So, how many of you have heard this name, Weeds. Some of you might get confused to what I am going to discuss in this article. But don’t get confused, I am not going to discuss what you have been perceiving till today, No! Now let’s not make this more lucid. Let’s dig in to the major topic that I am going to discuss with you right now.
If you are an agriculture scholar then you might have understood the relevant term before but if you haven’t, it’s obvious. Taking into account of farmers, farmers are often disturbed by the inhabitation of weeds that not only compete with their major crops for light, space and nutrients but also exhibit some allelochemicals that have allelopathic effect upon the growth and development of the major crop fields. Some may not be so noxious but other have detrimental effect upon the overall life function and economic yield of the field crops. So, what are weeds then?
Weeds are the plants that are unwanted in a given situation and may be harmful, dangerous or economically detrimental. They are serious threat to primary production and biodiversity that reduce farm and forest productivity, displace native species and contribute significantly to land and water degradation with second ranking in habitat loss and causing biodiversity decline. In short, “Any crop that are unwanted on a given situation can be considered a weed’’. So, how a weed infests and degrade the quality of your production then? let’s summarise.
- According to a statistic, in general weed cause 5% loss in agricultural production in most of developed countries, 10% loss in less developed countries and 25% loss in least developed countries.
- Weeds can deprive the crops from 47% N, 42% P, 50% K ,39% Ca and 24% Mg of their nutrient uptake.
- They act as alternate hosts that harbour insects, pests, diseases and other micro-organisms.
- In aquatic environment, weeds block the flow of water in water canals, water transport system and drainage system, rendering navigation difficult. The dense growth of aquatic weeds pollutes water by deoxygenating it and killing the fishes.
Every year, thousands of weeds infests the crop beyond their threshold level, reducing their economic yield. Under world conditions, about 30,000 species are grouped as weeds. But it is of concern, the high reproductive potential of weeds that has made them so adaptable in any environmental conditions around the world. Weeds generally propagate by two methods: sexual and asexual propagation.
- Reproduction by seed (Sexual): Requires pollination and fertilization of egg resulting in seed capable of producing new plant. For example, While Canada thistlehas been observed to produce 680 seeds/plant, Curly dock often produces more than 30,000 seeds/plant.
- Vegetative reproduction (asexual): Germinate by modification of underground stems(rhizomes), above-ground stems (Stolon), bulbs, corms and tubers. For example, Yellow nut-sedge (Cyperus esculentus) has been reported to produce more than 1,900 new plants and more than 6800 tubers in 1 year.
- Did you know, the immediate viability of weed seeds has been found to vary from 6-78% and remain viable for 2-100 years?
Weeds too have allelopathic effects on crops?
Weeds crop too show allelopathic effects (chemicals or exudate produced by one plant species) on the inhabiting crop determining the germination, growth or development of another plant species sharing the same habitat. Allelochemicals are produced by plants as end product, by-product and metabolites belonging to phenolic acids, flavonoids and other aromatic compounds. Some of the allelopathic effects of them are described below.
- Maize: Tubers of Cyperus esculentusaffects the dry matter production.
- Sorghum: Leaves and inflorescence of Parthenium affects germination and seedling growth.
- Wheat: Green and dried leaves of Argemone Mexicanaaffect germination and seedling growth.
- Sunflower: Seeds of Datura stramonium affect germination & growth.
Where there is problem, there is always a solution!
Plant being highly affected by the attacks of weeds have developed certain mechanism that has helped them to combat the deleterious effect of weeds on their survival. Let’s look at some of the examples here.
- Root exudation of maize inhibits the growth of Chenopodium album.
- The cold-water extracts of wheat straw when applied to weeds reduce germination and growth of Abutilon Spp.
Herbicide tolerance and resistance
Being a crop that has negative impact on the economic crops, human have tried many methods for their control. In past, in absence of the modern tools, manual weeding was the only mechanism of their control but with the Green revolution, human have tried the use of herbicides as their chemical control. But in absence of proper administration, weeds have developed resistance upon application of certain herbicides. According to Rubin (1991), Herbicide resistance is the naturally occurring inability of some weed biotypes within a population to survive an herbicide treatment that would, under conditions of use effectively control the weed population. Let’s look at some of the scenario;
- Senecio vulgaris resistance to triazine group of herbicides was noticed during 1970.
- Worldwide 183 weeds have developed resistance to herbicides till 1997.
- The highest resistance in 61 weed species was recorded for atrazine.
- USA alone found to have 49 herbicide resistant weeds, the highest in the world.
Basic principles of herbicide resistance
- Time, dose and method of application of herbicide variation.
- Variation in phenotypes of a population.
- Genetic variation by mutation or activation of pre-existing genes.
On high dose and repeated treatment of unqualified herbicides, weeds developed resistance. Resistance was exhibited in crop due to:
- Herbicide metabolism by crops making them inactive.
- Absence of certain metabolic process in crops compared to weeds thus tolerating the herbicides.
- Crops couples the herbicide molecule.
Regardless of the mechanism for resistance, becoming familiar with the herbicide mode of action can help design programs that prevent the introduction and spread of herbicide resistant weeds. Integrated approach can be the foremost principle in combating this problem. Dependence on single strategy would surely increase the likelihood of additional herbicide resistance problems in future. Some guidelines for this integrated approach have been discussed below:
- Crop rotation to minimize the undue effect of herbicides.
- Using rotation herbicides with different mode of action.
- Integrating cultural, mechanical and chemical (Optional) weed control methods.
On summarization, Weed do have negative effect on the overall cropping scheme but having uncontrolled and indeterminate application can make them a grave threat in future. So, a properly justified and well-designed integrated approach of weed control need to be developed and formulated as soon as possible other wise it’s not a rocket science that it will come out as a serious threat to human beings after global warming.