Can tillage effectively control weeds

Growers must continuously evaluate input costs, as each growing season presents a new set of challenges on the journey toward reaching profitability. Recently, supply chain issues have not only raised the prices of some herbicides, but also certain herbicides are not even available. Growers are forced to find, evaluate, and implement alternative weed strategies. Thus, some growers have asked if tillage can be an effective weed control strategy. To help answer that question, let’s do some analysis.

Tillage can work to control weeds when weed seeds have germinated but are still young, and when soil moisture is right at that “Goldilocks” stage of just enough moisture to germinate weeds but not wet enough to cause compaction or provide weeds enough moisture to bounce back. This situation often requires growers to delay planting a week to achieve adequate soil temperature to germinate weeds. Tillage can also help prevent future weed germination by burying weed seeds.

This is where it gets tricky. For simplicity, let’s list out the conditions or situations that rule out tillage as a weed control option:

• Soil is too wet, causing compaction that causes yield loss for multiple years
• Soil is too dry, slowing weed seed germination
• Soil is too cold, preventing weed seed germination
• After the cash crop has been planted in narrow rows
• After the cash crop reaches around 3 feet in wide rows

If the intent is to control weeds, tillage can offer some short term gains, but with long term losses. For example:

• If tillage has been used in the past to bury weed seeds, then another tillage pass to bury weed seeds on the soil surface will simply bring back up the previously buried weed seeds. Many weeds such as Marestail will last in the soil for 15+ years while maintaining viability. So with the first sign of sunlight, they germinate. 
• Tillage creates loose, bare soil; often at a time of year when the cash crop has not canopied. This creates the perfect seedbed for weed seeds to land and germinate, creating extensive weed issues later in the season.
• Tillage destroys the habitat for many beneficial insects who consume weed seeds. The ground beetle, for example, is a beneficial insect that consumes large quantities of small-size weed seeds such as lambsquarters, while ignoring cash crop seed planted into the ground.

Using tillage, even in perfect conditions listed above, will not simply terminate the growing weeds. There are a number of consequences to tillage, and not just the obvious erosion, significant fuel, labor and equipment costs. Here are three more significant consequences to tillage as a weed management strategy:

• Compaction, either caused by the process of tillage in wet soils or in decimating soil structure, allowing the next rain to compact the soil with its impact. This compaction will rob up to 15% of yield for the subsequent few years and still rob up to 5% of yields for many years into the future. That is really expensive.
• Reduced soil structure, leading to reduced water holding capacity, reduced infiltration, and long term yield loss. Not to mention, reduced field accessibility. Again, expensive.
• Erosion, even though I mentioned erosion already, the details are important. Erosion, even at lower rates, removes the best, most fertile and densest organic matter soil.

Numerous options exist that can have more success controlling weeds without the negative consequences. Increasing crop rotation and adding cover crops are two highly effective strategies that improve soil productivity while opening up opportunities to generate revenue from carbon credits. 

While tillage can solve weed issues in very specific situations, is the expense worth it? I’d lean towards “no,” especially when alternative strategies exist that provide a multitude of other benefits to your farm.

Ryan Stockwell, Grower Engagement Senior Manager and Wisconsin farmer