Critical Weed-Free Period for Soybeans

As you strive for higher soybean yields, do you need to worry about the critical weed-free period in soybeans? The answer is yes, absolutely. Once you hit the critical weed-free period, rapid, irreversible yield loss occurs. So, when is the critical-weed free period for soybeans? The 1^st to 3^rd trifoliate (V1 to V3) is a conservative estimate according to Professors Dr. Clarence Swanton, University of Guelph, and Dr. Peter Sikkema, University of Guelph-Ridgetown Campus. It’s hard to predict when exactly this takes place in soybeans as it will vary by field, and by year, depending on three factors:

1. Weed emergence relative to crop emergence – this is the most critical factor in yield loss
2. Weed density
3. Weed species – generally, annual grasses impact yield less than annual broadleaf weeds at the same density

Soybeans sense weeds before they emerge

In essence, the critical weed-free period in soybeans is V1 to V3. That means if your soybean crop senses weeds during this time, you are losing yield. However, prior to V1, the presence of weeds still impacts soybean growth and development. Swanton’s research shows that soybeans sense weeds even before they emerge and will change their morphology in response. Changes in gene expression, decreased photosynthetic pigment content, nitrogen content in roots and leaves as well as oxidative stress levels, amongst other effects, have been detected due to early weed presence in soybeans. 

V1 is the beginning of observable yield decline, a “free-fall in yield,” according to Swanton. If your field is not clean of weeds by this point you could be losing 0.5 to 2 bushels per day for each day that weeds are not controlled. This means that the soybeans must not sense weeds by the time they reach V1. Applying a pre-emerge residual herbicide offers a way to manage risk with weed control, such as weather risks and time management. Pre-emerge soybean herbicides also offer more options in the way of active ingredients and spectrum of weed control. Then you still have a plan B, which is your post-emerge herbicide, to clean up escapes and second flushes.

Timing is everything

If you choose to skip a residual herbicide and opt for a post-emerge program, your herbicide should be applied early enough for the weeds to be dead by V1. The issue with this plan is that ideally your soybean crop doesn’t see weeds until it’s past the V3 stage to maximize yield potential. A new trifoliate emerges approximately every 5 to 7 days, so you’re counting on your post-emerge application to keep the weeds out for about 3 weeks or come in with a second post-emerge application within that timeframe. And if there are gaps in your post-emerge program, you might not have a plan B.

Another watch-out for growers in Western Canada is that many post-emergent products are only labelled for application at V1 or later. This provides even greater incentive to plan a pre-emerge application strategy for your soybean fields.

At V3, yield is set, and generally speaking, weeds that emerge after this time are not yield limiting. The objective of controlling weeds past V3 shifts from managing yield to managing harvestability and weed seed return.

Dr. Swanton’s key takeaways on weed management in soybeans are:

1. Timing is everything
2. Yield loss can be calculated by the day
3. Yield loss is irreversible

As you prepare for each growing season, make sure you have a solid plan in place to keep your soybeans weed-free until V3 to maximize your soybean yield potential.