Crop Weed Competition By Mirza Hasanuzzaman

Crop-Weed Competition

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Crop-Weed Competition

Mirza Hasanuzzaman, PhD

Associate Professor

Department of Agronomy

Sher-e-Bangla Agricultural University

Concept

Competition is nothing but the struggle for existence and superiority. Competition exerts a powerful

force in the plant community, irrespective of the mechanism, tending towards limitation or extinction of

the weaker competitors. Competition is maximum when available resources for crop growth become

limited.

Competition is a negative interaction where individuals make simultaneous demands that exceed

limited resources and, while both suffer, one individual suffers less.

So, crop weed competition indicates competition between crop and weed in a natural ecosystem in

response to resources struggle for their existence and superiority.

Crop weed competition occurs in two broad aspects:

1. Direct competition- for nutrient, moisture, light and space

2. Indirect competition- through exudation and / or production of allelopathic chemicals.

By and large, weeds appears much more adapted to agroecosystem than our present day crop

plants.

Components of the overall competitive effect

In an infested field it is possible to identify different components of the overall competitive effect:

 Intraspecific competition between plants of the cultivated species;

 Interspecific competition between plants of the cultivated species and weed species;

 Interspecific competition between plants of the different weed species;

 Intraspecific competition between plants of the same weed species.

Competition between weeds and crops is expressed by altered growth and development of both

species. Interspecific competition occurs when two or more species coexist in time and space and

simultaneously demand a limited resource. Intraspecific competition occurs when two or more plants

of the same species coexist in time and space and simultaneously demand a limited resource.

Crop-Weed Competition

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Competition for nutrients

Plants compete mostly for nitrogen, phosphorus and potassium (but there are many others).

Phosphorus is usually the most limited nutrient in aquatic ecosystems. Nitrogen is usually the most

limited nutrient in terrestrial habitats. Potassium is often overlooked but some terrestrial weeds can

grow well in K-rich soils. Approximately competition for nutrients constitutes an important aspect of

weed crop competition. Weeds usually absorb mineral nutrients faster than many of our crop plants

and accumulate them in their tissue in relatively large amounts.

Fig. Effect of nitrogen addition and two weeds (barnyardgrass and the annual broadleaf

Monchorias) on rice yield (redrawn from data from Moody, 1981 in Zimdahl, 1999)

Table: Kilograms of Nutrients Required to Produce Equal Amounts of Dry Matter

Plant

Nitrogen

Phosphorus

Wheat

5.5

1.2

Lambrushquarter

7.6

1.6

Pig weed

5.1

1.4

 Species of Amaranthus, for example, often accumulate over 3% N in their dry matter.

 Chenopodium and Portulaca spp. are likewise potassium lovers with over 1.3% K

O in their

dry matter.

 Nutrient removal by weeds during the first 30 days of maize growth was 59 kg N, 10 kg P and

59 kg K per hectare, which was 7-10 times more than the nutrient removal by the crop

 Weed posses not only a capacity for heavy nutrient absorption and accumulation but also

gather tremendous quantities of dry matter.

Competition for moisture

Competition for water occurs below ground between roots. The ability to absorb water is related to

rooting volume. However, not only are the dimensions (breadth and depth) of rooting zones important:

so is the degree of water extraction.

In general, for producing equal amounts of dry matter, weeds transpire more water than do most of

our crop plants. In weedy fields, the soil moisture may be exhausted by the time the crop reaches the

fruiting stage, which is often the peak. The consumptive use of water of a common weed

Crop-Weed Competition

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Chenopodium album as 550 mm against 479 mm for wheat crop itself. It is because weed can remove

moisture from deeper depth of soil than crops.

Table: Water Required to Produce One Pound of Dry Matter (Dillman, 1931; Shantz et al., 1927)

Plant

Litres of water

Wheat

227

Lambrushquarter

300

Corn

159

Pigweed

132-139

Competition for light

Although it varies in duration, intensity, and quality, light regulates many aspects of plant growth and

development. Neighboring plants may reduce light supply by direct interception: shading. Leaves are

the site of light competition. Whenever a leaf is shaded by another, there is competition for light.

