days with a potential yield of 8.0 t/ha. The seeds were obtained from rice seed banks at the Teaching and Research Farm of University of Port Harcourt, Rivers State.

Herbicide used

ButaForce herbicide was used for the study. The herbicide was obtained at an Agrochemical store in Port Harcourt, Rivers State. The common name of the herbicide, its formulation, manufacturer and main marketing agent in Nigeria is shown in Table 1 and the chemical name Table 2.

Treatment and experimental design

Seven treatments were used for the experiment, which are itemized below:

(i) ButaForce Ò at 1.5 L/ha + SHW (21 DAS)

(ii) ButaForce Ò at 2.0 L/ha + SHW (21 DAS)

(iii) ButaForce Ò at 2.5 L/ha + SHW (21 DAS)

(iv) ButaForce Ò at 3.0 L/ha

(v) Weed free (weekly weeding)

(vi) Weeding  twice at 21 DAS and 42 DAS

 (vii) No weeding

These treatments were replicated three times to give a total twenty - one experimental plots, arranged in a Randomized Complete Block Design (RCBD).

Cultural details

The experimental land area of 29 m × 12 m (348 m²) of approximately 0.03 ha was cleared manually, stumps and excess vegetation packed away from the plots. The experimental area was divided into three blocks while each block was further divided into seven plots making it a total of twenty one plots. Each plot size was 3 m × 3 m. The plots were separated by 1 m while the blocks were separated with a pathway of 1 m. Planting was done on the 14^th and 15^th May 2018 and 2019 respectively.  The seeds were sown at a spacing of 30 cm ×  30 cm with three seeds per hole and later thinned to one seedling at  fourteen days after  sowing  (14 DAS)  to give a plant population of 100 plants /plot which is equivalent to  (111,111plants/ha). One day after sowing (1DAS) , twelve plots were sprayed with ButaForce Ò at 1.5, 2.0 and 3.0 L/ha using a hand-operated CP3 knapsack sprayer calibrated to deliver approximately 240 L/ha spray volume at a pressure of 210 kpa with red polijet nozzle (swath width½m). Supplementary hoe weeding was carried at 21 DAS in plots that were treated with ButaForce Ò at 1.5, 2.0 and 3.0 L/ha.  Three plots were manually weeded with a hoe   twice at 21 DAS and 42 DAS while another 3 plots were hoe weeded weekly. Basal application of urea fertilizer at 97.8 kg/ha was carried out at 21 DAS. This was done because the soil sample from the experimental site was found to be deficient of nitrogen (0.10 and 0.11% in 2018 and 2019 respectively) when compared to the critical level of nitrogen (0.15%) of southeastern soil established by Ibedu et al. (1988). Harvesting was carried out on 17^th and 18^th of September in 2018 and 2019 respectively with the use of sickle.

Data collection

Weed growth characteristics

Weed density and weed dry weight: Weed samples were collected at 21, 42, 63 and 84 DAS by placing 50 cm × 50 cm quadrats diagonally per plot twice. The weeds within each quadrat were removed by hand, counted and expressed in no/m². The weed dry weight was carried by using the same quadrat technique as weed density. The weeds were removed within the quadrat, sun dried to constant weight, weighed with an electronic scale, and expressed in g/m².

Weed control efficiency

Weed control efficiency was determined by using the method of Subramanian et al. (1991) as:

Where:

WCE (%) = Weed control efficiency

DWT  =  Dry weight

Subramanian et al. (1991)

Where: WI = weed index

Rice performance

Five plants from the middle row of each plot were randomly selected and tagged and used to determined plant height, number of tillers, number of leaves, and leaf area index.

Plant height

The height of each tagged plant was taken at 4 intervals (21, 42, 63 and 84 DAS) using a meter ruler. Plant height was determined by placing a meter ruler at the soil surface to the tip of the flag leaf of each tagged plant and the mean calculated and recorded in cm.

Number of productive tillers

The number of tillers was obtained by counting starting from 21 to 84 DAS.

Number of leaves

This was done by counting the number of leaves per plant.

 Leaf area index (LAI)

Leaf area index was determined by the following equation below:

LAI = T A x N /GA,

Where, TA = Total leaf area /plant N = number of plants/ gross plot, GA= Gross plot Area (Remison, 1997).

Panicle length

This was done by randomly from five panicles selected from harvested produce in each plot. It was measured from the neck-node to the tip of the apical grain and their average was taken as per panicle length.

Panicle weight (g)

The panicles selected for measuring length were weighed on an electrical weighing balance and then mean was worked out.

