Handbook of Integrated Weed Management for Major Field Crops

 In the initial phases of life on Earth, the prevalence of undesirable plants was not extensive. Humans developed the practice of differentiating between weeds and crops based on economic activities. It has been suggested that there are close to 30,000 species that can be classified as weeds. These have co-evolved with crops and are deemed undesirable because of their capacity to outcompete cultivated plants that are intended for food and fiber production. It is widely accepted that all primary cultivated plants have originated from wild weeds. Several plant species that were previously utilized for ornamental, therapeutic, or feed purposes are currently classified as weeds. The crop ecosystem has undergone significant changes due to the use of high levels of fertilizers, intensive cropping practices, and modifications in cropping patterns. Weeds possess specific characteristics that enable them to thrive and persist in environments where many cultivated plants would ultimately perish. The existence of weeds may have multiple effects on human existence. Apart from their economic implications, these cause hay fever in humans and can also exhibit toxicity in livestock and wildlife. The impact of weeds on crop productivity varies based on certain factors such as type of weed species, density of weeds, duration of weed competition, and method of cultivation.

A weed survey provides valuable insights into the prevalence of various weed species, the interplay between weeds and crops, and the dynamic patterns of these associations in field conditions. A roving survey is implemented within a vast geographical region that includes either a single district or multiple districts. A weed survey with identical objectives to the roving survey can be conducted on a farm. Periodic evaluation of the weed control treatments is necessary to assess their impact on both the crop and weed populations. Visual estimation is a technique employed to quickly gather data when dealing with a substantial number of plots and plant species while facing resource constraints. The European Weed Research Council (EWRC) has developed a rating system to assess the efficacy of weed control. Weed assessments should be conducted during the initial stages of crop growth when weed competition is at its peak and weeds are in their early and manageable stages for counting. The collection and preservation of weed specimens are essential for future reference and study, as not all weed species are readily accessible at all times and locations. An herbarium is a collection of dried, pressed, and preserved plants, including weeds, organized according to an accepted classification system such as Bentham & Hooker or Hutchinson. In addition to this, accurate identification of weed plants is a fundamental requirement for the development of efficient management strategies.

Parasitic flowering plants deprive the host plants of water, nutrients, and assimilates. Data also suggests that parasites transmit inhibitory compounds to hosts. Several species of the genera Striga and Orobanche (root parasites), as well as some species of Cuscuta and Loranthus (stem parasites), are widely distributed worldwide and cause substantial damage to economically important plants. The chapter elucidates the optimal strategies for addressing the issues of parasite infestations by incorporating a range of well-established methodologies. The effective management of parasitic weeds can be achieved through the implementation of various strategies. These include the cultivation of crop varieties that exhibit resistance or low susceptibility to the parasites, utilizing non-host crops as trap crops, employing pre-emergent and postemergent herbicides in combination with bio-herbicides, and manually removing the parasites. 

The field of weed ecology involves the examination of growth characteristics, adaptive traits, and survival mechanisms of weeds that allow them to effectively utilize environmental resources and successfully establish themselves in new habitats. The analysis of the ecological and biological characteristics of weeds enhances our understanding of the relationships between weeds and crops, as well as the factors contributing to their widespread establishment and management strategies. Climate, soil conditions, and biotic factors affect their geographic distribution, abundance, competitive abilities, behavioral tendencies, and survival. Despite the challenges posed by environmental factors and human interventions, weeds have a remarkable capacity to flourish and persist across many ecosystems. Several reasons may contribute to this phenomenon, including seed production in vast quantities, variable seed dormancy and longevity, propagation through vegetative organs, and mimicking the appearance of the crop.

Rice is farmed as a dry-seeded upland, low-land, puddle-seeded, transplanted, and deep-water paddy crop. More than 350 weed species from 150 genera and 60 plant families have been recorded. The chapter provides a comprehensive overview of the botanical characteristics and ecological impact of the various species of plants commonly referred to as weeds (grasses, sedges, broad leaf, ferns, and algal weeds) that are found in rice fields. It also elucidates the diverse methodologies employed in the management of weeds, encompassing preventive measures, cultural practices, mechanical techniques, and biological and chemical interventions. The prudent integration of these methodologies is imperative for achieving cost-efficient and environmentally viable weed control in rice cultivation.

Wheat holds significant agricultural importance as it is extensively cultivated across diverse climatic conditions and soil types. The transition from cultivating tall varieties to dwarf and semi-dwarf (specifically Mexican wheat) varieties has exacerbated the issue of weeds in significant wheat cultivation regions. Both grassy and broadleaved weeds are commonly found in this agricultural crop. The detrimental effects of weeds are most pronounced when they are permitted to compete with wheat crops during the initial 45 days. The implementation of weed management strategies encompasses both preventive measures and effective management practices. These may include the use of uncontaminated agricultural seeds sourced only from accredited state and national organizations, enhancement of crop vigor, altering the crop sequence, application of phosphoric fertilizers in the absence of nitrogen, and rotation strategy for herbicides. 

