Why early-season weed control matters

The basic principles of weed control have not changed much over the last decades, although one step was prone to being overlooked in the early, extremely successful results with Roundup Ready Systems. Sensational weed control with POST glyphosate applications created the false narrative that no weeds at harvest spelled success. Some producers skipped the essential PRE application. The onset of glyphosate-resistant weed populations refocused weed control efforts.

📸: @Photodjo via Canva/Featuring an agronomist inspecting soybean crops in the field.

Principles of herbicide-driven weed control

The general principles of herbicide-driven weed control, stated in many different ways, typically include the following goals:

• Identify the target weeds.
• Include multiple effective sites of action.
• Utilize a good residual soil-active herbicide (PRE or PPI).
• Use a timely POST herbicide.
• Treat at appropriate weed height.
• Consider adding a residual soil-active herbicide to POST.

Early-season weed control (starting clean) has become a more important focus for many reasons. Many of today's prevalent broadleaf weed populations have developed, or at least been selected for herbicide resistance. As the number of POST herbicide options dwindles, the need to control these weeds with a soil-residual herbicide increases. Weed control of the seed or seedling stage remains the most effective timing for weed control.

📸: BASF Contributing Writer/Featuring Row middles on the left have no PRE herbicide and contain abundant weeds, while the two-row middles to the right have no weed growth in V2 soybeans in 30" rows.

How plants sense neighbors

Plants have evolved to compete with their neighbors, whether it be the same species (crop) or another (weeds). The ideal environment for a plant would be one that allowed for unrestricted root and shoot growth, taking advantage of light, water, and nutrients without competition. Plants can adapt to adjacent competition by allocating resources to root growth, shoot growth, apical dominance growth, or altering leaf size.

Above ground, the plant can sense nearby photosynthetic activity (another plant) because the red spectrum of incoming sunlight is absorbed while the far-red spectrum is reflected. Most common crops seek full light and "gobble up" incoming red light in the daylight. If a plant senses reflected far-red light, it will increase stem elongation and not allocate resources to lateral growth, preferring to grow taller to get away from competition. Plant responses to "light seeking" can be observed at the forest or field edge (if one exists) where trees reach out over the field in search of light.

Competition for resources

Weeds reduce crop yields by competing for resources that promote plant growth. The main assets fought over include water, nutrients, and light. Physical space is also important, but only in the way it prevents root or leaf expansion into a competitive area. Think of a single plant with full access to all the light, nutrition, and water it needs, then start chipping away at maximum yield as you introduce competition from weeds or other crop plants.

📸: BASF Contributing Writer/Featuring a side by side comparison of waterhemp at different growth stages. In a matter of two weeks a young stand of waterhemp can form a continuous canopy if left unchecked. Growth rates of 4" a week are not uncommon.

Light

The competition for light is key to driving photosynthesis. Plants alter their structure and form a canopy by allocating resources to lateral and vertical growth to intercept the maximum amount of sunshine. If a plant has close adjacent neighbors, it will speed up cell elongation in the apical meristem to grow vertically. In an open space, it will grow laterally and vertically simultaneously. Soybeans have an ability to "compensate" by branching into open spaces and demonstrating limited lateral growth in high populations or when surrounded by weeds or an overshadowing remnant of cover crops.

Nutrients

Weeds deplete nutrients in the soil and water, particularly where they have established a root system in advance of the arrival of the crop root system. The uptake of nutrients occurs at a similar rate as crops, so with row middles filled with weeds, those nutrients are immobilized even after the weeds are killed. High-N consumers like corn exhibit multiple nutrient deficiencies when surrounded by weeds, as their healthy uptake rate demands a wide exploration of roots into soil without plant competition.

Water

Weeds use water just as crops do. Their water use dries the soil, which also discourages roots from adjacent plants from exploring the same soil volume. Unchecked weeds that form a canopy extract water at a similar rate to a closed-crop canopy. Crops growing in weed-free row middles extend their root systems laterally and have access to more water and nutrition.

What about root proliferation?

Roots extend preferentially into soil that is not occupied by other roots. There is some evidence that root exudates may signal the occupation of a soil volume. As mentioned, soil drying also discourages root exploration into weedy row middles.

📸: @Photodjo via Canva/Featuring an agronomist standing in a soybean field with clean row crops.

So what are the consequences?

Waiting until POST application timing to control weeds has several disastrous effects on crop canopy formation, particularly in soybeans. The side-by-side photos below (courtesy of BASF legend Mark Storr) are a classic example of what can happen without early-season weed control. The consequences are profound and yield-limiting. The scenario depicted on the left, where no PRE was used, allowed weeds to populate row middles until the weeds were removed by an application of Engenia^® herbicide and glyphosate. The existence of the weeds for 30 days suppressed canopy development as the soybeans avoided expanding into the weed-infested middles.

The competition for water and nutrients also limited vertical growth, and a lighter green color indicates a likely lower N content. The soybeans on the right were enabled by a PRE application of Zidua^® PRO herbicide that kept the row middles clear for soybean foliar expansion and root exploitation of water and nutrients. There is already evident regrowth of grasses in the row middles of the scenario without a PRE application. Unclosed canopy, weed re-infestation, nutrient deficiencies, and reduced yield are all consequences of poor or lacking early-season weed control.

📸: BASF employee/Featuring soybean canopies with POST only (L) and PRE followed by POST (Courtesy Mark Storr, BASF).

A timely POST application will build on the work that a PRE application has already performed. Adding a soil residual will provide additional protection as the canopy closes.

Coming next: Weed species emergence progression.

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This content is being brought to you in partnership with Grow Smart^® Live and contributing guest authors. BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.