Weed management

In short

A weed is any plant growing where it is not wanted - it competes with the crop for light, water, nutrients and space, harbours pests, and lowers both yield and produce quality. Weeds alone cause roughly one-third of total crop losses in India, typically 10-80% yield loss depending on crop and weed flora, which is more than insects and diseases combined. Weed management covers prevention, eradication and control by physical, cultural, chemical and biological means, combined as integrated weed management (IWM). The topic carries heavy weight in B.Sc (Ag) Agronomy papers, ICAR JRF/SRF and ARS, IBPS/SBI Agriculture Field Officer, NABARD and state Agriculture Officer exams, and in interviews with agrochemical companies (UPL, Dhanuka, Bayer, Syngenta, Corteva, PI Industries, Coromandel), seed firms and agri-input retail.

Explained

Weeds: definition, characteristics and classification

• Weed: a plant out of place, growing where it is not desired - Jethro Tull first used the term "weed" in the agronomic sense; the science of weeds is herbology.
• Characteristics: prolific seed production, long seed dormancy and viability, multiple dispersal mechanisms, vegetative propagation, fast early growth, C4 photosynthesis in many grasses, hardiness to stress.
• By life cycle: annuals (one season - *Phalaris minor*, *Echinochloa*, *Chenopodium*), biennials (*Daucus carota*), perennials (*Cyperus rotundus*, *Cynodon dactylon*, *Sorghum halepense*, *Saccharum spontaneum*).
• By morphology: grasses (Poaceae - *Echinochloa colona*, *Phalaris minor*, *Cynodon*), sedges (Cyperaceae - *Cyperus rotundus*, *C. difformis*), broadleaf/non-grass (*Parthenium*, *Chenopodium album*, *Convolvulus*).
• By habitat: cropland weeds, wasteland weeds, aquatic weeds (*Eichhornia crassipes* - water hyacinth or "Bengal terror", *Salvinia*, *Hydrilla*), problem/noxious weeds.
• Parasitic weeds: total root parasite Orobanche (broomrape - on mustard, tobacco, brinjal); partial root parasite Striga (witchweed - on sorghum, sugarcane, maize); total stem parasite Cuscuta (dodder - on lucerne, berseem, niger); partial stem parasite Loranthus/Dendrophthoe (on mango, citrus).
• Objectionable weeds: those whose seeds resemble crop seed and cannot be separated - *Cuscuta* in lucerne/berseem, wild rice in paddy, *Avena fatua* (wild oat) and *Phalaris minor* in wheat, *Argemone mexicana* in mustard (its seed oil causes epidemic dropsy).
• Alien invasive weeds in India: *Parthenium hysterophorus* (carrot grass, entered with PL-480 wheat in 1956, causes dermatitis and asthma), *Lantana camara*, *Eichhornia*, *Chromolaena odorata*, *Mikania micrantha*, *Prosopis juliflora*, *Ageratum conyzoides*.
• Allelopathy: release of chemicals (parthenin from *Parthenium*, juglone from walnut, sorgoleone from sorghum) that suppress neighbouring plants - used positively as smother/cover crops and in mulches.

Weed biology, crop-weed competition and losses

• Weeds compete for light, water, nutrients, CO₂ and space; competition is severest when demands coincide and when the weed is of the same morphology and photoperiod as the crop.
• Critical period of crop-weed competition (CPCWC): the shortest interval in crop life during which weeding gives yield equal to full-season weed-free conditions - broadly the first one-third to one-half of crop duration; indicative figures: transplanted rice 15-45 DAT, direct-seeded rice 15-45 DAS, wheat 30-45 DAS, maize 15-45 DAS, groundnut 20-45 DAS, soybean 20-45 DAS, cotton 15-60 DAS, sugarcane 30-120 DAP, onion 30-60 DAT.
• Weeds remove large quantities of nutrients - a heavy weed stand can take up 30-40 kg N/ha that the crop would have used.
• Mimicry: *Echinochloa crus-galli* var. *formosensis* mimics rice seedlings; *Phalaris minor* mimics wheat - makes hand weeding unreliable.
• Weed seed bank: cultivated soil commonly holds thousands to tens of thousands of viable weed seeds per square metre in the plough layer; only a small fraction germinates each season, so eradication is rarely economic.
• Other losses: reduced produce quality and market price, alternate hosts for pests and diseases (*Sorghum halepense* for sorghum shoot fly; *Chenopodium* for nematodes), obstruction to irrigation flow and harvesting, livestock poisoning and tainted milk, human health (allergy, dermatitis), 30-40% water loss from canals and tanks choked by aquatic weeds.