 Sorghum and pearl millet are prominent cereal crops extensively cultivated in India. These plant species are intentionally cultivated by agriculturalists with constrained financial means who reside in arid and semi-arid geographical areas. The presence of weeds significantly hinders the successful cultivation of these crops. The unregulated growth of undesirable plant species has been empirically documented to have a substantial negative impact on the agricultural output of sorghum and pearl millet, potentially resulting in a decline of approximately 30-40%. Therefore, this chapter elucidates the amalgamated approaches for weed management in the cultivation of sorghum and pearl millet, encompassing cultural, cropping, and chemical methods that are both economically viable and readily implementable for farmers residing in arid and semi-arid regions.

Maize exhibits a high susceptibility to adverse effects caused by weed competition. Research conducted on crop loss has revealed the existence of a crucial time frame window commencing approximately fourteen days post-planting and persisting for a duration of approximately six weeks. It is imperative to eradicate weeds within this specific timeframe to maximize crop yield. The existence of both annual and perennial weeds presents a substantial obstacle in the cultivation of maize. Certain weed species have the potential to result in complete crop failure, with the severity of infestation varying across different geographical regions. Optimal land preparation techniques are crucial for achieving a seedbed that is devoid of weeds during the sowing process. Additionally, the practice of deep ploughing can effectively manage rhizomatous perennial weeds. The development of high-yield cultivars is crucial for enhancing competitive advantage against weeds by facilitating the rapid establishment of a dense canopy. Increased plant density results in greater shading, which subsequently leads to the suppression of early weed growth. The application of a balanced fertilizer promotes improved nutrient utilization by crops compared to weeds. 

Sugarcane has considerable economic importance as a cash crop in India. Nevertheless, the yield of sugarcane cultivation in India is somewhat lower in comparison to other nations. The presence of weed infestation is a significant obstacle to the improvement of sugarcane farming in the majority of sugarcane-growing regions within the country. The issue of weed proliferation is intensified by the overutilization of fertilizers, increased plant spacing, frequent irrigation, and the prolonged duration needed for complete seedling germination. Therefore, the implementation of a comprehensive weed management strategy is crucial in the cultivation of sugarcane. This chapter entails the integration of many approaches that span the whole farming process, beginning with field preparation and persisting until the harvest stage. The objectives of these approaches are to facilitate the cultivation of sugarcane while concurrently inhibiting the proliferation of weeds.

Cotton is generally cultivated during the wet season. As a result, a large number of weeds thrive in this crop. It is very susceptible to weed competition, especially during the first 50-60 days post-planting, resulting in yield losses ranging from 16% to 63%. Consequently, it is critical to maintain a weed-free environment throughout this period in order to maximize crop output. The chapter highlights the significance of integrating cultural, mechanical, chemical, and biological control methods to create a synergistic effect that reduces weed competition, enhances crop yield, and sustains soil health.

In India's economy, oilseeds rank second only to cereal grains as the main source of vegetable oils. They are grown on 24 million hectares under various agroecological conditions, cropping techniques, and management practices. Weeds compete with crops for light, moisture, nutrients, and space and pose significant threats to oilseed crops, impacting yield and quality. This chapter provides a concise overview of the primary weed species found in oilseed crops, along with their associated reduction in crop yield and recommended methods for control. Integrated weed management establishes a balanced agricultural ecosystem, strengthening sustainability and productivity in oilseed cultivation. 

The cultivation of pulses in India is carried out across 23 million hectares, which encompasses a wide range of agroecological conditions, cropping techniques, and management practices. They are planted throughout both the Kharif and Rabi seasons. The average productivity of pulse crops in India is somewhat lower than the productivity levels reported in several other pulse-growing countries globally. The problem of weed infestation is a substantial hindrance to attaining the highest possible crop output. This chapter discusses the critical period of crop-weed competition in different pulse crops. The list of predominant weed species systematically classified based on their seasonal occurrences, along with corresponding herbicides recommended for their control, is also included. 

Herbicides are chemicals used to kill weeds or inhibit their growth. According to the latest compilation of registered pesticides under the Insecticide Act of 1968, a total of 33 herbicides have been officially registered in India. Herbicides are classified on the basis of their chemical composition, selectivity, mode of action, site, and time of application. The chapter provides a comprehensive overview of diverse instances of herbicides categorized accordingly. The typical herbicide formulations commonly found in the market include water-soluble powders, hydrophilic powder, water-soluble concentrates, emulsifiable concentrates, and granules. The application rates of herbicides are influenced by several factors, including soil composition, weed crop species/varieties, the growth stage of weeds, herbicide formulations, and the duration of weed control programs. The current chapter also examines different methodologies for efficiently managing the proliferation of undesired vegetation, such as foliar spraying, soil application, and seed treatment.