1 Ecologically based weed management! Chuck Mohler! Cornell University!
2 Outline! Understanding weeds! Behavior of wandering perennials! Tillage and emergence cues! Life and death of weed seeds in the soil! Season of emergence and crop rotation! Seed size and crop competitiveness! Weeds and nutrient management! Prevention!
3 Weeds are plants that thrive in disturbed environments! For example, in a farm field! We kill off natural vegetation & disturb the soil to make conditions suitable for crops! But this also creates habitats for weeds! Managing soil disturbance is critical for weed control! When? How?!
4 Understanding the biology of weeds is a key to their control! Managing the weeds without harming your crops depends on the biological differences between the weeds and crops.!
5 Ecological weed management requires multiple tactics! Conventional agriculture relies on a few big hammers (broad spectrum herbicides)!
6 Ecological weed management relies instead on many little hammers! Crop rotation! Enhanced crop competition! Mulches! Nutrient management! Timing and type of tillage! Cultivation!
7 Any little hammers Requires an integrated approach based on the biological characteristics of the weeds present in a particular field! Attack the whole life cycle of the weed! Multiple year strategy!
8 System design! 1. You need to get to know the weeds who they are, how they make a living! Also needs to know soil, crops, cover crops, pests etc. too! 2. Design the system to prevent the weeds from causing problems! And supply nutrients, insure crop health etc.! 3. Return to 1 (keep learning and tinkering)! Ecological management works best for people who find learning fun.!
9 Multiple ways to be a weed! Annuals! Summer annuals! Cool season annuals! Perennials! Stationary perennials! Wandering perennials!
10 Wandering perennials! Spread by thickened storage roots or by rhizomes (underground stems)!
11 Apical dominance in perennials! Tillage
12 Shoot above ground Shoot below ground New rhizomes Old rhizome fragment
13 Management of perennials! Key is exhaustion of reserves.! Time shoot removal relative to growth stage! Hit them repeatedly! Cultivated fallows! Frequently cultivated crops,! Short season crops (with post-harvest clean up)! Competitive crops!
14 Annual weeds! Live less than one year! Establish from seed each year! Seeds/seedlings are critical stages!
15 Seeds of most weeds are tiny why?! Disturbed environments are risky! Tiny seeds spread the risk over many offspring! Seedlings can be small because in a recently disturbed environment they have little competition.! Consequence: seedlings have limited resources!!
16 0.4 Velvetleaf Lambsquarters Redroot pigweed Small seeded species only emerge if near the soil surface! Depth (cm)
17 Germination cues! Seedlings compete poorly with established plants! So weed seeds need to know when other plants are absent! Respond to cues associated with! absence of plants! Near-surface conditions! soil disturbance!
18 Common germination cues! Light!! Warm soil temperature (90 F +)! Redroot pigweed, purslane! Day-night fluctuation in temperature! Common chickweed, curly dock! Nitrate! Lambsquarters, annual bluegrass! Most species respond to multiple cues! Lambsquarters: light, fluctuating temperatures and nitrate!
19 Consequences! Can use tillage to flush seeds out of the soil! Cultivated fallow! Conversely, soil cover suppresses germination of weed seeds! Mulch! Dense crop canopy! Absence of tillage suppresses germination! Stale seedbed!
20 Stale seedbed for carrots & parsnips! Prepare beds! Let the weeds emerge for 2 weeks! Flame, then plant! Flame again 10 days later! Crop emerges with few weeds!
21 Seed longevity! Loss per year (%) Species Cultivated Uncultivated Lambsquarters 31 8 Annual bluegrass Common chickweed Common groundsel High 45
22 1.0 Seeds Seed survival (%) Velvetleaf Pigweed survive better deep in the 0.0 soil! Depth in soil (inches)
23 a l u s r u f i p e s Death near the soil surface! Seed predation! Wetting and drying! Freeze-thaw! D e j. )
24 Plowing vs. minimum tillage?! Small seeded species with short lived seeds à plow them under! Most will die before they find their way to surface again! Example: hairy galinsoga! Large seeded species with long lived seeds à keep them near the surface! Their mortality will be greater at the surface! And most that are tilled under will come back to bother you later! Example: velvetleaf!
25 Weeds emerge at different times of year! Giant ragweed early spring! Lambsquarters spring! Redroot pigweed spring and summer! Purslane midsummer! Shepherd s-purse fall, spring! Downy brome fall (winter in warm climates)!
26 Rotate spring, summer and fall planted crops! This favors different suites of species in different years! Prevents build-up of any one species.! Fall germinating species get wiped out by spring tillage! Spring germinating species get wiped out by summer tillage!
27 More advantages of crop rotation! Can use different cultivation methods in different crops! Short season crops can be harvested before weeds go to seed. break the life cycle! Also allow extra soil disturbance to deplete perennials! Hay crops outcompete most annuals; repeated mowing of alfalfa sets back many perennials!
28 Crop seeds are mostly much larger than weed seeds!
29 Seed size controls growth rate! Initial growth rate Relative growth rate Seed weight Species (mg) (mg/d) (mg/mg/d) Lambsquarters Velvetleaf Cocklebur Sunflower Soybean
30 Enhance the crop s head start!! Plant when crop will emerge and grow quickly! Use transplants for small seeded vegetables! Don t delay between seedbed prep and planting! Or use a stale seedbed and kill the weeds right before planting!
31 Take advantage of the crop s head start! High density planting! Space plants for quick canopy closure! Trade-offs with cultivation! Use competitive cultivars! Put the competitive cultivars in the weediest fields/beds! In-row cultivation!
32 Weed dry weight (lb/a) Uniform Uniform Rows Random Using crop competition! Yield (bu/a) Rows Random Wheat density (plants/ft 2 )
33 Nutrients and weeds! A lot of mythology and not much data! Weeds are nutrient sponges! Avoid pulsed release of nutrients! Most agricultural weeds are highly responsive to N and P! Over fertilization leads to weed problems!
34 Response of corn to compost!
35 Height (cm) Lambsquarters Ragweed Foxtail Fitted curve Response of weeds to compost! Compost rate (ton/a)
36 Prevention!! Clean up after harvest to reduce seed production! Rogue out the big individuals! Avoid contaminated manure, mulch hay, cover crop seed!! Watch for and eliminate species that are new to the farm!