Light competition is most severe when there is high fertility and adequate moisture because plants

grow vigorously and have larger foliar areas. Plants with large leaf area indices (LAI) have a

competitive advantage with plants with smaller leaf areas.

Both light quality and quantity are important aspects of competition. Since the presence of dense leaf

canopies reduces the quantity and quality of light available to weeds, competition for light is greatest

when plant density is highest. Plant height defines an effective component of the competitive struggle

for light. It becomes most important element of weed crop competition when moisture and nutrients

are plentiful, and weeds have an edge over crop plants in respect of height. Light competition may

commence very early in the crop season if a dense weed growth smoother the crop seedlings. Once a

plant is shaded by another plant, increased light intensity cannot benefit it.

Critical period of weed growth

Critical period of weed growth can be defined as that shortest time span in the ontogeny of crop

growth when weeding with result in highest economic returns. The crop yield level obtained by

weeding during this short span should provide crop yield sufficiently close to that obtained by the full

crop season freedom from weeds. A fundamental principle of plant ecology is that early occupants on

a soil tend to exclude the later ones.

 On the basis of the plant ecology, crops required a weed free respite during the first one-forth

to one-third of its growing period.

 Sharma et al. (1977) found that in direct seeded rice, the critical weed competition period

occurred 10-20 days after crop emergence. For the transplanted rice the critical periods of

weed crop competition were identified. These were (i) 4-6 weeks after transplanting and (ii)

during the 12

weeks of crop growth.

 In maize for example, during the first 2-3 weeks of emergence, weeds often completed 15-

18% of their total growth, while maize put up only 2-3% of growth. Such observations have

provided a basis in favour of early season weeding to harvest acceptable yields.

 Singh et al. (1980) found that 4 to 16 weeks period after planting sugarcane critical for

competition weeds.

 In potato, weeding was found most essential between 2 and 4 weeks after planting. Delayed

weeding caused considerable shrinkage in tuber yields (Gupta et al., 1979).

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FAQs of Crop Weed Competition by Mirza Hasanuzzaman

What are the main types of competition in crop weed interactions?

Crop weed competition is categorized into direct and indirect competition. Direct competition occurs when crops and weeds compete for limited resources such as nutrients, moisture, and light. Indirect competition involves the release of allelopathic chemicals by weeds, which can inhibit the growth of nearby crops. Understanding these interactions is crucial for developing effective weed management strategies.

How does weed density affect crop yields?

Weed density significantly impacts crop yields by reducing the availability of essential resources. Higher weed populations can lead to greater competition for nutrients, water, and light, ultimately resulting in reduced crop growth and yield. The document emphasizes that even a small increase in weed dry matter can correspond to a proportional loss in crop dry matter, highlighting the importance of managing weed density effectively.

What is the critical period of weed competition for crops?

The critical period of weed competition refers to the specific time frame during a crop's growth when it is most vulnerable to weed interference. For many crops, this period occurs early in their development, often within the first few weeks after planting. Timely weeding during this critical period can lead to significant improvements in crop yield, as weeds can outcompete crops for resources if not controlled.

What factors influence the competitive ability of weeds?

Several factors influence the competitive ability of weeds, including their growth form, emergence timing, and root development. Weeds that establish quickly and develop extensive root systems tend to outcompete crops for water and nutrients. Additionally, the growth habit and density of weeds can significantly affect their competitive advantage, making it essential for farmers to understand these dynamics for effective weed management.

What role do allelopathic chemicals play in crop weed competition?

Allelopathic chemicals are substances released by plants that can inhibit the growth of neighboring plants. In the context of crop weed competition, these chemicals can negatively affect crop development by reducing germination rates and overall growth. The document discusses how understanding these chemical interactions can help in developing strategies to mitigate the impact of aggressive weed species on crops.

Crop Weed Competition by Mirza Hasanuzzaman

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