Paddy yield

The grains obtained after threshing and winnowing of the produce from each gross plot were sun dried, weighed per gross plot with a scale and the weight was expressed in kilogram per hectare (kg/ha).

Economic assessment

The economic assessment was done by using partial budgeting (Okoruwa et al., 2005).

Statistical analysis

Data were subjected to analysis of variance (ANOVA) at 5% level of probability   using  GENSTAT  12^th  Edition  while  treatments  mean were separated by using the least significant difference (LSD).

Soil analysis

The physiochemical characteristics of the soil before planting in both years are presented in Table 3. The soil of both years of study was loamy sand, slightly acidic with a moderate organic carbon content, available Phosphorus (P), exchangeable Potassium (K), Magnesium (Mg), Calcium (Ca) and low in total Nitrogen.

Rainfall

Table 4 shows the amount of rainfall data in 2018 and 2019 cropping seasons. The total amount of rainfall in 2018 cropping season (1362 mm) was higher than that of  2019 (1112.01 mm) by 22.48%.

Weed growth characteristics

Weed density

The effect of supplementary hoe weeding on the efficacy of Butaforce on weed density of low land rice is shown in Table 5. The treatments differed significantly (P < 0.05) throughout the sampling intervals (21, 42, 63 and 84 DAS). The highest weed density was recorded in no weeding plots throughout the observation periods except at 21DAS in 2018. The lowest weed density was recorded in plots that were weekly weeded throughout the periods of observation. All the ButaForceÒ rates with a supplementary hoe weeding had similar weed density at all the sampling intervals. Though at 21 DAS there were no significant differences between the supplementary hoe weeding and the recommended rates of ButaForceÒ at 3.0 L/ha at that period of sampling in both years, but all the herbicide plots that were supplemented with hoe weeding had lower weed density than the recommended rates of ButaForceÒ  at  3.0 L/ha.

Weed dry weight

The effect of supplementary hoe weeding on the efficacy of ButaForceÒ on weed  dry  weight  of  low  land  rice  is shown in Table 6. The treatments differed significantly (P < 0.05) throughout the sampling intervals (21, 42, 63 and 84 DAS) on weed dry weight. Plots that were weeded weekly had the lowest weed dry  weight  when  compared to other treatments in both years of study. The highest weed dry weight was produced in weedy plots at the four periods of observations except at 21 DAS in 2018 where the dry weight was statistically on par with plots that were hoe weeded twice.  All the ButaForceÒ rates with a supplementary hoe weeding had similar weed dry weight at all the sampling intervals. Though at 21DAS there were no significant differences between the supplementary hoe weeding and the recommended rates of ButaForceÒ   at 3.0 L/ha  at that period of sampling in both years, but all the herbicide plots that were supplemented with hoe weeding had lower weed dry weight than the recommended rates of ButaForceÒ  at 3.0 L/ha.

Weed control efficiency

The effect of supplementary hoe weeding on the efficacy of ButaForceÒon weed control efficiency of low land rice is shown in Table 7. The treatments differed significantly on weed control efficiency in both years of experimentation. Weed control efficiency was higher in plots that were hoe weeded weekly in all the sampling periods when compared to other treatments in both years of study.   Weed control efficiency was lower in weedy plots throughout the sampling periods in both years of study except at 21 DAS where it was higher in plots with ButaForceÒ   at  1.5  L/ha  +  SHW   (21  DAS)  and  Hoe weeded at 21 and 42 DAS in 2018 and Hoe weeded at 21 and  42 DAS in 2019.

Rice performance

Plant height

The effect of supplementary hoe weeding on the efficacy of ButaForceÒ on plant height of low land rice is shown in Table 8. All the weed control treatments significantly (P < 0.05) affected rice height at the various sampling periods. Plants grown on weekly weeded plots grew taller than that of other treatments at 21, 42, 63 and 84 DAS.  All the plots that received one supplementary hoe weeding at 21 DAS had identical plant heights in both years of study. Plots treated with ButaForceÒ at 3.0 L/ha at 21 DAS grew taller when compared to those plots that received one supplementary hoe weeding in both years. Plants in the weedy plots grew shorter throughout the sampling periods but it was at par with that of hoe weeded plots in both years of study at 21 DAS.

