Cotton plants are highly sensitive to abiotic and biotic stresses. The abiotic stresses on cotton may include a moistures stress (usually a drought stress), heat stress and salinity stress (L AW et al., 2001; L UO et al., 2008; M ASSACCI et al., 2008). Biotic stresses are more damaging for cottoncrop that the abiotic stresses. Cotton is among the crops which are attacked by hundreds of pests in the form of viruses, disease pathogens, insect pests and weeds which together can cause a yield loss of >80% in this crop (O ERKE , 2006). A number of weed species infest the cotton fields (D OGAN et al., 2014). Weeds such as Cynodon dactylon (L.) Pers., Trianthema portulacastrum L., Convolvolus arvensis L., Cyperus rotundus L., Conyza canadensis L. and Sorghum halepense (L.) Pers. can be quoted as the most important examples in this regard (K ALIVAS et al., 2012; D OGAN et al., 2014). The weeds can severely decrease cotton productivity and can negatively affect the lint quality. In this article, we have reviewed the important weeds of cottoncrop, the yieldlosses caused by these weeds and the salient weedcontrol methods in cotton.
Phytosociology is the study relates to all phenomena and effects regarding social life of plants (Braun-Blanquet, 1932). A plant may react with close proximity of neighbors (weed) by failure to survival with plastic development (Alam, 1991). (Saritha, 2013). Analysis of quantitative relationships among the plant species growing in an area reflects structural property of the community. Understanding the sociological structure of weeds in crop fields is a prerequisite necessary for its effective management. Ecological surveys of weedflora give us a comprehensive idea on weeds dominance (Ilorkar and khatin 2003). Knowledge on the nature and extent of infestation of weedflora in an agroecosystem through weed surveys is essential in formulating relevant weedcontrol strategies (Frick and Thomas, 1992). Identification and quantification of weed species present in different crop cultures and cropping systems allow us to form strategies for weedcontrol methods in important crops that can be adapted by marginal farmers. Since all the weed species are important to determine the nature of weed communities, it is desirable to know the quantitative characters like density, frequency and importance value of individual species. Documenting the diversity of weed species and their relative distribution facilitates the establishment of priorities for research and extension services (McClosky et al., 1998). A survey conducted in North Coastal Andhra Pradesh by Murty (2012) documented the distribution of different weed species in three major crop fields viz. Rice, Sugarcane and Groundnut. The cottoncrop cultivated fields of Visakhapatnam district are reported to be heavily infested with a large number of weeds, a common phenomenon to any agro ecosystem, causing heavy losses to the cottoncrop yields. With little information on the presence and distribution of weeds in the district floras published, a comprehensive study on the weed species in cotton crops of Visakhapatnam has been taken up in the present study as a first ever attempt for the Visakhapatnam district.
thrice at 20, 40, 60 days after sowing and unweeded control. The experiment was carried out in a randomised block design replicated thrice. The weedflora of the experimental field in red soil was dominated by Cynodon dactylon, Rottboellia exaltata, Parthenium hysterophorus, Trianthema portulacastrum and Commelina benghalensis. While in case of black soil predominant flora was Cynodon dactylon, Cyperus rotundus, Parthenium hysterophorus, Euphorbia geniculata, Tridax procumbens, Cyanotis cristata, Digera arvensis and Celosia argentea. The macronutrient contents in cottoncrop were higher in polymulch, mechanical weeding thrice at 20, 40 and 60 DAS and this was comparable to pre emergence application diuron at 1.0 kg ha -1 followed by post emergence
Chili (Capsicum annum L.) is an economically important vegetable and spice crop of Pakistan (Khan et al., 2006). Chili fruit is available in variety of colors and shapes (Cronin, 2002). In Pakistan, Chili is grown over 73800 hectares, with a national average of 2.5 tons ha-1 (Anonymous, 2014). Chili is an important cash crop of Pakistan (Naz et al., 2006). The Chili is also desired for its peculiar and nutrient content, especially vitamin C, A and E. (Rude et al., 2006). Chili plants are erect in nature that allow ample space for weeds emergence. The weeds compete for nutrients, sunlight, moisture and space that may result in yieldlosses as high as 60-70% (Patel et al., 2004) or even higher (Prakash et al., 2003, Shil and Adhikary, 2014). The young chili seedlings, generally, grow slowly as compared to weed species that decrease the supply of nutrients, moisture, sunlight and space foe the chili crop (Isik et al., 2009). Whereas, hand weeding has been an effective method to eradicate weeds, it is limited by labor availability, cost and time and labor required (Gul et al., 2011). Thus, chemical weedcontrol is a viable option to minimize infestation by the weeds (Shil and Adhikary, 2014). The post- emergence herbicides are less desirable due to their phytotixic effects and health hazards concerns (Jat et al. 2011). Thus, pre emergent herbicides are, commonly, used to control the weeds at early stages of crop development. (Rajkumara, 2009).
