Herbicide program development: Long-term planning

December 20, 2018 11:03 AM
Blog Post

This is the final article of a four-part series originally posted in 2016 on using multiple, effective herbicide sites of action (herbicide groups) at effective rates as part of a long-term weed management system. Read part one , part two, and part three now.

After we’ve successfully implemented a herbicide program with multiple, effective herbicide sites of action at effective rates, we need to consider other tactics that may supplement the herbicide program. 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 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.

In order 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 during busy periods, but 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.

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 (Figure 1). The rapid canopy closure can be very 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.

Graph of the effect of soybean row spacing on Amaranthus spp. control
Figure 1. Effect of soybean row spacing on Amaranthus spp. control. Means followed by the same letter are not different at P≤0.05. Dr. Kevin Bradley, U of Missouri1.

Research has also shown that increased soybean seeding rate in narrow-rows may reduce late-season weed densities as well, resulting in less seed production and fewer additions to the seed bank (Figure 2). 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.

Graph of the effect of soybean row spacing on Amaranthus spp. contro
Figure 2. Effect of soybean planting population on Amaranthus spp. control. Means followed by the same letter are not different at P≤0.05. Dr. Kevin Bradley, U of Missouri1.

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.

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 of their acres, but 'precision' use of these practices on problem fields or areas within a field can provide a real 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 ½ 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 tillage2. 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 multi-year 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.

1Bradley, K. 2014. Waterhemp Management in Soybean [PowerPoint slides].

2DeVore, JD, JK Norsworthy, and KR Brye. 2013. Influence of Deep Tillage, a Rye Cover Crop, and Various Soybean Production Systems on Palmer Amaranth Emergence in Soybean. Weed Technology: 27(2), 263-270.


Meaghan Anderson Field Agronomist in Central Iowa

Meaghan Anderson is a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach. Educational programming is available for farmers, agribusinesses, pesticide applicators, certified crop advisors, and other individuals interested in...