Methods of weed control

• Preventive (cheapest, most neglected): certified weed-free seed, clean implements and threshers, well-rotted FYM/compost (fresh manure carries viable seeds), weed-free irrigation channels and bunds, quarantine - Destructive Insects and Pests Act, 1914 and the Plant Quarantine (Regulation of Import into India) Order, 2003 regulate entry of weed seeds with imported planting material; seed purity standards for certified seed limit objectionable weed seeds.
• Eradication: complete removal of the weed and its propagules from an area - practicable only for small, high-value or quarantine situations.
• Control: reducing weed population/growth to an economically acceptable level - the normal farm objective.
• Physical/mechanical: hand weeding and hand hoeing (khurpi), interculture with hand hoe, wheel hoe, Japanese paddy weeder, cono-weeder (SRI), tractor-drawn cultivator/rotary weeder, mowing, dredging of aquatic weeds, burning, flooding (3-5 cm standing water in rice smothers *Echinochloa* but not sedges), soil solarisation with transparent polythene for 4-6 weeks in summer, mulching (straw, plastic, live mulch) to deny light.
• Cultural: stale seedbed technique, summer/deep ploughing to expose rhizomes, higher seed rate and narrower rows for quick canopy closure, crop rotation (sugarcane-paddy rotation breaks *Cyperus*; berseem suppresses *Phalaris*), intercropping and smother crops (cowpea, sunhemp, sesbania), green manuring, competitive and short-statured high-tillering varieties, proper timing and placement of fertiliser and irrigation (band or deep placement starves weeds).
• Biological: deliberate use of host-specific natural enemies - Zygogramma bicolorata (Mexican beetle) on *Parthenium*; Neochetina eichhorniae and N. bruchi weevils on water hyacinth; *Cactoblastis cactorum* and *Dactylopius* cochineal on *Opuntia*; *Teleonemia scrupulosa* on *Lantana*; fish (grass carp) for submerged aquatic weeds; bioherbicides/mycoherbicides (*Colletotrichum*, *Alternaria* formulations). Slow, host-specific, never eradicative, but permanent and cheap - best for perennial, non-cropped and aquatic situations.
• Chemical: herbicides - fast, labour-saving, effective on in-row and perennial weeds and under wet conditions when hand weeding is impossible; risks are phytotoxicity, residues, resistance, flora shift and drift.

Herbicides: classification, mode of action and application

• By selectivity: selective (kills weeds, spares crop - 2,4-D in wheat, atrazine in maize) and non-selective/total (glyphosate, paraquat, glufosinate).
• By time of application: pre-plant incorporation (PPI) - fluchloralin; pre-emergence (PE) applied before weed/crop emergence - pendimethalin, butachlor, atrazine, metribuzin, oxyfluorfen; early post-emergence - bispyribac-sodium; post-emergence (POE) - 2,4-D, clodinafop, sulfosulfuron, imazethapyr, quizalofop.
• By movement: contact (paraquat, propanil) kills only tissue touched; translocated/systemic (glyphosate, 2,4-D, sulfosulfuron) moves in xylem/phloem - essential for perennials with underground parts.
• By chemical family: phenoxy-acetic (2,4-D, MCPA), dinitroaniline (pendimethalin, fluchloralin), triazine (atrazine, metribuzin), substituted urea (isoproturon, diuron), acetanilide/chloroacetamide (butachlor, alachlor, pretilachlor, metolachlor), bipyridylium (paraquat, diquat), sulfonylurea (sulfosulfuron, metsulfuron, pyrazosulfuron), aryloxyphenoxypropionate "fops" (clodinafop, fenoxaprop, quizalofop) and cyclohexanedione "dims" (sethoxydim), imidazolinone (imazethapyr), glycine (glyphosate), diphenyl ether (oxyfluorfen), isoxazolidinone (clomazone), triketone (topramezone, tembotrione).
• Mode of action groups: photosynthesis inhibition at PS II (triazines, ureas, propanil), PS I electron diversion (paraquat), ALS/AHAS inhibition (sulfonylureas, imidazolinones), ACCase inhibition (fops and dims - grass-killers), EPSPS inhibition (glyphosate - blocks aromatic amino-acid synthesis), microtubule/cell-division inhibition (dinitroanilines), auxin mimics (2,4-D), carotenoid/HPPD inhibition (clomazone, topramezone), lipid/VLCFA synthesis (acetanilides), glutamine synthetase (glufosinate).
• Basis of selectivity: differential absorption and translocation, morphology (waxy, erect, narrow leaves shed spray), stage and placement (depth protection), and metabolic detoxification - maize detoxifies atrazine by glutathione conjugation; rice detoxifies propanil with aryl acylamidase (hence no carbamate/organophosphate insecticide near propanil spraying).
• Formulations: EC, SC/FS, WP, WG/WDG, SL, GR, SG; adjuvants - surfactants, stickers, wetters, safeners (fenclorim, benoxacor), and herbicide protectants/antidotes.
• Application equipment: knapsack/lever-operated sprayer, battery-operated and power sprayers; flat-fan nozzle for herbicides (even rectangular pattern), flood-jet for high-volume, hollow cone for insecticides/fungicides - never interchange, and keep a separate sprayer for herbicides (2,4-D traces damage cotton and vegetables).
• Good practice: calibrate before spraying, 375-500 L/ha spray volume for PE in field crops and 300-375 L/ha for POE (manufacturer label is final), spray in calm weather with a hood/shield to prevent drift, moist seedbed for soil-applied herbicides, avoid spraying before heavy rain, read the label, use PPE.

