Weed Management in Horticulture Crops (1+1)

CROP-WEED ASSOCIATION

CROP-WEED ASSOCIATION

Weeds possess many growth characteristics and adaptations which enable them to successfully exploit the numerous ecological niches left unoccupied by crop cultures. Weeds compete with themselves and with crop plant. Among the more important adaptations relevant to competitive advantage are properly synchronized germination, rapid establishment and growth of seedlings, tolerance to shading effects by the crop or by other weeds at the time of establishment, quick response to available soil moisture and nutrients, adaptation to the most severe climatic situations of the habitat, adaptations to the edaphic regime, relative immunity to post seeding soil disturbance, practices and resistance to herbicides that are used. In the initial stages of invasion by weeds of exposed ecological niches, only a very limited competition for resources by the crop and weed may occur, but as establishment of the crop-weed association is completed, competition for the available resources is more obvious.

Plant competition is a natural force whereby crop and weed plants tend to attain a maximum combined growth and yield, with the development of each species being to some extent at the expense of the other. It occurs when the demands of the plants for moisture, nutrients, light, and possibly carbon dioxide exceed the available supply. Competition may develop between crop and weed plants and also between individual plants of each. The ultimate outcome of competition usually results in the development of a characteristic crop-weed association. Crop plants and weeds may grow and mature in the state of mutual suppression that is often found in crops where no suitable herbicide is available to control the weeds. The weed suppresses the crop and result in reduction of yield. The crop also suppresses the weeds, a condition often found in row crop cultures. This is a logical sequence in a crop habitat where both cultural and herbicide methods provide effective control.

A principle of plant competition is that the first plants to occupy an area have an advantage over latecomers. This principle is of foremost consideration in practical weed control, where cropping practices are always directed to the establishment of the crop ahead of the weeds.

Competition and allelopathy are the main interactions which are of importance between crop and weed. Allelopathy is distinguished from competition because it depends on a chemical compound being added to the environment while competition involves removal or reduction of an essential factor or factors from the environment, which would have been otherwise utilized.

CROP WEED COMPETITION

Weeds appear much more adapted to agro-ecosystems than our crop plants. Without interference by man, weeds would easily wipe out the crop plants. This is because of their competition for nutrients, moisture, light and space which are the principle factors of production of crop. Generally, an increase in on kilogram of weed growth will decrease one kilogram of crop growth.

1. Competition for Nutrients
Weeds usually absorb mineral nutrients faster than many crop plants and accumulate them in their tissues in relatively larger amounts.

• Amaranthus sp. accumulates over 3% N on dry weight basis and is termed as “nitrophil”.
• Achyranths aspera, a ‘P’ accumulates over 1.5% P2O5
• Chenopodium sp & Portulaca sp. are ‘K’ lovers with over 1.3% K2O in dry matter

Mineral composition of certain common weeds on dry matter basis:

S.No	Species	N(%)		P2O5(%)	K2O(%)
1.	Achyranthus aspera	2.21		1.63	1.32
2.	Amaranthus viridis	3.16		0.06	4.51
3.	Chenopodium album	2.59		0.37	4.34
4.	Cynodon dactylon	1.72		0.25	1.75
5.	Cyperus rotundus	2.17		0.26	2.73
Crop plants
1.	Rice		1.13	0.34	1.10
2.	Sugarcane		0.33	0.19	0.67
3.	Wheat		1.33	0.59	1.44

• The associated weed is responsive to nitrogen and it utilizes more of the applied ‘N’ than the crop e.g. The ‘N’ uptake by Echinochloa crusgalli is more than rice.
• Nutrient removal by weeds leads to huge loss of nutrients in each crop season, which is often twice that of crop plants. For instance at early stages of maize cultivation, the weeds found to remove 9 times more of N, 10 times more of P and 7 times more of K.

2. Competition for moisture

• In general, for producing equal amounts of dry matter, weeds transpire more water than do most of our crop plants. It becomes increasingly critical with increasing soil moisture stress, as found in arid and semi-arid areas.
• As a rule, C4 plants utilize water more efficiently resulting in more biomass per unit of water. Cynodon dactylon had almost twice as high transpiration rate as pearl millet.
• In weedy fields soil moisture may be exhausted by the time the crop reaches the fruiting stage, i.e. the peak consumptive use period of the crop, causing significant loss in crop yields.

3. Competition for light

• It may commence very early in the cop season if a dense weed growth smothers the crop seedlings.
• It becomes important element of crop-weed competition when moisture and nutrients are plentiful.
• In dry land agriculture in years of normal rainfall the crop-weed competition is limited to nitrogen and light.
• Unlike competition for nutrients and moisture once weeds shade a crop plant, increased light intensity cannot benefit it.

4. Competition for space (CO2)

• Crop-weed competition for space is the requirement for CO2 and the competition may occur under extremely crowded plant community condition. A more efficient utilization of CO2 by C4 type weeds may contribute to their rapid growth over C3 type of crops.