Planting across Latham Country has been progressing at a rapid pace and it will not be long before post-emergence spraying will begin. When to spray, what to spray, should I spray . . . these are some of the questions growers need to consider before heading to the field. I believe “when to spray” is one of the most critical decisions a grower will make. Damaging or injuring a young plant can have lasting affects that may not be visible to the naked eye. Understanding growth stages and relating this to the labeled requirements is a key to successful growing season. Let’s take a look at corn first.

Labels typically refer to growth stages for application timing and the chart below is a good reference.

VE Stage – Corn emergence occurs when the coleoptiles reach and break through the soil surface. Normally, corn requires approximately 100-200 GDUs to emerge, which can be four to five days after planting. At this stage, growth is also taking place below the ground as the nodal root system begins to grow.

Emergence may occur as rapidly as four or five days after planting in warm moist soil, or may take three weeks or more in cool soils. A new leaf will appear about every three days during early growth, while later leaves developing during warmer conditions may appear in one to two days. Full season hybrids in the central Corn Belt typically can produce 21 to 22 leaves. Earlier maturing hybrids will produce fewer leaves.

Keep these numbers in mind as you plan out your season and prepare to spray your fields. Within a month after planting, a corn plant can go from the bag to V5-V7 if conditions are favorable.

Soybeans in a given field will not be in the same stage at the same time. When staging a field of soybeans, each V or R stage is defined when 50% or more of the plants in the field are in or beyond that stage. This makes it important to understand staging and development since not every plant in the field will be at the same stage when determining application timing. The chart below is a good reference for staging.

The general rule of thumb is to figure five days between growth stages in soybeans. The most important growth stage is R1 which is classified as one flower open at any node on the main stem. Soybean flowers are very sensitive and herbicide application should be avoided at this stage. R1 can begin before canopy closure and the temptation is there to make that final application before canopy closure. A good pre-plant program can help avoid the need for late season spraying and a few late escapes is not worth the consequences from spraying post-flower.

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