development, employment generation and economic development of Pakistan. Cottoncrop contributes 8.6% value addition in agriculture and is a source of employment (40% of the rural community). It also fetches a substantial amount of foreign exchange (60%) through exportable surplus of cotton fiber and fiber made products and about 10% of GDP (Anonymous, 2007). There is however no doubt that harsh climate, higher average day temperature and lower average night temperature is the major constraint in reducing the boll weight, which ultimately leads to lower yield. Also, weed infestation is considered to be one of the major problems among biotic stresses. Mechanical method of weedcontrol is not feasible due to its higher cost. Hence integration of different weedcontrol strategies including the use of herbicides in combination with mechanical weedcontrol practices is necessary for profitable production. The main advantage of integrated weed management is that the herbicides keep the cropweed free in early stages of growth and during later stages whereas the hand weeding and inter-culturing keep the weed population below economic threshold level.
and found that metolachlor (pre-em) at 1.25 kg ha -1 gave the best weedcontrol and seed cottonyield, while glyphosate and pendimethalin greatly decreased plant height and cottonyield when more than one application was given. Panwar et al. (2001) evaluated trifluralin, pendimethalin, acetachlor, fluazifop-p- butyl and pysithiobac in combination with manual weeding in cotton. Application of pendimethalin and trifluralin at 1000 g ha -1 and pyrithiobac at 100 g ha -1 reduced the weed population significantly over the weedy control. One hoeing at 45 days after sowing followed by 1500 g acetochlor ha -1 was the most effective in controlling the weeds and recorded significantly higher seed cottonyield than the weedy control. Hiremath and Rao (2001) conducted a study to see the effect of weed management in irrigated hybrid cotton found that diuron+ manual weedcontrol, inter-cultivation, standard farmer’s practice and diuron followed by glufosinate @ 0.525 kg ha -1 applied after 20 or 40 days of sowing produced similar seed cotton yields to that of weedy-free control. Askew et al. (2002) conducted field trial and reported that weeds were controlled and yield was increased by the application of herbicides at different levels. The pre-sowing and pre-emergence herbicides are not effective against all weeds, whereas, post-emergence herbicides can control weeds but it needs proper time and skill. The combination of pre and post emergence herbicides are required to be integrated for effective weedcontrol and increased in seed cottonyield. Aliet al. (2005) reported that maximum increase of 199.4% in seed cottonyield was obtained with Stomp 330E in combination with inter-culturing plus hand weeding while Round-up 490G/L @ 4.7 L ha -1 with 188.9% should increase over untreated check. Stomp 330E in combination with inter-culturing + hand weeding gave 90% broad leaf weeds (BLW) and 89% narrow leaf or grassy weeds (NLW) control, respectively, while Round-up 490G/L (directed spray) in combination with inter-culturing + hand weeding provided 93% control of BLW and 80% of NLW over untreated check.