Herbicide resistance, residues, flora shift and regulation

• Herbicide resistance: inherited ability of a weed biotype to survive a dose that kills the normal population - arises by repeated use of the same mode of action. The classic Indian case is isoproturon-resistant *Phalaris minor* in the rice-wheat belt of Punjab and Haryana (reported from the early 1990s), managed by rotating to clodinafop, sulfosulfuron, pinoxaden, metribuzin and by agronomic diversification; glyphosate-resistant and ALS-resistant biotypes are now also reported globally.
• Resistance management: rotate and mix modes of action, use full label dose at the right stage, keep non-chemical methods in the programme, follow rotation, prevent seed set of survivors.
• Cross-resistance (to herbicides of the same site of action) and multiple resistance (to several different sites) must be distinguished.
• Residues and carryover: atrazine and sulfonylureas can persist and injure the following sensitive crop - follow the label's plant-back interval; MRLs for pesticide residues in food are prescribed under the Food Safety and Standards Act, 2006 and FSSAI contaminants and residues regulations, with FSSAI as regulator.
• Flora shift: continuous use of one herbicide favours tolerant species - butachlor use in rice favours sedges; repeated 2,4-D use favours grasses.
• Regulation: herbicides are "insecticides" in law under the Insecticides Act, 1968 and Insecticides Rules, 1971; registration is by the Central Insecticides Board and Registration Committee (CIB&RC), Faridabad, with the Central Insecticides Laboratory for testing; manufacture, sale and use require licences from the state agriculture department, and only label-approved crop-herbicide combinations may be recommended. Some molecules are banned or restricted in India - the list is revised by notification, so verify the current CIB&RC banned/restricted list before advising. (A Pesticides Management Bill to replace the 1968 Act has been under consideration; the 1968 Act remains the operative law to the best of current knowledge - confirm before quoting.)
• Herbicide-tolerant crops: imazethapyr-tolerant non-GM mustard and rice lines and HT Bt cotton exist; no GM herbicide-tolerant crop is approved for commercial cultivation in India (only Bt cotton is approved for commercial cultivation), and GEAC regulates GM releases under the 1989 Rules of the Environment (Protection) Act, 1986.

