Editor’s note: This is the last of four-part series on weed management. This fourth article emphasizes long-term planning to achieve effective weed management and avoid development of herbicide resistance by weeds.

By Bob Hartzler and Meaghan Anderson

After you’ve successfully implemented your herbicide program using products that have multiple, effective herbicide sites of action and you apply those products at effective rates, you need to consider other tactics you can use to supplement and assist the herbicides.

The next “silver bullet” is not coming, so we need to protect existing herbicide technologies by using other weed management tactics to prolong herbicide efficacy.

The following suggestions in this fourth and final article in our series will help improve and strengthen your weed management strategy:

Prevent new weeds from getting established. The first step to prolonging herbicide efficacy is to prevent new weed species or new resistant biotypes from getting established in crop fields. This requires proper identification of Palmer amaranth and other emerging threats to crop production; management of important weeds in fencerows, waterways, and other areas adjacent to crop fields; and sanitation when moving equipment from infested fields to non-infested fields. It also requires a good understanding of weed biology to know the weaknesses of existing and emerging threats.

To prevent the spread of herbicide-resistant plants from one field to another, farmers should consider altering planting and harvest order or cleaning equipment prior to leaving an infested field. This requires time to do it during busy periods, but it can save significant dollars in the long run. Once a new weed or new resistance moves into a field, it is much more difficult to manage and may be impossible to eradicate.

Use cultural practices for controlling weeds. Cultural tactics, such as altered planting dates, narrow row spacing, increased soybean seeding rates and cover crops provide opportunities to enhance crop competition or reduce competitiveness of weeds. In fields with intense weed pressure, delayed planting allows early-emerging weeds to be controlled prior to crop planting. This provides a larger window for application of preemergence herbicides, and allows crop plants to emerge faster and compete better with weeds.

Narrow row spacing in soybeans results in earlier canopy closure and reduces late-season weed escapes. The rapid canopy closure can be helpful in managing waterhemp, a notoriously late-emerging weed. Notice that average late-season waterhemp density was the same in drilled and 15-inch-row soybeans, but the late-season waterhemp density was significantly greater in 30-inch rows.

Increase seeding rates in narrow rows. Research has also shown that increased soybean seeding rates in narrow rows may reduce late-season weed densities, resulting in less seed production and fewer additions to the seedbank. Late-season waterhemp density was the same in plots seeded with 160,000, 190,000 and 220,000 plants per acre. However, those densities were significantly lower than the late-season waterhemp density in the plots seeded with 130,000 plants per acre.

Cover crops must accumulate significant biomass to consistently suppress weeds; however, at lower levels of biomass, they may delay peak weed emergence. This delay in emergence will result in less competitive weeds that, combined with other tactics like narrow row spacing and an effective herbicide program, may improve weed management.

Use covers, mechanical tactics and tillage. Mechanical tactics like preplant tillage, inter-row cultivation or hand weeding may be feasible for farmers. It is unlikely that farmers can use cultivation or hand weeding on all their acres, but precision use of these practices on problem fields or areas within a field can provide a benefit in managing herbicide-resistant weeds. Tillage does not come without a downside. It increases erosion risks, reduces soil health, and requires significant time and money to implement.

Tillage moves newly produced weed seed from the soil surface deeper into the soil profile. Waterhemp seeds germinate best when they are within the upper half inch of soil due to their small seed. After a failure of weed control, deep tillage is an option to bury seed produced by those weeds. An Arkansas study found that deep tillage alone resulted in an 81% decrease in Palmer amaranth emergence over the two following years when compared to no tillage. This should be a last resort because further deep tillage passes will bring old problems back to light.

After crop emergence, inter-row cultivation and hand weeding may be the only options available for resistant weeds. Inter-row cultivation is rarely used today but will likely become more common in the future due to multiple-herbicide resistant waterhemp. It may not be necessary or practical across all acres or all years, but is a viable option for weed management in specific problem areas or in years when other weed control tactics fail.

Hand weeding is an expensive and time-consuming option but is a reality in other states where herbicide-resistant weeds are a common problem. Despite the time and expense, removing weeds will prevent millions of new seed from entering the seedbank.

Weed management will likely continue to rely heavily on herbicides but serious consideration of alternative tactics is important to preserve the efficacy of available herbicides. This will include planning herbicide programs to include multiple, herbicide groups at effective rates. It’s important to consider a multiyear plan to avoid falling into the habit of using the same herbicide program over multiple years.

It is equally important to consider what alternative tactics fit into your specific weed management system — tillage, narrow row soybeans, increased seeding rates, hand weeding or other techniques. These will become even more important in the future as waterhemp and other weeds continue to stack up resistances.

Previous articles emphasized using multiple, effective herbicide sites of action at effective herbicide application rates as part of a long-term weed management system. Read Part 1, Part 2 and Part 3.

Hartzler is an Iowa State University Extension weed control specialist; Anderson is an ISU Extension field agronomist. ISU Extension, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.