Abstract: Herbicide, Pendimethlin applied at 1.48 kg ai/ha–1 controlled the weeds and gave 53 percent higher grain yield (2858 kg ha–1) than weedy check (1877 kg ha–1) and was at par with weed free treatment (2867 kg ha–1). Similarly, oxadiazon applied at 0.45 kg ai/ha–1 showed better performance in controlling weeds and gave higher grain yield of 2610 kg ha–1. The lowest grain yield of 1877 kg ha–1 was recorded in weedy check. Weed density and biomass were also significantly effected with the different doses of pendimethalin and oxadiazon. The medium doses of both herbicides gave excellent performance. Pods per plant were significantly different with the different doses of herbicides whereas, the medium doses gave higher number of pods per plant. The oil content was not altered with the different doses of herbicides but comparatively higher oil percentage was found than weedy check.
Introduction
Amongst the oilseed crops, Soybean (Glycine max L. Merr.) is one of the most important oil and protein crop in the world. In Pakistan, soybean is a non-conventional oilseed crop and still it seeks sufficiently greater efforts to fit in the existing cropping pattern. Soybean is grown on 5137 hectares producing 2885 tones of grains with an average of 562 kg ha–1 (Anonymous, 1994). The potential yield of newly established varieties is far higher than the existing ones and this might be due to the negligence in the adoption of recent improved soybean production technology (Aslam et al., 1995).
In order to increase the soybean production in Pakistan, the development of appropriate weed management technique is an essential component of improved cropping system. New management practices include, hand weeding (manual), mechanical and chemical control measures used exclusively or in combined system. However, the autumn sown crop usually has frequent monsoon rains during the early growth period and aggressive completion with weeds. Weed control by labour is tedious and costly. In such situation herbicides offer the most practical, effective and economical weed control and crop yield is also increased the annual loss caused by weeds in country has amounted to Rs. 1150 million and is more than the losses caused by diseases (Haq, 1970). However for raising a successful soybean crop, the availability of satisfactory weed control techniques and their adaptability on farmers fields are a pre-requested. Weed infestation during crop growth period was evaluated and found that weeds competitions begin from 20 days after sowing and continued after maturity but the most severe competition was in between 30 to 60 days after sowing (Beckett et al., 1988).
The major objective of this study was to evaluate the most effective rate of pre-emergence herbicides for higher grain yields in autumn soybean.
Materials and Methods
The experiment was conducted under field conditions in a randomized complete block design with 8 treatments (Table 1) replicated 3 times at the National Agricultural Research Centre, Islamabad during autumn 1993 and 1994. Well inoculated seed of NARC-II with Rhizobium Japonicum species, was planted with single hand drill with 45 cm row spacing. Plot size was 5×1.6 m, Recommended dose of NP fertilizer (25:50 Kg ha–1) was broadcasted at the time of sowing.
| Table 1: | Pre-emergence herbicides with different doses used for weed control in soybean during autumn 1993 & 1994. |
Pendimethalin (Stomp 330E) at 1.00, 1.48 and 1.96 kg ai/ha–1 and oxadiazon at 0.35, 0.45 and 0.55 kg ai/ha–1 were applied with hand operated Knap-Sack sprayer with flate-fan nozzle soon after sowing.
| Table 2: | Weeds flora recorded in autumn soybean crop during 1993 and 1994 |
| A = Annual Sdg = Sedges P = Perennial Bl = Broad leaves | |
| Table 3: | Effect of different doses of herbicides on weed density, biomass, yield and yield components in soybean (1993 and 1994) |
| Means in column followed by the same letter are not significantly different at 5 percent level of probability | |
Weed free (till harvest) and weedy check plots were included in the experiment for comparison. Weeds samples for weed density and weed biomass were taken by quadrate of 25×25 cm from each plot just 45 days after sowing. Plant height, pods plant–1 and gain yield per plot was recorded at the time of maturity. Grain yield determination was based on the dry weight basis of threshed grain and adjusted at 13 percent moisture level and computed to kg ha–1. Data recorded for weed density, weed biomass, grain yield and other yield components were analyzed following MSTATC Package adopted by Bricker (1991).