Integrated weed management and problem-weed programmes

• IWM: a planned combination of preventive, cultural, mechanical, biological and chemical methods to keep weeds below the economic threshold with least cost, least herbicide and least environmental risk.
• Steps: survey and identify the weed flora → map the critical period → choose a preventive and cultural base (clean seed, stale seedbed, rotation, spacing) → apply a pre-emergence herbicide → follow with one post-emergence spray or interculture → prevent seed set of escapes → record and rotate modes of action next season.
• Rice: pretilachlor or pyrazosulfuron or bensulfuron+pretilachlor (PE) followed by bispyribac-sodium (POE) in direct-seeded rice; butachlor/pretilachlor in transplanted rice plus 3-5 cm standing water; cono-weeding in SRI.
• Wheat: pendimethalin PE or clodinafop (grasses), metsulfuron/2,4-D (broadleaf), sulfosulfuron or pinoxaden or ready-mixes for mixed flora; zero-till and timely sowing reduce *Phalaris*.
• Maize: atrazine PE 0.5-1.0 kg a.i./ha (never in intercropped legumes - atrazine is phytotoxic to pulses), with topramezone/tembotrione as POE.
• *Cyperus rotundus*: repeated summer ploughing to desiccate tubers, glyphosate 1.0-2.0 kg a.i./ha on actively growing shoots in non-crop or pre-plant situations, halosulfuron in sugarcane/maize.
• *Parthenium*: uproot before flowering (with gloves), *Cassia* competitive replacement, *Zygogramma bicolorata* release, glyphosate or metribuzin on wasteland; a national awareness week is observed in August.
• Water hyacinth: mechanical removal plus *Neochetina* weevils; 2,4-D where permitted; composting the biomass.
• Economic/ecological thresholds guide whether intervention pays - control is justified when the value of yield saved exceeds the cost of control.

Key points

• Weed - a plant growing where it is not wanted; weed science is herbology; weeds cause about one-third of all crop losses in India.
• Critical period of crop-weed competition - the first one-third to one-half of crop duration, when weed-free conditions must be maintained.
• Three levels of action - prevention (stop entry), eradication (total removal, rarely economic), control (keep below threshold).
• Perennials - *Cyperus rotundus* (purple nutsedge, world's worst weed), *Cynodon dactylon*, *Saccharum spontaneum*; need translocated herbicides, not contact ones.
• Parasitic weeds - Orobanche (total root), Striga (partial root), Cuscuta (total stem), Loranthus (partial stem).
• Objectionable weeds - seed resembles crop seed: Cuscuta in berseem/lucerne, *Phalaris minor* and *Avena fatua* in wheat, *Argemone* in mustard.
• Parthenium hysterophorus - alien invasive since 1956, allelopathic (parthenin), causes dermatitis and asthma; controlled by *Zygogramma bicolorata*.
• Selectivity - differential absorption, morphology, placement and metabolic detoxification (atrazine in maize, propanil in rice).
• Pre-emergence - applied to moist soil before emergence (pendimethalin, butachlor, atrazine); post-emergence - on growing weeds (2,4-D, clodinafop, bispyribac).
• Contact vs systemic - paraquat kills only what it touches; glyphosate translocates to rhizomes and tubers.
• Mode-of-action handles - ACCase (fops/dims, grasses), ALS (sulfonylureas, imidazolinones), PS II (atrazine, isoproturon), EPSPS (glyphosate), PS I (paraquat).
• Flat-fan nozzle - the standard herbicide nozzle; keep a dedicated herbicide sprayer.
• Herbicide resistance - inherited survival of a lethal dose; India's classic case is isoproturon-resistant *Phalaris minor* in the rice-wheat belt.
• Flora shift - repeated use of one herbicide favours tolerant species (butachlor → sedges; 2,4-D → grasses).
• Biological control - *Neochetina* weevils on water hyacinth, *Zygogramma* on Parthenium, *Cactoblastis*/*Dactylopius* on *Opuntia*; slow but permanent.
• Regulation - herbicides fall under the Insecticides Act, 1968 and Rules, 1971, registered by CIB&RC; residues in food are governed by FSSAI under the FSS Act, 2006.
• Allelopathy - chemical suppression of one plant by another; exploited in smother crops, mulches and bioherbicides.
• IWM - preventive and cultural base + one PE + one POE/interculture + stop seed set + rotate modes of action.

Formulas and examples

Example 1 - Commercial product from a.i. dose. Recommendation: pendimethalin 1.0 kg a.i./ha; available as pendimethalin 30% EC. Field = 2.5 ha.

• Product per ha = 1.0 × 100 / 30 = 3.33 L/ha.
• For 2.5 ha = 3.33 × 2.5 = 8.33 L of 30% EC.
• At a spray volume of 500 L/ha, water needed = 500 × 2.5 = 1,250 L, i.e. 1,250 − 8.33 ≈ 1,242 L water (treated as 1,250 L in practice).
• Number of 15-L knapsack loads = 1,250 / 15 ≈ 84 loads; herbicide per load = 8.33 / 84 ≈ 0.10 L (100 mL).