Irrespective of the system of planting, there would be an optimum plant density per unit area that the resources and environment could support. Therefore, based on soil, rainfall, water resources, and the prevailing weeds, the optimum density need to be researched and evolved. Yield advantages cannot always be war- ranted ( Roche et al., 2003, 2006 ). However, studies clearly indicated the weed suppression by narrow row spacing ( Balkcom et al., 2010; Jost and Cothren, 2000 ). As narrow row offers more competition this system would offer ef ﬁcient and sustainable weedcontrol ( Culpepper and York, 2000 ), thereby, multiple applications of post- emergent herbicides can be minimised. However, to increase the adoption, management options need to be revised for narrow row cotton. There should be assured irrigation at the beginning of the cropping season as the moisture requirement of a thick plant stand will be high ( Knowles and Cramer, 1999 ). In addition, in sandy soils, a high density system may not yield desired results due to less soil moisture storage in tune with rapid plant growth ( Knowles and Cramer, 1999 ). The interaction of other weed management op- tions and herbicides should be evaluated. Rotating cotton with crops of high residue cover would reduce early emergence of weeds and integrating this with narrow row spacing would yield desired results as late emerging weeds may have to face intense competi- tion from cotton ( Price et al., 2012, 2016 ).
Southeastern Ultisols are highly prone to compaction and the development of hardpans due to the coarse topsoil and comparatively clay-heavy subsoil in the Coastal Plain region (Simoes et al., 2009). The Southeast typically experiences short-term droughts during cash crop growing seasons. The wetting and drying cycles caused by short-term droughts and standard vehicle traffic contribute to increased bulk density and the development of hardpans (Mapa et al., 1986; Williams and Weil, 2004; Blanco- Canqui et al., 2015). Soil compaction can limit cropyield potential by restricting root growth and elongation (Raper et al., 2000; Simoes et al., 2009). Tillage breaks apart compacted soil layers to promote root penetration, so compaction-prone soils have occasionally experienced yieldlosses under no-till management. In-row subsoiling is frequently utilized in conservation tillage systems, but some studies have found that cover crop implementation can alleviate soil compaction, and deep-rooted Daikon radishes are marketed in part based on potential compaction alleviation (Raper et al., 2000; Blanco-Canqui et al., 2015). Raper et al. (2000) found that spring soil strength and bulk density in a silt loam soil was reduced under shallow in-row tillage and cover crop systems, with similar cotton yields between conventionally tilled and reduced tillage with cover crops.
weed species unlike most of crops, are well adapted to high soil moisture conditions (Bukan, 2005) while others are not. During July, increasing weed density and wet-logged condition developed because of uneven fields are mostly found in the climate of this region due to small farmer’s community practices. Weeds compete with crop for water, nutrients and light and have been a matter of great concern to the cotton growers. According to Makhan-Kova and Voceodin (1984), the losses in cotton yields due to weeds could be in the range of 50 to 70%. They exhibit allelopathy, competition and parasitism (Hussain, 1980; 1983; Hussain et al., 1985; Hussain and Khan, 1987). This competition increases in the wet, hot, and humid monsoon season (July) and the ability of weeds to compete successfully with crops for light, water and nutrients depends on several interrelated factors. These include the timing of weed emergence in relation to crop emergence, the growth form of the weeds, and the density of the weeds present in the crop. The different environmental conditions determine the specific weed spectrum, composition and population of each region (Memon et al., 2007). The reduction in yield due to weed- crop competition mainly depends on weed species and their densities as well as crop species. As the distribution and infestation intensity of each weed is different, so the extent of cropyield reduction will mainly depend on the number and kind of weeds found in the field (Frisbie et al., 1989).