Results and Discussion
Twelve weed species were recorded in the experimental area during the autumn season 1993 and 1994 (Table 2). They were collected as distributed in 8 botanical families comprising grassy, broad leaved and sedges of annual or perianal habitat. Problematic weeds like deela (Cyperus rotundusl), baroo (Sorghum halepense) and khabbal (Cynodon dactylon) appeared in herbicide treated plot due to their propagation through underground stem (Rhizome) and consequently lower mortality rate was observed. However, efficacy of herbicide treatments proved excellent in controlling weeds when compared with weedy check.
The pooled data of two years of weed population and dry matter accumulation (Table 3) was significantly influenced by herbicide treatments. There was an acute weed infestation in weedy check plots as the density of mixed weeds was enormous (165.70/m–2) in contrast to herbicide treated plots (52.67-82.67/m–2). Similar trend was observed in dry matter accumulation by weeds as exhibited in Table 3. Medium dose at 1.48 kg ai ha–1 of pendimethalin and oxadiazon (0.45 kg ai ha–1) effectively controlled most of the weeds species as estimated 45 days after the sowing of soybean. Other doses of both tested herbicides were less effective in controlling weeds population and their biomass. Results are in line with the results achieved by Ghafoor et al. (1990), Khan et al. (1991) and Khan et al. (1992).
The weeds control efficacy of the tested herbicides has been further evaluated considering the plant height at the time of harvest that was found physically different but statistically similar plant height in weed free and medium dose of pendimethalin treatments. In other treated and untreated plots, plant height was significantly lower indicating weed interference as not allowing the crop plant to develop with its full potential. Different doses of both herbicides have shown their Marked effect on the plant height. The effectiveness of weed control treatment on pods bearing capability given in the Table 3 revealed that average pods plant–1 with one treatment is significantly different from average values of pod plant–1 with other treatments. The lowest average value of (37.67) pods plant–1 was recorded in weedy check which might be due to higher values of weed density and dry matter accumulation. Development of pods plant–1 in medium dose of pendimethalin with respect to weed free plots were physically different but statistically similar showing the higher efficacy of the pendimethalin (1.48 kg ai/ha–1) dose. Among the different doses of oxadiazon, the medium dose (0.45 kg ai/ha–1) gave better results and was in line with Balyan and Bhan (1984).
Analysis for oil content (Table 3) revealed that different herbicides and their doses have shown non-significant difference. However, herbicidal treatments caused an increase of 2.9-4.7 percent in the oil content as compared to weedy check (20.96%).
Seed yield (Kg ha–1 of soybean variety NARC-lI) was also affected significantly by the herbicide treatments (Table 3). The maximum yield of 2867 kg ha–1 was recorded in weed free (till harvest) treatment as compared to other treatments and was 53 percent more over the weedy check. Results are in conformity accordance with those of Chandler et al. (1984). Comparing the efficiency of different doses of pendimethalin through grading the means of seed yield, it was exhibited that the medium dose (1.48 kg ai/ha–1) gave higher seed yield than other two doses of this herbicide but was found at par with the weed free treatment carrying non-significant difference. This showed that medium dose of (1.48 kg ai/ha–1) pendimethalin was more effective and results are matching with the results of Khan et al. (1992) and Cheema et al. (1994). It was also indicated that on comparing yield among the different doses of oxadiazon, the medium dose (0.45 kg ai/ha–1) produced higher yield than remaining two doses which showed its superiority due to its higher weeds mortality rate. Results are similar with the investigation of Balyan and Bhan (1984).
It could be very clearly concluded based on the two years study, that the medium doses of pendimethalin (1.48 kg ai/ha–1) and oxadiazon (0.45 kg ai/ha–1) have excellent performance in controlling weed flora in autumn sown soybean and surely increase the seed yield giving an efficient weed control strategy for soybean for its successful and beneficial crop husbandry in Pakistan.