Agronomy Blog

Shade out the Rows

Canopy closure is critical since these weeds are very sensitive to shading. Waterhemp, in particular, prefers unfiltered sunlight for germination and growth, so accelerating canopy closure is necessary.  Two management strategies to help with fast canopy closure are: 

1. Early planting 
2. Narrow row soybeans. - A 15” row will canopy about 25 days sooner than a 30” row. 

In Iowa, narrow rows can offer a 5-10% yield advantage, which is a great benefit above and beyond weed control.  If switching to narrow rows, consult your Agronomy Field Advisor on a variety with a good disease package and low lodging scores for maximum success. If you are in wide rows, really think twice about cutting a pre-emergent herbicide pass.  The longer the canopy is open, the longer you need to keep the soil weed-free. If you’re sticking with a two-pass post system, timing is everything. And if your budget allows, consider including a residual product to add some residual holding power.

Cover Crops

Cover crops can be used to control and suppress weeds. Research has shown around a 40% reduction in Waterhemp emergence with the presence of a cereal rye cover crop and around a 50% reduction in Giant Ragweed biomass with approximately 1.7 tons/ac of cereal rye biomass. These weeds don’t like early competition from a cover crop.  Cover crops can be tricky, so ask a Liqui-Grow Agronomy Field Advisor how to best implement cover crops on your own farm! 

Tillage

Tillage has long been used as a method for weed control, based on the idea that burying weed seeds reduces their chances of germination. While deep tillage can be highly effective, it’s not always the best practice for every farmer. If deep tillage is used, it’s recommended to avoid further tillage for 4 to 5 years to keep the buried seeds from resurfacing and germinating. However, giant ragweed presents a unique challenge since it can germinate from depths of up to 4 inches below the soil surface, making most modern tillage practices ineffective against it. Additionally, spring tillage tends to stimulate germination of both giant ragweed and waterhemp, offering little benefit in terms of long-term control.

Herbicide Use

Lastly, chemical control is a critical piece of the puzzle. Over the past couple of years, tighter profit margins on soybeans have led many farmers to think about skipping residual or pre-emergence herbicides. 2025 was not the kind of year to skip pre-emergent herbicides because excessive rain made timely applications nearly impossible for those trying to implement a two-pass post-emergence plan. 

Products like glufosinate and 2,4-D can be effective against waterhemp and giant ragweed when the weeds are less than 3” to 6” tall. Utilizing a pre-emerge helps slow them down and gives you time to get back into the field, in case the weather isn’t cooperating. If a pre isn’t an option, use your spring tillage pass strategically. Minimize the time between tillage and planting, and follow up quickly with your first post-emerge spray. Horizontal tillage tools (like field cultivators) generally offer better weed control than vertical tillage.

One last, but essential point is that these weeds are becoming increasingly intelligent. To give you a sense of just how tough these weeds really are, waterhemp and giant ragweed both cross-pollinate, which helps them overcome these herbicides and creates resistance, such as ALS-resistant giant ragweed. These weeds can also regrow from each leaf, known as the leaf axis, making it even more challenging to maintain full control.  To fight back, use a pre-emergence herbicide with PPO inhibitors (Group 14) to help manage these populations. Giant ragweed has developed glyphosate resistance through two different mechanisms: a “non-rapid response” type that slowly turns chlorotic before recovering, and a “rapid response” type that shows necrosis on mature leaves but also bounces back.