By MSR&PC Research Director Sergio Cabello Leiva. 

Pre-emergence herbicides are not optional in Minnesota soybeans. They are the foundation of a working program. If you skip them or cut them back, you will fight weeds all season, especially resistant ones like waterhemp. 

Start clean and stay ahead 

Soybeans do not compete well early in the season. If weeds come up with the crop, the yield is already at risk before we consider a POST herbicide application (in most cases). A strong PRE herbicide keeps fields clean during those first critical weeks and gives you flexibility on your POST timing. This is even more important with early planting. When beans go in early, there is a longer gap before POST. That also means more competition from early emerging weeds, so residual herbicide selection becomes more critical. 

Know what your herbicides do 

Group 14 herbicides like Valor and Authority are strong on small broadleaf weeds and give you a quick start. They help control early flushes but do not last as long. 

Group 15 herbicides such as Zidua and Dual II Magnum provide longer residual control and are highly effective against small-seeded weeds such as waterhemp and lambsquarter. These herbicides are the backbone of most programs. 

But there are limits. Group 15 herbicides are largely ineffective against giant ragweed. In early-planted soybean fields with ragweed pressure, relying solely on Group 15 is a mistake. Residual programs need to specifically target that species. Premixes that include additional modes of action, especially Group 2 herbicides such as imazethapyr, chlorimuron or cloransulam provide more reliable control. Products that combine multiple residuals offer a broader spectrum and help manage resistance. 

Table 1. Waterhemp Response to PRE Herbicides
SOA	Active ingredient	Product	Min rate (lb ai/ac)	Product rate	Effectiveness
14	sulfentrazone	Spartan	0.125–0.156	4–5 fl oz	Good
14	flumioxazin	Valor EZ	0.063	2 fl oz	Good
14	fomesafen	Flexstar	0.235	1 pt	Fair
15	pyroxasulfone	Zidua SC	0.082	3 fl oz	Good
15	S-metolachlor	Dual II Magnum	1.15	1.2 pt	Good
15	dimethenamid-P	Outlook	0.656	14 fl oz	Good
15	acetochlor	Warrant	1.13	48 fl oz	Fair–Good
5	metribuzin	Tricor DF	0.28–0.38	6–8 oz	Rate dependent
Table Adapted from Waterhemp Research Summary: What we have learned from 5 years of chemical waterhemp control in soybean. Ryan DeWerff, Research Specialist, University of Wisconsin-Madison

Do not go too low on herbicide rates (consider your soil) 

Most of the time, using the lowest labeled rate for PRE herbicides often backfires. Residual herbicides are strongly influenced by soil: 

• Higher organic matter, higher CEC, and clay soils need higher rates, while coarse soil (sandy) needs lower rates 

• Lower rates run out faster and let weeds break through  

Better rates mean more consistent control, especially in challenging conditions. 

Watch the weather 

PRE herbicides need rain to work. About half to one inch within a week after application is ideal. Without it, control will be uneven. If that happens, be ready to move earlier with your POST pass. 

What MSR&PC checkoff-supported research shows 

Research supported by the Minnesota Soybean Research & Promotion Council and conducted by Next Gen Ag and the University of Minnesota strongly supports the use of PRE herbicides. Results show that these applications are critical for managing both waterhemp and giant ragweed, especially as part of a layered program. 

Build a system 

The most effective programs are built, not improvised. Start with a strong PRE using multiple residuals, ideally three effective sites of action to broaden control and manage resistance. Then follow with a timely POST application and be prepared for a second POST pass in early-planted soybeans if the season stretches and new flushes occur. 

A good PRE program makes everything else easier. A weak one makes everything harder. 

The Minnesota Soybean Research & Promotion Council does not endorse specific products or companies, although brand names, trademarks or service names may be mentioned. This publication is for educational purposes only and does not serve as a recommendation. Herbicide options, application rates and other decisions should be evaluated on a case-by-case basis by agronomists and farmers.  

Sergio Cabello Leiva can be reached at scabelloleiva@agmgmtsolutions.com.