Weed control in no-till systems
A no-till planted field does not have to mean a field full of weeds. No-till systems use herbicide applications instead of cultivation for weed control. A greater reliance on herbicides requires attention to detail--types of weeds, weather trends, cropping patterns, soil type--in order to control weed populations without adversely affecting the environment. The first few years of no-tillage may require higher herbicide inputs. However, many long-term growers claim once no-till is established, herbicide costs generally decrease and become competitive with conventional systems.
Successful weed control in no-tillage requires:
- accurately identifying weeds
- proper timing of herbicide treatments
- scouting and monitoring fields for problem areas
- spot spraying herbaceous and woody perennials
- keeping fence rows and field borders free of aggressive weeds
- assuring even distribution of crop residue after harvest
Shift in Weed Species
Without deep tillage weed seeds stay near the soil surface instead of being buried too deep for germination. Small-seeded broadleaf weeds and annual grasses can germinate under crop residue as shown in Figure 1. Large-seeded weeds, such as velvetleaf and common cocklebur, need deeper soil placement to germinate. With continued no-tillage large-seeded weed populations tend to decline. Both simple perennial and creeping perennial weeds multiply in a no-tillage system. Creeping perennials also tend to be a problem in conventional tillage systems although tillage controls simple perennials.
For the best results apply early preplant herbicide treatments before germination of summer annual weeds. The timing of these treatments range from 10 to 45 days prior to planting of the crop. Applying herbicides early in the season, when rains are more frequent, insures activation of the herbicides. Early application also helps spread out the spring work load. With timely herbicide applications, the fields will be free of weeds at planting, eliminating the need for a burndown treatment. Early killing of weeds also discourages insects attracted to weedy fields, such as black cutworm moths.
Burndown herbicides control emerged weeds before or just after planting, but prior to crop emergence. These herbicides can be combined with early preplant or preemergence treatments. Paraquat (e.g. Gramoxone Inteon) works well in combination with residual herbicides. A tank mix of glyphosate (eg. Roundup Weathermax) and 2,4-D broadens the spectrum of weed control. The weed species present, size, life cycle, and herbicide efficacy determines what and how much herbicide to apply.
The success of preemergence herbicide treatments depends on rainfall to activate the herbicide soon after spraying. Without moisture, herbicide performance suffers. Herbicides that are soil-applied after planting are often preferred over treatment prior to planting. Planter row-cleaners or "trash-whippers" can move herbicide-treated soil from the row to the row middles, creating weed problems in the untreated zone.
Two types of preemergence herbicide programs are common in no-tillage:
- a full season program--where herbicides control weeds throughout most of the growing season, or
- a short residual program--where the crop shades out later germinating weeds. Postemergence treatments may be necessary for weed escapes.
Weeds in no-tilled fields tend to germinate throughout the season. Maintain season-long weed control with early pre-plant treatments of "full season" residual herbicides or split treatments of herbicides with medium longevity. Split applications often include applying 1/3 of the labeled herbicide rate early and the remaining 2/3 at planting. Many early preplant herbicides have burndown activity on small weeds (1" to 3"). Adding a crop oil or surfactant to the herbicide mix improves their control. Weeds greater than 3", such as winter or early summer annuals, may be present at the time of early preplant applications. Abundant weed growth may require a burndown herbicide, such as glyphosate + 2,4-D, paraquat, or 2,4-D.