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Deep tillage: To turn or not to turn?

I am not suggesting every peanut, corn, cotton or soybean field needs to be plowed in 2024, but deep tillage once every 3-4 years can be a useful tool for managing troublesome weeds.

Eric Prostko

February 2, 2024

3 Min Read
Conventional tillage with a switch plow.Eric Prostko/UGA Extension

My North Cackalakian colleague and I have been conducting our 25th Annual Weed Science Revival since early January. 

If you have been to one of these county meetings, I hope you remember hearing us talk about “The Program,” which includes the following:

  1. Start clean (tillage/cover crops/herbicides).

  2. Planting in narrower row spacings appropriate for the crop.

  3. Apply multiple residual herbicides throughout the year.

  4. Be timelier with postemergence herbicide applications.

  5. Remove weed escapes before they produce seed. 

Of all of these, tillage, more specifically deep/inversion tillage, seems to cause the most heartburn for some folks.

I am certainly aware of the negative aspects of deep tillage including soil erosion, increased diesel fuel costs, equipment maintenance/repairs and time. These are very legitimate concerns for all who are interested in protecting the environment and keeping crop production costs in check. But every farmer should know by now that the use of herbicides exclusively for weed control in any crop is not sustainable long term.

So, how does deep tillage or plowing influence weed control? The short and simple answer is that many weeds do not have enough energy to emerge from the soil when seeds are buried below a certain depth.  Remember seed germination and weed emergence are two separate events. 

Related:6 tweaks to make large fields as weed free as small plots

Based upon a quick literature review, I have listed in Table 1 the weed seed burial depths at which emergence was ≤ 10%. That specific depth can vary by weed species and soil type. Generally, larger weed seeds need to be buried deeper than smaller weed seeds. Since deep tillage can also result in the upward movement of previously buried weed seed closer to the soil surface, it should not be implemented every year.

Prostko-Feb-2024--3.jpg

It is also very important to note that not all weeds respond to deep tillage. For example, purple moonflower or purple morning glory emergence from seed buried at a 6” depth was still > 40%.  Purple moonflower seed is relatively large in comparison to other weed seeds so plants developing from that seed have lots of energy to emerge from deeper burial depths (Figure 2).  

Weed seed longevity in the soil is a topic for another day. For now, just know that once weed seed is deposited to the soil, there is lots going on.  Dormancy, burial depth, microbial decomposition (bacteria, fungi), and predation (birds, rodents, insects, slugs) will all have an influence on weed seed longevity. 

Prostko_Feb_2024_-2.jpg

On the flip side, I know of at least one elite grower who has not tilled any of his cotton and peanut fields for ~23 years and he does not have major weed problems.  But, he is Johnny-on-the-spot when it comes to overall crop management. Thus, strip-till/cover crop systems can work with a little extra effort. 

Related:Conservation tillage works for these peanut farmers

Let me be very clear. I am not suggesting that every peanut, corn, cotton or soybean field in Georgia needs to be plowed in 2024. I am only bringing to your attention that deep tillage once every 3-4 years can be a useful tool for managing troublesome weed populations in fields where tillage has been absent for a while. 

As always, good weed hunting!

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