Leaf area index

Table 9 shows the effect of supplementary hoe weeding on the efficacy of ButaForceÒ on leaf area index of low land rice. The leaf area index  differed  significantly  in  all the sampling periods in both years of study. Plots hoe weeded weekly consistently  produced  the  greatest  leaf area index at 21, 42, 63 and 84 DAS in both years of study.    The   lowest   leaf  area  index  was  observed  in weedy plots. Although plots treated with ButaForceÒ at 3.0 L/ha tended to have the greatest leaf area index at 21 DAS but the leaf area index did not differed significantly from that plots that received supplemented one hoe weeding in 2018. While in 2019 plots treated with ButaForceÒ at 3.0 L/ha differ significantly from that of plots treated with ButaForceÒ at 1.5 L/ha with a supplementary hoe weeding (21 DAS) but statistically similar with that of ButaForceÒ at 2.0 L/ha + SHW (21DAS). In the 2018 cropping season, all the plots with one supplementary hoe weeding did not differ significantly from one another at all sampling intervals except at 42 DAS. However, in 2019 there were no significant differences among the supplementary hoe weeding throughout the sampling periods.

Number of tillers

Table 10 shows the effect of supplementary hoe weeding on the efficacy of ButaForceÒ on number of tillers on low land rice. There were significant differences in the number of tillers among the weed control treatments at the various intervals of sampling in both years of experimentation. The highest number of tillers was recorded in weekly weeding plots throughout the sampling intervals. The weedy plots had the lowest number of tillers at the various sampling intervals but at 21 DAS it has the same values  on  the  number  of  tillers with plots that were hoe weeded twice in both sampling periods. At 21 DAS, plots that were treated at the recommended rates of ButaForceÒ at 3.0 L/ha had higher numbers of tillers that differed from those with supplemented hoe weeding plots.

Panicle length and panicle weight

Table 11 shows the effect of supplementary hoe weeding on the efficacy of ButaForceÒ  on panicle length and panicle weight.  There were significant differences among the weed control treatment on panicle length in both years of study. The weekly weeded plots had the longest length of panicle while the weedy plots had the shortest length in both years of study.  Panicle length was longer in Plots treated with ButaForceÒ at 2.5 L/ha than the others supplementary hoe weeding Panicle weight was heavier in weekly weeding and lighter in weedy check.  Plots treated with ButaForceÒ  at 2.5 L/ha had a heavier weight of panicle than other plots that received one supplementary weeding.

Yield and weed index

Table 12 shows the effect of supplementary hoe weeding on the efficacy of ButaForceÒ  on yield and weed index of   low   land   rice.    There  were  significant  differences among the weed control treatments on paddy yield in both years. In 2018, the  weekly  weeded  plots  recorded significantly higher yields, which was comparable to the weeding   twice   plots   and    three   supplementary  hoe weedings at 21DAS while in 2019 it was comparable to hoe weeded plots at 21 and 42 DAS treated with  ButaForceÒ at 2.0L/ha and 2.5 L/ha  with one supplementary hoe weeding each, respectively Weed index differed significantly among the weed control treatments in both years.  The highest weed index was recorded in weedy plots while the lowest was recorded in the weekly weeded plots.  Weed index in 2018 cropping season in all the supplementary hoe weeded plots were comparable.

Economic assessment

The economic evaluation of the different weed control treatments for the production of lowland rice is presented in Table 13. The highest cost of production was recorded in plots that were manually hoe weeded weekly at 21 and 42 DAS in both years of study while the weedy plots had the lowest cost of production. Sale revenue was higher in plots weeded weekly and weeded twice at 21 and 42 DAS of both years while the lower revenue was produced in plots in the weedy check. The highest profit in both years of study was obtained in plots treated with ButaForceÒ at 2.5 L/ha with supplementary hoe weeding (21 DAS) while the weedy check had the lowest profit with negative values which signified no gain or loss.

The   physiochemical   characteristics  of  the  soil  before planting showed that the soil was sandy loam, slightly acidic with moderate organic carbon content, available P, exchangeable K, Mg, Ca and Na but was low in nitrogen content when compared to their critical levels (Ibedu et al., 1988).  The low value of N obtained from the soil could be attributed to excessive rainfall, and leaching of nutrients and high temperature.