Example 2 - Atrazine in maize, part-field dose. Atrazine 0.75 kg a.i./ha using atrazine 50% WP on 0.4 ha (1 acre).

• Product per ha = 0.75 × 100 / 50 = 1.5 kg/ha → for 0.4 ha = 0.6 kg (600 g).
• Spray volume 500 L/ha → water = 500 × 0.4 = 200 L; concentration = 0.6/200 × 100 = 0.3%.
• Per 15-L knapsack: 600 g ÷ (200/15 = 13.33 loads) = 45 g per load.

Example 3 - Weed control efficiency and weed index. Transplanted rice trial: unweeded control weed dry weight 180 g/m², yield 3,200 kg/ha; herbicide treatment weed dry weight 36 g/m², yield 5,440 kg/ha; hand-weeded (weed-free) yield 5,600 kg/ha.

• WCE = (180 − 36) × 100 / 180 = 80%.
• WI = (5,600 − 5,440) × 100 / 5,600 = 2.86% - the herbicide nearly matched weed-free yield.
• Yield increase over control = (5,440 − 3,200)/3,200 × 100 = 70%.
• HEI = [(5,440 − 3,200)/3,200] ÷ [(180 − 36)/180] = 0.70 ÷ 0.80 = 0.875.

Example 4 - Economics of herbicide versus hand weeding (1 ha wheat). Clodinafop 15% WP at 60 g a.i./ha: product = 60 × 100/15 = 400 g/ha; at ₹900/kg → ₹360. Spraying labour 1 person-day at ₹400 = ₹760/ha total. Two hand weedings need 50 person-days at ₹400 = ₹20,000/ha. Yield: herbicide 4,400 kg/ha, hand weeding 4,500 kg/ha, unweeded 3,000 kg/ha; wheat at ₹2,425/q (indicative - MSP is revised every season, so use the current notified MSP).

• Herbicide: extra yield 1,400 kg = 14 q × ₹2,425 = ₹33,950; net gain = 33,950 − 760 = ₹33,190/ha.
• Hand weeding: extra yield 1,500 kg = 15 q × ₹2,425 = ₹36,375; net gain = 36,375 − 20,000 = ₹16,375/ha.
• Herbicide is about twice as profitable here, and feasible when labour is scarce.

Example 5 - Sprayer calibration. Knapsack with flat-fan nozzle, swath 1 m, walking speed 50 m/min, target volume 500 L/ha.

• Area per minute = 1 × 50 = 50 m².
• Required output = 50 × 500 / 10,000 = 2.5 L/min.
• Time to spray 1 ha = 10,000/50 = 200 min; water used = 200 × 2.5 = 500 L (checks out).
• Collecting nozzle discharge for 1 min: if it gives 2.0 L/min instead, walking speed must drop to 50 × 2.0/2.5 = 40 m/min to deliver 500 L/ha.

Where it is used

• Agrochemical industry: product development, label claim trials, bio-efficacy and residue studies for CIB&RC registration, and technical field support at UPL, Dhanuka, Bayer, Syngenta, Corteva, PI Industries, Coromandel, Rallis and Insecticides India - the commonest entry role for a B.Sc (Ag) graduate as Territory/Agronomy Officer.
• Banking and development finance: IBPS/SBI Agriculture Field Officer and NABARD roles appraising crop loans, Kisan Credit Card limits and scale-of-finance that include weedicide cost per hectare.
• State extension and KVKs: Agriculture Officer and ADO roles running demonstrations, Parthenium eradication drives, herbicide-use advisories under ATMA, and the Krishi Vigyan Kendra network of ICAR.
• Research: ICAR Directorate of Weed Research, Jabalpur (the national institute for weed science), SAU agronomy departments, AICRP on Weed Management - resistance monitoring, bioherbicides, long-term herbicide trials.
• Conservation agriculture and contract farming: zero-till wheat after rice, Happy Seeder residue mulch, direct-seeded rice - all depend on herbicide-based weed management since interculture is not possible.
• Non-crop and infrastructure: canal and reservoir de-weeding, railway tracks, airfields, plantations, highway verges, and seed certification agencies enforcing objectionable-weed-seed limits.