Cotton growers rely heavily upon herbicides for weedcontrol. A typical herbicide program for cotton in the Southeast begins with application of two or more herbicides at planting. Most fields also require postemergence herbicides for adequate weedcontrol. Several herbicides can be applied postemergence over the top of cotton for annual and perennial grass control without injury to the crop. Postemergence control of broadleaf weeds has typically been achieved with directed herbicide applications. For effective weedcontrol and adequate crop safety with postemergence-directed herbicides, a height differential between the crop and weeds is required. This height differential is often difficult to achieve as cotton usually grows more slowly early in the season than broadleaf weeds. Additionally, directed application of herbicides to small cotton is a slow and tedious operation requiring special equipment and precise operation. Growers prefer to apply herbicides over the top of cotton. Fluometuron (Cotoran, Meturon, and others) and the organic arsenicals MSMA and DSMA can be applied overtop but weed scientists caution that these herbicides applied in this manner can delay cotton maturity and reduce yield.
production (Saqib et al., 2012). The annual losses of Pakistan in wheat yield due to weeds are about 17-25 % (Abbas, 2006). In wheat due to weeds 24-39.95% economic yieldlosses occurred. Weed may reduce 40-50% grain yield in wheat crop (Oad et al., 2007). Extensive and indiscriminate use of herbicides are becoming serious ecological and environmental hazards that led to resistance of weeds and shifts in weed population (Johnson and Mortimer, 2008). To reduce reliance on use of herbicide and launched efficient and effective weedcontrol methods, cultural methods of weedcontrol (Chauhan and Johnson, 2010; Khaliq et al., 2013) is employed that offer several approaches of prevention, evasion and control of weeds (Wilson et al., 2009) either by controlling weeds growth or by making crop aggressive (Gibson et al., 2002).
underground root portions like tubers and stolens were effectively removed by mechanical methods of weedcontrol than the chemical application. This was due to the imposement of first manual weeding on 20 DAS which avoided the competition by weeds with crop for nutrient and moisture (Prabhu, 2010). Shobana (2002) reported that Cynodondactylon, was perennial in nature which was not much controlled by pendimethalin application. At this stage, manual weeding twice controlled the grass and sedge weed efficiently and favored the growth of cotton which influenced the crop and covered the field surface area much earlier than the weed.
In both cropping practices, the characteristics of crop stand explained a large share of the varia- tion in the species composition. Age of crop stand, i.e. the difference between sowing and sampling dates, was an important variable among the crop stand characteristics. Apparently, competitive abil- ity of crop stand is an important factor for reducing weed problems, especially under organic cropping where application of herbicides is avoided (Bond and Lennartson 1999). Pea breeding programmes in Finland have been successful in increasing the protein content of peas and developing high-yield- ing semileaﬂess aﬁla-type varieties (Hovinen 1988). However, such varieties are poor competi- tors against weeds. Competitive ability of pea stands should be taken into account when breeding new cultivars. Meanwhile, effective direct weedcontrol is a prerequisite for a high pea yield.
Weed competition leads to substantial harvest losses, increasing production cost and also intensify the problems of diseases and insect pests by serving as the alternate host. There is no simple method to control weeds of all groups. Mechanical weedcontrol method was partially effective because most of the weeds growing in intra rows escaped from weeding and incessant rains make the manual weeding impossible which resulted in an efficient weedcontrol and low sugarcane yield (Srivastava and Chauhan, 2002).
tillage (Bradley, 1993). In Texas, yields of cotton were 41% higher for reduced tillage than for conventional tillage (Harmon et al., 1989). Triplett (1985) reported that poor weedcontrol is a limiting factor in the adoption of conservation tillage. Tillage system can affect the density and spectrum of weed population, and broadleaf species become dominant in conventional tillage (Clements et al., 1996). Broome et al. (2000) noted that selection of herbicides and timing of application are important factors in controlling perennial species in a reduced tillage system. Weeds can cause signiﬁcant economic losses (Swanton et al., 1993), so the success of conservation tillage depends on the development of agronomically and economically viable weed management systems (Derksen et al., 1996).