All the weed control treatment significantly reduced weed infestation judging from their lower weed density and weed dry weight when compared to weedy check probably due to their effectiveness in controlling weeds.   Weed density and dry weight were low in weekly weeded plots as a result of the constant weekly hoe weeding.  Unweeded plots had the highest weed density and weed dry weight probably because no treatment was applied to them.  However, at 21 DAS, plots labeled as hoe weeded at 21 and 42 DAS were not weeded before collecting weed data as at that period, hence it was weedy as the no weeding plots. Among the plots that received supplementary hoe weeding, ButaForceÒ at 2.5 L/ha had the lowest weed density and dry weight than the others probably because it was applied at a higher rate. ButaForceÒ at 3.0 L/ha applied without supplementary hoe weeding could only have effective control of weeds at 21 DAS. As the rice growth stages progress there was a gradual decline of the herbicide rate in controlling weeds probably due to decrease of the herbicide concentration by gradual dissipation of the herbicide from the soil due to leaching.  Although weed density and dry weight were reduced in all the plots that received supplementary hoe weeding, the reduction was more pronounced  with  ButaForceÒ at higher rates of 2.5L/.ha.  Imoloame (2017) reported similar findings that integration of herbicide rate with one supplementary hoe weeding provided better weed control in maize. Peer et al. (2013) also noted similar finding in other crops that herbicide supplemented with hoe weeding gave adequate weed control.

Generally, weed control efficiency was higher in weekly weeded plots when compared to other treatments probably as a result of constant weeding which made it free from weeds. The significantly higher plant height obtained in the weekly weeded plots could be attributed to efficient and effective weed control of the treatment. This is in line with Akbar et al. (2011) who reported taller plants in weed- free plots than in weedy plots. Leaf area index was higher in weekly weeded plots.  High leaf area index indicates that the crop had a good canopy cover which shade out weeds from sunlight from penetrating the soil surface which could have stimulated weed growth. The weed - suppressing ability for other crops due to crop canopies has been reported by Busaari (1996) and Binang et al. (2016). The weedy plot had the lowest leaf area index which implies poor canopy formation which allows sunlight to penetrate the soil surface to stimulate rapid weed germination and weed growth. The highest tiller number and tiller dry weight was recorded in the weed -free plot while the lowest tiller number and tiller dry weight was recorded in the weedy plot. The number of tillers observed in the weekly weeded plots might be attributed to high weed control efficiency of the treatment as a result of reduced weed pressure.

Panicle length and panicle weight were longer and heavier in weekly weeded plots when compared to other treatments probably as a result of the weed-free condition of the plots. The rice plant was able to out-compete the weeds for available growth resources. The paddy yield was higher in weekly weeded plots probably as a result of no weed competition and higher leaf area index which produced good canopy closer for capturing sunlight for photosynthesis which promotes more yield. The high tillers produced from the weekly weeded plots also smothered the weeds giving the rice crop a competitive advantage.  The weedy plots had the lowest paddy yield probably as a result of severe weed competition for water, carbon dioxide, sunlight, and space. Uncontrolled weed growth resulted to a paddy yield loss of 86.67 and 83.94% in 2018 and 2019 cropping seasons respectively.  This result is in collaboration with that of Rodenburg and Johnson (2009) who reported 28- 89% yield loss in direct – seeded lowland rice due to uncontrolled weed growth. The yield variation observed in both years might be attributed to differences in rainfall. Rainfall was higher in 2018 than in 2019 and this could be the probable reason for the higher yield recorded in 2018 than in 2019.

The differences observed in the sale revenue of the various weed control treatments could be attributed to differences in yield. Although plots hoe weeded weekly and weeding twice had the highest sale revenue, their cost of production was higher than others  probably  as  a result of expensive labour involved due to scarcity during the time of the weed control. This finding is in line with that of Adigun and Lagoke (2003) who noted that the cost of hoe weeding is expensive. The highest profit was obtained in plots treated with 2.5 L/ha +SHW, followed by 2.0 L/ha + SHW and  ButaForceÒ at 1.5 L/ha + SHW,  and  ButaForceÒ at 3.0 L/ha without supplementary hoe weeding  in both years of study  probably because their cost of production was lower than that of hoe weeded plots. Imoloame (2009) also observed a similar finding that herbicide use in most crops production is more profitable than manual hoe weeding.

Weekly weeding plots had the lowest weed index and highest weed control efficiency. The performance of lowland rice was better in weed-free plots than other treatments. Weed suppressive ability was better in plots treated with ButaForceÒ at 2.5 L/ha with supplementary hoe weeding than other supplementary hoe weeding in both years; judging from it high weed control efficiency and lower weed index. SHW enhanced the yield in plots treated with ButaForceÒ   rate lower than the recommended rate of 3.0 L/ha with about a 6% yield advantage and a 3.0% profit margin in 2018 while in 2019 they had 8.43% yield advantage and a 6.63% profit margin. Since ButaForceÒ at 2.5 L/ha with supplementary hoe weeding had the highest profit, it is therefore recommended to rice farmers in the study area.

The authors have not declared any conflict of interests.