Explaining poor weed control

The classic explanation of “too hot, too cold, too wet, too dry, too bad” surprisingly doesn’t cover all of the possible factors for why a herbicide doesn’t deliver 100% satisfaction. A recent field walk illustrated why physical obstruction and weed density are two factors that are every bit as influential as environmental conditions when it comes to herbicide performance.

Scenario # 1 (Two minutes for obstruction)

Green pigweed is at the 2-4 leaf stage in a soybean crop just approaching the 1st trifoliate stage. The field is sprayed with Reflex + Turbcharge. Control at first glance is excellent but upon closer inspection some green leaf tissue is noticed near a corn husk. When the corn husk is lifted it reveals a number nicely shield and therefore unharmed pigweed.

pigweed escaping reflex

Scenario # 2, Density matters

In a demonstration plot, velvetleaf was at an incredibly high population of over 400 plants/m2. The majority of herbicides failed to deliver adequate control. The image below illustrates how an otherwise effective contact herbicide can look less then stellar because the density of weeds are so high that they end of obstructing spray droplets from penetrating the plants below. Yet another reasons why minimizing weed seed return is important for successful weed management.velvetleaf density and impact on control