Weed management plays an important role in sugar beet production. Wide row spacing and slow development in early growth stages result in late canopy closure. Up to 100% of the cropyield may be lost because of weed competition if weedcontrol is poor or is not performed at all (Kropff & Spitters 1991). Effective weedcontrol is needed mainly during the critical period of sugar beet development, which is approximately the period during the first 60 days after emergence. Then, sugar beet does not tolerate co-existence with weeds without losing yield (May & Wilson 2006; Jalali & Salehi 2013). Weeds need to be removed until the 8-leaf stage of sugar beet. Emerging weeds after the 8-leaf stage did not cause any significant sugar beet yieldlosses (Jursík et al. 2008). In European sugar beet production, Chenopodium album L., amaranthus retroflexus L., Galium aparine L., Matricaria chamomilla L., M. inodora L., stellaria media (L.) Vill., and Po- lygonum convolvulus L. are the most abundant weed species (Petersen 2008).
with chlorbromuron, cyanazine, bifenox and atrazine plus dicamba and tank mixtures of linuron, chlorbromuron and metribuzin with glyphosate reduced the control of wheat grass in soybeans. However, addition of residual herbicide S-metolachlor, pyrithiobac, or trifloxysulfuron did not affect glyphosate control efficacy on weed, e.g., goosegrass, hemp sesbania, johnsongrass, Palmer amaranth (A. palmeri S. Wat.), redroot pigweed (A. retroflexus L.), sicklepod, or smellmelon (Cucumis melo L.) (Scroggs et al. 2007). Scroggs et al. (2005) reported that no reduction in control of barnyardgrass, hemp sesbania, johnsongrass, pitted morningglory, or sicklepod when glyphosate was coapplied with the plant growth-regulator mepiquat pentaborate, a foliar sodium calcium borate micronutrient solution, and a foliar nitrogen fertilizer solution. Generally, chemical agents have been shown to cause antagonism when coapplied with glyphosate. Bailey et al. (2002) documented that control efficacy of glyphosate to lambsquarters, large crabgrass (Digitaria sanguinalis (L.) Scop.), morningglory spp., and smooth pigweed was reduced when manganese was added to herbicide solutions of glyphosate. Similar results were noted in reduction of weedcontrol, including barnyardgrass, browntop millet (Urochloa ramosa (L.) Nguyen), Palmer amaranth, johnsongrass, ivyleaf morningglory (I. hedera-
Field experiment was conducted at Research Farm, Faculty of Agriculture, Gomal University, Dera Ismail Khan, Pakistan during Rabi 2007-08 to study the effect of herbicides and tillage on weeds and wheat. The experiment was carried out in randomized complete block design with split plot arrangement replicated four times. The treatments included in the experiment were 3 tillage depths viz. shallow tillage (5-7.5 cm with one cultivator and rotavator), medium tillage (10 cm with 2 cultivator and rotavator), and deep tillage (20-25 cm with disk plough, cultivator and rotavator) assigned to main plots, while five weedcontrol treatments i.e. 2,4-D amine @ 1 L ha -1 , Topik 15 WP @
stands, increased weed infestation and compensation of troublesome weed species are frequently found, among others, which results in a deterioration of most crop and ear components and consequently in reduced yields [3,4]. Therefore, optimal agronomic solutions that will mitigate the negative effects of continuous cropping are constantly sought. One of such methods is the introduc- tion of cover crops as plants that play a phytosanitary role, supplement mineral fertilization and improve soil organic matter balance. Cover crops are an environ- mentally-friendly method to reduce weed infestation in crops, irrespective of the cover plant [5–8]. Grow- ing cover crops as green manure that is ploughed un- der, which improves the quality of the soil environment with relatively low investment, is gaining special impor- tance [9,10]. The studies of some authors show that the ploughing-in of such crops before winter reduces weed infestation, increases cereal crop yields and is a more beneficial method for counteracting the negative effects of continuous cereal cropping compared to soil mulch- ing with cover crops for the winter period [9,11].