Weeds menace in agriculture
Weeds are the major biological constraints that halt potential productivity of any crop. Weeds can be defined as the plants growing out of place which interfere with intended land use and water resources. They are having specific features or characters which make them different from the other growing plants such as seed dormancy, abundant seed production and rapid establishment with highly competitive nature. The multiple adaptations and plasticity of weeds provide them an advantage over cultivated crops. Also, the practice of intensive agriculture with high yield and dwarf varieties provided favorable conditions for better growth of weeds compared to the earlier taller cultivars
(Sharma and Kaur, 2012). They are perennial problems and numerous resources spent on them by the farmers to reduce their impact. In India, the highest loss is caused by weeds
i.
e. about 33% followed 26% by pathogens, 20% by insects, 7% by storage pests, 6% by rodents and 8% by others
(Oerke, 2006). The losses due to weeds depend on type of weed species, weed density, duration of infestation, cropping system, soil and environmental conditions
(Chhokar et al., 2012). It has been calculated that weeds cause a total economic loss in arable crops equivalent to approximately USD 11-13 billion per annum
(Rao, 2018).
Major weed flora of wheat
Wheat (
Triticum aestivum) is invaded with composite weed flora consisting of grass along with broadleaf weeds. Amongst grasses,
Phalaris minor,
Avena ludoviciana,
Poa annua and
Polypogon monspeliensis are dominant and
Chenopodium album,
C.
murale,
Rumex dentatus,
Rumex spinosus,
Coronopus didymus,
Anagallis arvensis,
Medicago denticulata,
Melilotus indica,
Malva parviflora and
Convolvulus arvensis amongst broad leaf weeds are dominant weed species
(Punia et al., 2017). A shift in the weed flora in wheat has been observed with time. During 1960s,
Carthamus oxycantha was the major weed to wheat but it got eliminated with increase in irrigated area
(Chhokar et al., 2012).
With the advancement of high yielding dwarf wheat varieties along with higher doses of fertilizers and assured irrigation, resulted in replacement of several broadleaf weeds by
P.
minor and
Avena ludoviciana. With time
P.
minor became the major weed in the RWCS and
A.
ludoviciana in irrigated, well drained, lighter textured soils in the other cropping systems. Morphological similarities between these grass weeds and wheat along with closer row spacing of the crop makes manual weed removal very difficult.
P.
minor now became a nuisance for sustainable wheat production in the rice-wheat growing regions of different states in India. As, area under zero till conditions is increasing, it also led to the infestation of broad leaf weeds such as
R.
dentatus (Chhokar et al., 2012).
Losses caused by weeds in wheat crop
Weeds are a serious problem responsible for lowering the wheat productivity to the tune of 15-80% or even higher based upon their type, intensity of infestation and management practices but in extreme cases there could be complete crop failure
(Yaduraju et al., 2006). The weeds in different wheat growing regions causing a yield loss of about 20-30% and may reach up to 60%
(Mongia et al., 2005, Chhokar et al., 2008). Weed infestation also increases cost of cultivation, impair crop produce quality, interfere with farm operations and spread several insect pests, diseases and nematodes by acting as alternate hosts
(Rana and Rana, 2016).
Weed management practices
Cultural practices
Cultural practices comprise of crop rotation, increased crop density, crop geometry, stale bed and manipulation of sowing time. By adopting these practices, we can reduce the herbicide use in crop production. The following cultural practices can be adopted for effective management of weeds in wheat.
Planting time
Planting time considerably influences the germination and establishment of weeds. Early sowing of wheat in the last week of October to first week of November reduces in-crop infestation of
P.
minor as warmer temperature at that time is not favorable for the germination of this weed
(Bhullar et al., 2017). Under these situations,
P.
minor seeds will germinate with the first irrigation and encounter strong competition by wheat. The early sowing of wheat may encourage infestation of wild oat in non-rice growing area but it can be controlled effectively with available herbicides.
