Once corn reaches R5, there can be a preconceived notion that it’s home free if any stress occurs. People think yield is made at that point. However, the truth is that a significant amount of grain yield is still being determined.

The R5 stage in corn occurs 30 to 40 days following silking. This stage is defined when nearly all kernels are dented at the crown and hard starch or solid endosperm is forming. The dent at the kernel top happens when starch content increases and moisture content decreases.

The R5 growth stage of corn takes 30 to 33 calendar days and 337 to 360 growing degree days to complete, and can depend on hybrid maturity. Corn also advances much more quickly through the beginning stages of R5 compared to the later stages of R5.

Understanding and staging the progression of the R5 growth stage in corn is determined through the identification of the milk line that separates solid endosperm and liquid endosperm. As the plant approaches maturity, more moisture is lost from the kernel, and the milk line progresses toward the cob.

The milk line is identified in the graphic accompanying this article. This ear and kernels are shown at the beginning of the R5 stage. Starch that the plant produced accumulates above the milk line. The portion of the kernel below the milk line nearest to the cob is still liquid endosperm, which is not yet converted into starch. As the R5 stage progresses toward R6, or black layer, the milk line moves down the kernel closer to the cob.

Key questions

Identifying the milk line is important to answer three questions. Here is a closer look:

What is approximate kernel moisture? When R5 begins, kernel moisture is typically at about 60%. By the time it reaches a quarter milk line within the kernel, moisture drops to around 52%. If the milk line is halfway to the kernel tip, grain moisture is about 40%. By three-quarters milk line, the average is around 37%. By physiological maturity or black layer, it’s around 35%. Actual moisture content at black layer can vary with hybrids and environment from about 32% to nearly 40%.

What is approximate kernel dry matter accumulation? Kernel dry matter percentage as a percent of total dry weight starts out at about 45% when R5 begins. It moves to 65%, 90%, 97% and 100% at a quarter milk line, half milk line, three-quarters milk line and black layer, respectively.

How much time is left before corn reaches maturity? The plant typically takes only about three days to go from the beginning of R5 to the quarter milk line stage. It takes an additional six days to go from quarter to half milk line. The closer to black layer, the slower the progress. Going from half to three-quarters milk line requires about 10 days, and from three quarters milk line to black layer takes a full two weeks. Overall, from beginning of R5 to maturity is about 33 days.

Actual time may vary depending upon the hybrid and the environment. However, this is a guide that lets you know what to expect. If you irrigate, milk line progression can also help gauge when irrigation should be terminated.

Quinn is a Purdue University Extension corn specialist. Email him at [email protected].