Advantages of Pyroligneous Acid Combined with Reduced-Dose Herbicides: Healthier Soils and Higher-Quality Rice
Rice production faces ongoing challenges due to weed competition, heavy reliance on chemical inputs, and concerns about maintaining soil health. A 2015 study published in the International Journal of Agriculture and Biology examined whether mixing reduced rates of post-emergence herbicides with pyroligneous acid, commonly known as wood vinegar (WV) and rice vinegar (RV), could effectively control major rice weeds while preserving soil fertility, improving grain quality, and maintaining strong yields. The findings offer a glimpse into how farmers might minimize synthetic inputs without sacrificing productivity.
Introduction
Conventional rice cultivation often uses an array of chemical herbicides to combat weeds such as
Echinochloa crus-galli (barnyard grass) and Monochoria vaginalis, among others. However, this approach can lead to environmental concerns, health risks, and the potential for herbicide resistance. Pyroligneous acids, which result from the condensation of smoke during charcoal production, have emerged as promising natural additives or 'bio-helpers.' They contain organic acids (mainly acetic acid) and other active compounds that may improve plant growth, act against pests, and possibly boost soil health by encouraging beneficial microbial processes.
This study specifically assessed whether herbicide activity could be maintained or even enhanced by mixing herbicides at half the recommended rate with a modest concentration (500×) of wood or rice vinegar, rather than applying the full rate of herbicide alone.
Experimental Design and Application
The field experiment took place on paddy soil in Daegu, South Korea. Researchers tested three post-emergence herbicides:
- Azimsulfuron + Cafenstrole (AC)
- Mefenacet + Pyrazosulfuron-ethyl (MPE)
- Bentazon + Cyhalofop-butyl (BCB)
Each herbicide was applied in four different ways:
- 100% recommended rate (RR)
- 50% recommended rate
- 50% recommended rate + wood vinegar (WV) at 500× dilution
- 50% recommended rate + rice vinegar (RV) at 500× dilution
A non-treated, hand-weeding control was also included. The recommended rates followed each herbicide’s manufacturer guidelines, while wood and rice vinegar solutions were prepared at 500× dilution. These were applied to the field 15 to 25 days after transplanting, depending on whether the herbicide was in granular or liquid form.
Key Findings
1. Weed Control Efficacy
All mixtures of herbicide plus pyroligneous acid effectively suppressed E. crus-galli, M. vaginalis, and Scirpus juncoides (a perennial sedge). In many cases, the weed control was comparable to that achieved by 100% herbicide rates. For instance:
- Echinochloa crus-galli: Reduced to near zero fresh and dry weight across both full-rate and half-rate + vinegar treatments.
- Monochoria vaginalis: Not entirely eliminated but still controlled effectively, with significantly less weed biomass than in the non-treated plots.
- Scirpus juncoides: Also successfully minimized, including combinations of half-rate herbicides with vinegar.
2. Growth Characteristics and Chlorophyll Content
Although weeds were effectively suppressed, rice growth did not suffer from the reduced herbicide amounts. In many treatments, plant height, tiller number, and chlorophyll (SPAD values) were comparable or slightly superior to the plots receiving 100% herbicide. The presence of wood or rice vinegar often helped maintain or even boost chlorophyll levels, suggesting that the pyroligneous acid may have mitigated stress on the crop.
3. Yield and Grain Quality
Grain yield was sustained or even slightly improved in the mixed treatments, particularly those involving MPE or AC. Crucially:
- Half-rate AC + WV increased spikelet counts per panicle and improved rice milling outcomes relative to full-rate AC alone.
- Half-rate MPE combinations generally maintained high yields, driven by better spikelet fertility.
- Although yield in BCB-based mixtures varied, half-rate BCB with wood or rice vinegar still performed as well as or better than the non-treated control.
Grain quality indicators—such as protein content, whole grain yield, and percentage of damaged grain—remained stable or improved with some vinegar mixtures. For example, the combination of half-rate AC and wood vinegar led to higher whole grain yield and fewer damaged grains.
4. Soil Chemical Properties
Post-harvest soil tests showed that reduced herbicide usage, combined with pyroligneous acid, tended to increase soil organic matter and available phosphorus relative to initial soil conditions. Soil acidity also shifted slightly, though the direction and magnitude varied among treatments. Overall, the data suggest that using half the herbicide dose plus pyroligneous acid neither harmed the soil nor depleted essential nutrients.
Implications
These findings highlight the potential benefits of integrating pyroligneous acids into weed management strategies for paddy rice. By halving the recommended herbicide dose and supplementing it with wood or rice vinegar, farmers could:
- Maintain or improve weed control, especially for barnyard grass and perennial sedges.
- Avoid major yield losses while potentially boosting certain rice quality attributes.
- Support healthier soil conditions, as indicated by better organic matter levels and nutrient availability.
- Potentially reduce costs and environmental impacts by cutting herbicide usage in half.
Given the global push toward more sustainable farming practices, these results underscore a practical method to reduce reliance on chemical herbicides while preserving, and in some cases enhancing, crop performance. Additional research into long-term field trials and different soil types could further clarify the role of pyroligneous acids in integrated weed management.