Crop rotation
Crop rotation helps in lowering down the weed infestation by disrupting the life cycle of weeds by rotating the crops which require different management practices. For example, the seed bank of
P.
minor can be eliminated within a span of 2 to 3 years by replacing wheat with berseem (
Egyptian clover), potato, rapeseed and mustard, winter maize, autumn sugarcane, sunflower in winter season, or by replacement of rice with cotton, maize and sugarcane in summer season. The adoption of rice-potato/vegetable pea-wheat, rice-potato-sunflower and rice-berseem in place of rice-wheat system significantly reduce the infestation of
P.
minor compared to rice-wheat system. Including fodder crops such as berseem in cropping system significantly reduced the seed bank of
P.
minor within three years
(Banga et al., 1997). In potato-based crop rotations, manual removal of
P.
minor plants at potato harvest reduced
P.
minor infestation by >90% in succeeding wheat
(Walia and Brar, 2004):
Sowing method and seed rate
Sowing of wheat at narrow spacing (15 cm) is helpful in reducing the infestation of P. minor and other grass and broad leaf weeds. Also, bi directional sowing of wheat at 22.5 × 22.5 cm reduced density of
P.
minor as compared to broadcast, closer (15.0 cm) and normal sowing (22.5 cm)
(Singh and Singh, 1996). Increased seed rate of wheat (150 kg/ha) also reduced biomass of
P.
minor by >40% and increased grain yield of wheat compared to wheat sown at the recommended seed rate (100 kg/ha)
(Bhullar and Walia, 2004). The use of higher crop seed rate shifts the use of resources in favor of crop plants and reduces availability for weed plants
(Yaduraju and Ahuja, 1997). Planting techniques of wheat also affect the dry matter accumulation of
Phalaris minor (Brar and Walia, 2007):
Competitive variety
The different wheat cultivars have their different competitive ability to compete with weeds. The different characteristics of wheat cultivars such as greater early vigour along with high tillering capacity help them to compete with weeds. The tall wheat varieties resulted in better weed control as compared to dwarf ones although they cannot be adopted because of their poor yield. It was reported that wheat cultivars namely HD 3086, PBW 677, PBW 725, HD 2967, PBW 621 and PBW 550 resulted in more suppression of
P.
minor due to their quick growing and vigorous nature as compared to DBW 17 and WH 542
(Sharma and Kaur, 2012).
Stale seed bed
Under this technique, double pre-sowing irrigation is given before field preparation for the wheat to stimulate the germination of the weeds. Then these germinated weeds are killed with the help of non-selective herbicide or uprooted by shallow cultivation before sowing. This practice played a significant role in reducing the weed seed bank in the soil.
Kumar et al., (2013) studied the effect of stale seedbed technique and hand weeding on weed density and dry weight at 60 DAS and grain yield of DSR rice:
Soil mulch
The soil mulch is having multiple effects including the weed control. After the seed bed preparation with conventional tillage, the upper layer of the soil is allowed to dry thus forming a soil mulch before wheat sowing and wheat seeds are sown at a depth of 4 to 5 cm
(Ramakrishna et al., 2006). The most of the weed seeds are present in the top soil and they will not germinate until the first irrigation.
Mechanical
The mechanical tillage is associated with cultivating tillage and plays a significant role in weed control. The cultivating tillage is performed after the crop sowing and weeds are removed by shallow tillage with the help of hoes or harrows. It consists inter row, intra row and whole crop cultivation. It helps in tearing weeds into small pieces, uproots them and mixed them into the soil
(Kurstjens and Kropff, 2001). It controls the weeds propagated with seeds during their early developmental stages.
Herbicide
Globally, herbicides are the key tool for management of weeds in wheat crop due to its high economical efficiency and quick knockdown effect. But sole dependence on herbicides resulted in appearance of multiple problems associated with injudicious and frequent use of herbicides such as weed shift, residue accumulation in soil, environmental pollution and accelerated herbicide resistance that limit the potential wheat production globally
(Chhokar et al., 2012). Herbicides act on the weeds and inhibit enzyme activities responsible for essential plant processes like amino acid synthesis (sulfosulfuron, metsulfuron), fatty acid synthesis (pinoxaden, clodinafop, fenoxaprop), photosynthesis (isoproturon, metribuzin) or cell division (pendimethalin) leading to their death.
Punia et al., (2017) evaluated the various herbicides alone and in combination for controlling the different weed population in wheat crop:
Herbicide resistance
Herbicide resistance is defined as the inherited ability of a weed biotype to survive and reproduce after exposure to rate of herbicide which would normally give good control of wild type or to which the original population was susceptible. However, with continuous use of herbicides with same mode of action resulted in evolution of herbicide resistance in different weed species
(Delye et al., 2013).
