Weed Management in Horticultural Crops

PROBLEMATIC WEEDS

Weed control practices often have an effect on the weeds, on a year by year basis. Before the development of herbicides, growers relied heavily on tillage as a tool for controlling and suppressing weeds. Once herbicides became a valuable tool, some of the problem weeds found in predominantly tillage based management practices began to fade and new problematic weeds began to fill the gap. As our habits change, specific weeds will exploit the new niches we create and become the more dominant species.
Some of the weeds like Cyperus rotundus, Cynodon dactylon, Eleusine indica etc., are listed as world’s worst weeds. Cyperus rotundus is the most problematic weed present in 92 countries, followed by Cynodon dactylon in 80 countries. Both weeds are perennial, mainly propagated by vegetative means and also by seeds. Cyperus rotundus is a problem weed in 52 crops while Eleusine indica in 46 crops.

Table: Some of the world’s worst weeds are listed below

Weed Ranking	Common Name	Botanical Name	Occurrence in no. of
			Crops	Countries
1	Nut grass	Cyperus rotundus	52	92
2	Bermuda grass	Cynodon dactylon	40	80
3	Barnyard grass	Echinochloa colonum	36	61
4	Jungle grass	Echinochloa crusgalli	35	60
5	Goose grass	Eleusine indica	46	60
6	Johnson grass	Sorghum halepense	30	53
7	-	Imperata cylindrica	35	73
8	Water hyacinth	Eichornia crassipes	-	-
9	-	Portulaca oleracea	45	81
10	Fat hen	Chenopodium album	40	47
11	Large crab grass	Digitaria saugunialis	33	56
12	Field bind weed	Convolvulus arvensis	32	44

Perennial and Problematic weed management
Prevention

The most basic and effective of all methods to control perennial weeds is prevention. As discussed earlier, there are several means of weed seed dispersal, most of which can be prevented. Ensuring clean crop seed, animal feed, and hay is the most important measure in preventing seed dispersal. Other methods of prevention include cleaning field machinery and harvest equipment when moving between fields, proper long-term manure storage to reduce seed viability after passing through animals' digestive tracts, and maintenance of weed-free irrigation water.

Crop rotation can be another effective method to prevent the establishment of perennial weeds. The most effective crop rotations for this purpose include not only crops that compete well with perennial weeds, but also those that allow the use of herbicides to control perennial seedlings.
Mechanical weed control

Cultivation, when combined with other management tactics, can be used to control seedlings before energy-storing vegetative tissue has accumulated. Mechanical control no longer is effective after energy has been stored in underground vegetative tissue. In fact, cultivation of established perennials can spread weeds by cutting roots and moving them to new areas.

Perennial weeds are more common in reduced-tillage fields, where there is little soil disturbance to disrupt the development of below-ground storage organs. Once perennial weeds are established in reduced-tillage fields, cultivation is ineffective and might increase the spread of vegetative roots.

In pasture and forage crops, frequent mowing or cutting can prevent weed seed production and reduce the amount of energy stored in below-ground structures. Most important, maintenance of a vigorous crop stand through proper fertility and water management, seeding density, and variety selection will allow the competitive ability of the crop to suppress perennial weed growth. This simple "hands-off" approach requires little additional input or management, but can greatly reduce weed seed production and root growth.
Chemical weed control

Perennial weed control with herbicides must be repeated for 2 to 3 years and combined with other management tactics such as mowing. The key is to get the herbicide into the roots. Herbicide activity relies on foliar absorption and transport from the leaves to the root system. Young leaves move nutrients from the root in an upward, above-ground direction, while more mature leaves transport photosynthetic products to the root system for storage. Thus, the most effective herbicide activity occurs as the product is transported to the roots with the products of photosynthesis.

Herbicides are most effective on perennial weeds in the early fall, when weeds are transporting energy to the roots before winter dormancy. Treatment just before and during flower bud initiation also is effective, as the herbicide will be carried with photosynthetic products to the roots. To ensure the presence of sufficient mature foliage, apply post emergent herbicides either 1 to 2 weeks before cultivation or mowing, or after weed regrowth is at least 8 inches tall.

Biological weed control

Biological control is a slow process, and results are not guaranteed. Therefore, it is used most appropriately as a component of an integrated weed management system that relies on multiple tactics for perennial weed control. For example, the fungus Concholiobolus lunatus kills barnyard grass seedlings with fewer than two leaves, but growth of larger plants is only slowed and plants recover. However, when the fungus is combined with a sub lethal dose of Atrazine (a dose that injures but does not kill the barnyard grass), larger barnyard grass plants can be controlled better than when Atrazine is used alone.

Integrated weed management

Management of perennial weeds is most successful when multiple tactics are employed, such as the combination of chemical, mechanical, and cultural control. Integrated weed management, when combined with prevention and control of weeds outside of crop production areas, provides the best long-term management of perennial weeds.