Escalating herbicide resistance and unpredictable weather patterns are reshaping weed management strategies across U.S. row crop systems. Agronomists increasingly recommend a planned two-pass herbicide program to maintain clean fields, preserve yield potential, and slow the spread of resistant weed populations.

Kurtis Pilkington, customer solutions agronomist with Bayer CropScience, emphasizes that keeping fields weed-free throughout the entire growing season is essential - particularly when dealing with resistant species such as waterhemp, which continues emerging well into late summer.

A typical two-pass system begins with a pre-emergent residual herbicide, applied pre-plant or early post-emergence, designed to provide a clean start and residual protection. This is followed by a post-emergent application later in the season to control weed escapes and late-emerging flushes.

The strategy addresses two major production risks: herbicide resistance and weather variability. Pre-emergent herbicides depend on sufficient moisture for activation. In dry conditions, performance may decline, allowing early-season weed competition during the crop's critical weed-free period.

For corn, that period typically extends from emergence through the six-leaf stage. For soybeans, it runs from emergence to the third trifoliate stage. Weed competition during these windows can significantly reduce yields, impacting overall farm revenue and return on investment.

A second pass provides insurance against these uncertainties. By incorporating multiple modes of action across application timings, growers strengthen control and reduce the selection pressure that accelerates resistance development. This approach aligns with integrated pest management (IPM) principles and supports long-term sustainable agriculture goals.

University research supports the agronomic and financial benefits. Trials conducted by the University of Guelph in glyphosate-resistant corn demonstrated that a two-pass system - residual pre-emergence followed by late post-emergence - delivered the most effective and profitable weed management. Likewise, a review of 61 corn herbicide trials at the University of Missouri found that two-pass programs produced the highest corn yields 67% of the time.

Beyond immediate yield gains, a two-pass system plays a critical role in weed seedbank management. Preventing late-season weeds from setting seed reduces pressure in future growing seasons, lowering long-term control costs and protecting soil productivity.

Harvest efficiency is another advantage. Eliminating green weed stems improves combine performance and reduces the likelihood of spreading weed seed via equipment - an important operational consideration within the broader agricultural supply chain.

Waterhemp illustrates why a single-pass strategy often falls short. Its extended germination window means new plants can emerge after an initial application loses effectiveness. A planned second pass targets these later flushes before they compete aggressively for nutrients, light, and moisture.

Still, flexibility remains essential. Agronomists recommend thorough field scouting to assess weed spectrum and resistance patterns before finalizing product selection. In some seasons, if the first application achieves strong control, a second pass may be adjusted or optimized based on field conditions.

Producers must also carefully review herbicide labels to confirm proper application timing, tank-mix compatibility, and the need for adjuvants or surfactants to maximize performance and maintain crop safety.

While two-pass programs may involve higher upfront input costs, the broader economic analysis often favors the investment. In a period marked by volatile commodity prices, rising production expenses, and tighter margins, protecting yield potential remains one of the most reliable ways to sustain farm profitability.

As resistance intensifies and climate variability continues to challenge field performance, the two-pass herbicide strategy is emerging as a cornerstone of modern U.S. crop management - balancing agronomic effectiveness with long-term stewardship.