Evolution of resistance in weeds against herbicide is a global hazard and at present, there are 510 herbicide resistant weeds (unique cases), in 262 weed species (152 dicots and 110 monocots). 70 countries have been reported having cases of herbicide resistant weeds in 93 crops
(Heap 2021). Across the world, 341 unique cases of herbicide resistance amongst 75 weed species have been reported in wheat crop. Out of which 68, 19, 10 and 9 weeds have evolved herbicide resistance against ALS inhibitors, ACCase inhibitors, synthetic auxins and PSII inhibitors (ureas and amide), respectively
(Heap 2021).
Herbicide resistance mechanisms
i) Target site resistance
The target site resistance (TSR) involves a change in the molecular target of the herbicide action which decreases its binding for the herbicide. This type of herbicide resistance can also occur by amplification or over-expression of the target gene which results in reducing the herbicidal activity
(Powles and Yu 2010,
Sammons and Gaines 2014). The extent of decrease in herbicidal binding to its target site depends on structural alteration in the target protein, resulting into high or moderate resistance
(Yu et al., 2014). Long et al (2019) reported that resistance in
Sisymbrium orientale against ALS inhibitor herbicides was due to mutation at Pro197 and Trp574 in ALS gene.
ii) Non-target site herbicide resistance
Non-target site herbicide resistance (NTSR) includes all other resistance mechanisms except alteration in the target site which reduces the amount of herbicide that reaches the target site or that changes the effect of the herbicide instead of its target site inhibition. It comprises reduced uptake or translocation of herbicide, increased metabolism of herbicide, herbicide sequestration and/or decreased rate of herbicide activation
(Devine and Shukla 2000). In metabolism based NTSR, there is increase in the activity of enzyme complexes which detoxify or metabolize the herbicide such as cytochrome P450s. esterases, glutathione S-transferases (GSTs) or uridine 5-diphospho-glucosyl transferases
(Powles and Yu 2010). This type of NTSR is polygenic
i.
e. controlled by many genes and may results in evolution of herbicide resistance with completely different modes of action
(Delye et al., 2013, Preston 2003). However in many herbicide resistant weeds, there are also reports of monogenic inheritance of NTSR
(Huffman et al., 2015).
Resistance due to plant detoxification involves four-phases
i.
e. detoxification, conjugation, transport and degradation
(Liu et al., 2018). In detoxification, oxidation process is carried out by cytochrome P450 mono-oxygenases or oxidases and in second phase there is conjugation of xenobiotic by the addition of thiols or sugars, or directly by GSTs and glycosyltransferases. Transport involves movement of conjugated molecule into the vacuole or in extracellular space by most common group of transporters
i.
e. ATP-binding cassette (ABC) transporters and the last phase comprises degradation of the conjugated molecule
(Liu et al., 2018, Gardin 2015). The evolution of herbicide detoxification mechanism of NTSR is a serious threat to crop production as it can leads to multiple herbicide resistance.
Duhoux et al (2015) reported NTSR in
Lolium spp. against ALS inhibitor herbicides was due to increased activity of cytochrome P450 and glycosyl-transferase.
Status of herbicide resistance in India
Earlier herbicide resistance was limited to only
Phalaris minor in wheat against substituted ureas during 1990s in north-western India
(Malik and Singh 1995). It was found that resistance emergence was associated with delay in herbicide application, poor selection of spray nozzle and continuous reliance on a single herbicide or similar mode of action
(Singh 2007,
Chhokar et al., 2012, Bhullar et al 2017). This subsequently resulted in multiple resistance in some
P.
minor biotypes against three modes of action: photosynthesis at the photosystem II site A (PS-II), acetyl-CoA carboxylase (ACCase) and acetolactate synthase (ALS)
(Chhokar and Sharma 2008). The situation gets further deteriorated with addition of herbicide resistance in
Rumex dentatus,
Chenopodium album,
Polypogon monspliensis,
Avena fatua (Singh, 2016,
Singh et al., 2017, Chhokar, 2014).
Chhokar et al., (2013) reported the first case of herbicide resistance against broad leaf weed
Rumex dentatus. It was found resistant to metsulfuron and also exhibited cross resistance to florasulam, pyroxsulam, iodosulfuron and triasulfuron whereas
Chenopodium album was found resistant to metsulfuron only
(Chhokar et al., 2017). Recently resistance in
R.
dentatus has been confirmed to metsulfuron methyl from the rice-wheat belt of Punjab state
(Dhanda et al., 2020).
Integrated weed management
Integrated weed management (IWM) involves combination of all weed control measures like chemical, cultural and mechanical means to get effective control of weeds and reduced the dependence on solely herbicides. The IWM can be an effective tool in managing weeds and to delay the evolution of resistance in weed species.