Interview questions

• Q: Define a weed and state why weeds are more damaging than insects or diseases in India? A: A weed is any plant growing where it is not wanted; weeds compete through the whole season for light, water, nutrients and space, go largely unnoticed, and account for about a third of total crop losses - more than insects and diseases together.
• Q: What is the critical period of crop-weed competition and why does it matter? A: It is the shortest period in crop life that must be kept weed-free to get full-season weed-free yield, usually the first one-third to one-half of crop duration; weeding only in this window saves cost without losing yield.
• Q: Difference between pre-emergence and post-emergence herbicides? A: Pre-emergence herbicides are applied to a moist, prepared soil before crop or weed emergence and act on germinating seedlings; post-emergence herbicides are sprayed on already emerged, actively growing weeds and need leaf coverage.
• Q: Why would you choose glyphosate over paraquat for *Cynodon dactylon*? A: Glyphosate is systemic and translocates to the rhizomes, killing the whole plant, whereas paraquat is contact-acting and only burns the top growth, which regrows from the rhizome.
• Q: Why is atrazine safe in maize but not in cowpea intercropped with maize? A: Maize detoxifies atrazine by conjugating it with glutathione, while legumes lack that capacity and are injured - selectivity here is metabolic, not morphological.
• Q: What is herbicide resistance and give an Indian example? A: It is the inherited ability of a weed biotype to survive a normally lethal dose; isoproturon-resistant *Phalaris minor* in the Punjab-Haryana rice-wheat belt is the classic Indian case, managed by rotating to clodinafop, sulfosulfuron or pinoxaden and diversifying agronomy.
• Q: Name the parasitic weeds and their hosts? A: Orobanche (total root parasite - mustard, tobacco, brinjal), Striga (partial root parasite - sorghum, sugarcane), Cuscuta (total stem parasite - lucerne, berseem) and Loranthus (partial stem parasite - mango, citrus).
• Q: Which nozzle do you use for herbicides and why? A: A flat-fan nozzle, because it lays an even rectangular band of coarse droplets at low pressure, giving uniform soil or foliage coverage with little drift; cone nozzles are for insecticides and fungicides.
• Q: How much of a 30% EC product gives 1 kg a.i./ha? A: 1 × 100/30 = 3.33 litres per hectare.
• Q: What is flora shift and how do you prevent it? A: Repeated use of one herbicide eliminates susceptible species and lets tolerant ones dominate - butachlor in rice favours sedges; prevented by rotating modes of action and keeping non-chemical methods in the programme.
• Q: Which Act and which authority regulate herbicides in India? A: Herbicides are covered as "insecticides" under the Insecticides Act, 1968 and Insecticides Rules, 1971, registered by the Central Insecticides Board and Registration Committee; residues in food come under FSSAI and the FSS Act, 2006.
• Q: Why is biological control not the main method in field crops? A: It is slow, host-specific and only suppresses a single weed species to a tolerable level, so it suits perennial, aquatic and non-cropped weeds like water hyacinth and Parthenium rather than mixed annual flora that must be controlled within weeks.

Common mistakes

• Confusing eradication with control, or claiming eradication is the normal field objective - the seed bank makes it uneconomic.
• Dividing instead of multiplying in the a.i. conversion: the formula is a.i. dose × 100 ÷ % a.i., so a lower-strength formulation needs more product.
• Quoting a herbicide dose without saying whether it is kg a.i./ha or kg of formulated product/ha - a frequent interview trap.
• Spraying a contact herbicide on rhizomatous perennials, or applying a pre-emergence herbicide on dry soil or after weeds have emerged.
• Using one sprayer for 2,4-D and then for other crops, using a cone nozzle for herbicides, or not calibrating - all cause failure or crop damage.
• Treating the critical period as a fixed number of days for all crops, and writing *Cyperus rotundus* as a grass instead of a sedge.
• Saying "herbicides are not covered by the Insecticides Act" or assuming GM herbicide-tolerant crops are approved for commercial cultivation in India - only Bt cotton is.

Related topics

Principles of agronomy · Cereal crop production · Irrigation water management · Plant protection and pesticide chemistry · Conservation agriculture and zero tillage · Cropping systems and crop rotation