Growing Degree Days Through August 31st

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

Unless you planted corn later in June, and some people appear to have made that choice, cornfields across Delaware should be into the reproductive stages. The easiest reproductive stage to decipher is probably R5, which is the dent stage (Figure 1). As corn grains dry down, moisture loss causes a dent near the crown as the kernel shrinks a little. Prior to dent you must estimate liquid in the kernel to determine whether you are in blister, milk, or dough stages. A guide to grain fill stages can be found here: https://www.agry.purdue.edu/ext/corn/news/timeless/GrainFill.html.

The dent stage is typically reached at 2190 to 2450 growing degree days (GDD), which most corn planted in April has reached. The ear in figure 1 was planted in Georgetown on April 24^th, which is between 2196 and 2277 GDD. For the temperatures this ear has seen, the kernels look fairly good, with minimal aborted grains. Through July, temperatures were often above 87°F during the day and sometimes 72°F at night, which can cause the plant to stress during pollination and grain fill (Figure 2). Corn planted later in May could have seen some relief in the last week as kernels have pollinated and started to fill. Only scouting will tell how the weather has interacted with your planting dates.

Figure 1: Corn in dent stage (R5) planted on April 24^th in Georgetown.

Rainfall has continued to benefit the northern end of the state, with both Newark (21.5”) and Dover (18.8”) leading the state in total rainfall since April 14^th. Smaller rainfalls have contributed a little to soil moisture in Georgetown, Dagsboro and Delmar in the last week. However, dryland fields in the southern part of the state were hit hard during grain fill, and the results may show up in September. The southern end of the state has received about 5 inches less than the Newark region, so hopefully you kept the irrigation running.

Table 1: Accumulated growing degree-days based on planting dates through July 31^st

R1 = 1400 GDD, R5 (Dent) = 2190-2450, R6 (Blacklayer) = 2700

Figure 2: Statewide temperatures since April 1^st.

Figure 3: Statewide rainfall accumulation since April 14^th.