Cotton (Gossypium hirsutum L.) is the most preferred natural fiber and contributes to a one-third of the total fiber traded on a global scale. Emerging weeds in cotton production zones and their management is a major crop production challenge across the world. Poor weed management in cotton can lead to a significant yield reduction, depending on weed management, yield reductions can range from 10 to 90%. Rapidly emerging herbicide-resistant weeds further adds to the complexity of cotton weed management. A major change that occurred in the cotton farming is the rapid adoption of genetically modified herbicide-tolerant cultivars (GM cultivars), of which glyphosate-tolerant cotton accounts a major proportion. Herbicide-tolerant cultivars offer myriad of benefits, such as flexibility and efficiency in weed management, and better economic returns; however, over-reliance on a single herbicide without diversity has led to the emergence of many problematic weeds in cotton growing zones. Many weeds rapidly evolved glyphosate resistance due to the intense selection pressure from glyphosate. The continuous use of glyphosate led to the shift of weeds, the dominant weeds Cirsium vulgare (L.) Scop., Setaria sp., and Abutilon theophrasti Medik. were shifted to Sorghum halepense (L.) Pers., Amaranthus sp., Conyza canadensis (L.) Cronquist, and Chenopodium album (L.)
Weed flora and their competitiveness in cotton
Cotton is a perennial plant and can completely cover the land area with canopy, although the time taken for this can vary depending on the environment, management, plant density, and row spacing. There was a lint yield reduction of 5.5% for every one plant per 15 m row. The certain weed species are highly competitive in cotton. For example, Xanthium pensylvanicum is more competitive than A. retroflexus and A. hybridus. One plant of both Amaranthus species and C. obtusifolia per 7.5 m row reduced seed cotton yield by 9%. C. obtusifolia did not reduce yield in cotton per 3 m row, whereas, 5 to 7% yield loss was observed when Anoda cristata L. Schltdl., Xanthium strumarium L. Xanst., Digitaria sanguinalis (L.) Scop. Datura stramonium (L.) Datst., C. album, Amaranthus retroflexus, and Ambrosia artemisiifolia (L.) Ambel. were present at the same density. Some weeds can be competitive throughout the crop growing phases, although certain weeds are competitive at the initial crop growing phases. However, weeds like A. palmeri can be detrimental even at later stages of crop growth, as substantial seed production is possible even under competitive environments. A. palmeri can significantly shade cotton and prevent it from reaching its full yield potential. Competitive interference of A. palmeri indicated that 10 plants per 9.1 m row resulted in a 45% reduction in canopy volume at 10 weeks after planting and reduced cotton biomass to 50% by 8 weeks after planting. Similarly, A. hybridus can grow taller than cotton and can reach up to 3 m thereby, shade the crop plants.
Weed competitive cultivars
Crop cultivars differ in their competitiveness ability to suppress weeds. Integration of competitive cultivars in cropping systems would reduce herbicide usage and the overall weed management costs. In general, weed competitiveness of cultivars is correlated with plant height, seedling vigor, early canopy closure, leaf orientation, leaf area development, and branching and tillering pattern. During the growth process, cotton plants will branch and expand their canopy leading to complete canopy closure. Once the canopy is closed, there would be less light penetration into the inter-row spaces and weeds will not be able to compete with cotton as early in the crop establishment period. However, as cotton establishes slowly and less competitive when grown in 1 m row spacing, weed competition at an early phase could significantly reduce plant growth and yield Therefore, from a weed management point of view, cultivars with enhanced seedling vigor would be highly desirable to suppress weeds, as that would ensure weed suppression in the initial crop growing phases. The early vigor characteristics were genetically controlled and correlated to seed weight and cotyledon area.
Increasing plant density and reduced row spacing
Increasing plant density is a non-chemical tactic that can be easily integrated with cropping to suppress many dominant weeds. Under high weed pressure situations, there can often be crop yield benefits, better weed control, and reductions in cost of weed control by adopting dense crop stands. Increasing plant density would lead to early canopy closure and thereby limit light penetration into the inter-row spaces and lead to the suppression of many dominant weeds. The weeds that proliferate under a noncompetitive environment, but perform poorly under increased competition, could be effectively suppressed by integrating thistactic. Cyperus sp. And Echinochloa sp. were suppressed in rice by adopting this tactic these results are important as Cyperus sp. and Echinochloa sp. are also dominant weeds of cotton growing regions across the world. In the case of cotton planted at 1 m row spacing, the planting density followed is 8-12 plants m 2 and 6-9 plants m 2 for irrigated and dryland cotton. In cotton, a varying plant density is achieved by narrowing row spaces, skipping crop rows or by planting in pairs row. There will be differential morphological changes with increased density as a competitive response and search for more sunlight, plants will increase in height in the thicker stand over thinner stand at the early crop growing phase and help the crop to have a comparative advantage over weeds at the beginning phases of crop growth. The plant density of 200,000 to 300,000 ha 1 is maintained, and yield of more than 1800 kg ha 1 is achieved by this cultivation technique, it is called “short-dense-early” technique. The advantages are, early crop, better weed control over the standard row system, water saving, less cultivation cost, and avoidance of terminal drought.
This article is collectively authored by Muhammad Nazim*1, Muqarrab Ali1, Qurat-ul-Ain Sadiq2 and Afnan Sehar3-1 Department of Agronomy, Muhammad Nawaz Sharif, University of Agriculture Multan, Pakistan. 2 Department of Soil and Enivironmental Sciences, Muhammad Nawaz Shareef, University of Agriculture Multan, Pakistan. 3Department of Plant Pathology, University of Poonch Rawalakot, Azad Kashmir, Pakistan.